Author name code: freytag
ADS astronomy entries on 2022-09-14
author:"Freytag, Bernd" 
------------------------------------------------------------------------  

Title: Probing red supergiant dynamics through photo-center
    displacements measured by Gaia
Authors: Chiavassa, A.; Kudritzki, R.; Davies, B.; Freytag, B.;
   de Mink, S. E.
Bibcode: 2022A&A...661L...1C
Altcode: 2022arXiv220505156C
  Context. Red supergiant (RSGs) are cool massive stars in a late phase of
  their evolution when the stellar envelope becomes fully convective. They
  are the brightest stars in the universe at infrared light and can be
  detected in galaxies far beyond the Local Group, allowing for accurate
  determination of chemical composition of galaxies. The study of their
  physical properties is extremely important for various phenomena
  including the final fate of massive stars as type II supernovae and
  gravitational wave progenitors. <BR /> Aims: We explore the well-studied
  nearby young stellar cluster χ Per, which contains a relatively large
  population of RSG stars. Using Gaia EDR3 data, we find the distance
  of the cluster (d = 2.260 ± 0.020 kpc) from blue main sequence stars
  and compare with RSG parallax measurements analysing the parallax
  uncertainties of both groups. We then investigate the variability of
  the convection-related surface structure as a source for parallax
  measurement uncertainty. <BR /> Methods: We use state-of-the-art
  three-dimensional radiative hydrodynamics simulations of convection
  with CO5BOLD and the post-processing radiative transfer code OPTIM3D to
  compute intensity maps in the Gaia G photometric system. We calculate
  the variabiltiy, as a function of time, of the intensity-weighted mean
  (or the photo-center) from the synthetic maps. We then select the RSG
  stars in the cluster and compare their uncertainty on parallaxes to the
  predictions of photocentre displacements. <BR /> Results: The synthetic
  maps of RSG show extremely irregular and temporal variable surfaces
  due to convection-related dynamics. Consequentially, the position
  of the photo-center varies during Gaia measurements between 0.033
  and 0.130 AU (≈1 to ≈5% of the corresponding simulation stellar
  radius). We argue that the variability of the convection-related
  surface structures accounts for a substantial part of the Gaia EDR3
  parallax error of the RSG sample of χ Per. <BR /> Conclusions:
  We suggest that the variation of the uncertainty on Gaia parallax
  could be exploited quantitatively using appropriate RHD simulations
  to extract, in a unique way, important information about the stellar
  dynamics and parameters of RSG stars. <P />Movies are available at <A
  href="https://www.aanda.org/10.1051/0004-6361/202243568/olm">https://www.aanda.org</A>

Title: Explaining the winds of AGB stars: Recent progress
Authors: Höfner, Susanne; Freytag, Bernd
Bibcode: 2022arXiv220409728H
Altcode:
  The winds observed around asymptotic giant branch (AGB) stars are
  generally attributed to radiation pressure on dust, which is formed
  in the extended dynamical atmospheres of these pulsating, strongly
  convective stars. Current radiation-hydrodynamical models can explain
  many of the observed features, and they are on the brink of delivering
  a predictive theory of mass loss. This review summarizes recent
  results and ongoing work on winds of AGB stars, discussing critical
  ingredients of the driving mechanism, and first results of global 3D
  RHD star-and-wind-in-a-box simulations. With such models it becomes
  possible to follow the flow of matter, in full 3D geometry, all the
  way from the turbulent, pulsating interior of an AGB star, through
  its atmosphere and dust formation zone into the region where the wind
  is accelerated by radiation pressure on dust. Advanced instruments,
  which can resolve the stellar atmospheres, where the winds originate,
  provide essential data for testing the models.

Title: The extended atmosphere and circumstellar environment of the
    cool evolved star VX Sagittarii as seen by MATISSE
Authors: Chiavassa, A.; Kravchenko, K.; Montargès, M.; Millour, F.;
   Matter, A.; Freytag, B.; Wittkowski, M.; Hocdé, V.; Cruzalèbes, P.;
   Allouche, F.; Lopez, B.; Lagarde, S.; Petrov, R. G.; Meilland, A.;
   Robbe-Dubois, S.; Hofmann, K. -H.; Weigelt, G.; Berio, P.; Bendjoya,
   P.; Bettonvil, F.; Domiciano de Souza, A.; Heininger, M.; Henning, Th.;
   Isbell, J. W.; Jaffe, W.; Labadie, L.; Lehmitz, M.; Meisenheimer, K.;
   Soulain, A.; Varga, J.; Augereau, J. -C.; van Boekel, R.; Burtscher,
   L.; Danchi, W. C.; Dominik, C.; Drevon, J.; Gámez Rosas, V.;
   Hogerheijde, M. R.; Hron, J.; Klarmann, L.; Kokoulina, E.; Lagadec,
   E.; Leftley, J.; Mosoni, L.; Nardetto, N.; Paladini, C.; Pantin, E.;
   Schertl, D.; Stee, P.; Szabados, L.; Waters, R.; Wolf, S.; Yoffe, G.
Bibcode: 2022A&A...658A.185C
Altcode: 2021arXiv211210695C
  Context. VX Sgr is a cool, evolved, and luminous red star whose
  stellar parameters are difficult to determine, which affects
  its classification. <BR /> Aims: We aim to spatially resolve the
  photospheric extent as well as the circumstellar environment. <BR
  /> Methods: We used interferometric observations obtained with
  the MATISSE instrument in the L (3-4 μm), M (4.5-5 μm), and N
  (8-13 μm) bands. We reconstructed monochromatic images using
  the MIRA software. We used 3D radiation-hydrodynamics simulations
  carried out with CO<SUP>5</SUP>BOLD and a uniform disc model to
  estimate the apparent diameter and interpret the stellar surface
  structures. Moreover, we employed the radiative transfer codes OPTIM3D
  and RADMC3D to compute the spectral energy distribution for the L,
  M, and N bands, respectively. <BR /> Results: MATISSE observations
  unveil, for the first time, the morphology of VX Sgr across the L, M,
  and N bands. The reconstructed images show a complex morphology with
  brighter areas whose characteristics depend on the wavelength probed. We
  measured the angular diameter as a function of the wavelength and
  showed that the photospheric extent in the L and M bands depends on the
  opacity through the atmosphere. In addition to this, we also concluded
  that the observed photospheric inhomogeneities can be interpreted
  as convection-related surface structures. The comparison in the N
  band yielded a qualitative agreement between the N-band spectrum
  and simple dust radiative transfer simulations. However, it is not
  possible to firmly conclude on the interpretation of the current
  data because of the difficulty in constraing the model parameters
  using the limited accuracy of our absolute flux calibration. <BR />
  Conclusions: MATISSE observations and the derived reconstructed images
  unveil the appearance of VX Sgr's stellar surface and circumstellar
  environment across a very large spectral domain for the first time. <P
  />Based on the observations made with VLTI-ESO Paranal, Chile under
  the programme IDs 0103.D-0153(D, E, G). The data are available at <A
  href="http://oidb.jmmc.fr/index.html">oidb.jmmc.fr</A>

Title: Atmosphere of Betelgeuse before and during the Great Dimming
    event revealed by tomography
Authors: Kravchenko, K.; Jorissen, A.; Van Eck, S.; Merle, T.;
   Chiavassa, A.; Paladini, C.; Freytag, B.; Plez, B.; Montargès, M.;
   Van Winckel, H.
Bibcode: 2021A&A...650L..17K
Altcode: 2021arXiv210408105K
  Context. Despite being the best studied red supergiant star in our
  Galaxy, the physics behind the photometric variability and mass loss
  of Betelgeuse is poorly understood. Moreover, recently the star has
  experienced an unusual fading with its visual magnitude reaching a
  historical minimum. The nature of this event was investigated by several
  studies where mechanisms, such as episodic mass loss and the presence
  of dark spots in the photosphere, were invoked. <BR /> Aims: We aim
  to relate the atmospheric dynamics of Betelgeuse to its photometric
  variability, with the main focus on the dimming event. <BR /> Methods:
  We used the tomographic method which allowed us to probe different
  depths in the stellar atmosphere and to recover the corresponding
  disk-averaged velocity field. The method was applied to a series of
  high-resolution HERMES observations of Betelgeuse. Variations in the
  velocity field were then compared with photometric and spectroscopic
  variations. <BR /> Results: The tomographic method reveals that the
  succession of two shocks along our line-of-sight (in February 2018 and
  January 2019), the second one amplifying the effect of the first one,
  combined with underlying convection and/or outward motion present at
  this phase of the 400 d pulsation cycle, produced a rapid expansion of
  a portion of the atmosphere of Betelgeuse and an outflow between October
  2019 and February 2020. This resulted in a sudden increase in molecular
  opacity in the cooler upper atmosphere of Betelgeuse and, thus, in the
  observed unusual decrease of the star's brightness. <P />The reduced
  HERMES spectra are only available at the CDS via anonymous ftp to <A
  href="http://cdsarc.u-strasbg.fr/">cdsarc.u-strasbg.fr</A>
  (ftp://130.79.128.5) or via <A
  href="http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/650/L17">http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/650/L17</A>

Title: Spatially resolved spectroscopy across stellar surfaces. IV. F,
G, and K-stars: Synthetic 3D spectra at hyper-high resolution
Authors: Dravins, Dainis; Ludwig, Hans-Günter; Freytag, Bernd
Bibcode: 2021A&A...649A..16D
Altcode: 2021arXiv210303880D
  Context. High-precision stellar analyses require hydrodynamic 3D
  modeling. Such models predict changes across stellar disks of spectral
  line shapes, asymmetries, and wavelength shifts. For testing models in
  stars other than the Sun, spatially resolved observations are feasible
  from differential spectroscopy during exoplanet transits, retrieving
  spectra of those stellar surface segments that successively become
  hidden behind the transiting planet, as demonstrated in Papers I, II,
  and III. <BR /> Aims: Synthetic high-resolution spectra over extended
  spectral regions are now available from 3D models. Similar to other ab
  initio simulations in astrophysics, these data contain patterns that
  have not been specifically modeled but may be revealed after analyses
  to be analogous to those of a large volume of observations. <BR />
  Methods: From five 3D models spanning T<SUB>eff</SUB> = 3964-6726 K
  (spectral types ~K8 V-F3 V), synthetic spectra at hyper-high resolution
  (λ/Δλ &gt;1 000 000) were analyzed. Selected Fe I and Fe II lines at
  various positions across stellar disks were searched for characteristic
  patterns between different types of lines in the same star and for
  similar lines between different stars. <BR /> Results: Spectral-line
  patterns are identified for representative photospheric lines of
  different strengths, excitation potentials, and ionization levels,
  thereby encoding the hydrodynamic 3D structure. Line profiles and
  bisectors are shown for various stars at different positions across
  stellar disks. Absolute convective wavelength shifts are obtained
  as differences to 1D models, where such shifts do not occur. <BR />
  Conclusions: Observable relationships for line properties are retrieved
  from realistically complex synthetic spectra. Such patterns may also
  test very detailed 3D modeling, including non-LTE effects. While present
  results are obtained at hyper-high spectral resolution, the subsequent
  Paper V examines their practical observability at realistically lower
  resolutions, and in the presence of noise.

Title: Spatially resolved spectroscopy across stellar
surfaces. V. Observational prospects: toward Earth-like exoplanet
    detection
Authors: Dravins, Dainis; Ludwig, Hans-Günter; Freytag, Bernd
Bibcode: 2021A&A...649A..17D
Altcode: 2021arXiv210304996D
  Context. High-precision stellar analyses require hydrodynamic 3D
  modeling. Testing such models is feasible by retrieving spectral line
  shapes across stellar disks, using differential spectroscopy during
  exoplanet transits. Observations were presented in Papers I, II, and
  III, while Paper IV explored synthetic data at hyper-high spectral
  resolution for different classes of stars, identifying characteristic
  patterns for Fe I and Fe II lines. <BR /> Aims: Anticipating future
  observations, the observability of patterns among photospheric lines
  of different strength, excitation potential and ionization level are
  examined from synthetic spectra, as observed at ordinary spectral
  resolutions and at different levels of noise. Time variability in 3D
  atmospheres induces changes in spectral-line parameters, some of which
  are correlated. An adequate calibration could identify proxies for
  the jitter in apparent radial velocity to enable adjustments to actual
  stellar radial motion. <BR /> Methods: We used spectral-line patterns
  identified in synthetic spectra at hyper-high resolution in Paper IV
  from 3D models spanning T<SUB>eff</SUB> = 3964-6726 K (spectral types
  ~K8 V-F3 V) to simulate practically observable signals at different
  stellar disk positions at various lower spectral resolutions, down
  to λ/Δλ = 75 000. We also examined the center-to-limb temporal
  variability. <BR /> Results: Recovery of spatially resolved line
  profiles with fitted widths and depths is shown for various noise
  levels, with gradual degradation at successively lower spectral
  resolutions. Signals during exoplanet transit are simulated. In
  addition to Rossiter-McLaughlin type signatures in apparent radial
  velocity, analogous effects are shown for line depths and widths. In
  a solar model, temporal variability in line profiles and apparent
  radial velocity shows correlations between jittering in apparent
  radial velocity and fluctuations in line depth. <BR /> Conclusions:
  Spatially resolved spectroscopy using exoplanet transits is feasible
  for main-sequence stars. Overall line parameters of width, depth and
  wavelength position can be retrieved already with moderate efforts,
  but a very good signal-to-noise ratio is required to reveal the more
  subtle signatures between subgroups of spectral lines, where finer
  details of atmospheric structure are encoded. Fluctuations in line depth
  correlate with those in wavelength, and because both can be measured
  from the ground, searches for low-mass exoplanets should explore these
  to adjust apparent radial velocities to actual stellar motion.

Title: Horizontal spreading of planetary debris accreted by white
    dwarfs
Authors: Cunningham, Tim; Tremblay, Pier-Emmanuel; Bauer, Evan B.;
   Toloza, Odette; Cukanovaite, Elena; Koester, Detlev; Farihi, Jay;
   Freytag, Bernd; Gänsicke, Boris T.; Ludwig, Hans-Günter; Veras,
   Dimitri
Bibcode: 2021MNRAS.503.1646C
Altcode: 2021arXiv210209564C; 2021MNRAS.tmp..586C
  White dwarfs with metal-polluted atmospheres have been studied widely
  in the context of the accretion of rocky debris from evolved planetary
  systems. One open question is the geometry of accretion and how
  material arrives and mixes in the white dwarf surface layers. Using the
  three-dimensional (3D) radiation hydrodynamics code CO<SUP>5</SUP>BOLD,
  we present the first transport coefficients in degenerate star
  atmospheres that describe the advection-diffusion of a passive scalar
  across the surface plane. We couple newly derived horizontal diffusion
  coefficients with previously published vertical diffusion coefficients
  to provide theoretical constraints on surface spreading of metals in
  white dwarfs. Our grid of 3D simulations probes the vast majority of
  the parameter space of convective white dwarfs, with pure-hydrogen
  atmospheres in the effective temperature range of 6000-18 000 K and
  pure-helium atmospheres in the range of 12 000-34 000 K. Our results
  suggest that warm hydrogen-rich atmospheres (DA; ${\gtrsim} 13\, 000$
  K) and helium-rich atmospheres (DB and DBA; ${\gtrsim} 30\, 000$ K) are
  unable to efficiently spread the accreted metals across their surface,
  regardless of the time dependence of accretion. This result may be at
  odds with the current non-detection of surface abundance variations in
  white dwarfs with debris discs. For cooler hydrogen- and helium-rich
  atmospheres, we predict a largely homogeneous distribution of metals
  across the surface within a vertical diffusion time-scale. This is
  typically less than 0.1 per cent of disc lifetime estimates, a quantity
  that is revisited in this paper using the overshoot results. These
  results have relevance for studies of the bulk composition of evolved
  planetary systems and models of accretion disc physics.

Title: Investigating mass loss from RSG and AGB stars using the new
    VLTI-MATISSE imaging instrument
Authors: Wittkowski, Markus; Chiavassa, Andrea; Baron, Fabien; Freytag,
   Bernd; Höfner, Susanne; Paladini, Claudia
Bibcode: 2021csss.confE.310W
Altcode:
  It is currently an open question in stellar astrophysics which physical
  processes initiate the mass loss of red supergiants. Observations of
  Betelgeuse during its recent great dimming event in 2019/2020 suggested
  a discrete highly localised mass ejection event, possibly connected
  to photospheric motion caused by a stochastic occurence of an extreme
  convection cell and possibly enhanced by pulsation (Dupree et al. 2020,
  Harper et al. 2020, Montarges et al., submitted), which may also explain
  the mass-loss history of the red hypergiant VY CMa, and of RSGs in
  general (Humphreys et al. 2020). There is indication that corundum,
  metallic iron, or other large transparent grains auch as Ca-Al-rich
  silicates may be present as close as down to about 2 stellar radii,
  which may serve as seeds for Mg-Fe-rich silicates at larger radii
  (Gail 2020). Here, we image the extended molecular layers and inner
  dust shell of the red supergaint AH Scorpii (AH Sco), which is known
  to exhibit a strong and narrow classical 9.7 mu silicate feature. We
  use the newly available infrared beam combiner MATISSE at the Very
  Large Telescope Interferometer (VLTI) to image AH Sco in the L band
  with a spectral resolution of 506, focusing on the SiO (2-0) bandhead,
  as well as in the N band focusing on the silicate dust. Our spatial
  resolution ranges from 3 mas at 4 mu to 10 mas at 12.5 mu. We also
  provide an outlook on comparing the levitation of the atmospheres and
  the dust condensation sequence between RSG and AGB stars, using the
  same observational setup for the Mira star R Aqr.

Title: 3D spectroscopic analysis of helium-line white dwarfs
Authors: Cukanovaite, Elena; Tremblay, Pier-Emmanuel; Bergeron,
   Pierre; Freytag, Bernd; Ludwig, Hans-Günter; Steffen, Matthias
Bibcode: 2021MNRAS.501.5274C
Altcode: 2020arXiv201112693C; 2020MNRAS.tmp.3465C
  In this paper, we present corrections to the spectroscopic parameters
  of DB and DBA white dwarfs with -10.0 ≤ log (H/He) ≤ -2.0, 7.5
  ≤ log g ≤ 9.0, and $12\, 000$ ≲ T<SUB>eff</SUB> $\lesssim 34\,
  000\,\mathrm{ K}$ , based on 282 3D atmospheric models calculated with
  the CO<SUP>5</SUP>BOLD radiation-hydrodynamics code. These corrections
  arise due to a better physical treatment of convective energy transport
  in 3D models when compared to the previously available 1D model
  atmospheres. By applying the corrections to an existing Sloan Digital
  Sky Survey (SDSS) sample of DB and DBA white dwarfs, we find significant
  corrections both for effective temperature and surface gravity. The
  3D log g corrections are most significant for T<SUB>eff</SUB> ≲
  18 000 K, reaching up to -0.20 dex at log g = 8.0. However, in this
  low effective temperature range, the surface gravity determined from
  the spectroscopic technique can also be significantly affected by
  the treatment of the neutral van der Waals line broadening of helium
  and by non-ideal effects due to the perturbation of helium by neutral
  atoms. Thus, by removing uncertainties due to 1D convection, our work
  showcases the need for improved description of microphysics for DB
  and DBA model atmospheres. Overall, we find that our 3D spectroscopic
  parameters for the SDSS sample are generally in agreement with Gaia
  Data Release 2 absolute fluxes within 1σ-3σ for individual white
  dwarfs. By comparing our results to DA white dwarfs, we determine
  that the precision and accuracy of DB/DBA atmospheric models are
  similar. For ease of user application of the correction functions,
  we provide an example PYTHON code.

Title: VizieR Online Data Catalog: HERMES spectra of Betelgeuse
    (Kravchenko+, 2021)
Authors: Kravchenko, K.; Jorissen, A.; van Eck, S.; Merle, T.;
   Chiavassa, A.; Paladini, C.; Freytag, B.; Plez, B.; Montarges, M.;
   van Winckel, H.
Bibcode: 2021yCat..36509017K
Altcode:
  The observations of Betelgeuse were performed with the high-resolution
  fibre-fed cross-dispersed echelle spectrograph HERMES mounted on the
  1.2m Mercator telescope at the Roque de Los Muchachos Observatory, La
  Palma (Spain). The spectral resolution of HERMES is R=86000, and the
  wavelength coverage is from 3800 to 9000Å. The spectra were obtained
  between November 2015 and September 2020. <P />(2 data files).

Title: Tomography of cool giant and supergiant star
    atmospheres. III. Validation of the method on VLTI/AMBER observations
    of the Mira star S Ori
Authors: Kravchenko, Kateryna; Wittkowski, Markus; Jorissen, Alain;
   Chiavassa, Andrea; Van Eck, Sophie; Anderson, Richard I.; Freytag,
   Bernd; Käufl, Ulli
Bibcode: 2020A&A...642A.235K
Altcode: 2020arXiv200903955K
  Context. Asymptotic giant branch (AGB) stars are characterized by
  substantial mass loss, however the mechanism behind it not yet fully
  understood. The knowledge of the structure and dynamics of AGB-star
  atmospheres is crucial to better understanding the mass loss. The
  recently established tomographic method, which relies on the design
  of spectral masks containing lines that form in given ranges of
  optical depths in the stellar atmosphere, is an ideal technique for
  this purpose. <BR /> Aims: We aim to validate the capability of the
  tomographic method in probing different geometrical depths in the
  stellar atmosphere and recovering the relation between optical and
  geometrical depth scales. <BR /> Methods: We applied the tomographic
  method to high-resolution spectro-interferometric VLTI/AMBER
  observations of the Mira-type AGB star S Ori. The interferometric
  visibilities were extracted at wavelengths contributing to the
  tomographic masks and fitted to those computed from a uniform disk
  model. This allows us to measure the geometrical extent of the
  atmospheric layer probed by the corresponding mask. We then compared
  the observed atmospheric extension with others measured from available
  1D pulsation CODEX models and 3D radiative-hydrodynamics CO5BOLD
  simulations. <BR /> Results: While the average optical depths probed by
  the tomographic masks in S Ori decrease (with ⟨log τ<SUB>0</SUB>⟩
  = -0.45, - 1.45, and - 2.45 from the innermost to the central and
  outermost layers), the angular diameters of these layers increase,
  from 10.59 ± 0.09 mas through 11.84 ± 0.17 mas, up to 14.08 ± 0.15
  mas. A similar behavior is observed when the tomographic method is
  applied to 1D and 3D dynamical models. <BR /> Conclusions: This study
  derives, for the first time, a quantitative relation between optical and
  geometrical depth scales when applied to the Mira star S Ori, or to 1D
  and 3D dynamical models. In the context of Mira-type stars, knowledge
  of the link between the optical and geometrical depths opens the way to
  deriving the shock-wave propagation velocity, which cannot be directly
  observed in these stars. <P />Based on ESO observing program 084.D-0595.

Title: Optical interferometry and Gaia measurement uncertainties
    reveal the physics of asymptotic giant branch stars
Authors: Chiavassa, A.; Kravchenko, K.; Millour, F.; Schaefer, G.;
   Schultheis, M.; Freytag, B.; Creevey, O.; Hocdé, V.; Morand, F.; Ligi,
   R.; Kraus, S.; Monnier, J. D.; Mourard, D.; Nardetto, N.; Anugu, N.;
   Le Bouquin, J. -B.; Davies, C. L.; Ennis, J.; Gardner, T.; Labdon,
   A.; Lanthermann, C.; Setterholm, B. R.; ten Brummelaar, T.
Bibcode: 2020A&A...640A..23C
Altcode: 2020arXiv200607318C
  Context. Asymptotic giant branch (AGB) stars are cool luminous evolved
  stars that are well observable across the Galaxy and populating Gaia
  data. They have complex stellar surface dynamics, which amplifies
  the uncertainties on stellar parameters and distances. <BR /> Aims:
  On the AGB star CL Lac, it has been shown that the convection-related
  variability accounts for a substantial part of the Gaia DR2 parallax
  error. We observed this star with the MIRC-X beam combiner installed
  at the CHARA interferometer to detect the presence of stellar surface
  inhomogeneities. <BR /> Methods: We performed the reconstruction of
  aperture synthesis images from the interferometric observations at
  different wavelengths. Then, we used 3D radiative hydrodynamics (RHD)
  simulations of stellar convection with CO5BOLD and the post-processing
  radiative transfer code OPTIM3D to compute intensity maps in the
  spectral channels of MIRC-X observations. Then, we determined the
  stellar radius using the average 3D intensity profile and, finally,
  compared the 3D synthetic maps to the reconstructed ones focusing on
  matching the intensity contrast, the morphology of stellar surface
  structures, and the photocentre position at two different spectral
  channels, 1.52 and 1.70 μm, simultaneously. <BR /> Results: We measured
  the apparent diameter of CL Lac at two wavelengths (3.299 ± 0.005
  mas and 3.053 ± 0.006 mas at 1.52 and 1.70 μm, respectively) and
  recovered the radius (R = 307 ± 41 and R = 284 ± 38 R<SUB>⊙</SUB>)
  using a Gaia parallax. In addition to this, the reconstructed images are
  characterised by the presence of a brighter area that largely affects
  the position of the photocentre. The comparison with 3D simulation
  shows good agreement with the observations both in terms of contrast
  and surface structure morphology, meaning that our model is adequate
  for explaining the observed inhomogenities. <BR /> Conclusions: This
  work confirms the presence of convection-related surface structures
  on an AGB star of Gaia DR2. Our result will help us to take a step
  forward in exploiting Gaia measurement uncertainties to extract the
  fundamental properties of AGB stars using appropriate RHD simulations.

Title: Focus on Betelgeuse
Authors: Dupree, Andrea; Chiavassa, Andrea; Freytag, Bernd; Harper,
   Graham M.; Kervella, Pierre; Lebre, Agnes; Montarges, Miguel; Ohnaka,
   Keiichi; Quirrenbach, Andreas; Richards, Anita; Schmitt, Henrique R.;
   Strassmeier, Klaus G.; Uitenbroek, Han; Wheeler, J. Craig; Wittkowski,
   Markus; Matthews, Lynn D.
Bibcode: 2020hst..prop16216D
Altcode:
  Multiple ultraviolet spectra of the nearby red supergiant, Betelgeuse,
  using STIS will enable spatially resolved measures of chromospheric
  structure and mass inflows and outflows. An HST campaign of 3 cycles
  will be complemented by multi-frequency photometry, spectroscopy,
  interferometry, and polarimetry at radio, infrared, and optical
  wavelengths in order to map surface structures and their variability,
  and the extended outer atmosphere over both the short (400-day)
  and long secondary (2000-day) periods of this supergiant. These
  observations, coupled with detailed modeling and simulations, will
  probe the structure, the dynamics, and the mass loss from Betelgeuse in
  unprecedented detail and provide crucial insights into the atmospheric
  physics and wind-driving mechanisms of red supergiants.

Title: VLTI-PIONIER imaging of the red supergiant V602 Carinae
Authors: Climent, J. B.; Wittkowski, M.; Chiavassa, A.; Baron, F.;
   Marcaide, J. M.; Guirado, J. C.; Freytag, B.; Höfner, S.; Haubois,
   X.; Woillez, J.
Bibcode: 2020A&A...635A.160C
Altcode: 2020arXiv200209243C
  Context. Red supergiant stars possess surface features and extended
  molecular atmospheres. Photospheric convection may be a crucial
  factor of the levitation of the outer atmospheric layers. However,
  the mechanism responsible is still poorly understood. <BR /> Aims:
  We image the stellar surface of V602 Carinae (V602 Car) to constrain
  the morphology and contrast of the surface features and of the
  extended atmospheric layers. <BR /> Methods: We observed V602 Car
  with the Very Large Telescope Interferometer PIONIER instrument
  (1.53-1.78 μm) between May and July 2016, and April and July
  2019 with different telescope configurations. We compared the image
  reconstructions with 81 temporal snapshots of 3D radiative-hydrodynamics
  (RHD) CO<SUP>5</SUP>BOLD simulations in terms of contrast and
  morphology, using the Structural Similarity Index. <BR /> Results:
  The interferometric data are compatible with an overall spherical
  disk of angular diameter 4.4 ± 0.2 mas, and an extended molecular
  layer. In 2016, the reconstructed image reveals a bright arc-like
  feature toward the northern rim of the photospheric surface. In 2019,
  an arc-like feature is seen at a different orientation and a new
  peak of emission is detected on the opposite side. The contrasts of
  the reconstructed surface images are 11% ± 2% and 9% ± 2% for 2016
  and 2019, respectively. The morphology and contrast of the two images
  are consistent with 3D RHD simulations, within our achieved spatial
  resolution and dynamic range. The extended molecular layer contributes
  10-13% of the total flux with an angular diameter of 6-8 mas. It is
  present but not clearly visible in the reconstructed images because it
  is close to the limits of the achieved dynamic range. The presence of
  the molecular layer is not reproduced by the 3D RHD simulations. <BR
  /> Conclusions: 3D RHD simulations predict substructures similar to
  the observed surface features of V602 Car at two different epochs. We
  interpret the structure on the stellar surface as being related to
  instationary convection. This structure is further convolved to larger
  observed patches on the stellar surface with our observational spatial
  resolution. Even though the simulations reproduce the observed features
  on the stellar surface, convection alone may not be the only relevant
  process that is levitating the atmosphere. <P />Based on observations
  made with the Very Large Telescope Interferometer at the Paranal
  Observatory under program IDs 097.D-0286, 60.A-9138 (NAOMI science
  verification) and 2103.D-5029.

Title: Using Gaia to measure the atmospheric dynamics in AGB stars
Authors: Chiavassa, Andrea; Freytag, Bernd; Schultheis, Mathias
Bibcode: 2019IAUS..343..373C
Altcode:
  We use 3D radiative-hydrodynamics simulations of convection with
  CO5BOLD and the post-processing radiative transfer code Optim3D to
  compute intensity maps in the Gaia G band [325-1030 nm]. We calculate
  the intensity-weighted mean of all emitting points tiling the visible
  stellar surface (i.e., the photo-center) and evaluate its motion as
  a function of time. We show that the convection-related variability
  accounts for a substantial part to the Gaia DR2 parallax error of
  our sample of semiregular variables. Finally, we denote that Gaia
  parallax variations could be exploited quantitatively to extract
  stellar parameters using appropriate RHD simulations corresponding to
  the observed star.

Title: Lumpy stars and bumpy winds
Authors: Liljegren, Sofie; Höfner, Susanne; Freytag, Bernd; Bladh,
   Sara
Bibcode: 2019IAUS..343..134L
Altcode:
  The wind-driving process of AGB stars is thought to be a two-step
  process: first matter is levitated by shock waves, and then accelerated
  outwards by radiation pressure on newly condensed dust grains. When
  modelling such a wind, spherical symmetry is usually assumed. This is
  in stark contrast with recent observations, which shows significant
  non-spherical structures. Giant convection cells cover the surface
  of the star, and matter is being ejected into the atmosphere where it
  condenses into lumpy dust clouds. We try to quantify the differences
  between what is simulated in the 3D star-in-a-box models (CO5BOLD code)
  and the 1D dynamical atmosphere and wind models (DARWIN code). The
  impact of having a non-spherical star on the wind properties is also
  investigated. We find that the inherent non-spherical behaviour of AGB
  stars might induce a dust-driven weak wind already early on the AGB,
  and including that the star is anisotropic when simulating the wind
  leads to large time variations in the density of the outflow. Such
  variations might be observable as small-scale structures in the
  circumstellar envelope.

Title: Precision Monitoring of Cool Evolved Stars: Constraining
    Effects of Convection and Pulsation
Authors: Wittkowski, M.; Bladh, S.; Chiavassa, A.; de Wit, W. -J.;
   Eriksson, K.; Freytag, B.; Haubois, X.; Höfner, S.; Kravchenko, K.;
   Paladini, C.; Paumard, T.; Rau, G.; Wood, P. R.
Bibcode: 2019Msngr.178...34W
Altcode:
  Mass loss from cool evolved stars is an important ingredient of
  the cosmic matter cycle, enriching the Universe with newly formed
  elements and dust. However, physical processes that are not considered
  in current models represent uncertainties in our general understanding
  of mass loss. Time-series of interferometric data provide the strongest
  tests of dynamical processes in the atmospheres of these stars. Here,
  we present a pilot study of such measurements obtained with the GRAVITY
  instrument on the Very Large Telescope Interferometer.

Title: TIGvival: High-resolution spectroscopic monitoring of LPV stars
Authors: Wolter, Uwe; Engels, Dieter; Aringer, Bernhard; Freytag, Bernd
Bibcode: 2019IAUS..343..548W
Altcode:
  TIGvival is a spectroscopic monitoring program of long-period
  variables (LPV) using our robotic telescope TIGRE. Since 2013, we
  obtain low-noise, high-resolution spectra (R= 20 000) that cover the
  optical regime (3800 Å to 8800 Å). We are now continuously monitoring
  7 LPVs with different periods and chemical properties. Our 350+ spectra
  evenly sample the target cycles, as far as ground-based observations
  allow. Analyzing the TIGvival spectra of Mira as a sample case,
  our measurements indicate that the strength of the TiO-absorption is
  phase-shifted with respect to the visual light curve.

Title: The atmospheric dynamics of AGB stars revealed by Gaia through
    numerical simulations
Authors: Chiavassa, A.; Freytag, B.; Schultheis, M.
Bibcode: 2019sf2a.conf..137C
Altcode:
  A considerable fraction of the detected intrinsically variable stars
  in Gaia data are Long-Period Variables. These objects have large
  luminosity amplitudes and variability timescales. They have complex
  stellar surface dynamics that affect the measurements and amplify
  the uncertainties on stellar parameters. We explore the impact of
  the convection-related surface structure in AGBs on the photocentric
  variability. We quantify these effects to characterise the observed
  parallax errors and estimate fundamental stellar parameters and
  dynamical properties. For this purpose, we use state-of-the-art
  three-dimensional (3D) radiative hydrodynamics simulations of convection
  with CO5BOLD and the post-processing radiative transfer code OPTIM3D
  to compute intensity maps in the Gaia G band [325 -- 1030~nm]. Then,
  we calculate the intensity-weighted mean of all emitting points tiling
  the visible stellar surface (i.e. the photocentre) and evaluate its
  motion as a function of time. We show that the convection-related
  variability accounts for a substantial part of the Gaia DR2 parallax
  error of our sample of semi-regular variables. We prospect the roadmap
  to extract quantitatively fundamental properties of AGB stars directly
  from Gaia errors exploiting appropriate RHD simulations.

Title: 3D modelling of AGB stars with CO5BOLD
Authors: Freytag, Bernd; Höfner, Susanne; Liljegren, Sofie
Bibcode: 2019IAUS..343....9F
Altcode:
  Local three-dimensional radiation-hydrodynamics simulations of patches
  of the surfaces of solar-type stars, that are governed by small-scale
  granular convection, have helped analyzing and interpreting observations
  for decades. These models contributed considerably to the understanding
  of the atmospheres and indirectly also of the interiors and the active
  layers above the surface of these stars. Of great help was of course
  the availability of a close-by prototype of these stars - the sun.

Title: Tomography of the red supergiant star μ Cep
Authors: Kravchenko, K.; Chiavassa, A.; Van Eck, S.; Jorissen, A.;
   Merle, T.; Freytag, B.
Bibcode: 2019IAUS..343..441K
Altcode:
  A tomographic method, aiming at probing velocity fields at depth in
  stellar atmospheres, is applied to the red supergiant star μ Cep and to
  snapshots of 3D radiative-hydrodynamics simulation in order to constrain
  atmospheric motions and relate them to photometric variability.

Title: Constraining convection across the AGB with
    high-angular-resolution observations
Authors: Paladini, Claudia; Baron, Fabien; Jorissen, A.; Le Bouquin,
   J. -B.; Freytag, B.; Van Eck, S.; Wittkowski, M.; Hron, J.; Chiavassa,
   A.; Berger, J. -P.; Siopis, C.; Mayer, A.; Sadowski, G.; Kravchenko,
   K.; Shetye, S.; Kerschbaum, F.; Kluska, J.; Ramstedt, S.
Bibcode: 2019IAUS..343...27P
Altcode:
  We present very detailed images of the photosphere of an AGB star
  obtained with the PIONIER instrument, installed at the Very Large
  Telescope Interferometer (VLTI). The images show a well defined
  stellar disc populated by a few convective patterns. Thanks to the high
  precision of the observations we are able to derive the contrast and
  granulation horizontal scale of the convective pattern for the first
  time in a direct way. Such quantities are then compared with scaling
  relations between granule size, effective temperature, and surface
  gravity that are predicted by simulations of stellar surface convection.

Title: Tomography of cool giant and supergiant star
    atmospheres. II. Signature of convection in the atmosphere of the
    red supergiant star μ Cep
Authors: Kravchenko, K.; Chiavassa, A.; Van Eck, S.; Jorissen, A.;
   Merle, T.; Freytag, B.; Plez, B.
Bibcode: 2019A&A...632A..28K
Altcode: 2019arXiv191004657K
  Context. Red supergiants are cool massive stars and are the largest
  and the most luminous stars in the Universe. They are characterized
  by irregular or semi-regular photometric variations, the physics of
  which is not clearly understood. <BR /> Aims: The paper aims to derive
  the velocity field in the red supergiant star μ Cep and to relate
  it to the photometric variability with the help of the tomographic
  method. <BR /> Methods: The tomographic method allows one to recover
  the line-of-sight velocity distribution over the stellar disk and within
  different optical-depth slices. This method was applied to a series of
  high-resolution spectra of μ Cep, and these results are compared to
  those obtained from 3D radiative-hydrodynamics CO5BOLD simulations of
  red supergiants. Fluctuations in the velocity field are compared with
  photometric and spectroscopic variations, the latter were derived from
  the TiO band strength and serve, at least partly, as a proxy of the
  variations in effective temperature. <BR /> Results: The tomographic
  method reveals a phase shift between the velocity and spectroscopic and
  photometric variations. This phase shift results in a hysteresis loop
  in the temperature - velocity plane with a timescale of a few hundred
  days, which is similar to the photometric one. The similarity between
  the hysteresis loop timescale measured in μ Cep and the timescale
  of acoustic waves disturbing the convective pattern suggests that
  such waves play an important role in triggering the hysteresis
  loops. <P />The movie associated to Fig. 13 is available at <A
  href="https://www.aanda.org/10.1051/0004-6361/201935809/olm">http://www.aanda.org</A>
  <P />Based on observations made with the Mercator Telescope, operated
  on the island of La Palma by the Flemish Community, at the Spanish
  Observatorio del Roque de los Muchachos of the Instituto de Astrof-sica
  de Canarias.

Title: Calibration of the mixing-length theory for structures of
    helium-dominated atmosphere white dwarfs
Authors: Cukanovaite, E.; Tremblay, P. -E.; Freytag, B.; Ludwig,
   H. -G.; Fontaine, G.; Brassard, P.; Toloza, O.; Koester, D.
Bibcode: 2019MNRAS.490.1010C
Altcode: 2019MNRAS.tmp.2282C; 2019arXiv190910532C
  We perform a calibration of the mixing-length parameter at the bottom
  boundary of the convection zone for helium-dominated atmospheres of
  white dwarfs. This calibration is based on a grid of 3D DB (pure-helium)
  and DBA (helium-dominated with traces of hydrogen) model atmospheres
  computed with the CO5BOLD radiation-hydrodynamics code, and a grid
  of 1D DB and DBA envelope structures. The 3D models span a parameter
  space of hydrogen-to-helium abundances in the range -10.0 ≤ log
  (H/He) ≤-2.0, surface gravities in the range 7.5 ≤ log g ≤ 9.0,
  and effective temperatures in the range 12 000 K ≲ T<SUB>eff</SUB> ≲
  34 000 K. The 1D envelopes cover a similar atmospheric parameter range,
  but are also calculated with different values of the mixing-length
  parameter, namely 0.4 ≤ ML2/α ≤ 1.4. The calibration is performed
  based on two definitions of the bottom boundary of the convection zone:
  the Schwarzschild and the zero convective flux boundaries. Thus, our
  calibration is relevant for applications involving the bulk properties
  of the convection zone including its total mass, which excludes the
  spectroscopic technique. Overall, the calibrated ML2/α is smaller
  than what is commonly used in evolutionary models and theoretical
  determinations of the blue edge of the instability strip for pulsating
  DB and DBA stars. With calibrated ML2/α we are able to deduce more
  accurate convection zone sizes needed for studies of planetary debris
  mixing and dredge-up of carbon from the core. We highlight this by
  calculating examples of metal-rich 3D DBAZ models and finding their
  convection zone masses. Mixing-length calibration represents the first
  step of in-depth investigations of convective overshoot in white dwarfs
  with helium-dominated atmospheres.

Title: Convective overshoot and macroscopic diffusion in
    pure-hydrogen-atmosphere white dwarfs
Authors: Cunningham, Tim; Tremblay, Pier-Emmanuel; Freytag, Bernd;
   Ludwig, Hans-Günter; Koester, Detlev
Bibcode: 2019MNRAS.488.2503C
Altcode: 2019MNRAS.tmp.1723C; 2019arXiv190611252C
  We present a theoretical description of macroscopic diffusion
  caused by convective overshoot in pure-hydrogen DA white dwarfs
  using 3D, closed-bottom, radiation hydrodynamics CO<SUP>5</SUP>BOLD
  simulations. We rely on a new grid of deep 3D white dwarf models
  in the temperature range 11 400 ≤ T_{eff} ≤ 18 000 K where
  tracer particles and a tracer density are used to derive macroscopic
  diffusion coefficients driven by convective overshoot. These diffusion
  coefficients are compared to microscopic diffusion coefficients from 1D
  structures. We find that the mass of the fully mixed region is likely to
  increase by up to 2.5 orders of magnitude while inferred accretion rates
  increase by a more moderate order of magnitude. We present evidence
  that an increase in settling time of up to 2 orders of magnitude is to
  be expected, which is of significance for time-variability studies of
  polluted white dwarfs. Our grid also provides the most robust constraint
  on the onset of convective instabilities in DA white dwarfs to be in
  the effective temperature range from 18 000 to 18 250 K.

Title: Focus on Betelgeuse
Authors: Dupree, Andrea; Chiavassa, Andrea; Freytag, Bernd; Harper,
   Graham M.; Kervella, Pierre; Lebre, Agnes; Montarges, Miguel; Ohnaka,
   Keiichi; Quirrenbach, Andreas; Richards, Anita; Schmitt, Henrique R.;
   Strassmeier, Klaus G.; Uitenbroek, Han; Wheeler, J. Craig; Wittkowski,
   Markus; Matthews, Lynn D.
Bibcode: 2019hst..prop15873D
Altcode:
  Multiple ultraviolet spectra of the nearby red supergiant, Betelgeuse,
  using STIS will enable spatially resolved measures of chromospheric
  structure and mass inflows and outflows. An HST campaign of 3 cycles
  will be complemented by multi-frequency photometry, spectroscopy,
  interferometry, and polarimetry at radio, infrared, and optical
  wavelengths in order to map surface structures and their variability,
  and the extended outer atmosphere over both the short (400-day)
  and long secondary (2000-day) periods of this supergiant. These
  observations, coupled with detailed modeling and simulations, will
  probe the structure, the dynamics, and the mass loss from Betelgeuse in
  unprecedented detail and provide crucial insights into the atmospheric
  physics and wind-driving mechanisms of red supergiants.

Title: Exploring the origin of clumpy dust clouds around cool
    giants. A global 3D RHD model of a dust-forming M-type AGB star
Authors: Höfner, Susanne; Freytag, Bernd
Bibcode: 2019A&A...623A.158H
Altcode: 2019arXiv190204074H
  Context. Dust grains forming in the extended atmospheres of AGB
  stars are critical for the heavy mass loss of these cool luminous
  giants, as they provide radiative acceleration for the stellar
  winds. Characteristic mid-IR spectral features indicate that the
  grains consist mainly of silicates and corundum. The latter species
  seems to form in a narrow zone within about 2 stellar radii,
  preceding the condensation of silicate dust, which triggers the
  outflow. Recent high-angular-resolution observations show clumpy,
  variable dust clouds at these distances. <BR /> Aims: We explore
  possible causes for the formation of inhomogeneous dust layers,
  using 3D dynamical simulations. <BR /> Methods: We modeled the
  outer convective envelope and the dust-forming atmosphere of an
  M-type AGB star with the CO5BOLD radiation-hydrodynamics code. The
  simulations account for frequency-dependent gas opacities, and include
  a time-dependent description of grain growth and evaporation for
  corundum (Al<SUB>2</SUB>O<SUB>3</SUB>) and olivine-type silicates
  (Mg<SUB>2</SUB>SiO<SUB>4</SUB>). <BR /> Results: In the inner,
  gravitationally bound, and corundum-dominated layers of the
  circumstellar envelope, a patchy distribution of the dust emerges
  naturally, due to atmospheric shock waves that are generated by
  large-scale convective flows and pulsations. The formation of silicate
  dust at somewhat larger distances probably indicates the outer limit of
  the gravitationally bound layers. The current models do not describe
  wind acceleration, but the cloud formation mechanism should also work
  for stars with outflows. Timescales of atmospheric dynamics and grain
  growth are similar to observed values. In spherical averages of dust
  densities, more easily comparable to unresolved observations and 1D
  models, the variable 3D morphology manifests itself as cycle-to-cycle
  variations. <BR /> Conclusions: Grain growth in the wake of large-scale
  non-spherical shock waves, generated by convection and pulsations,
  is a likely mechanism for producing the observed clumpy dust clouds,
  and for explaining their physical and dynamical properties.

Title: Pure-helium 3D model atmospheres of white dwarfs
Authors: Cukanovaite, E.; Tremblay, P. -E.; Freytag, B.; Ludwig,
   H. -G.; Bergeron, P.
Bibcode: 2018MNRAS.481.1522C
Altcode: 2018arXiv180900590C; 2018MNRAS.tmp.2259C
  We present the first grid of 3D simulations for the pure-helium
  atmospheres of DB white dwarfs. The simulations were computed with the
  co<SUP>5</SUP>bold radiation-hydrodynamics code and cover effective
  temperatures and surface gravities between 12 000 K ≲ T<SUB>eff</SUB>
  ≲ 34 000 K and 7.5 ≤ log g (cgs units) ≤ 9.0, respectively. In
  this introductory work, synthetic spectra calculated from the 3D
  simulations are compared to appropriate 1D model spectra under a
  differential approach. This results in the derivation of 3D corrections
  for the spectroscopically derived atmospheric parameters of DB stars
  with respect to the 1D ML2/α = 1.25 mixing-length parametrization. No
  significant T<SUB>eff</SUB> corrections are found for the V777 Her
  instability strip region, and therefore no 3D revision is expected
  for the empirical blue and red edges of the strip. However, large log
  g corrections are found in the range 12 000 K &lt; T<SUB>eff</SUB>
  &lt; 23 000 K for all log g values covered by the 3D grid. These
  corrections indicate that 1D model atmospheres overpredict log g,
  reminiscent of the results found from 3D simulations of pure-hydrogen
  white dwarfs. The next step will be to compute 3D simulations with mixed
  helium and hydrogen atmospheres to comprehend the full implications
  for the stellar parameters of DB and DBA white dwarfs.

Title: Atmospheres and wind properties of non-spherical AGB stars
Authors: Liljegren, S.; Höfner, S.; Freytag, B.; Bladh, S.
Bibcode: 2018A&A...619A..47L
Altcode: 2018arXiv180805043L
  Context. The wind-driving mechanism of asymptotic giant branch
  (AGB) stars is commonly attributed to a two-step process: first,
  gas in the stellar atmosphere is levitated by shockwaves caused by
  stellar pulsation, then accelerated outwards by radiative pressure on
  newly formed dust, inducing a wind. Dynamical modelling of such winds
  usually assumes a spherically symmetric star. <BR /> Aims: We explore
  the potential consequences of complex stellar surface structures, as
  predicted by three-dimensional (3D) star-in-a-box modelling of M-type
  AGB stars, on the resulting wind properties with the aim to improve the
  current wind models. <BR /> Methods: Two different modelling approaches
  are used; the CO<SUP>5</SUP>BOLD 3D star-in-a-box code to simulate
  the convective, pulsating interior and lower atmosphere of the star,
  and the DARWIN one-dimensional (1D) code to describe the dynamical
  atmosphere where the wind is accelerated. The gas dynamics of the
  inner atmosphere region at distances of R ∼ 1-2 R<SUB>⋆</SUB>,
  which both modelling approaches simulate, are compared. Dynamical
  properties and luminosity variations derived from CO<SUP>5</SUP>BOLD
  interior models are used as input for the inner boundary in DARWIN wind
  models in order to emulate the effects of giant convection cells and
  pulsation, and explore their influence on the dynamical properties. <BR
  /> Results: The CO<SUP>5</SUP>BOLD models are inherently anisotropic,
  with non-uniform shock fronts and varying luminosity amplitudes, in
  contrast to the spherically symmetrical DARWIN wind models. DARWIN
  wind models with CO<SUP>5</SUP>BOLD-derived inner boundary conditions
  produced wind velocities and mass-loss rates comparable to the standard
  DARWIN models, however the winds show large density variations on
  time-scales of 10-20 yr. <BR /> Conclusions: The method outlined in
  this paper derives pulsation properties from the 3D star-in-a-box
  CO<SUP>5</SUP>BOLD models, to be used in the DARWIN models. If the
  current grid of CO<SUP>5</SUP>BOLD models is extended, it will be
  possible to construct extensive DARWIN grids with inner boundary
  conditions derived from 3D interior modelling of convection and
  pulsation, and avoid the free parameters of the current approach.

Title: Heading Gaia to measure atmospheric dynamics in AGB stars
Authors: Chiavassa, A.; Freytag, B.; Schultheis, M.
Bibcode: 2018A&A...617L...1C
Altcode: 2018arXiv180802548C
  Context. Asymptotic giant branch (AGB) stars are characterised by
  complex stellar surface dynamics that affect the measurements and
  amplify the uncertainties on stellar parameters. The uncertainties in
  observed absolute magnitudes have been found to originate mainly from
  uncertainties in the parallaxes. The resulting motion of the stellar
  photocentre could have adverse effects on the parallax determination
  with Gaia. <BR /> Aims: We explore the impact of the convection-related
  surface structure in AGBs on the photocentric variability. We quantify
  these effects to characterise the observed parallax errors and estimate
  fundamental stellar parameters and dynamical properties. <BR /> Methods:
  We use three-dimensional (3D) radiative hydrodynamics simulations of
  convection with CO5BOLD and the post-processing radiative transfer code
  OPTIM3D to compute intensity maps in the Gaia G band [325-1030 nm]. From
  those maps, we calculate the intensity-weighted mean of all emitting
  points tiling the visible stellar surface (i.e. the photocentre) and
  evaluate its motion as a function of time. We extract the parallax
  error from Gaia data-release 2 (DR2) for a sample of semi-regular
  variables in the solar neighbourhood and compare it to the synthetic
  predictions of photocentre displacements. <BR /> Results: AGB stars
  show a complex surface morphology characterised by the presence of few
  large-scale long-lived convective cells accompanied by short-lived
  and small-scale structures. As a consequence, the position of the
  photocentre displays temporal excursions between 0.077 and 0.198 AU
  (≈5 to ≈11% of the corresponding stellar radius), depending on the
  simulation considered. We show that the convection-related variability
  accounts for a substantial part of the Gaia DR2 parallax error of our
  sample of semi-regular variables. Finally, we present evidence for a
  correlation between the mean photocentre displacement and the stellar
  fundamental parameters: surface gravity and pulsation. We suggest
  that parallax variations could be exploited quantitatively using
  appropriate radiation-hydrodynamics (RHD) simulations corresponding
  to the observed star.

Title: Constraining Convection in Evolved Stars with the VLTI
Authors: Paladini, C.; Baron, F.; Jorissen, A.; Le Bouquin, J. -B.;
   Freytag, B.; Van Eck, S.; Wittkowski, M.; Hron, J.; Chiavassa, A.;
   Berger, J. -P.; Siopis, C.; Mayer, A.; Sadowski, G.; Kravchenko, K.;
   Shetye, S.; Kerschbaum, F.; Kluska, J.; Ramstedt, S.
Bibcode: 2018Msngr.172...24P
Altcode:
  We used the Precision Integrated-Optics Near-infrared Imaging ExpeRiment
  (PIONIER) at the Very Large Telescope Interferometer (VLTI) to image
  the stellar surface of the S-type Asymptotic Giant Branch (AGB) star
  π<SUP>1</SUP> Gruis. The angular resolution of two milliarcseconds
  allowed us to observe the surface of this giant star in unprecedented
  detail. At the observed wavelength the stellar disc appears circular
  and dust-free. Moreover, the disc is characterised by a few bubbles of
  a convective nature. We determine the contrast, and the characteristic
  horizontal length-scale of the convective granules. The latter is
  determined, for the first time, directly from the image, without
  involving the usual geometric modelling that has been used in the
  literature. The measurements fall along empirical scaling relations
  between stellar parameters and convective sizes, which are determined on
  the basis of three-dimensional stellar convection models. Our results
  open up a new era for the characterisation of stellar convection in
  stars other than the Sun.

Title: Simulation of the small-scale magnetism in main-sequence
    stellar atmospheres
Authors: Salhab, R. G.; Steiner, O.; Berdyugina, S. V.; Freytag, B.;
   Rajaguru, S. P.; Steffen, M.
Bibcode: 2018A&A...614A..78S
Altcode:
  Context. Observations of the Sun tell us that its granular and
  subgranular small-scale magnetism has significant consequences for
  global quantities such as the total solar irradiance or convective
  blueshift of spectral lines. <BR /> Aims: In this paper, properties
  of the small-scale magnetism of four cool stellar atmospheres,
  including the Sun, are investigated, and in particular its effects
  on the radiative intensity and flux. <BR /> Methods: We carried out
  three-dimensional radiation magnetohydrodynamic simulations with the
  CO<SUP>5</SUP>BOLD code in two different settings: with and without
  a magnetic field. These are thought to represent states of high and
  low small-scale magnetic activity of a stellar magnetic cycle. <BR
  /> Results: We find that the presence of small-scale magnetism
  increases the bolometric intensity and flux in all investigated
  models. The surplus in radiative flux of the magnetic over the magnetic
  field-free atmosphere increases with increasing effective temperature,
  T<SUB>eff</SUB>, from 0.47% for spectral type K8V to 1.05% for the solar
  model, but decreases for higher effective temperatures than solar. The
  degree of evacuation of the magnetic flux concentrations monotonically
  increases with T<SUB>eff</SUB> as does their depression of the visible
  optical surface, that is the Wilson depression. Nevertheless, the
  strength of the field concentrations on this surface stays remarkably
  unchanged at ≈1560 G throughout the considered range of spectral
  types. With respect to the surrounding gas pressure, the field strength
  is close to (thermal) equipartition for the Sun and spectral type F5V
  but is clearly sub-equipartition for K2V and more so for K8V. The
  magnetic flux concentrations appear most conspicuous for model K2V
  owing to their high brightness contrast. <BR /> Conclusions: For mean
  magnetic flux densities of approximately 50 G, we expect the small-scale
  magnetism of stars in the spectral range from F5V to K8V to produce a
  positive contribution to their bolometric luminosity. The modulation
  seems to be most effective for early G-type stars.

Title: Spectroscopic Properties of a Two-Dimensional Cepheid Model
Authors: Vasilyev, Valeriy; Ludwig, Hans-Günter; Freytag, Bernd;
   Lemasle, Bertrand; Marconi, Marcella
Bibcode: 2018pas6.conf..222V
Altcode: 2018pas..conf..222V
  The chemical composition of Cepheid variables can provide information on
  the chemo-dynamical evolution of the Galaxy and beyond. The standard
  method for determining atmospheric parameters and abundances of
  Cepheids is based on one-dimensional plane-parallel hydrostatic
  model atmospheres, where convection is treated by Mixing Length
  Theory. We check the validity of the quasi-static approach against
  a two-dimensional dynamical Cepheid model computed with CO5BOLD. The
  spectroscopic investigation of the two-dimensional Cepheid model allowed
  to derive projection factors and to explain the residual line-of-sight
  velocity of Galactic Cepheids, long known as the ``K-term'', by line
  shifts of convective origin. Moreover, hydrostatic 1D model atmospheres
  can provide unbiased estimates of stellar parameters and abundances
  of Cepheids for particular phases of their pulsations.

Title: Tomography of the Red Supergiant Star MU Cep
Authors: Kravchenko, Kateryna; Chiavassa, A.; Van Eck, S.; Jorissen,
   A.; Merle, T.; Freytag, B.
Bibcode: 2018iss..confE..20K
Altcode: 2018arXiv180907581K
  We present a tomographic method allowing to recover the velocity field
  at different optical depths in a stellar atmosphere. It is based on
  the computation of the contribution function to identify the depth
  of formation of spectral lines in order to construct numerical masks
  probing different optical depths. These masks are cross-correlated
  with observed spectra to extract information about the average shape
  of lines forming at a given optical depth and to derive the velocity
  field projected on the line of sight. We applied this method to series
  of spectra of the red supergiant star mu Cep and derived velocities in
  different atmospheric layers. The resulting velocity variations reveal
  complex atmospheric dynamics and indicate that convective cells are
  present in the atmosphere of the mu Cep. The mu Cep velocities were
  compared with those obtained by applying the tomographic masks to series
  of snapshot spectra from 3D radiative-hydrodynamics CO5BOLD simulations.

Title: Using the CIFIST grid of CO<SUP>5</SUP>BOLD 3D model
    atmospheres to study the effects of stellar granulation on photometric
    colours. I. Grids of 3D corrections in the UBVRI, 2MASS, HIPPARCOS,
    Gaia, and SDSS systems
Authors: Bonifacio, P.; Caffau, E.; Ludwig, H. -G.; Steffen, M.;
   Castelli, F.; Gallagher, A. J.; Kučinskas, A.; Prakapavičius, D.;
   Cayrel, R.; Freytag, B.; Plez, B.; Homeier, D.
Bibcode: 2018A&A...611A..68B
Altcode: 2017arXiv171200024B
  Context. The atmospheres of cool stars are temporally and spatially
  inhomogeneous due to the effects of convection. The influence of
  this inhomogeneity, referred to as granulation, on colours has never
  been investigated over a large range of effective temperatures and
  gravities. Aim. We aim to study, in a quantitative way, the impact of
  granulation on colours. <BR /> Methods: We use the CIFIST (Cosmological
  Impact of the FIrst Stars) grid of CO5BOLD (COnservative COde for the
  COmputation of COmpressible COnvection in a BOx of L Dimensions, L = 2,
  3) hydrodynamical models to compute emerging fluxes. These in turn are
  used to compute theoretical colours in the UBV RI, 2MASS, HIPPARCOS,
  Gaia and SDSS systems. Every CO5BOLD model has a corresponding one
  dimensional (1D) plane-parallel LHD (Lagrangian HydroDynamics) model
  computed for the same atmospheric parameters, which we used to define
  a "3D correction" that can be applied to colours computed from fluxes
  computed from any 1D model atmosphere code. As an example, we illustrate
  these corrections applied to colours computed from ATLAS models. <BR />
  Results: The 3D corrections on colours are generally small, of the order
  of a few hundredths of a magnitude, yet they are far from negligible. We
  find that ignoring granulation effects can lead to underestimation of
  Teff by up to 200 K and overestimation of gravity by up to 0.5 dex, when
  using colours as diagnostics. We have identified a major shortcoming in
  how scattering is treated in the current version of the CIFIST grid,
  which could lead to offsets of the order 0.01 mag, especially for
  colours involving blue and UV bands. We have investigated the Gaia and
  HIPPARCOS photometric systems and found that the (G - H<SUB>p</SUB>),
  (BP - RP) diagram is immune to the effects of granulation. In addition,
  we point to the potential of the RVS photometry as a metallicity
  diagnostic. <BR /> Conclusions: Our investigation shows that the
  effects of granulation should not be neglected if one wants to use
  colours as diagnostics of the stellar parameters of F, G, K stars. A
  limitation is that scattering is treated as true absorption in our
  current computations, thus our 3D corrections are likely an upper
  limit to the true effect. We are already computing the next generation
  of the CIFIST grid, using an approximate treatment of scattering. <P
  />The appendix tables are only available at the CDS via anonymous ftp
  to <A href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A>
  (<A href="http://cdsarc.u-strasbg.fr">http://130.79.128.5</A>) or via <A
  href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/611/A68">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/611/A68</A>

Title: Spectroscopic properties of a two-dimensional time-dependent
    Cepheid model. II. Determination of stellar parameters and abundances
Authors: Vasilyev, V.; Ludwig, H. -G.; Freytag, B.; Lemasle, B.;
   Marconi, M.
Bibcode: 2018A&A...611A..19V
Altcode: 2017arXiv171100236V
  Context. Standard spectroscopic analyses of variable stars are based
  on hydrostatic 1D model atmospheres. This quasi-static approach has
  not been theoretically validated. Aim. We aim at investigating the
  validity of the quasi-static approximation for Cepheid variables. We
  focus on the spectroscopic determination of the effective temperature
  T<SUB>eff</SUB>, surface gravity log g, microturbulent velocity
  ξ<SUB>t</SUB>, and a generic metal abundance log A, here taken
  as iron. <BR /> Methods: We calculated a grid of 1D hydrostatic
  plane-parallel models covering the ranges in effective temperature and
  gravity that are encountered during the evolution of a 2D time-dependent
  envelope model of a Cepheid computed with the radiation-hydrodynamics
  code CO5BOLD. We performed 1D spectral syntheses for artificial iron
  lines in local thermodynamic equilibrium by varying the microturbulent
  velocity and abundance. We fit the resulting equivalent widths
  to corresponding values obtained from our dynamical model for 150
  instances in time, covering six pulsational cycles. In addition,
  we considered 99 instances during the initial non-pulsating stage
  of the temporal evolution of the 2D model. In the most general case,
  we treated T<SUB>eff</SUB>, log g, ξ<SUB>t</SUB>, and log A as free
  parameters, and in two more limited cases, we fixed T<SUB>eff</SUB>
  and log g by independent constraints. We argue analytically that our
  approach of fitting equivalent widths is closely related to current
  standard procedures focusing on line-by-line abundances. <BR />
  Results: For the four-parametric case, the stellar parameters are
  typically underestimated and exhibit a bias in the iron abundance of
  ≈-0.2 dex. To avoid biases of this type, it is favorable to restrict
  the spectroscopic analysis to photometric phases ϕ<SUB>ph</SUB>
  ≈ 0.3…0.65 using additional information to fix the effective
  temperature and surface gravity. <BR /> Conclusions: Hydrostatic 1D
  model atmospheres can provide unbiased estimates of stellar parameters
  and abundances of Cepheid variables for particular phases of their
  pulsations. We identified convective inhomogeneities as the main driver
  behind potential biases. To obtain a complete view on the effects
  when determining stellar parameters with 1D models, multidimensional
  Cepheid atmosphere models are necessary for variables of longer period
  than investigated here.

Title: Tomography of cool giant and supergiant star
    atmospheres. I. Validation of the method
Authors: Kravchenko, K.; Van Eck, S.; Chiavassa, A.; Jorissen, A.;
   Freytag, B.; Plez, B.
Bibcode: 2018A&A...610A..29K
Altcode: 2017arXiv171108327K
  Context. Cool giant and supergiant star atmospheres are characterized
  by complex velocity fields originating from convection and pulsation
  processes which are not fully understood yet. The velocity fields
  impact the formation of spectral lines, which thus contain information
  on the dynamics of stellar atmospheres. Aim. The tomographic method
  allows to recover the distribution of the component of the velocity
  field projected on the line of sight at different optical depths in
  the stellar atmosphere. The computation of the contribution function
  to the line depression aims at correctly identifying the depth of
  formation of spectral lines in order to construct numerical masks
  probing spectral lines forming at different optical depths. <BR />
  Methods: The tomographic method is applied to one-dimensional (1D)
  model atmospheres and to a realistic three-dimensional (3D) radiative
  hydrodynamics simulation performed with CO5BOLD in order to compare
  their spectral line formation depths and velocity fields. <BR />
  Results: In 1D model atmospheres, each spectral line forms in
  a restricted range of optical depths. On the other hand, in 3D
  simulations, the line formation depths are spread in the atmosphere
  mainly because of temperature and density inhomogeneities. Comparison of
  cross-correlation function profiles obtained from 3D synthetic spectra
  with velocities from the 3D simulation shows that the tomographic
  method correctly recovers the distribution of the velocity component
  projected on the line of sight in the atmosphere.

Title: Large granulation cells on the surface of the giant star
    π<SUP>1</SUP> Gruis
Authors: Paladini, C.; Baron, F.; Jorissen, A.; Le Bouquin, J. -B.;
   Freytag, B.; van Eck, S.; Wittkowski, M.; Hron, J.; Chiavassa, A.;
   Berger, J. -P.; Siopis, C.; Mayer, A.; Sadowski, G.; Kravchenko, K.;
   Shetye, S.; Kerschbaum, F.; Kluska, J.; Ramstedt, S.
Bibcode: 2018Natur.553..310P
Altcode:
  Convection plays a major part in many astrophysical processes,
  including energy transport, pulsation, dynamos and winds on evolved
  stars, in dust clouds and on brown dwarfs. Most of our knowledge
  about stellar convection has come from studying the Sun: about two
  million convective cells with typical sizes of around 2,000 kilometres
  across are present on the surface of the Sun—a phenomenon known
  as granulation. But on the surfaces of giant and supergiant stars
  there should be only a few large (several tens of thousands of times
  larger than those on the Sun) convective cells, owing to low surface
  gravity. Deriving the characteristic properties of convection (such as
  granule size and contrast) for the most evolved giant and supergiant
  stars is challenging because their photospheres are obscured by dust,
  which partially masks the convective patterns. These properties can be
  inferred from geometric model fitting, but this indirect method does
  not provide information about the physical origin of the convective
  cells. Here we report interferometric images of the surface of the
  evolved giant star π<SUP>1</SUP> Gruis, of spectral type S5,7. Our
  images show a nearly circular, dust-free atmosphere, which is very
  compact and only weakly affected by molecular opacity. We find that
  the stellar surface has a complex convective pattern with an average
  intensity contrast of 12 per cent, which increases towards shorter
  wavelengths. We derive a characteristic horizontal granule size of about
  1.2 × 10<SUP>11</SUP> metres, which corresponds to 27 per cent
  of the diameter of the star. Our measurements fall along the scaling
  relations between granule size, effective temperature and surface
  gravity that are predicted by simulations of stellar surface convection.

Title: VizieR Online Data Catalog: 3D correction in 5 photometric
    systems (Bonifacio+, 2018)
Authors: Bonifacio, P.; Caffau, E.; Ludwig, H. -G.; Steffen, M.;
   Castelli, F.; Gallagher, A. J.; Kucinskas, A.; Prakapavicius, D.;
   Cayrel, R.; Freytag, B.; Plez, B.; Homeier, D.
Bibcode: 2018yCat..36110068B
Altcode:
  We have used the CIFIST grid of CO5BOLD models to investigate the
  effects of granulation on fluxes and colours of stars of spectral
  type F, G, and K. <P />We publish tables with 3D corrections that
  can be applied to colours computed from any 1D model atmosphere. For
  Teff&gt;=5000K, the corrections are smooth enough, as a function
  of atmospheric parameters, that it is possible to interpolate the
  corrections between grid points; thus the coarseness of the CIFIST
  grid should not be a major limitation. However at the cool end there
  are still far too few models to allow a reliable interpolation. <P
  />(20 data files).

Title: Spectroscopic properties of a two-dimensional time-dependent
    Cepheid model. I. Description and validation of the model
Authors: Vasilyev, V.; Ludwig, H. -G.; Freytag, B.; Lemasle, B.;
   Marconi, M.
Bibcode: 2017A&A...606A.140V
Altcode: 2017arXiv170903905V
  Context. Standard spectroscopic analyses of Cepheid variables are based
  on hydrostatic one-dimensional model atmospheres, with convection
  treated using various formulations of mixing-length theory. <BR />
  Aims: This paper aims to carry out an investigation of the validity of
  the quasi-static approximation in the context of pulsating stars. We
  check the adequacy of a two-dimensional time-dependent model of a
  Cepheid-like variable with focus on its spectroscopic properties. <BR />
  Methods: With the radiation-hydrodynamics code CO5BOLD, we construct
  a two-dimensional time-dependent envelope model of a Cepheid with
  T<SUB>eff</SUB> = 5600 K, log g = 2.0, solar metallicity, and a
  2.8-day pulsation period. Subsequently, we perform extensive spectral
  syntheses of a set of artificial iron lines in local thermodynamic
  equilibrium. The set of lines allows us to systematically study effects
  of line strength, ionization stage, and excitation potential. <BR />
  Results: We evaluate the microturbulent velocity, line asymmetry,
  projection factor, and Doppler shifts. The microturbulent velocity,
  averaged over all lines, depends on the pulsational phase and varies
  between 1.5 and 2.7 km s<SUP>-1</SUP>. The derived projection factor
  lies between 1.23 and 1.27, which agrees with observational results. The
  mean Doppler shift is non-zero and negative, -1 km s<SUP>-1</SUP>,
  after averaging over several full periods and lines. This residual
  line-of-sight velocity (related to the "K-term") is primarily caused
  by horizontal inhomogeneities, and consequently we interpret it as the
  familiar convective blueshift ubiquitously present in non-pulsating
  late-type stars. Limited statistics prevent firm conclusions on the line
  asymmetries. <BR /> Conclusions: Our two-dimensional model provides
  a reasonably accurate representation of the spectroscopic properties
  of a short-period Cepheid-like variable star. Some properties are
  primarily controlled by convective inhomogeneities rather than by
  the Cepheid-defining pulsations. Extended multi-dimensional modelling
  offers new insight into the nature of pulsating stars.

Title: Global 3D radiation-hydrodynamics models of AGB stars. Effects
    of convection and radial pulsations on atmospheric structures
Authors: Freytag, B.; Liljegren, S.; Höfner, S.
Bibcode: 2017A&A...600A.137F
Altcode: 2017arXiv170205433F
  Context. Observations of asymptotic giant branch (AGB) stars with
  increasing spatial resolution reveal new layers of complexity of
  atmospheric processes on a variety of scales. <BR /> Aims: To analyze
  the physical mechanisms that cause asymmetries and surface structures
  in observed images, we use detailed 3D dynamical simulations of AGB
  stars; these simulations self-consistently describe convection and
  pulsations. <BR /> Methods: We used the CO5BOLD radiation-hydrodynamics
  code to produce an exploratory grid of global "star-in-a-box" models
  of the outer convective envelope and the inner atmosphere of AGB
  stars to study convection, pulsations, and shock waves and their
  dependence on stellar and numerical parameters. <BR /> Results:
  The model dynamics are governed by the interaction of long-lasting
  giant convection cells, short-lived surface granules, and strong,
  radial, fundamental-mode pulsations. Radial pulsations and shorter
  wavelength, traveling, acoustic waves induce shocks on various scales
  in the atmosphere. Convection, waves, and shocks all contribute
  to the dynamical pressure and, thus, to an increase of the stellar
  radius and to a levitation of material into layers where dust can
  form. Consequently, the resulting relation of pulsation period and
  stellar radius is shifted toward larger radii compared to that of
  non-linear 1D models. The dependence of pulsation period on luminosity
  agrees well with observed relations. The interaction of the pulsation
  mode with the non-stationary convective flow causes occasional amplitude
  changes and phase shifts. The regularity of the pulsations decreases
  with decreasing gravity as the relative size of convection cells
  increases. The model stars do not have a well-defined surface. Instead,
  the light is emitted from a very extended inhomogeneous atmosphere with
  a complex dynamic pattern of high-contrast features. <BR /> Conclusions:
  Our models self-consistently describe convection, convectively generated
  acoustic noise, fundamental-mode radial pulsations, and atmospheric
  shocks of various scales, which give rise to complex changing structures
  in the atmospheres of AGB stars.

Title: Convective overshoot and metal accretion onto white dwarfs.
Authors: Tremblay, P. -E.; Ludwig, H. -G.; Freytag, B.; Koester, D.;
   Fontaine, G.
Bibcode: 2017MmSAI..88..104T
Altcode:
  A large fraction of white dwarfs host evolved planetary systems and show
  evidence of accretion from planetary debris. The accretion-diffusion
  model is the preferred method to understand the metal pollution in these
  otherwise hydrogen- and helium-rich white dwarf atmospheres. In this
  scenario, the accreted material first settles on the atmosphere. If
  the outer stellar layers are unstable to convection, the metals are
  then rapidly mixed up within the convection zone. In the classical
  1D approach, it is generally assumed that the convection zone has a
  sharp bottom boundary, below which microscopic diffusion is unhampered
  and slowly removes metals from the visible layers. More realistic
  3D radiation-hydrodynamics simulations of white dwarfs with CO5BOLD
  demonstrate, however, that the bottom of the convection zone does not
  have a sharp boundary, and that velocities decay exponentially below
  the unstable convective layers with a velocity scale height of the
  order of one pressure scale height. This has a potentially dramatic
  effect on the inferred mass of accreted materiel, hence on the chemical
  composition and size of planetary debris around white dwarfs.

Title: Boundary conditions in CO5BOLD
Authors: Freytag, Bernd
Bibcode: 2017MmSAI..88...12F
Altcode:
  The declaration of boundary conditions is a crucial step in the setup
  of a CO5BOLD simulation (and many others) due to the physical nature
  of the problem, that is reflected in the mathematical description
  by partial differential equations, discrete versions of which are
  integrated by the numerical solver(s). While parameters controlling
  the flux of energy through the computational box are most important
  for all simulations of convective flows, the detailed specifications
  describing the behavior of energy, gas and dust densities, velocities,
  and magnetic fields at or just beyond the boundaries influence the
  flow, dynamics, and stratification within the box. Recent refinements
  of the treatment of boundary conditions in CO5BOLD resulted in reliably
  working implementations of open and closed versions for top, bottom, and
  ``inner'' boundaries even under conditions with strong velocity fields
  (waves, shocks, or downdrafts). They are implemented and available in
  the current version of CO5BOLD - but have to be activated properly
  with parameters adapted to the type of the star under consideration
  (by defining for instance the depth of the damping layers for the
  closed-bottom boundary or by specifying the damping constants for the
  open-bottom boundary).

Title: Using CO5BOLD models to predict the effects of granulation
    on colours .
Authors: Bonifacio, P.; Caffau, E.; Ludwig, H. -G.; Steffen, M.;
   Castelli, F.; Gallagher, A. J.; Prakapavičius, D.; Kučinskas, A.;
   Cayrel, R.; Freytag, B.; Plez, B.; Homeier, D.
Bibcode: 2017MmSAI..88...90B
Altcode:
  In order to investigate the effects of granulation on fluxes and
  colours, we computed the emerging fluxes from the models in the
  CO5BOLD grid with metallicities [M/H]=0.0,-1.0,-2.0 and -3.0. These
  fluxes have been used to compute colours in different photometric
  systems. We explain here how our computations have been performed and
  provide some results.

Title: Enhanced methods for computing spectra from CO5BOLD models
    using Linfor3D. Molecular bands in metal-poor stars
Authors: Gallagher, A. J.; Steffen, M.; Caffau, E.; Bonifacio, P.;
   Ludwig, H. -G.; Freytag, B.
Bibcode: 2017MmSAI..88...82G
Altcode: 2016arXiv161004427G
  Molecular features such as the G-band, CN-band and NH-band are important
  diagnostics for measuring a star's carbon and nitrogen abundances,
  especially in metal-poor stars where atomic lines are no longer visible
  in stellar spectra. Unlike atomic transitions, molecular features
  tend to form in bands, which cover large wavelength regions in a
  spectrum. While it is a trivial matter to compute carbon and nitrogen
  molecular bands under the assumption of 1D, it is extremely time
  consuming in 3D. In this contribution to the 2016 COBOLD workshop we
  review the improvements made to the 3D spectral synthesis code Linfor3D,
  and discuss the new challenges found when computing molecular features
  in 3D.

Title: Non-magnetic photospheric bright points in 3D simulations of
    the solar atmosphere
Authors: Calvo, F.; Steiner, O.; Freytag, B.
Bibcode: 2016A&A...596A..43C
Altcode: 2016arXiv161204278C
  Context. Small-scale bright features in the photosphere of the Sun,
  such as faculae or G-band bright points, appear in connection with
  small-scale magnetic flux concentrations. <BR /> Aims: Here we report
  on a new class of photospheric bright points that are free of magnetic
  fields. So far, these are visible in numerical simulations only. We
  explore conditions required for their observational detection. <BR />
  Methods: Numerical radiation (magneto-)hydrodynamic simulations of the
  near-surface layers of the Sun were carried out. The magnetic field-free
  simulations show tiny bright points, reminiscent of magnetic bright
  points, only smaller. A simple toy model for these non-magnetic bright
  points (nMBPs) was established that serves as a base for the development
  of an algorithm for their automatic detection. Basic physical properties
  of 357 detected nMBPs were extracted and statistically evaluated. We
  produced synthetic intensity maps that mimic observations with various
  solar telescopes to obtain hints on their detectability. <BR /> Results:
  The nMBPs of the simulations show a mean bolometric intensity contrast
  with respect to their intergranular surroundings of approximately 20%, a
  size of 60-80 km, and the isosurface of optical depth unity is at their
  location depressed by 80-100 km. They are caused by swirling downdrafts
  that provide, by means of the centripetal force, the necessary pressure
  gradient for the formation of a funnel of reduced mass density that
  reaches from the subsurface layers into the photosphere. Similar,
  frequently occurring funnels that do not reach into the photosphere,
  do not produce bright points. <BR /> Conclusions: Non-magnetic bright
  points are the observable manifestation of vertically extending vortices
  (vortex tubes) in the photosphere. The resolving power of 4-m-class
  telescopes, such as the DKIST, is needed for an unambiguous detection
  of them. <P />The movie associated to Fig. 1 is available at <A
  href="http://www.aanda.org/10.1051/0004-6361/201628649/olm">http://www.aanda.org</A>

Title: Application of a Theory and Simulation-based Convective
    Boundary Mixing Model for AGB Star Evolution and Nucleosynthesis
Authors: Battino, U.; Pignatari, M.; Ritter, C.; Herwig, F.;
   Denisenkov, P.; Den Hartogh, J. W.; Trappitsch, R.; Hirschi, R.;
   Freytag, B.; Thielemann, F.; Paxton, B.
Bibcode: 2016ApJ...827...30B
Altcode: 2016arXiv160506159B
  The s-process nucleosynthesis in Asymptotic giant branch (AGB) stars
  depends on the modeling of convective boundaries. We present models
  and s-process simulations that adopt a treatment of convective
  boundaries based on the results of hydrodynamic simulations and
  on the theory of mixing due to gravity waves in the vicinity of
  convective boundaries. Hydrodynamics simulations suggest the presence
  of convective boundary mixing (CBM) at the bottom of the thermal
  pulse-driven convective zone. Similarly, convection-induced mixing
  processes are proposed for the mixing below the convective envelope
  during third dredge-up (TDU), where the {}<SUP>13</SUP>{{C}} pocket for
  the s process in AGB stars forms. In this work, we apply a CBM model
  motivated by simulations and theory to models with initial mass M =
  2 and M=3 {M}<SUB>⊙ </SUB>, and with initial metal content Z = 0.01
  and Z = 0.02. As reported previously, the He-intershell abundances of
  {}<SUP>12</SUP>{{C}} and {}<SUP>16</SUP>{{O}} are increased by CBM at
  the bottom of the pulse-driven convection zone. This mixing is affecting
  the {}<SUP>22</SUP>{Ne}(α, n){}<SUP>25</SUP>{Mg} activation and the
  s-process efficiency in the {}<SUP>13</SUP>{{C}}-pocket. In our model,
  CBM at the bottom of the convective envelope during the TDU represents
  gravity wave mixing. Furthermore, we take into account the fact that
  hydrodynamic simulations indicate a declining mixing efficiency that is
  already about a pressure scale height from the convective boundaries,
  compared to mixing-length theory. We obtain the formation of the
  {}<SUP>13</SUP>{{C}}-pocket with a mass of ≈ {10}<SUP>-4</SUP>
  {M}<SUB>⊙ </SUB>. The final s-process abundances are characterized
  by 0.36\lt [{{s}}/{Fe}]\lt 0.78 and the heavy-to-light s-process ratio
  is -0.23\lt [{hs}/{ls}]\lt 0.45. Finally, we compare our results with
  stellar observations, presolar grain measurements and previous work.

Title: Near-infrared spectro-interferometry of Mira variables
    and comparisons to 1D dynamic model atmospheres and 3D convection
    simulations
Authors: Wittkowski, M.; Chiavassa, A.; Freytag, B.; Scholz, M.;
   Höfner, S.; Karovicova, I.; Whitelock, P. A.
Bibcode: 2016A&A...587A..12W
Altcode: 2016arXiv160102368W
  <BR /> Aims: We aim at comparing spectro-interferometric observations
  of Mira variable asymptotic giant branch (AGB) stars with the latest
  1D dynamic model atmospheres based on self-excited pulsation models
  (CODEX models) and with 3D dynamic model atmospheres including
  pulsation and convection (CO5BOLD models) to better understand the
  processes that extend the molecular atmosphere to radii where dust can
  form. <BR /> Methods: We obtained a total of 20 near-infrared K-band
  spectro-interferometric snapshot observations of the Mira variables o
  Cet, R Leo, R Aqr, X Hya, W Vel, and R Cnc with a spectral resolution
  of about 1500. We compared observed flux and visibility spectra with
  predictions by CODEX 1D dynamic model atmospheres and with azimuthally
  averaged intensities based on CO5BOLD 3D dynamic model atmospheres. <BR
  /> Results: Our visibility data confirm the presence of spatially
  extended molecular atmospheres located above the continuum radii with
  large-scale inhomogeneities or clumps that contribute a few percent
  of the total flux. The detailed structure of the inhomogeneities or
  clumps show a variability on time scales of 3 months and above. Both
  modeling attempts provided satisfactory fits to our data. In particular,
  they are both consistent with the observed decrease in the visibility
  function at molecular bands of water vapor and CO, indicating a
  spatially extended molecular atmosphere. Observational variability
  phases are mostly consistent with those of the best-fit CODEX models,
  except for near-maximum phases, where data are better described by
  near-minimum models. Rosseland angular diameters derived from the
  model fits are broadly consistent between those based on the 1D and
  the 3D models and with earlier observations. We derived fundamental
  parameters including absolute radii, effective temperatures, and
  luminosities for our sources. <BR /> Conclusions: Our results provide
  a first observational support for theoretical results that shocks
  induced by convection and pulsation in the 3D CO5BOLD models of AGB
  stars are roughly spherically expanding and of similar nature to
  those of self-excited pulsations in 1D CODEX models. Unlike for red
  supergiants, the pulsation- and shock-induced dynamics can levitate
  the molecular atmospheres of Mira variables to extensions that are
  consistent with observations. <P />Based on observations made with
  the VLT Interferometer (VLTI) at Paranal Observatory under program
  IDs 082.D-0723, 084.D-0839, 088.D-0160, 090.D-0817, and 091.D-0765.

Title: Pathways for Observing Stellar Surfaces Using 3D Hydrodynamical
    Simulations of Evolved Stars
Authors: Chiavassa, A.; Freytag, B.
Bibcode: 2015EAS....71..237C
Altcode: 2015arXiv151203590C
  Evolved stars are among the largest and brightest stars and they are
  ideal targets for the new generation of sensitive, high resolution
  instrumentation that provides spectrophotometric, interferometric,
  astrometric, and imaging observables. The interpretation of the complex
  stellar surface images requires numerical simulations of stellar
  convection that take into account multi-dimensional time-dependent
  radiation hydrodynamics with realistic input physics. We show how the
  evolved star simulations are obtained using the radiative hydrodynamics
  code CO5BOLD and how the accurate observables are computed with
  the post-processing radiative transfer code Optim3D. The synergy
  between observations and theoretical work is supported by a proper
  and quantitative analysis using these simulations, and by strong
  constraints from the observational side.

Title: On the Evolution of Magnetic White Dwarfs
Authors: Tremblay, P. -E.; Fontaine, G.; Freytag, B.; Steiner, O.;
   Ludwig, H. -G.; Steffen, M.; Wedemeyer, S.; Brassard, P.
Bibcode: 2015ApJ...812...19T
Altcode: 2015arXiv150905398T
  We present the first radiation magnetohydrodynamic simulations of the
  atmosphere of white dwarf stars. We demonstrate that convective energy
  transfer is seriously impeded by magnetic fields when the plasma-β
  parameter, the thermal-to-magnetic-pressure ratio, becomes smaller
  than unity. The critical field strength that inhibits convection
  in the photosphere of white dwarfs is in the range B = 1-50 kG,
  which is much smaller than the typical 1-1000 MG field strengths
  observed in magnetic white dwarfs, implying that these objects have
  radiative atmospheres. We have employed evolutionary models to study the
  cooling process of high-field magnetic white dwarfs, where convection
  is entirely suppressed during the full evolution (B ≳ 10 MG). We
  find that the inhibition of convection has no effect on cooling rates
  until the effective temperature (T<SUB>eff</SUB>) reaches a value of
  around 5500 K. In this regime, the standard convective sequences start
  to deviate from the ones without convection due to the convective
  coupling between the outer layers and the degenerate reservoir of
  thermal energy. Since no magnetic white dwarfs are currently known
  at the low temperatures where this coupling significantly changes the
  evolution, the effects of magnetism on cooling rates are not expected
  to be observed. This result contrasts with a recent suggestion
  that magnetic white dwarfs with T<SUB>eff</SUB> ≲ 10,000 K cool
  significantly slower than non-magnetic degenerates.

Title: 3D Hydrodynamical Simulations of Evolved Stars and Observations
    of Stellar Surfaces
Authors: Chiavassa, A.; Freytag, B.
Bibcode: 2015ASPC..497...11C
Altcode: 2014arXiv1410.3868C
  Evolved stars are among the largest and brightest stars and they are
  ideal targets for the new generation of sensitive, high resolution
  instrumentation that provides spectrophotometric, interferometric,
  astrometric, and imaging observables. The interpretation of the complex
  stellar surface images requires numerical simulations of stellar
  convection that take into account multi-dimensional time-dependent
  radiation hydrodynamics with realistic input physics. We show how the
  evolved star simulations are obtained using the radiative hydrodynamics
  code CO<SUP>5</SUP>BOLD and how the accurate observables are computed
  with the post-processing radiative transfer code OPTIM3D. The synergy
  between observations and theoretical work is supported by a proper
  and quantitative analysis using these simulations, and by strong
  constraints from the observational side.

Title: 3D Model Atmospheres for Extremely Low-mass White Dwarfs
Authors: Tremblay, P. -E.; Gianninas, A.; Kilic, M.; Ludwig, H. -G.;
   Steffen, M.; Freytag, B.; Hermes, J. J.
Bibcode: 2015ApJ...809..148T
Altcode: 2015arXiv150701927T
  We present an extended grid of mean three-dimensional (3D) spectra
  for low-mass, pure-hydrogen atmosphere DA white dwarfs (WDs). We use
  CO5BOLD radiation-hydrodynamics 3D simulations covering T<SUB>eff</SUB>
  = 6000-11,500 K and log g = 5-6.5 (g in cm s<SUP>-2</SUP>) to derive
  analytical functions to convert spectroscopically determined 1D
  temperatures and surface gravities to 3D atmospheric parameters. Along
  with the previously published 3D models, the 1D to 3D corrections are
  now available for essentially all known convective DA WDs (i.e., log g
  = 5-9). For low-mass WDs, the correction in temperature is relatively
  small (a few percent at the most), but the surface gravities measured
  from the 3D models are lower by as much as 0.35 dex. We revisit
  the spectroscopic analysis of the extremely low-mass (ELM) WDs, and
  demonstrate that the 3D models largely resolve the discrepancies seen
  in the radius and mass measurements for relatively cool ELM WDs in
  eclipsing double WD and WD + millisecond pulsar binary systems. We
  also use the 3D corrections to revise the boundaries of the ZZ Ceti
  instability strip, including the recently found ELM pulsators.

Title: VLTI/AMBER Studies of the Atmospheric Structure and Fundamental
    Parameters of Red Giant and Supergiant Stars
Authors: Arroyo-Torres, B.; Wittkowski, M.; Marcaide, J. M.; Abellan,
   F. J.; Chiavassa, A.; Fabregat, J.; Freytag, B.; Guirado, J. C.;
   Hauschildt, P. H.; Marti-Vidal, I.; Quirrenbach, A.; Scholz, M.;
   Wood, P. R.
Bibcode: 2015ASPC..497...91A
Altcode:
  We present recent near-IR interferometric studies of red giant and
  supergiant stars, which are aimed at obtaining information on the
  structure of the atmospheric layers and constraining the fundamental
  parameters of these objects. <P />The observed visibilities of six red
  supergiants (RSGs), and also of one of the five red giants observed,
  indicate large extensions of the molecular layers, as previously
  observed for Mira stars. These extensions are not predicted by
  hydrostatic PHOENIX model atmospheres, hydrodynamical (RHD) simulations
  of stellar convection, or self-excited pulsation models. All these
  models based on parameters of RSGs lead to atmospheric structures
  that are too compact compared to our observations. We discuss how
  alternative processes might explain the atmospheric extensions for
  these objects. <P />As the continuum appears to be largely free of
  contamination by molecular layers, we can estimate reliable Rosseland
  angular radii for our stars. Together with distances and bolometric
  fluxes, we estimate the effective temperatures and luminosities of our
  targets, locate them in the HR diagram, and compare their positions
  to recent evolutionary tracks.

Title: Studying the Generation of Shock Waves in AGB Stars with
    3-Dimensional Radiation-Hydrodynamics Simulations
Authors: Freytag, B.
Bibcode: 2015ASPC..497...23F
Altcode:
  In the Sun, low-amplitude small-scale acoustic waves are just
  detectable in the photosphere and start to become dynamically relevant
  only in the lower chromosphere. The generation of these waves by
  non-stationary convective flows can be studied in detail by local
  3-D radiation-hydrodynamics simulations. Using this technique for
  global models of AGB stars reveals roughly similar phenomena but on a
  larger scale and with much higher amplitude. Convection cells spanning
  a significant fraction of the entire surface produce strong waves
  that cause a network of smaller shocks in the inner photosphere and
  occasional global shocks, travelling outward in large arcs. Material
  falling back interacts with the surface convection cells. A new
  generation of 3-D RHD simulations of these layers with CO5BOLD is
  presented and analyzed with particular attention given to acoustic
  waves and shock fronts.

Title: Calibration of the Mixing-Length Free Parameter for White
    Dwarf Structures
Authors: Tremblay, P. -E.; Ludwig, H. -G.; Freytag, B.; Fontaine,
   G.; Steffen, M.; Brassard, P.
Bibcode: 2015ASPC..493...89T
Altcode:
  We present a comparison of our grid of 3D radiation-hydrodynamical
  simulations for 70 pure-hydrogen DA white dwarfs, in the surface
  gravity range 7.0 ≤log g≤ 9.0, with 1D envelope models based on the
  mixing-length theory (MLT) for convection. We perform a calibration
  of the mixing-length parameter for the lower part of the convection
  zone. The 3D simulations are often restricted to the upper convective
  layers, and in those cases, we rely on the asymptotic entropy value of
  the adiabatic 3D upflows to calibrate 1D envelopes. Our results can be
  applied to 1D structure calculations, and in particular for pulsation
  and convective mixing studies. We demonstrate that while the 1D MLT
  only provides a bottom boundary of the convection zone based on the
  Schwarzschild criterion, the 3D stratifications are more complex. There
  is a large overshoot region below the convective layers that is likely
  critical for chemical diffusion applications.

Title: What causes the large extensions of red supergiant
    atmospheres?. Comparisons of interferometric observations with 1D
    hydrostatic, 3D convection, and 1D pulsating model atmospheres
Authors: Arroyo-Torres, B.; Wittkowski, M.; Chiavassa, A.; Scholz,
   M.; Freytag, B.; Marcaide, J. M.; Hauschildt, P. H.; Wood, P. R.;
   Abellan, F. J.
Bibcode: 2015A&A...575A..50A
Altcode: 2015arXiv150101560A
  <BR /> Aims: This research has two main goals. First, we present
  the atmospheric structure and the fundamental parameters of three
  red supergiants (RSGs), increasing the sample of RSGs observed by
  near-infrared spectro-interferometry. Additionally, we test possible
  mechanisms that may explain the large observed atmospheric extensions
  of RSGs. <BR /> Methods: We carried out spectro-interferometric
  observations of the RSGs V602 Car, HD 95687, and HD 183589 in the
  near-infrared K-band (1.92-2.47 μm) with the VLTI/AMBER instrument at
  medium spectral resolution (R ~ 1500). To categorize and comprehend
  the extended atmospheres, we compared our observational results to
  predictions by available hydrostatic PHOENIX, available 3D convection,
  and new 1D self-excited pulsation models of RSGs. <BR /> Results:
  Our near-infrared flux spectra of V602 Car, HD 95687, and HD 183589
  are well reproduced by the PHOENIX model atmospheres. The continuum
  visibility values are consistent with a limb-darkened disk as
  predicted by the PHOENIX models, allowing us to determine the angular
  diameter and the fundamental parameters of our sources. Nonetheless,
  in the case of V602 Car and HD 95686, the PHOENIX model visibilities
  do not predict the large observed extensions of molecular layers,
  most remarkably in the CO bands. Likewise, the 3D convection models
  and the 1D pulsation models with typical parameters of RSGs lead
  to compact atmospheric structures as well, which are similar to
  the structure of the hydrostatic PHOENIX models. They can also not
  explain the observed decreases in the visibilities and thus the
  large atmospheric molecular extensions. The full sample of our RSGs
  indicates increasing observed atmospheric extensions with increasing
  luminosity and decreasing surface gravity, and no correlation with
  effective temperature or variability amplitude. <BR /> Conclusions:
  The location of our RSG sources in the Hertzsprung-Russell diagram is
  confirmed to be consistent with the red limits of recent evolutionary
  tracks. The observed extensions of the atmospheric layers of our
  sample of RSGs are comparable to those of Mira stars. This phenomenon
  is not predicted by any of the considered model atmospheres including
  available 3D convection and new 1D pulsation models of RSGs. This
  confirms that neither convection nor pulsation alone can levitate the
  molecular atmospheres of RSGs. Our observed correlation of atmospheric
  extension with luminosity supports a scenario of radiative acceleration
  on Doppler-shifted molecular lines. <P />Based on observations made with
  the VLT Interferometer (VLTI) at Paranal Observatory under programme
  ID 091.D-0275.Figures 2-6 are available in electronic form at <A
  href="http://www.aanda.org/10.1051/0004-6361/201425212/olm">http://www.aanda.org</A>

Title: Calibration of the Mixing-length Theory for Convective White
    Dwarf Envelopes
Authors: Tremblay, P. -E.; Ludwig, H. -G.; Freytag, B.; Fontaine,
   G.; Steffen, M.; Brassard, P.
Bibcode: 2015ApJ...799..142T
Altcode: 2014arXiv1412.1789T
  A calibration of the mixing-length parameter in the local mixing-length
  theory (MLT) is presented for the lower part of the convection zone in
  pure-hydrogen-atmosphere white dwarfs. The parameterization is performed
  from a comparison of three-dimensional (3D) CO5BOLD simulations with
  a grid of one-dimensional (1D) envelopes with a varying mixing-length
  parameter. In many instances, the 3D simulations are restricted to the
  upper part of the convection zone. The hydrodynamical calculations
  suggest, in those cases, that the entropy of the upflows does not
  change significantly from the bottom of the convection zone to regions
  immediately below the photosphere. We rely on this asymptotic entropy
  value, characteristic of the deep and adiabatically stratified layers,
  to calibrate 1D envelopes. The calibration encompasses the convective
  hydrogen-line (DA) white dwarfs in the effective temperature range
  6000 &lt;= T <SUB>eff</SUB> (K) &lt;=15, 000 and the surface gravity
  range 7.0 &lt;= log g &lt;= 9.0. It is established that the local
  MLT is unable to reproduce simultaneously the thermodynamical, flux,
  and dynamical properties of the 3D simulations. We therefore propose
  three different parameterizations for these quantities. The resulting
  calibration can be applied to structure and envelope calculations,
  in particular for pulsation, chemical diffusion, and convective mixing
  studies. On the other hand, convection has no effect on the white dwarf
  cooling rates until there is a convective coupling with the degenerate
  core below T <SUB>eff</SUB> ~ 5000 K. In this regime, the 1D structures
  are insensitive to the MLT parameterization and converge to the mean
  3D results, hence they remain fully appropriate for age determinations.

Title: On the atmospheric structure and fundamental parameters of
    red supergiants
Authors: Wittkowski, M.; Arroyo-Torres, B.; Marcaide, J. M.; Abellan,
   F. J.; Chiavassa, A.; Freytag, B.; Scholz, M.; Wood, P. R.; Hauschildt,
   P. H.
Bibcode: 2015IAUS..307..280W
Altcode:
  We present near-infrared spectro-interferometric studies of red
  supergiant (RSG) stars using the VLTI/AMBER instrument, which are
  compared to previously obtained similar observations of AGB stars. Our
  observations indicate spatially extended atmospheric molecular layers of
  water vapor and CO, similar as previously observed for Mira stars. Data
  of VY~CMa indicate that the molecular layers are asymmetric, possibly
  clumpy. Thanks to the spectro-interferometric capabilities of the
  VLTI/AMBER instrument, we can isolate continuum bandpasses, estimate
  fundamental parameters of our sources, locate them in the HR diagram,
  and compare their positions to recent evolutionary tracks. For the
  example of VY CMa, this puts it close to evolutionary tracks of initial
  mass 25-32 M <SUB>⊙</SUB>. Comparisons of our data to hydrostatic
  model atmospheres, 3d simulations of convection, and 1d dynamic model
  atmospheres based on self-excited pulsation models indicate that
  none of these models can presently explain the observed atmospheric
  extensions for RSGs. The mechanism that levitates the atmospheres of
  red supergiant is thus a currently unsolved problem.

Title: Properties of small-scale magnetism of stellar atmospheres
Authors: Steiner, Oskar; Salhab, René; Freytag, Bernd; Rajaguru,
   Paul; Schaffenberger, Werner; Steffen, Matthias
Bibcode: 2014PASJ...66S...5S
Altcode: 2014PASJ..tmp...95S
  The magnetic field outside of sunspots is concentrated in the
  intergranular space, where it forms a delicate filigree of bright
  ribbons and dots as seen on broad band images of the Sun. We expect this
  small-scale magnetic field to exhibit a similar behavior in stellar
  atmospheres. In order to find out more about it, we perform numerical
  simulations of the surface layers of stellar atmospheres. Here, we
  report on preliminary results from simulations in the range between
  4000 K and 6500 K effective temperature with an initial vertical,
  homogeneous magnetic field of 50 G strength. We find that the field
  strength of the strongest magnetic flux concentrations increases with
  decreasing effective temperature at the height level where the average
  Rosseland optical depth is one. On the other hand, at the same level,
  the field is less strong than the thermal equipartition value in the
  coolest model but assumes superequipartition in the models hotter
  than 5000 K. While the Wilson depression of the strongest field
  concentrations is about one pressure scale height in the coolest
  model, it is more than four times the pressure scale height in the
  hottest one. We also find that the relative contribution of the bright
  filigree to the bolometric, vertically directed radiative intensity is
  most significant for the T<SUB>eff</SUB> = 5000 K model (0.6%-0.79%)
  and least significant for the hottest and coolest models (0.1%-0.46%
  and 0.14%-0.32%, respectively). This behavior suggests that the effect
  of the small-scale magnetic field on the photometric variability is more
  significant for K dwarf stars than for F-type and also M-type stars.

Title: White Dwarfs in the UKIRT Infrared Deep Sky Survey Data
    Release 9
Authors: Tremblay, P. -E.; Leggett, S. K.; Lodieu, N.; Freytag, B.;
   Bergeron, P.; Kalirai, J. S.; Ludwig, H. -G.
Bibcode: 2014ApJ...788..103T
Altcode:
  We have identified 8 to 10 new cool white dwarfs from the Large Area
  Survey (LAS) Data Release 9 of the United Kingdom InfraRed Telescope
  (UKIRT) Infrared Deep Sky Survey (UKIDSS). The data set was paired
  with the Sloan Digital Sky Survey to obtain proper motions and a broad
  ugrizYJHK wavelength coverage. Optical spectroscopic observations were
  secured at Gemini Observatory and confirm the degenerate status for
  eight of our targets. The final sample includes two additional white
  dwarf candidates with no spectroscopic observations. We rely on improved
  one-dimensional model atmospheres and new multi-dimensional simulations
  with CO5BOLD to review the stellar parameters of the published LAS
  white dwarf sample along with our additional discoveries. Most of the
  new objects possess very cool atmospheres with effective temperatures
  below 5000 K, including two pure-hydrogen remnants with a cooling age
  between 8.5 and 9.0 Gyr, and tangential velocities in the range 40 km
  s<SUP>-1</SUP> &lt;=v <SUB>tan</SUB> &lt;= 60 km s<SUP>-1</SUP>. They
  are likely thick disk 10-11 Gyr old objects. In addition, we find
  a resolved double degenerate system with v <SUB>tan</SUB> ~ 155 km
  s<SUP>-1</SUP> and a cooling age between 3.0 and 5.0 Gyr. These white
  dwarfs could be disk remnants with a very high velocity or former halo
  G stars. We also compare the LAS sample with earlier studies of very
  cool degenerates and observe a similar deficit of helium-dominated
  atmospheres in the range 5000 &lt; T <SUB>eff</SUB> (K) &lt; 6000. We
  review the possible explanations for the spectral evolution from
  helium-dominated toward hydrogen-rich atmospheres at low temperatures.

Title: White Dwarfs In The UKIRT Infrared Deep Sky Survey Data
    Release 9
Authors: Tremblay, P. -E.; Leggett, S. K.; Lodieu, N.; Freytag, B.;
   Bergeron, P.; Kalirai, J. S.; Ludwig, H. -G.
Bibcode: 2014arXiv1405.0266T
Altcode:
  We have identified eight to ten new cool white dwarfs from the Large
  Area Survey (LAS) Data Release 9 of the United Kingdom InfraRed
  Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS). The data
  set was paired with the Sloan Digital Sky Survey (SDSS) to obtain
  proper motions and a broad ugrizYJHK wavelength coverage. Optical
  spectroscopic observations were secured at Gemini Observatory
  and confirm the degenerate status for eight of our targets. The
  final sample includes two additional white dwarf candidates with no
  spectroscopic observations. We rely on improved 1D model atmospheres
  and new multi-dimensional simulations with CO5BOLD to review the
  stellar parameters of the published LAS white dwarf sample along with
  our additional discoveries. Most of the new objects possess very cool
  atmospheres with effective temperatures below 5000 K, including two
  pure-hydrogen remnants with a cooling age between 8.5 and 9.0 Gyr,
  and tangential velocities in the range 40 km/s &lt; vtan &lt; 60
  km/s. They are likely thick disk 10-11 Gyr-old objects. In addition
  we find a resolved double degenerate system with vtan ~ 155 km/s and a
  cooling age between 3.0 and 5.0 Gyr. These white dwarfs could be disk
  remnants with a very high velocity or former halo G stars. We also
  compare the LAS sample with earlier studies of very cool degenerates
  and observe a similar deficit of helium-dominated atmospheres in the
  range 5000 &lt; Teff (K) &lt; 6000. We review the possible explanations
  for the spectral evolution from helium-dominated towards hydrogen-rich
  atmospheres at low temperatures.

Title: 3D modeling of stellar atmospheres and the impact on the
    understanding of the reliability of elemental abundances in stars
    as tracers of galactic chemical evolution
Authors: Ludwig, H. -G.; Steffen, M.; Bonifacio, P.; Caffau, E.;
   Kučinskas, A.; Freytag, B.
Bibcode: 2014IAUS..298..343L
Altcode:
  We present a critical review of the construction of 3D model atmospheres
  with emphasis on modeling challenges. We discuss the basic physical
  processes which give rise to the effects which set 3D models apart
  from 1D standard models. We consider elemental abundances derived
  from molecular features, and the determination of the microturbulence
  with 3D models. The examples serve as illustration of the limitations
  inherent to 1D, however, also to 3D modeling. We find that 3D models
  can provide constraints on the microturbulence parameter, and predict
  substantial corrections for abundances derived from molecular species.

Title: 3D Model Atmospheres of White Dwarfs
Authors: Tremblay, Pier-Emmanuel; Ludwig, H.; Steffen, M.; Freytag, B.
Bibcode: 2014AAS...22331507T
Altcode:
  We present the first grid of 3D model atmospheres for
  hydrogen-atmosphere (DA) white dwarfs. These CO5BOLD
  radiation-hydrodynamics simulations, unlike the previous 1D
  calculations, do not rely on the mixing-length theory for the
  treatment of convection. The simulations have been employed to compute
  model spectra and we compared our improved Balmer line profiles to
  spectroscopic data from the Sloan Digital Sky Survey and the White
  Dwarf Catalog. The 3D surface gravities are found to be as much as
  0.3 dex lower than the values derived from 1D models. The white dwarfs
  with a radiative and a convective atmosphere have derived mean masses
  that are the same within 0.01 Msun with our new models, in much better
  agreement with our understanding of stellar evolution.

Title: Synthetic spectral libraries
Authors: Allard, F.; Homeier, D.; Freytag, B.
Bibcode: 2014ASInC..11...33A
Altcode:
  Within the next few years, several instruments aiming at imaging
  extrasolar planets will see first light. In parallel, low mass
  planets are being searched around red dwarfs which offer more favorable
  conditions, both for radial velocity detection and transit studies, than
  solar-type stars. We review recent advancements and issues concerning
  the construction of synthetic spectral libraries for very low mass
  stars, brown dwarfs and exoplanets. The revised solar oxygen abundances
  and cloud models allow to reproduce the photometric and spectroscopic
  properties of this transition to a degree never achieved before, but
  problems remain in the important M-L transition characteristic of the
  Teff range of characterizable exoplanets.

Title: Weather on the Nearest Brown Dwarfs: Resolved Simultaneous
    Multi-wavelength Variability Monitoring of WISE J104915.57-531906.1AB
Authors: Biller, Beth A.; Crossfield, Ian J. M.; Mancini, Luigi;
   Ciceri, Simona; Southworth, John; Kopytova, Taisiya G.; Bonnefoy,
   Mickaël; Deacon, Niall R.; Schlieder, Joshua E.; Buenzli, Esther;
   Brandner, Wolfgang; Allard, France; Homeier, Derek; Freytag, Bernd;
   Bailer-Jones, Coryn A. L.; Greiner, Jochen; Henning, Thomas; Goldman,
   Bertrand
Bibcode: 2013ApJ...778L..10B
Altcode: 2013arXiv1310.5144B
  We present two epochs of MPG/ESO 2.2 m GROND simultaneous six-band
  (r'i'z' JHK) photometric monitoring of the closest known L/T transition
  brown dwarf binary WISE J104915.57-531906.1AB. We report here the first
  resolved variability monitoring of both the T0.5 and L7.5 components. We
  obtained 4 hr of focused observations on the night of 2013 April 22
  (UT), as well as 4 hr of defocused (unresolved) observations on the
  night of 2013 April 16 (UT). We note a number of robust trends in our
  light curves. The r' and i' light curves appear to be anti-correlated
  with z' and H for the T0.5 component and in the unresolved light
  curve. In the defocused dataset, J appears correlated with z' and H
  and anti-correlated with r' and i', while in the focused dataset we
  measure no variability for J at the level of our photometric precision,
  likely due to evolving weather phenomena. In our focused T0.5 component
  light curve, the K band light curve displays a significant phase
  offset relative to both H and z'. We argue that the measured phase
  offsets are correlated with atmospheric pressure probed at each band,
  as estimated from one-dimensional atmospheric models. We also report
  low-amplitude variability in i' and z' intrinsic to the L7.5 component.

Title: Spectroscopic analysis of DA white dwarfs with 3D model
    atmospheres
Authors: Tremblay, P. -E.; Ludwig, H. -G.; Steffen, M.; Freytag, B.
Bibcode: 2013A&A...559A.104T
Altcode: 2013arXiv1309.0886T
  We present the first grid of mean three-dimensional (3D) spectra for
  pure-hydrogen (DA) white dwarfs based on 3D model atmospheres. We use
  CO<SUP>5</SUP>BOLD radiation-hydrodynamics 3D simulations instead of the
  mixing-length theory for the treatment of convection. The simulations
  cover the effective temperature range of 6000 &lt; T<SUB>eff</SUB>
  (K) &lt; 15 000 and the surface gravity range of 7 &lt; log g &lt;
  9 where the large majority of DAs with a convective atmosphere
  are located. We rely on horizontally averaged 3D structures (over
  constant Rosseland optical depth) to compute ⟨3D⟩ spectra. It
  is demonstrated that our ⟨3D⟩ spectra can be smoothly connected
  to their 1D counterparts at higher and lower T<SUB>eff</SUB> where
  the 3D effects are small. Analytical functions are provided in order
  to convert spectroscopically determined 1D effective temperatures
  and surface gravities to 3D atmospheric parameters. We apply our
  improved models to well studied spectroscopic data sets from the
  Sloan Digital Sky Survey and the White Dwarf Catalog. We confirm
  that the so-called high-log g problem is not present when employing
  ⟨3D⟩ spectra and that the issue was caused by inaccuracies in
  the 1D mixing-length approach. The white dwarfs with a radiative
  and a convective atmosphere have derived mean masses that are
  the same within ~0.01 M<SUB>⊙</SUB>, in much better agreement
  with our understanding of stellar evolution. Furthermore, the 3D
  atmospheric parameters are in better agreement with independent
  T<SUB>eff</SUB> and log g values from photometric and parallax
  measurements. <P />Appendices are only available in electronic form
  at <A href="http://www.aanda.org">http://www.aanda.org</A>

Title: A Search for Variability in a Young Planet
Authors: Biller, Beth; Crossfield, Ian; Deacon, Niall; Buenzli,
   Esther; Allard, France; Bonnefoy, Mickael; Schlieder, Joshua; Goldman,
   Bertrand; Homeier, Derek; Freytag, Bernd; Brandner, Wolfgang; Henning,
   Thomas; Bailer-Jones, Coryn
Bibcode: 2013sptz.prop10061B
Altcode:
  Variability attributed to cloud structure already appears to be a
  persistent feature for L and T type field brown dwarfs. Directly
  imaged planets occupy the same temperature regime as L and T type
  brown dwarfs and are likely to be equally variable. We propose
  mid-IR variability monitoring for the young free-floating planet
  CFHTWIR-Oph33 (&lt;0.5 Myr, ~6 MJup). Such variability is predicted due
  to rotationally-modulated cloud features. These observations will probe
  weather conditions on a directly imaged exoplanet for the first time,
  providing benchmark data for ongoing development of multidimensional
  cloud modeling approaches. These variability monitoring observations
  are only possible with Spitzer and are a pilot study for future
  variability monitoring with JWST-MIRI of directly imaged exoplanets
  around young stars.

Title: Granulation properties of giants, dwarfs, and white dwarfs
    from the CIFIST 3D model atmosphere grid
Authors: Tremblay, P. -E.; Ludwig, H. -G.; Freytag, B.; Steffen, M.;
   Caffau, E.
Bibcode: 2013A&A...557A...7T
Altcode: 2013arXiv1307.2810T
  Three-dimensional model atmospheres for giants, dwarfs, and white
  dwarfs, computed with the CO5BOLD code and part of the CIFIST grid,
  have been used for spectroscopic and asteroseismic studies. Unlike
  existing plane-parallel 1D structures, these simulations predict
  the spatially and temporally resolved emergent intensity so that
  granulation can be analysed, which provides insights on how convective
  energy transfer operates in stars. The wide range of atmospheric
  parameters of the CIFIST 3D simulations (3600 &lt; T<SUB>eff</SUB>
  (K) &lt; 13 000 and 1 &lt; log g &lt; 9) allows the comparison of
  convective processes in significantly different environments. We
  show that the relative intensity contrast is correlated with both
  the Mach and Péclet numbers in the photosphere. The horizontal size
  of granules varies between 3 and 10 times the local pressure scale
  height, with a tight correlation between the factor and the Mach
  number of the flow. Given that convective giants, dwarfs, and white
  dwarfs cover the same range of Mach and Péclet numbers, we conclude
  that photospheric convection operates in a very similar way in those
  objects. <P />Table 1 and Appendix A are available in electronic form
  at <A href="http://www.aanda.org">http://www.aanda.org</A>

Title: 3D hydrodynamical simulations to interpret observations of
    stellar surfaces of red supergiant stars
Authors: Chiavassa, A.; Freytag, B.; Plez, B.
Bibcode: 2013EAS....60..145C
Altcode:
  As red supergiants are the largest and brightest stars, they are
  ideal targets for the new generation of sensitive, high resolution
  instrumentation that provides spectrophotometric, interferometric,
  astrometric, and imaging observables. The interpretation of the complex
  stellar surface images requires numerical simulations of stellar
  convection that take into account multi-dimensional time-dependent
  radiation hydrodynamics with realistic input physics. We show the
  results obtained with the synergy between the radiative hydrodynamics
  code CO5BOLD and the post-processing radiative transfer code
  Optim3D. Such simulations support a proper and quantitative analysis
  of these observations, and the observations provide the theoretical
  work with strong constraints.

Title: Global radiation-hydrodynamics simulations of red supergiant
    stars
Authors: Freytag, B.; Chiavassa, A.
Bibcode: 2013EAS....60..137F
Altcode:
  The small-scale surface granulation on cool main-sequence stars and
  white dwarfs influences the overall appearance of these objects
  only weakly. And it is only indirectly observable by analyzing
  e.g. line-shapes or temporal fluctuations - except for the Sun. The
  large-scale and high-contrast convective surface cells and accompanying
  sound waves on supergiants and low-gravity AGB stars on the other hand
  have a strong impact on the outer atmospheric layers and are directly
  detectable by interferometric observations. Necessary to interpret
  modern observations with their high resolution in frequency, time,
  and/or space are detailed numerical multi-dimensional time-dependent
  radiation-hydrodynamical simulations. Local simulations of small patches
  of convective surface layers and the atmosphere of main-sequence stars
  have matured over three decades and have reached an impressive level
  of agreement with observations and also between different computational
  codes. However, global simulations of the entire convective surface and
  atmosphere of a red supergiants are considerably more demanding - and
  limited - and have become available only for about one decade. Still,
  they show how the surface is shaped by the interaction of small
  surface granules, that sit on top of large envelope convection cells,
  and waves, that can travel as shocks into the outer atmosphere. The
  route to more complete future models will be discussed, that comprise
  the outer atmosphere of the stars and that could explain some of
  the little-understood phenomena like chromosphere, molsphere, or
  wind-formation.

Title: Pure-hydrogen 3D model atmospheres of cool white dwarfs
Authors: Tremblay, P. -E.; Ludwig, H. -G.; Steffen, M.; Freytag, B.
Bibcode: 2013A&A...552A..13T
Altcode: 2013arXiv1302.2013T
  A sequence of pure-hydrogen CO5BOLD 3D model atmospheres of DA white
  dwarfs is presented for a surface gravity of log g = 8 and effective
  temperatures from 6000 to 13 000 K. We show that convective properties,
  such as flow velocities, characteristic granulation size and intensity
  contrast of the granulation patterns, change significantly over this
  range. We demonstrate that these 3D simulations are not sensitive to
  numerical parameters unlike the 1D structures that considerably depend
  on the mixing-length parameters. We conclude that 3D spectra can be used
  directly in the spectroscopic analyses of DA white dwarfs. We confirm
  the result of an earlier preliminary study that 3D model spectra provide
  a much better characterization of the mass distribution of white dwarfs
  and that shortcomings of the 1D mixing-length theory are responsible
  for the spurious high-log g determinations of cool white dwarfs. In
  particular, the 1D theory is unable to account for the cooling effect
  of the convective overshoot in the upper atmospheres.

Title: Convective line shifts for the Gaia RVS from the CIFIST 3D
    model atmosphere grid
Authors: Allende Prieto, C.; Koesterke, L.; Ludwig, H. -G.; Freytag,
   B.; Caffau, E.
Bibcode: 2013A&A...550A.103A
Altcode: 2013arXiv1301.3703A
  Context. To derive space velocities of stars along the line of sight
  from wavelength shifts in stellar spectra requires accounting for a
  number of second-order effects. For most stars, gravitational redshifts,
  convective blueshifts, and transverse stellar motion are the dominant
  contributors. <BR /> Aims: We provide theoretical corrections for the
  net velocity shifts due to convection expected for the measurements
  from the Gaia Radial Velocity Spectrometer (RVS). <BR /> Methods: We
  used a set of three-dimensional time-dependent simulations of stellar
  surface convection computed with CO5BOLD to calculate spectra of
  late-type stars in the Gaia RVS range and to infer the net velocity
  offset that convective motions will induce in radial velocities
  derived by cross-correlation. <BR /> Results: The net velocity shifts
  derived by cross-correlation depend both on the wavelength range and
  spectral resolution of the observations. Convective shifts for Gaia
  RVS observations are less than 0.1 km s<SUP>-1</SUP> for late-K-type
  stars, and they increase with stellar mass, reaching about 0.3 km
  s<SUP>-1</SUP> or more for early F-type dwarfs. This tendency is the
  result of an increase with effective temperature in both temperature
  and velocity fluctuations in the line-forming region. Our simulations
  also indicate that the net RVS convective shifts can be positive
  (i.e. redshifts) in some cases. Overall, the blueshifts weaken
  slightly with increasing surface gravity, and are enhanced at low
  metallicity. Gravitational redshifts amount to 0.7 km s<SUP>-1</SUP>
  and dominate convective blueshifts for dwarfs, but become much
  weaker for giants. <P />Appendix A is available in electronic form
  at <A href="http://www.aanda.org">http://www.aanda.org</A>Model
  spectra from the 1D and 3D calculations are only available
  in electronic form at the CDS via anonymous ftp to <A
  href="http://cdsarc.u-strasbg.fr">cdsarc.u-strasbg.fr</A><A
  href="http://130.79.128.5">130.79.128.5</A> or via <A
  href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/550/A103">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/550/A103</A>

Title: Radiation hydrodynamics simulations of brown dwarf atmospheres
    with CO5BOLD
Authors: Freytag, B.; Allard, F.; Homeier, D.
Bibcode: 2013MmSAI..84.1070F
Altcode:
  The interplay of radiative and hydrodynamic processes governs
  the stratified atmospheres of cool stars and substellar objects,
  with the underlying convection zone playing an important role. At
  sufficiently low temperatures, matter can condense into dust grains,
  whose formation, growths, destruction, and gravitational settling
  critically depend on the thermal structure of the atmosphere. Equally
  important are horizontal and vertical mixing and transport of material,
  that can compensate the rain-out of dust, induced on small scales
  by convective motions, overshoot, gravity waves, and turbulence, but
  also on larger scales by a global wind system strongly influenced by
  the rapid rotation of a typical brown dwarf. Local and first simple
  global radiation-hydrodynamics simulations of cool objects with the
  CO5BOLD code are presented.

Title: First steps with CO5BOLD using HLLMHD and PP reconstruction .
Authors: Steiner, O.; Rajaguru, S. P.; Vigeesh, G.; Steffen, M.;
   Schaffenberger, W.; Freytag, B.
Bibcode: 2013MSAIS..24..100S
Altcode:
  We report on first experiences with real-life applications using
  the MHD-module of CO5BOLD together with the piecewise parabolic
  reconstruction scheme and present preliminary results of stellar
  magnetic models with T<SUB>eff</SUB> = 4000 K to T<SUB>eff</SUB> =
  5770 K.

Title: Granulation in DA white dwarfs from CO5BOLD 3D model
    atmospheres
Authors: Tremblay, P. -E.; Ludwig, H. -G.; Freytag, B.; Steffen, M.
Bibcode: 2013MSAIS..24...61T
Altcode:
  Time-dependent 3D simulations of pure-hydrogen DA white dwarf
  atmospheres have been computed in recent years. Synthetic Balmer lines
  spectra drawn from these radiation-hydrodynamics (RHD) simulations
  have been shown to predict surface gravities significantly lower than
  the standard 1D models, in much better agreement with the expectation
  that white dwarfs cool at constant mass. We have now computed a grid
  of CO5BOLD pure-hydrogen 3D model atmospheres for surface gravities
  from log g = 7 to log g = 8.5 and effective temperatures from 6000 to
  13,000 K. Over this range, we observe a significant variation of the
  intensity contrast of the surface granulation patterns, which indicates
  the strength of the 3D effects. Furthermore, the size and appearance
  of granules are also varying considerably. An explanation of these
  behaviours can lead to a better understanding of the physical processes
  responsible for the energy transfer in white dwarf atmospheres.

Title: Progress in modeling very low mass stars, brown dwarfs,
    and planetary mass objects.
Authors: Allard, F.; Homeier, D.; Freytag, B.; Schaffenberger, W.;
   Rajpurohit, A. S.
Bibcode: 2013MSAIS..24..128A
Altcode: 2013arXiv1302.6559A
  We review recent advancements in modeling the stellar to substellar
  transition. The revised molecular opacities, solar oxygen abundances
  and cloud models allow to reproduce the photometric and spectroscopic
  properties of this transition to a degree never achieved before,
  but problems remain in the important M-L transition characteristic
  of the effective temperature range of characterizable exoplanets. We
  discuss of the validity of these classical models. We also present
  new preliminary global Radiation HydroDynamical M dwarfs simulations.

Title: Atmospheres from very low-mass stars to extrasolar planets.
Authors: Allard, F.; Homeier, D.; Freytag, B.
Bibcode: 2013MmSAI..84.1053A
Altcode:
  Within the next few years, several instruments aiming at imaging
  extrasolar planets will see first light. In parallel, low mass planets
  are being searched around red dwarfs which offer more favorable
  conditions, both for radial velocity detection and transit studies,
  than solar-type stars. We review recent advancements in modeling the
  stellar to substellar transition. The revised solar oxygen abundances
  and cloud models allow to reproduce the photometric and spectroscopic
  properties of this transition to a degree never achieved before, but
  problems remain in the important M-L transition characteristic of the
  T_{eff } range of characterisable exoplanets.

Title: Advances in the hydrodynamics solver of CO5BOLD
Authors: Freytag, Bernd
Bibcode: 2013MSAIS..24...26F
Altcode:
  Many features of the Roe solver used in the hydrodynamics module
  of CO5BOLD have recently been added or overhauled, including the
  reconstruction methods (by adding the new second-order ``Frankenstein's
  method''), the treatment of transversal velocities, energy-flux
  averaging and entropy-wave treatment at small Mach numbers, the CTU
  scheme to combine the one-dimensional fluxes, and additional safety
  measures. All this results in a significantly better behavior at low
  Mach number flows, and an improved stability at larger Mach numbers
  requiring less (or no) additional tensor viscosity, which then leads
  to a noticeable increase in effective resolution.

Title: Oxygen spectral line synthesis: 3D non-LTE with
    CO<SUP>5</SUP>BOLD hydrodynamical model atmospheres.
Authors: Prakapavičius, D.; Steffen, M.; Kučinskas, A.; Ludwig,
   H. -G.; Freytag, B.; Caffau, E.; Cayrel, R.
Bibcode: 2013MSAIS..24..111P
Altcode: 2013arXiv1303.2016P
  In this work we present first results of our current project aimed at
  combining the 3D hydrodynamical stellar atmosphere approach with non-LTE
  (NLTE) spectral line synthesis for a number of key chemical species. We
  carried out a full 3D-NLTE spectrum synthesis of the oxygen IR 777 nm
  triplet, using a modified and improved version of our NLTE3D package to
  calculate departure coefficients for the atomic levels of oxygen in a
  CO<SUP>5</SUP>BOLD 3D hydrodynamical solar model atmosphere. Spectral
  line synthesis was subsequently performed with the Linfor3D code. In
  agreement with previous studies, we find that the lines of the
  oxygen triplet produce deeper cores under NLTE conditions, due to
  the diminished line source function in the line forming region. This
  means that the solar oxygen IR 777 nm lines should be stronger in NLTE,
  leading to negative 3D NLTE-LTE abundance corrections. Qualitatively
  this result would support previous claims for a relatively low solar
  oxygen abundance. Finally, we outline several further steps that need
  to be taken in order to improve the physical realism and numerical
  accuracy of our current 3D-NLTE calculations.

Title: 3D Model Atmospheres of DA White Dwarfs
Authors: Tremblay, P. -E.; Ludwig, H. -G.; Steffen, M.; Freytag, B.
Bibcode: 2013ASPC..469..155T
Altcode:
  The spectroscopically determined surface gravities of cool
  hydrogen-atmosphere DA white dwarfs are significantly higher than
  the mean value of log g ∼ 8 found for hotter objects with radiative
  atmospheres (T<SUB>eff</SUB> &gt; 13000 K). It was recently suggested
  that a problem with the treatment of convective energy transport in
  the 1D mixing-length theory was the explanation for this high-log
  g problem. We have now computed a grid of pure-hydrogen 3D model
  atmospheres with the CO<SUP>5</SUP>BOLD code for surface gravities
  from log g = 7 to 8.5 and effective temperatures from 6000 to 13000
  K. Over this range, the intensity contrast of the surface granulation
  pattern, which describes the strength of the 3D effects, is varying
  significantly. We confirm the result of an earlier investigation that
  3D model spectra provide a much better characterization of the mass
  distribution of cool white dwarfs.

Title: VizieR Online Data Catalog: Model 1D (LHD) and 3D (CO5BOLD)
    spectra (Allende Prieto+, 2013)
Authors: Allende Prieto, C.; Koesterke, L. Ludwig H. -G.; Freytag,
   B.; Caffau, E.
Bibcode: 2012yCat..35500103A
Altcode: 2012yCat..35509103A
  Model spectral fluxes for late-type stars computed from 3D
  hydrodynamical simulations of surface convection performed with the
  CO5BOLD code. Their 1D hydrostatic counterparts are included, based on
  the LHD code, sharing the same microphysics as the CO5BOLD models. The
  fluxes for both the 3D and 1D models are calculated with the same
  opacities and radiative transfer code (ASSET). <P />(6 data files).

Title: Atmospheres From Very Low-Mass Stars to Extrasolar Planets
Authors: Allard, F.; Homeier, D.; Freytag, B.; Sharp, C. M.
Bibcode: 2012EAS....57....3A
Altcode: 2012arXiv1206.1021A
  Within the next few years, several instruments aiming at imaging
  extrasolar planets will see first light. In parallel, low mass planets
  are being searched around red dwarfs which offer more favorable
  conditions - both for radial velocity detection and transit studies -
  than solar-type stars. We review recent advancements in modeling the
  stellar to substellar transition. The revised solar oxygen abundances
  and cloud models allow to reproduce the photometric and spectroscopic
  properties of this transition to a degree never achieved before, but
  problems remain at the stellar-brown dwarf transition typical of the
  T<SUB>eff</SUB> range of characterizable exoplanets.

Title: Models of very-low-mass stars, brown dwarfs and exoplanets
Authors: Allard, F.; Homeier, D.; Freytag, B.
Bibcode: 2012RSPTA.370.2765A
Altcode: 2011arXiv1112.3591A
  Within the next few years, GAIA and several instruments aiming at imag-
  ing extrasolar planets will see first light. In parallel, low mass
  planets are being searched around red dwarfs which offer more favourable
  conditions, both for radial velocity de- tection and transit studies,
  than solar-type stars. Authors of the model atmosphere code which has
  allowed the detection of water vapour in the atmosphere of Hot Jupiters
  re- view recent advancement in modelling the stellar to substellar
  transition. The revised solar oxygen abundances and cloud model allow
  for the first time to reproduce the pho- tometric and spectroscopic
  properties of this transition. Also presented are highlight results of
  a model atmosphere grid for stars, brown dwarfs and extrasolar planets.

Title: Imaging the heart of astrophysical objects with optical
    long-baseline interferometry
Authors: Berger, J. -P.; Malbet, F.; Baron, F.; Chiavassa, A.;
   Duvert, G.; Elitzur, M.; Freytag, B.; Gueth, F.; Hönig, S.; Hron, J.;
   Jang-Condell, H.; Le Bouquin, J. -B.; Monin, J. -L.; Monnier, J. D.;
   Perrin, G.; Plez, B.; Ratzka, T.; Renard, S.; Stefl, S.; Thiébaut,
   E.; Tristram, K. R. W.; Verhoelst, T.; Wolf, S.; Young, J.
Bibcode: 2012A&ARv..20...53B
Altcode: 2012arXiv1204.4363B
  The number of publications of aperture-synthesis images based on optical
  long-baseline interferometry measurements has recently increased
  due to easier access to visible and infrared interferometers. The
  interferometry technique has now reached a technical maturity level
  that opens new avenues for numerous astrophysical topics requiring
  milli-arcsecond model-independent imaging. In writing this paper
  our motivation was twofold: (1) review and publicize emblematic
  excerpts of the impressive corpus accumulated in the field of optical
  interferometry image reconstruction; (2) discuss future prospects
  for this technique by selecting four representative astrophysical
  science cases in order to review the potential benefits of using
  optical long-baseline interferometers. For this second goal we have
  simulated interferometric data from those selected astrophysical
  environments and used state-of-the-art codes to provide the
  reconstructed images that are reachable with current or soon-to-be
  facilities. The image-reconstruction process was "blind" in the
  sense that reconstructors had no knowledge of the input brightness
  distributions. We discuss the impact of optical interferometry in
  those four astrophysical fields. We show that image-reconstruction
  software successfully provides accurate morphological information on
  a variety of astrophysical topics and review the current strengths
  and weaknesses of such reconstructions. We investigate how to improve
  image reconstruction and the quality of the image possibly by upgrading
  the current facilities. We finally argue that optical interferometers
  and their corresponding instrumentation, existing or to come, with
  six to ten telescopes, should be well suited to provide images of
  complex sceneries.

Title: Stellar to Substellar Model Atmospheres
Authors: Allard, France; Homeier, Derek; Freytag, Bernd
Bibcode: 2012IAUS..282..235A
Altcode:
  The spectral transition from Very Low Mass stars (VLMs) to brown
  dwarfs (BDs) and planetary mass objects (Planemos) requires model
  atmospheres that can treat line, molecule, and dust-cloud formation
  with completeness and accuracy. One of the essential problems is
  the determination of the surface velocity field throughout the main
  sequence down to the BD and planemo mass regimes. We present local 2D
  and 3D radiation hydrodynamic simulations using the CO5BOLD code with
  binned Phoenix gas opacities, forsterite dust formation (and opacities)
  and rotation. The resulting velocity field vs depth and Teff has been
  used in the general purpose model atmosphere code Phoenix, adapted in
  static 1D spherical symmetry for these cool atmospheres. The result is
  a better understanding of the spectral transition from the stellar to
  substellar regimes. However, problems remain in reproducing the colors
  of the dustiest brown dwarfs. The global properties of rotation can
  change the averaged spectral properties of these objects. Our project
  for the period 2011-2015 is therefore to develop scaled down global
  3D simulations of convection, cloud formation and rotation thanks to
  funding by the Agence Nationale de la Recherche in France.

Title: Simulations of the solar near-surface layers with the CO5BOLD,
    MURaM, and Stagger codes
Authors: Beeck, B.; Collet, R.; Steffen, M.; Asplund, M.; Cameron,
   R. H.; Freytag, B.; Hayek, W.; Ludwig, H. -G.; Schüssler, M.
Bibcode: 2012A&A...539A.121B
Altcode: 2012arXiv1201.1103B
  Context. Radiative hydrodynamic simulations of solar and stellar surface
  convection have become an important tool for exploring the structure and
  gas dynamics in the envelopes and atmospheres of late-type stars and for
  improving our understanding of the formation of stellar spectra. <BR
  /> Aims: We quantitatively compare results from three-dimensional,
  radiative hydrodynamic simulations of convection near the solar surface
  generated with three numerical codes (CO<SUP>5</SUP>BOLD, MURaM,
  and Stagger) and different simulation setups in order to investigate
  the level of similarity and to cross-validate the simulations. <BR
  /> Methods: For all three simulations, we considered the average
  stratifications of various quantities (temperature, pressure, flow
  velocity, etc.) on surfaces of constant geometrical or optical depth,
  as well as their temporal and spatial fluctuations. We also compared
  observables, such as the spatially resolved patterns of the emerging
  intensity and of the vertical velocity at the solar optical surface
  as well as the center-to-limb variation of the continuum intensity
  at various wavelengths. <BR /> Results: The depth profiles of the
  thermodynamical quantities and of the convective velocities as well as
  their spatial fluctuations agree quite well. Slight deviations can be
  understood in terms of differences in box size, spatial resolution
  and in the treatment of non-gray radiative transfer between the
  simulations. <BR /> Conclusions: The results give confidence in the
  reliability of the results from comprehensive radiative hydrodynamic
  simulations.

Title: Simulations of stellar convection with CO5BOLD
Authors: Freytag, B.; Steffen, M.; Ludwig, H. -G.; Wedemeyer-Böhm,
   S.; Schaffenberger, W.; Steiner, O.
Bibcode: 2012JCoPh.231..919F
Altcode: 2011arXiv1110.6844F
  High-resolution images of the solar surface show a granulation
  pattern of hot rising and cooler downward-sinking material - the
  top of the deep-reaching solar convection zone. Convection plays a
  role for the thermal structure of the solar interior and the dynamo
  acting there, for the stratification of the photosphere, where most
  of the visible light is emitted, as well as for the energy budget of
  the spectacular processes in the chromosphere and corona. Convective
  stellar atmospheres can be modeled by numerically solving the coupled
  equations of (magneto)hydrodynamics and non-local radiation transport
  in the presence of a gravity field. The CO5BOLD code described in this
  article is designed for so-called "realistic" simulations that take
  into account the detailed microphysics under the conditions in solar
  or stellar surface layers (equation-of-state and optical properties of
  the matter). These simulations indeed deserve the label "realistic"
  because they reproduce the various observables very well - with only
  minor differences between different implementations. The agreement
  with observations has improved over time and the simulations are now
  well-established and have been performed for a number of stars. Still,
  severe challenges are encountered when it comes to extending these
  simulations to include ideally the entire star or substellar object:
  the strong stratification leads to completely different conditions in
  the interior, the photosphere, and the corona. Simulations have to cover
  spatial scales from the sub-granular level to the stellar diameter and
  time scales from photospheric wave travel times to stellar rotation
  or dynamo cycle periods. Various non-equilibrium processes have to be
  taken into account. Last but not least, realistic simulations are based
  on detailed microphysics and depend on the quality of the input data,
  which can be the actual accuracy limiter. This article provides an
  overview of the physical problem and the numerical solution and the
  capabilities of CO5BOLD, illustrated with a number of applications.

Title: Model Atmospheres From Very Low Mass Stars to Brown Dwarfs
Authors: Allard, F.; Homeier, D.; Freytag, B.
Bibcode: 2011ASPC..448...91A
Altcode: 2011csss...16...91A; 2010arXiv1011.5405A
  Since the discovery of brown dwarfs in 1994, and the discovery of dust
  cloud formation in the latest Very Low Mass Stars (VLMs) and Brown
  Dwarfs (BDs) in 1996, the most important challenge in modeling their
  atmospheres as become the understanding of cloud formation and advective
  mixing. For this purpose, we have developed radiation hydrodynamic
  2D model atmosphere simulations to study the formation of forsterite
  dust in presence of advection, condensation, and sedimentation across
  the M-L-T VLMs to BDs sequence (T<SUB>eff</SUB> = 2800 K to 900 K,
  Freytag et al. 2010). We discovered the formation of gravity waves as
  a driving mechanism for the formation of clouds in these atmospheres,
  and derived a rule for the velocity field versus atmospheric depth and
  T<SUB>eff</SUB>, which is relatively insensitive to gravity. This rule
  has been used in the construction of the new model atmosphere grid,
  BT-Settl, to determine the micro-turbulence velocity, the diffusion
  coefficient, and the advective mixing of molecules as a function of
  depth. This new model grid of atmospheres and synthetic spectra has
  been computed for 100,000 K &gt; T<SUB>eff</SUB> &gt; 400 K, 5.5 &gt;
  logg &gt; -0.5, and [M/H]= +0.5 to -1.5, and the reference solar
  abundances of Asplund et al. (2009). We found that the new solar
  abundances allow an improved (close to perfect) reproduction of the
  photometric and spectroscopic VLMs properties, and, for the first
  time, a smooth transition between stellar and substellar regimes --
  unlike the transition between the NextGen models from Hauschildt et
  al. 1999a,b, and the AMES-Dusty models from Allard et al. 2001. In the
  BDs regime, the BT-Settl models propose an improved explanation for
  the M-L-T spectral transition. In this paper, we therefore present
  the new BT-Settl model atmosphere grid, which explains the entire
  transition from the stellar to planetary mass regimes.

Title: Radiation-Hydrodynamics Simulations of Cool Stellar and
    Substellar Atmospheres
Authors: Freytag, B.; Allard, F.; Ludwig, H. -G.; Homeier, D.;
   Steffen, M.
Bibcode: 2011ASPC..448..855F
Altcode: 2011csss...16..855F
  In the atmospheres of brown dwarfs, not only molecules but much
  larger and heavier "dust" particles can form. The latter should sink
  under the influence of gravity into deeper layers and vanish from the
  atmosphere, clearing it from condensable material. However, observed
  spectra can only be reproduced by models assuming the presence of dust
  and its resulting greenhouse effect in the visible layers. Apparently,
  hydrodynamical mixing can counteract the gravitational settling. We
  present new 2D and 3D radiation-hydrodynamics simulations with CO5BOLD
  of the upper part of the convection zone and the atmosphere of cool
  stars and brown dwarfs in a range of temperatures and gravities
  that enable the formation of dust clouds in the visible layers. We
  find that the differences between 2D and 3D models are remarkably
  small. Lowering the gravity has a somewhat similar effect on the surface
  intensity contrast as increasing the effective temperature. The biggest
  uncertainties of the simulations come from approximations made in the
  description of the dust chemistry. Global circulation and rotation
  likely play an important role.

Title: Radiation Hydrodynamics Simulations of Dust Clouds in the
    Atmospheres of Substellar Objects
Authors: Freytag, B.; Allard, F.; Homeier, D.; Ludwig, H.; Steffen, M.
Bibcode: 2011ASPC..450..125F
Altcode:
  The temperature structure and the motions in the atmospheres of cool
  stars are affected by the underlying convection zone. The radiation
  hydrodynamics code CO5BOLD has been developed to simulate (small patches
  of the) convective surface layers of these stars. Updated opacity tables
  based on PHOENIX data and a description for the formation, destruction,
  advective transport, and settling of dust have made the code fit to
  handle the conditions in brown dwarf atmospheres. Currently, objects
  from 8500 K down to about 900 K have been simulated. Recently, incident
  radiation has been included, allowing simulations with conditions
  found on hot planets. In non-irradiated brown dwarf models we encounter
  mixing by gravity waves and in the cooler models convection within the
  clouds. The qualitative effects of incident radiation are surprisingly
  small, as long as the effective temperature of the object stays well
  below the dust condensation temperature. Beyond that point, there are
  no layers where dust could form, anymore.

Title: Radiative hydrodynamics simulations of red supergiant
    stars. IV. Gray versus non-gray opacities
Authors: Chiavassa, A.; Freytag, B.; Masseron, T.; Plez, B.
Bibcode: 2011A&A...535A..22C
Altcode: 2011arXiv1109.3619C
  Context. Red supergiants are massive evolved stars that contribute
  extensively to the chemical enrichment of our Galaxy. It has been
  shown that convection in those stars produces large granules that
  cause surface inhomogeneities and shock waves in the photosphere. The
  understanding of their dynamics is crucial for unveiling the unknown
  mass-loss mechanism, their chemical composition, and their stellar
  parameters. <BR /> Aims: We present a new generation of red supergiant
  simulations with a more sophisticated opacity treatment performed with
  3D radiative-hydrodynamics code CO5BOLD. <BR /> Methods: In the code
  the coupled equations of compressible hydrodynamics and non-local
  radiation transport are solved in the presence of a spherical
  potential. The stellar core is replaced by a special spherical
  inner boundary condition, where the gravitational potential is
  smoothed and the energy production by fusion is mimicked by a simply
  producing heat corresponding to the stellar luminosity. All outer
  boundaries are transmitting for matter and light. The post-processing
  radiative transfer code OPTIM3D is used to extract spectroscopic
  and interferometric observables. <BR /> Results: We show that if
  one relaxes the assumption of frequency-independent opacities, this
  leads to a steeper mean thermal gradient in the optical thin region
  that strongly affects the atomic strengths and the spectral energy
  distribution. Moreover, the weaker temperature fluctuations reduce the
  incertitude on the radius determination with interferometry. We show
  that 1D models of red supergiants must include a turbulent velocity
  that is calibrated on 3D simulations to obtain the effective surface
  gravity that mimics the effect of turbulent pressure on the stellar
  atmosphere. We provide an empirical calibration of the ad hoc micro-
  and macroturbulence parameters for 1D models using the 3D simulations:
  we find that there is no clear distinction between the different
  macroturbulent profiles needed in 1D models to fit 3D synthetic lines.

Title: Photocentric and Photometric Variability of Red Supergiant
    Stars
Authors: Chiavassa, A.; Pasquato, E.; Jorissen, A.; Sacuto, S.;
   Babusiaux, C.; Freytag, B.; Ludwig, H. -G.; Cruzalèbes, P.; Rabbia,
   Y.; Spang, A.; Chesneau, O.
Bibcode: 2011ASPC..445..169C
Altcode:
  Red supergiant stars are characterized by large convection-related
  surface structures that cause surface inhomogeneities and shock
  waves. We explore the impact of granulation on photocentric and
  photometric variability using 3D simulations of convection with Co5BOLD
  and the post-processing radiative transfer code OPTIM 3D to compute
  spectra and intensity maps in the Gaia G band (325 - 1030 nm).

Title: Solution to the problem of the surface gravity distribution
    of cool DA white dwarfs from improved 3D model atmospheres
Authors: Tremblay, P. -E.; Ludwig, H. -G.; Steffen, M.; Bergeron,
   P.; Freytag, B.
Bibcode: 2011A&A...531L..19T
Altcode: 2011arXiv1106.6007T
  The surface gravities of cool (T<SUB>eff</SUB> &lt; 13 000 K)
  hydrogen-atmosphere DA white dwarfs, determined from spectroscopic
  analyses, are found to be significantly higher than the canonical value
  of log g ~ 8 expected for these stars. It was recently concluded that
  a problem with the treatment of convective energy transport within
  the framework of the mixing-length theory was the most plausible
  explanation for this high-log g problem. We pursue the investigation
  of this discrepancy by computing model spectra of cool convective
  white dwarfs from a small sequence (11 300 K &lt; T<SUB>eff</SUB>
  &lt; 12 800 K) of 3D hydrodynamical model atmospheres, which feature
  a sophisticated treatment of convection and radiative transfer. Our
  approach is to proceed with a differential analysis between 3D and
  standard 1D models. We find that the 3D spectra predict significantly
  lower surface gravities, with corrections of the right amplitude
  as a function of effective temperature to obtain values of log g ~
  8 on average. We conclude that the surface gravity distribution of
  cool convective DA white dwarfs is much closer to that of hotter
  radiative objects when using, for the treatment of the convection,
  3D models instead of the mixing-length framework. <P />Figure 2 is
  available in electronic form at http://www.aanda.org

Title: Radiative hydrodynamic simulations of red supergiant
    stars. III. Spectro-photocentric variability, photometric variability,
    and consequences on Gaia measurements
Authors: Chiavassa, A.; Pasquato, E.; Jorissen, A.; Sacuto, S.;
   Babusiaux, C.; Freytag, B.; Ludwig, H. -G.; Cruzalèbes, P.; Rabbia,
   Y.; Spang, A.; Chesneau, O.
Bibcode: 2011A&A...528A.120C
Altcode: 2010arXiv1012.5234C
  Context. It has been shown that convection in red supergiant stars (RSG)
  gives rise to large granules that cause surface inhomogeneities and
  shock waves in the photosphere. The resulting motion of the photocentre
  (on time scales ranging from months to years) could possibly have
  adverse effects on the parallax determination with Gaia. <BR /> Aims:
  We explore the impact of the granulation on the photocentric and
  photometric variability. We quantify these effects in order to better
  characterise the error that could possibly alter the parallax. <BR
  /> Methods: We use 3D radiative-hydrodynamics (RHD) simulations of
  convection with CO5BOLD and the post-processing radiative transfer
  code Optim3D to compute intensity maps and spectra in the Gaia G band
  [325-1030 nm]. <BR /> Results: We provide astrometric and photometric
  predictions from 3D simulations of RSGs that are used to evaluate
  the possible degradation of the astrometric parameters of evolved
  stars derived by Gaia. We show in particular from RHD simulations
  that a supergiant like Betelgeuse exhibits a photocentric noise
  characterised by a standard deviation of the order of 0.1 AU. The
  number of bright giant and supergiant stars whose Gaia parallaxes
  will be altered by the photocentric noise ranges from a few tens to
  several thousands, depending on the poorly known relation between the
  size of the convective cells and the atmospheric pressure scale height
  of supergiants, and to a lower extent, on the adopted prescription
  for galactic extinction. In the worst situation, the degradation
  of the astrometric fit caused by this photocentric noise will be
  noticeable up to about 5 kpc for the brightest supergiants. Moreover,
  parallaxes of Betelgeuse-like supergiants are affected by an error of
  the order of a few percents. We also show that the photocentric noise,
  as predicted by the 3D simulation, does account for a substantial part
  of the supplementary "cosmic noise" that affects Hipparcos measurements
  of Betelgeuse and Antares.

Title: Solar Chemical Abundances Determined with a CO5BOLD 3D Model
    Atmosphere
Authors: Caffau, E.; Ludwig, H. -G.; Steffen, M.; Freytag, B.;
   Bonifacio, P.
Bibcode: 2011SoPh..268..255C
Altcode: 2010SoPh..tmp...66C; 2010arXiv1003.1190C
  In the last decade, the photospheric solar metallicity as determined
  from spectroscopy experienced a remarkable downward revision. Part
  of this effect can be attributed to an improvement of atomic data and
  the inclusion of NLTE computations, but also the use of hydrodynamical
  model atmospheres seemed to play a role. This "decrease" with time of
  the metallicity of the solar photosphere increased the disagreement
  with the results from helioseismology. With a CO<SUP>5</SUP>BOLD 3D
  model of the solar atmosphere, the CIFIST team at the Paris Observatory
  re-determined the photospheric solar abundances of several elements,
  among them C, N, and O. The spectroscopic abundances are obtained by
  fitting the equivalent width and/or the profile of observed spectral
  lines with synthetic spectra computed from the 3D model atmosphere. We
  conclude that the effects of granular fluctuations depend on the
  characteristics of the individual lines, but are found to be relevant
  only in a few particular cases. 3D effects are not responsible for
  the systematic lowering of the solar abundances in recent years. The
  solar metallicity resulting from this analysis is Z=0.0153, Z/X=0.0209.

Title: Photocentric variability of red supergiant stars and
    consequences on Gaia measurements
Authors: Chiavassa, A.; Pasquato, E.; Jorissen, A.; Sacuto, S.;
   Babusiaux, C.; Freytag, B.; Ludwig, H. -G.; Cruzalèbes, P.; Rabbia,
   Y.; Spang, A.; Chesneau, O.
Bibcode: 2010sf2a.conf..339C
Altcode:
  Red supergiant stars are characterized by large convection-related
  surface structures that cause surface inhomogeneities and shock
  waves. We explore the impact of granulation on the photocentric motion
  using 3D simulations of convection with CO5BOLD and post-processing
  radiative transfer code Optim3D to compute spectra and intensity maps
  in the Gaia G band [325 -- 1030~nm]. We found that the Gaia parallax
  for Betelgeuse-like supergiants are characterized by a systematic
  error of a few percents.

Title: The metal-poor end of the Spite plateau. I. Stellar parameters,
    metallicities, and lithium abundances
Authors: Sbordone, L.; Bonifacio, P.; Caffau, E.; Ludwig, H. -G.;
   Behara, N. T.; González Hernández, J. I.; Steffen, M.; Cayrel, R.;
   Freytag, B.; van't Veer, C.; Molaro, P.; Plez, B.; Sivarani, T.; Spite,
   M.; Spite, F.; Beers, T. C.; Christlieb, N.; François, P.; Hill, V.
Bibcode: 2010A&A...522A..26S
Altcode: 2010arXiv1003.4510S
  Context. The primordial nature of the Spite plateau is at odds with
  the WMAP satellite measurements, implying a primordial Li production
  at least three times higher than observed. It has also been suggested
  that A(Li) might exhibit a positive correlation with metallicity below
  [Fe/H] ~ -2.5. Previous samples studied comprised few stars below
  [Fe/H] = -3. <BR /> Aims: We present VLT-UVES Li abundances of 28
  halo dwarf stars between [Fe/H] = -2.5 and -3.5, ten of which have
  [Fe/H] &lt;-3. <BR /> Methods: We determined stellar parameters and
  abundances using four different T<SUB>eff</SUB> scales. The direct
  infrared flux method was applied to infrared photometry. Hα wings were
  fitted with two synthetic grids computed by means of 1D LTE atmosphere
  models, assuming two different self-broadening theories. A grid of Hα
  profiles was finally computed by means of 3D hydrodynamical atmosphere
  models. The Li i doublet at 670.8 nm has been used to measure A(Li)
  by means of 3D hydrodynamical NLTE spectral syntheses. An analytical
  fit of A(Li)<SUB>3D, NLTE</SUB> as a function of equivalent width,
  T<SUB>eff</SUB>, log g, and [Fe/H] has been derived and is made
  available. <BR /> Results: We confirm previous claims that A(Li)
  does not exhibit a plateau below [Fe/H] = -3. We detect a strong
  positive correlation with [Fe/H] that is insensitive to the choice of
  T<SUB>eff</SUB> estimator. From a linear fit, we infer a steep slope
  of about 0.30 dex in A(Li) per dex in [Fe/H], which has a significance
  of 2-3σ. The slopes derived using the four T<SUB>eff</SUB> estimators
  are consistent to within 1σ. A significant slope is also detected
  in the A(Li)-T<SUB>eff</SUB> plane, driven mainly by the coolest
  stars in the sample (T<SUB>eff</SUB> &lt; 6250), which appear to be
  Li-poor. However, when we remove these stars the slope detected in
  the A(Li)-[Fe/H] plane is not altered significantly. When the full
  sample is considered, the scatter in A(Li) increases by a factor
  of 2 towards lower metallicities, while the plateau appears very
  thin above [Fe/H] = -2.8. At this metallicity, the plateau lies at
  &lt;A(Li)<SUB>3D, NLTE</SUB>&gt; = 2.199±0.086. <BR /> Conclusions:
  The meltdown of the Spite plateau below [Fe/H] ~ -3 is established,
  but its cause is unclear. If the primordial A(Li) were that derived
  from standard BBN, it appears difficult to envision a single depletion
  phenomenon producing a thin, metallicity independent plateau above
  [Fe/H] = -2.8, and a highly scattered, metallicity dependent
  distribution below. That no star below [Fe/H] = -3 lies above the
  plateau suggests that they formed at plateau level and experienced
  subsequent depletion. <P />Based on observations made with the ESO Very
  Large Telescope at Paranal Observatory, Chile (Programmes 076.A-0463
  and 077.D-0299).Full Table 3 is available in electronic form at the
  CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via <A
  href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/522/A26">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/522/A26</A>IDL
  code (appendix) is only available in electronic form at <A
  href="http://www.aanda.org">http://www.aanda.org</A>

Title: Brown Dwarf Model Atmospheres Based on Multi-Dimensional
    Radiation Hydrodynamics
Authors: Allard, France; Freytag, Bernd
Bibcode: 2010HiA....15..756A
Altcode:
  The atmospheres of Brown Dwarfs (BDs) are the site of molecular
  opacities and cloud formation, and control their cooling rate,
  radius and brightness evolution. Brown dwarfs evolve from stellar-like
  properties (magnetic activity, spots, flares, mass loss) to planet-like
  properties (electron degeneracy of the interior, cloud formation,
  dynamical molecular transport) while retaining, due to their fully
  convective interior, larger rotational velocities (≤ 30 km/s i.e. P
  &lt; 4 hrs versus 11 hrs for Jupiter). Model atmospheres treating all
  this complexity are therefore essential to understand the evolution
  properties, and to interpret the observations of these objects. While
  the pure gas-phase based NextGen model atmospheres (Allard et al. 1997,
  Hauschildt et al. 1999) have allowed the understanding of the several
  populations of Very Low Mass Stars (VLMs), the AMES-Dusty models
  (Allard et al. 2001) based on equilibrium chemistry have reproduced
  some near-IR photometric properties of M and L-type brown dwarfs, and
  played a key role in the determination of the mass of brown dwarfs
  and Planetary Mass Objects (PMOs) in the eld and in young stellar
  clusters. In this paper, we present a new model atmosphere grid for
  VLMs, BDs, PMOs named BT-Settl, which includes a cloud model and
  dynamical molecular transport based on mixing information from 2D
  Radiation Hydrodynamic (RHD) simulations (Freytag et al. 2009). We
  also present the status of our 3D RHD simulations including rotation
  (Coriolis forces) of a cube on the surface of a brown dwarf. The
  BT-Settl model atmosphere grid will be available shortly via the
  Phoenix web simulator (http://phoenix.ens-lyon.fr/simulator/).

Title: CO5BOLD: COnservative COde for the COmputation of COmpressible
    COnvection in a BOx of L Dimensions with l=2,3
Authors: Freytag, Bernd; Steffen, Matthias; Wedemeyer-Böhm, Sven;
   Ludwig, Hans-Günter; Leenaarts, Jorrit; Schaffenberger, Werner;
   Allard, France; Chiavassa, Andrea; Höfner, Susanne; Kamp, Inga;
   Steiner, Oskar
Bibcode: 2010ascl.soft11014F
Altcode:
  CO5BOLD - nickname COBOLD - is the short form of "COnservative
  COde for the COmputation of COmpressible COnvection in a BOx of L
  Dimensions with l=2,3". <P />It is used to model solar and stellar
  surface convection. For solar-type stars only a small fraction of the
  stellar surface layers are included in the computational domain. In
  the case of red supergiants the computational box contains the entire
  star. Recently, the model range has been extended to sub-stellar objects
  (brown dwarfs). <P />CO5BOLD solves the coupled non-linear equations
  of compressible hydrodynamics in an external gravity field together
  with non-local frequency-dependent radiation transport. Operator
  splitting is applied to solve the equations of hydrodynamics (including
  gravity), the radiative energy transfer (with a long-characteristics
  or a short-characteristics ray scheme), and possibly additional 3D
  (turbulent) diffusion in individual sub steps. The 3D hydrodynamics
  step is further simplified with directional splitting (usually). The 1D
  sub steps are performed with a Roe solver, accounting for an external
  gravity field and an arbitrary equation of state from a table. <P
  />The radiation transport is computed with either one of three
  modules: <P />MSrad module: It uses long characteristics. The lateral
  boundaries have to be periodic. Top and bottom can be closed or open
  ("solar module"). <P />LHDrad module: It uses long characteristics
  and is restricted to an equidistant grid and open boundaries at all
  surfaces (old "supergiant module"). <P />SHORTrad module: It uses
  short characteristics and is restricted to an equidistant grid and
  open boundaries at all surfaces (new "supergiant module"). <P />The
  code was supplemented with an (optional) MHD version [Schaffenberger
  et al. (2005)] that can treat magnetic fields. There are also modules
  for the formation and advection of dust available. The current version
  now contains the treatment of chemical reaction networks, mostly used
  for the formation of molecules [Wedemeyer-Böhm et al. (2005)], and
  hydrogen ionization [Leenaarts &amp; Wedemeyer-Böhm (2005)], too. <P
  />CO5BOLD is written in Fortran90. The parallelization is done with
  OpenMP directives.

Title: Implicit Hydrodynamic Simulations of Stellar Interiors
Authors: Viallet, M.; Baraffe, I.; Mulet-Marquis, C.; Lévêque, E.;
   Walder, R.; Freytag, B.; Winisdoerffer, C.
Bibcode: 2010ASPC..429..167V
Altcode: 2010arXiv1003.5555V
  We report on the development of an implicit multi-D hydrodynamic code
  for stellar evolution. We present two test-cases relevant for the first
  scientific goal of the code: the simulation of convection in pulsating
  stars. First results on a realistic stellar model are also presented.

Title: Galactic evolution of oxygen. OH lines in 3D hydrodynamical
    model atmospheres
Authors: González Hernández, J. I.; Bonifacio, P.; Ludwig, H. -G.;
   Caffau, E.; Behara, N. T.; Freytag, B.
Bibcode: 2010A&A...519A..46G
Altcode: 2010arXiv1005.3754G
  Context. Oxygen is the third most common element in the Universe. The
  measurement of oxygen lines in metal-poor unevolved stars, in
  particular near-UV OH lines, can provide invaluable information
  about the properties of the Early Galaxy. <BR /> Aims: Near-UV OH
  lines constitute an important tool to derive oxygen abundances in
  metal-poor dwarf stars. Therefore, it is important to correctly model
  the line formation of OH lines, especially in metal-poor stars, where
  3D hydrodynamical models commonly predict cooler temperatures than
  plane-parallel hydrostatic models in the upper photosphere. <BR />
  Methods: We have made use of a grid of 52 3D hydrodynamical model
  atmospheres for dwarf stars computed with the code CO<SUP>5</SUP>BOLD,
  extracted from the more extended CIFIST grid. The 52 models cover
  the effective temperature range 5000-6500 K, the surface gravity
  range 3.5-4.5 and the metallicity range -3 &lt; [Fe/H] &lt; 0. <BR />
  Results: We determine 3D-LTE abundance corrections in all 52 3D models
  for several OH lines and ion{Fe}{i} lines of different excitation
  potentials. These 3D-LTE corrections are generally negative and reach
  values of roughly -1 dex (for the OH 3167 with excitation potential
  of approximately 1 eV) for the higher temperatures and surface
  gravities. <BR /> Conclusions: We apply these 3D-LTE corrections
  to the individual O abundances derived from OH lines of a sample
  the metal-poor dwarf stars reported in Israelian et al. (1998, ApJ,
  507, 805), Israelian et al. (2001, ApJ, 551, 833) and Boesgaard et
  al. (1999, AJ, 117, 492) by interpolating the stellar parameters of the
  dwarfs in the grid of 3D-LTE corrections. The new 3D-LTE [O/Fe] ratio
  still keeps a similar trend as the 1D-LTE, i.e., increasing towards
  lower [Fe/H] values. We applied 1D-NLTE corrections to 3D ion{Fe}{i}
  abundances and still see an increasing [O/Fe] ratio towards lower
  metallicites. However, the Galactic [O/Fe] ratio must be revisited
  once 3D-NLTE corrections become available for OH and Fe lines for a
  grid of 3D hydrodynamical model atmospheres.

Title: VizieR Online Data Catalog: Fe Abundances in metal-poor stars
    (Sbordone+ 2010)
Authors: Sbordone, L.; Bonifacio, P.; Caffau, E.; Ludwig, H. -G.;
   Behara, N. T.; Gonzalez Hernandez, J. I.; Steffen, M.; Cayrel, R.;
   Freytag, B.; van't Veer, C.; Molaro, P.; Plez, B.; Sivarani, T.; Spite,
   M.; Spite, F.; Beers, T. C.; Christlieb, N.; Francois, P.; Hill, V.
Bibcode: 2010yCat..35220026S
Altcode: 2010yCat..35229026S
  Line-by-line abundances for FeI and FeII lines used to estimate
  metallicity and gravity for the program stars. The first column lists
  the star name, then the ion (FeI or FeII) The the wavelength in nm,
  the loggf, the measured EW (pm) and the derived abundance assuming the
  four stellar parameter sets used in the article, respectively 3D, BA,
  ALI and IRFM. <P />(3 data files).

Title: Radiative hydrodynamics simulations of red supergiant
    stars. II. Simulations of convection on Betelgeuse match
    interferometric observations
Authors: Chiavassa, A.; Haubois, X.; Young, J. S.; Plez, B.; Josselin,
   E.; Perrin, G.; Freytag, B.
Bibcode: 2010A&A...515A..12C
Altcode: 2010arXiv1003.1407C
  Context. The red supergiant (RSG) Betelgeuse is an irregular variable
  star. Convection may play an important role in understanding this
  variability. Interferometric observations can be interpreted using
  sophisticated simulations of stellar convection. <BR /> Aims: We compare
  the visibility curves and closure phases obtained from our 3D simulation
  of RSG convection with CO5BOLD to various interferometric observations
  of Betelgeuse from the optical to the H band to characterize and
  measure the convection pattern on this star. <BR /> Methods: We use a
  3D radiative-hydrodynamics (RHD) simulation to compute intensity maps
  in different filters and thus derive interferometric observables using
  the post-processing radiative transfer code OPTIM3D. The synthetic
  visibility curves and closure phases are compared to observations. <BR
  /> Results: We provide a robust detection of the granulation pattern on
  the surface of Betelgeuse in both the optical and the H band based on
  excellent fits to the observed visibility points and closure phases. We
  determine that the Betelgeuse surface in the H band is covered by
  small to medium scale (5-15 mas) convection-related surface structures
  and a large (≈30 mas) convective cell. In this spectral region,
  H<SUB>2</SUB>O molecules are the main absorbers and contribute to
  both the small structures and the position of the first null of the
  visibility curve (i.e., the apparent stellar radius).

Title: The solar photospheric abundance of carbon. Analysis of atomic
    carbon lines with the CO5BOLD solar model
Authors: Caffau, E.; Ludwig, H. -G.; Bonifacio, P.; Faraggiana, R.;
   Steffen, M.; Freytag, B.; Kamp, I.; Ayres, T. R.
Bibcode: 2010A&A...514A..92C
Altcode: 2010arXiv1002.2628C
  Context. The analysis of the solar spectra using hydrodynamical
  simulations, with a specific selection of lines, atomic data, and method
  for computing deviations from local thermodynamical equilibrium, has
  led to a downward revision of the solar metallicity, Z. We are using
  the latest simulations computed with the CO5BOLD code to reassess
  the solar chemical composition. Our previous analyses of the key
  elements, oxygen and nitrogen, have not confirmed any extreme downward
  revision of Z, as derived in other works based on hydrodynamical
  models. <BR /> Aims: We determine the solar photospheric carbon
  abundance with a radiation-hydrodynamical CO5BOLD model and compute
  the departures from local thermodynamical equilibrium by using the
  Kiel code. <BR /> Methods: We measured equivalent widths of atomic C
  I lines on high-resolution, high signal-to-noise ratio solar atlases
  of disc-centre intensity and integrated disc flux. These equivalent
  widths were analysed with our latest solar 3D hydrodynamical simulation
  computed with CO5BOLD. Deviations from local thermodynamic equilibrium
  we computed in 1D with the Kiel code, using the average temperature
  structure of the hydrodynamical simulation as a background model. <BR />
  Results: Our recommended value for the solar carbon abundance relies
  on 98 independent measurements of observed lines and is A(C)=8.50
  ± 0.06. The quoted error is the sum of statistical and systematic
  errors. Combined with our recent results for the solar oxygen and
  nitrogen abundances, this implies a solar metallicity of Z = 0.0154
  and Z/X = 0.0211. <BR /> Conclusions: Our analysis implies a solar
  carbon abundance that is about 0.1 dex higher than what was found in
  previous analyses based on different 3D hydrodynamical computations. The
  difference is partly driven by our equivalent width measurements
  (we measure, on average, larger equivalent widths than the other work
  based on a 3D model), in part because of the different properties of
  the hydrodynamical simulations and the spectrum synthesis code. The
  solar metallicity we obtain from the CO5BOLD analyses is in slightly
  better agreement with the constraints of helioseismology than the
  previous 3D abundance results.

Title: The role of convection, overshoot, and gravity waves for the
    transport of dust in M dwarf and brown dwarf atmospheres
Authors: Freytag, B.; Allard, F.; Ludwig, H. -G.; Homeier, D.;
   Steffen, M.
Bibcode: 2010A&A...513A..19F
Altcode: 2010arXiv1002.3437F
  Context. Observationally, spectra of brown dwarfs indicate the presence
  of dust in their atmospheres while theoretically it is not clear what
  prevents the dust from settling and disappearing from the regions of
  spectrum formation. Consequently, standard models have to rely on ad
  hoc assumptions about the mechanism that keeps dust grains aloft in
  the atmosphere. <BR /> Aims: We apply hydrodynamical simulations to
  develop an improved physical understanding of the mixing properties of
  macroscopic flows in M dwarf and brown dwarf atmospheres, in particular
  of the influence of the underlying convection zone. <BR /> Methods: We
  performed two-dimensional radiation hydrodynamics simulations including
  a description of dust grain formation and transport with the CO5BOLD
  code. The simulations cover the very top of the convection zone and
  the photosphere including the dust layers for a sequence of effective
  temperatures between 900 K and 2800 K, all with log g = 5 assuming solar
  chemical composition. <BR /> Results: Convective overshoot occurs in the
  form of exponentially declining velocities with small scale heights, so
  that it affects only the region immediately above the almost adiabatic
  convective layers. From there on, mixing is provided by gravity waves
  that are strong enough to maintain thin dust clouds in the hotter
  models. With decreasing effective temperature, the amplitudes of the
  waves become smaller but the clouds become thicker and develop internal
  convective flows that are more efficient in transporting and mixing
  material than gravity waves. The presence of clouds often leads to a
  highly structured appearance of the stellar surface on short temporal
  and small spatial scales (presently inaccessible to observations). <BR
  /> Conclusions: We identify convectively excited gravity waves as an
  essential mixing process in M dwarf and brown dwarf atmospheres. Under
  conditions of strong cloud formation, dust convection is the dominant
  self-sustaining mixing component.

Title: The metal-poor end of the Spite plateau: gravity sensitivity
    of the Hα wings fitting.
Authors: Sbordone, L.; Bonifacio, P.; Caffau, E.; Ludwig, H. -G.;
   Behara, N.; Gonzalez-Hernandez, J. I.; Steffen, M.; Cayrel, R.;
   Freytag, B.; Van't Veer, C.; Molaro, P.; Plez, B.; Sivarani, T.; Spite,
   M.; Spite, F.; Beers, T. C.; Christlieb, N.; François, P.; Hill, V.
Bibcode: 2010IAUS..268..355S
Altcode:
  We recently presented (Sbordone et al., 2009a) the largest sample to
  date of lithium abundances in extremely metal-poor (EMP) Halo dwarf and
  Turn-Off (TO) stars. One of the most crucial aspects in estimating Li
  abundances is the T<SUB>eff</SUB> determination, since the Li I 670.8
  nm doublet is highly temperature sensitive. In this short contribution
  we concentrate on the T<SUB>eff</SUB> determination based on Hα wings
  fitting, and on its sensitivity to the chosen stellar gravity.

Title: Solar abundances and 3D model atmospheres
Authors: Ludwig, Hans-Günter; Caffau, Elisabetta; Steffen, Matthias;
   Bonifacio, Piercarlo; Freytag, Bernd; Cayrel, Roger
Bibcode: 2010IAUS..265..201L
Altcode: 2009arXiv0911.4248L
  We present solar photospheric abundances for 12 elements from optical
  and near-infrared spectroscopy. The abundance analysis was conducted
  employing 3D hydrodynamical (CO<SUP>5</SUP>BOLD) as well as standard
  1D hydrostatic model atmospheres. We compare our results to others
  with emphasis on discrepancies and still lingering problems, in
  particular exemplified by the pivotal abundance of oxygen. We argue
  that the thermal structure of the lower solar photosphere is very
  well represented by our 3D model. We obtain an excellent match of
  the observed center-to-limb variation of the line-blanketed continuum
  intensity, also at wavelengths shortward of the Balmer jump.

Title: The metal-poor end of the Spite plateau
Authors: Sbordone, L.; Bonifacio, P.; Caffau, E.; Ludwig, H. -G.;
   Behara, N.; Gonzalez-Hernandez, J. I.; Steffen, M.; Cayrel, R.;
   Freytag, B.; Van't Veer, C.; Molaro, P.; Plez, B.; Sivarani, T.; Spite,
   M.; Spite, F.; Beers, T. C.; Christlieb, N.; François, P.; Hill, V.
Bibcode: 2010IAUS..265...75S
Altcode:
  We present the largest sample available to date of lithium abundances in
  extremely metal poor (EMP) Halo dwarfs. Four T<SUB>eff</SUB> estimators
  are used, including IRFM and Hα wings fitting against 3D hydrodynamical
  synthetic profiles. Lithium abundances are computed by means of 1D and
  3D-hydrodynamical NLTE computations. Below [Fe/H]~-3, a strong positive
  correlation of A(Li) with [Fe/H] appears, not influenced by the choice
  of the Teff estimator. A linear fit finds a slope of about 0.30 dex in
  A(Li) per dex in [Fe/H], significant to 2-3 σ, and consistent within
  1 σ among all the T<SUB>eff</SUB> estimators. The scatter in A(Li)
  increases significantly below [Fe/H]~-3. Above, the plateau lies at
  &lt;A(Li)<SUB>3D, NLTE</SUB>&gt; = 2.199 ± 0.086. If the primordial
  A(Li) is the one derived from standard Big Bang Nucleosynthesis
  (BBN), it appears difficult to envision a single depletion phenomenon
  producing a thin, metallicity independent plateau above [Fe/H] = -2.8,
  and a highly scattered, metallicity dependent distribution below.

Title: VLTI/AMBER spectro-interferometric imaging of VX Sagittarii's
    inhomogenous outer atmosphere
Authors: Chiavassa, A.; Lacour, S.; Millour, F.; Driebe, T.;
   Wittkowski, M.; Plez, B.; Thiébaut, E.; Josselin, E.; Freytag, B.;
   Scholz, M.; Haubois, X.
Bibcode: 2010A&A...511A..51C
Altcode: 2009arXiv0911.4422C
  <BR /> Aims: We aim to explore the photosphere of the very cool
  late-type star VX Sgr and in particular the characterization of
  molecular layers above the continuum forming photosphere. <BR />
  Methods: We obtained interferometric observations with the VLTI/AMBER
  interferometer using the fringe tracker FINITO in the spectral domain
  1.45-2.50 μm with a spectral resolution of ≈35 and baselines ranging
  from 15 to 88 m. We performed independent image reconstruction for
  different wavelength bins and fit the interferometric data with a
  geometrical toy model. We also compared the data to 1D dynamical
  models of Miras atmosphere and to 3D hydrodynamical simulations of
  red supergiant (RSG) and asymptotic giant branch (AGB) stars. <BR />
  Results: Reconstructed images and visibilities show a strong wavelength
  dependence. The H-band images display two bright spots whose positions
  are confirmed by the geometrical toy model. The inhomogeneities are
  qualitatively predicted by 3D simulations. At ≈2.00 μm and in the
  region 2.35-2.50 μm, the photosphere appears extended and the radius
  is larger than in the H band. In this spectral region, the geometrical
  toy model locates a third bright spot outside the photosphere that
  can be a feature of the molecular layers. The wavelength dependence of
  the visibility can be qualitatively explained by 1D dynamical models
  of Mira atmospheres. The best-fitting photospheric models show a good
  match with the observed visibilities and give a photospheric diameter
  of Theta=8.82 ± 0.50 mas. The H<SUB>2</SUB>O molecule seems to be the
  dominant absorber in the molecular layers. <BR /> Conclusions: We show
  that the atmosphere of VX Sgr seems to resemble Mira/AGB star model
  atmospheres more closely than do RSG model atmospheres. In particular,
  we see molecular (water) layers that are typical of Mira stars. <P
  />Based on the observations made with VLTI-ESO Paranal, Chile under
  the programs IDs 081.D-0005(A, B, C, D, E, F, G, H).

Title: Radiative hydrodynamics simulations of red supergiant
    stars. I. interpretation of interferometric observations
Authors: Chiavassa, A.; Plez, B.; Josselin, E.; Freytag, B.
Bibcode: 2009A&A...506.1351C
Altcode: 2009arXiv0907.1860C
  Context: It has been proposed that convection in red supergiant
  (RSG) stars produces large-scale granules causing observable surface
  inhomogeneities. This convection is also extremely vigorous and is
  suspected to be one of the main causes of mass-loss in RSGs. It should
  thus be understood in detail. Evidence has accumulated of asymmetries
  in the photospheres of RSGs, but detailed studies of granulation are
  still lacking. Interferometric observations provide an innovative way
  of addressing this question, but they are still often interpreted using
  smooth symmetrical limb-darkened intensity distributions, or simple,
  spotted, ad hoc models. <BR />Aims: We explore the impact of the
  granulation on visibility curves and closure phases using the radiative
  transfer code OPTIM3D. We simultaneously assess how 3D simulations
  of convection in RSG with CO^5BOLD can be tested by comparing with
  these observations. <BR />Methods: We use 3D radiative hydrodynamical
  (RHD) simulations of convection to compute intensity maps at various
  wavelengths and time, from which we derive interferometric visibility
  amplitudes and phases. We study their behaviour with time, position
  angle, and wavelength, and compare them to observations of the RSG α
  Ori. <BR />Results: We provide average limb-darkening coefficients for
  RSGs. We describe the prospects for the detection and characterization
  of granulation (i.e., contrast, size) on RSGs. We demonstrate that our
  RHD simulations provide an excellent fit to existing interferometric
  observations of α Ori, in contrast to limb darkened disks. This
  confirms the existence of large convective cells on the surface of
  Betelgeuse.

Title: Convective mixing and dust clouds in the atmospheres of
    brown dwarfs
Authors: Freytag, Bernd; Allard, France; Ludwig, Hans-Günter; Homeier,
   Derek; Steffen, Matthias; Sharp, Christopher
Bibcode: 2009AIPC.1094..489F
Altcode: 2009csss...15..489F
  Observed spectra of brown dwarfs demonstrate that their atmospheres
  are influenced by dust. To investigate the mechanism that controls
  the formation and gravitational settling of dust grains as well as the
  mixing of fresh condensable material into the atmosphere, we performed
  2D radiation-hydrodynamics simulations with CO5BOLD. The models comprise
  the upper part of the convection zone and the atmosphere with the
  dust cloud layers. We find that direct convective overshoot does not
  play a major role. Instead, the mixing in the clouds is controlled by
  gravity waves.

Title: The CIFIST 3D model atmosphere grid.
Authors: Ludwig, H. -G.; Caffau, E.; Steffen, M.; Freytag, B.;
   Bonifacio, P.; Kučinskas, A.
Bibcode: 2009MmSAI..80..711L
Altcode: 2009arXiv0908.4496L
  Grids of stellar atmosphere models and associated synthetic spectra
  are numerical products which have a large impact in astronomy due to
  their ubiquitous application in the interpretation of radiation from
  individual stars and stellar populations. 3D model atmospheres are
  now on the verge of becoming generally available for a wide range
  of stellar atmospheric parameters. We report on efforts to develop
  a grid of 3D model atmospheres for late-type stars within the CIFIST
  Team at Paris Observatory. The substantial demands in computational
  and human labor for the model production and post-processing render
  this apparently mundane task a challenging logistic exercise. At
  the moment the CIFIST grid comprises 77 3D model atmospheres with
  emphasis on dwarfs of solar and sub-solar metallicities. While the
  model production is still ongoing, first applications are already
  worked upon by the CIFIST Team and collaborators.

Title: Simulations of dust clouds in the atmospheres of substellar
    objects.  Theory toddles after observations
Authors: Freytag, B.; Allard, F.; Ludwig, H. -G.; Homeier, D.;
   Steffen, M.
Bibcode: 2009MmSAI..80..670F
Altcode:
  The atmospheres of brown dwarfs allow the formation of dust grains
  and their rain-out into deeper, invisible layers. However, observed
  spectra of L dwarfs can only be reproduced when static 1D models
  account for dust formation and its resulting greenhouse effect in the
  visible layers. Time-dependent hydrodynamical processes can mix up the
  material giving rise to complex unsteady weather phenomena on these
  objects. We performed radiation hydrodynamics simulations in two and
  three dimensions of the atmospheres of brown dwarfs with CO5BOLD,
  including a treatment of dust particles. We find that exponential
  overshoot (close to the gas convection zone), gravity waves (weak
  omni-present mixing), and convection within dust layers (in the thick
  clouds in cooler models) contribute to the atmospheric mixing, which
  is far from being a stationary process. The presence of dust in the
  atmospheres is accompanied by large temporal and spatial intensity
  fluctuations.

Title: Models of surface convection and dust clouds in brown dwarfs
Authors: Freytag, B.; Allard, F.; Ludwig, H. -G.; Homeier, D.;
   Steffen, M.
Bibcode: 2008PhST..133a4005F
Altcode:
  The influence of dust grains on the atmospheres of brown dwarfs is
  visible in observed spectra. To investigate what prevents the dust
  grains from falling down, or how fresh condensable material is mixed
  up in the atmosphere to allow new grains to form, we performed 2D
  radiation-hydrodynamics simulations with CO5BOLD of the upper part
  of the convection zone and the atmosphere containing the dust cloud
  layers. We find that unlike in models of Cepheids, the convective
  overshoot does not play a major role. Instead, the mixing in the dust
  clouds is controlled by gravity waves.

Title: he models comprise the upper part of the convection zone
    and the atmosphere with %the dust cloud layers. We find that direct
    convective overshoot does not play a major role. Instead, the mixing
    in the clouds is controlled by gravity waves.
Authors: Freytag, B.; Allard, F.; Ludwig, H. -G.; Homeier, D.; Steffen,
   M.; Sharp, C.
Bibcode: 2008sf2a.conf..469F
Altcode:
  To investigate the mechanism that controls the formation and
  gravitational settling of dust grains as well as the mixing of fresh
  condensable material into the atmosphere of brown dwarfs, we performed
  2D radiation-hydrodynamics simulations with CO5BOLD.

Title: The photospheric solar oxygen project. I. Abundance analysis
    of atomic lines and influence of atmospheric models
Authors: Caffau, E.; Ludwig, H. -G.; Steffen, M.; Ayres, T. R.;
   Bonifacio, P.; Cayrel, R.; Freytag, B.; Plez, B.
Bibcode: 2008A&A...488.1031C
Altcode: 2008arXiv0805.4398C
  Context: The solar oxygen abundance has undergone a major downward
  revision in the past decade, the most noticeable one being the
  update including 3D hydrodynamical simulations to model the solar
  photosphere. Up to now, such an analysis has only been carried out
  by one group using one radiation-hydrodynamics code. <BR />Aims:
  We investigate the photospheric oxygen abundance considering lines
  from atomic transitions. We also consider the relationship between
  the solar model used and the resulting solar oxygen abundance, to
  understand whether the downward abundance revision is specifically
  related to 3D hydrodynamical effects. <BR />Methods: We performed
  a new determination of the solar photospheric oxygen abundance by
  analysing different high-resolution high signal-to-noise ratio atlases
  of the solar flux and disc-centre intensity, making use of the latest
  generation of CO5BOLD 3D solar model atmospheres. <BR />Results: We
  find 8.73 ≤ log (N_O/N_H) +12 ≤ 8.79. The lower and upper values
  represent extreme assumptions on the role of collisional excitation
  and ionisation by neutral hydrogen for the NLTE level populations
  of neutral oxygen. The error of our analysis is ± (0.04± 0.03)
  dex, the last being related to NLTE corrections, the first error
  to any other effect. The 3D “granulation effects” do not play a
  decisive role in lowering the oxygen abundance. <BR />Conclusions:
  Our recommended value is log (N_O/N_H) = 8.76 ± 0.07, considering our
  present ignorance of the role of collisions with hydrogen atoms on the
  NLTE level populations of oxygen. The reasons for lower O abundances in
  the past are identified as (1) the lower equivalent widths adopted and
  (2) the choice of neglecting collisions with hydrogen atoms in the
  statistical equilibrium calculations for oxygen. <P />This paper is
  dedicated to the memory of Hartmut Holweger.

Title: Radiation hydrodynamics simulations of stellar surface
    convection
Authors: Freytag, Bernd; Steffen, Matthias; Ludwig, Hans-Guenter;
   Wedemeyer-Boehm, Sven
Bibcode: 2008asd..soft...36F
Altcode:
  The code is used to perform radiation hydrodynamics simulations of
  the convective surface layers and the photosphere of cool stars.

Title: The solar photospheric abundance of europium. Results from
    CO5BOLD 3D hydrodynamical model atmospheres
Authors: Mucciarelli, A.; Caffau, E.; Freytag, B.; Ludwig, H. -G.;
   Bonifacio, P.
Bibcode: 2008A&A...484..841M
Altcode: 2008arXiv0803.0863M
  Context: Europium is an almost pure r-process element, which may be
  useful as a reference in nucleocosmochronology. <BR />Aims: Determine
  the photospheric solar abundance using CO5BOLD 3D hydrodynamical
  model atmospheres. <BR />Methods: Disc-centre and integrated-flux
  observed solar spectra are used. The europium abundance is derived
  using equivalent-width measurements. As a reference, one-dimensional
  model atmospheres are in addition used. <BR />Results: The europium
  photospheric solar abundance (0.52 ± 0.02) agrees with previous
  determinations. We determine the photospheric isotopic fraction of
  <SUP>151</SUP>Eu to be 49% ± 2.3% using the intensity spectra, and 50%
  ± 2.3% using the flux spectra. This compares well to the meteoritic
  isotopic fraction 47.8%. We explore 3D corrections for dwarfs and
  sub-giants in the temperature range ~5000 K to ~6500 K and solar and
  1/10-solar metallicities and find them to be negligible for all models
  investigated. <BR />Conclusions: Our photospheric Eu abundance agrees
  well with previous determinations based on 1D models. This is in line
  with our conclusion that 3D effects for this element are negligible
  in the case of the Sun.

Title: Three-dimensional simulations of the atmosphere of an AGB star
Authors: Freytag, B.; Höfner, S.
Bibcode: 2008A&A...483..571F
Altcode:
  Context: Winds of asymptotic giant branch stars are assumed to be
  driven by radiation pressure on dust. Previously, this process has been
  modeled with detailed time-dependent simulations of atmospheres and
  winds assuming spherically symmetric flows. In such models kinetic
  energy is injected by a variable inner boundary (“piston”)
  simulating the effects of stellar pulsation. However, the dynamical
  processes in these atmospheres - convection and pulsations - are
  actually three-dimensional. <BR />Aims: We present and analyze first
  3D radiation hydrodynamics simulations of the convective interior
  and the atmosphere of a typical AGB star. In particular, we check
  whether the piston description in the 1D wind models is compatible
  with the 3D results. <BR />Methods: We used two different RHD codes,
  one (CO5BOLD) to produce 3D models of the outer convective envelope
  and the inner atmosphere of an AGB star, the other to describe the
  atmosphere and the wind acceleration region, including dust formation
  and non-grey radiative transfer, but assuming spherically symmetric
  flows. From the movements of stellar surface layers in the 3D models,
  we derived a description for the variable inner boundary in the 1D
  models. <BR />Results: The 3D models show large convection cells and
  pulsations that give rise to roughly spherically expanding shock waves
  in the atmosphere, levitating material into regions which are cool
  enough to allow for dust formation. The atmospheric velocity fields
  have amplitudes and time scales close to the values that are necessary
  to start dust formation in the 1D wind models. <BR />Conclusions: The
  convection cells in the 3D simulations are so large that the associated
  shock fronts appear almost spherical, justifying the assumption of
  spherical symmetry and the use of a piston boundary condition in the
  context of wind models. Nevertheless, certain non-radial structures
  exist in the dust shell developing in the 3D simulations which should
  be detectable with current interferometric techniques.

Title: Atmospheric dynamics of red supergiant stars and applications
    to Interferometry
Authors: Chiavassa, A.; Plez, B.; Josselin, E.; Freytag, B.
Bibcode: 2008arXiv0802.1403C
Altcode:
  We have written a 3D radiative transfer code that computes emerging
  spectra and intensity maps. We derive from radiative hydrodynamic
  (RHD) simulations of RSG stars carried out with CO5BOLD (Freytag
  et al. 2002) observables expected for red supergiant stars (RSG)
  especially for interferometric observations, with emphasis on small
  scale structures. We show that the convection-related surface structures
  are detectable in the H band with today's interferometers and that
  the diameter measurement should not be too dependent on the adopted
  model. The simulations are a great improvement over parametric models
  for the interpretation of interferometric observations.

Title: Numerical Simulations of Stellar Surface Convection and
    Related Phenomena
Authors: Freytag, B.
Bibcode: 2008EAS....28....9F
Altcode:
  Sophisticated radiative transfer methods have been used for decades
  to model one-dimensional static stellar atmospheres. They predict
  an outward decrease of the atmospheric temperature that is now
  observable with simple one-baseline interferometers via measurements
  of limb darkening. However, the surface layers of many stars are
  affected by convection which requires a treatment by time-dependent
  multi-dimensional radiation hydrodynamics simulations. Solar granulation
  is directly observable with “ordinary” telescopes. The simulated
  granule pattern and evolution compares well with the observed ones. The
  upcoming radio interferometer ALMA could be used to probe the convection
  induced shock-pattern in the chromosphere that is predicted by
  simulations and that is not easily observable otherwise. The typical
  granular scale on other near main-sequence stars is too small to
  be accessible by interferometers. However, scaling arguments and
  recent numerical simulations predict very large structures on cool
  supergiants and AGB stars. These stars were and are candidates for
  optical/near-infrared interferometry. The complexity of the predicted
  surface phenomena requires good (or at least some) spatial resolution
  in conjunction with temporal and frequency resolution. To fully exploit
  and interpret these data the simulations have to be improved in terms
  of treatment of microphysics (especially opacities in the radiative
  transfer step) and spatial resolution.

Title: Rotating `star-in-a-box' experiments
Authors: Steffen, M.; Freytag, B.
Bibcode: 2007AN....328.1054S
Altcode:
  Using the radiation hydrodynamics code CO<SUP>5</SUP>BOLD in its
  `star-in-a-box' setup, we have performed exploratory simulations
  of global convection in a rotating reference frame. The goal is to
  study the interaction of convection and rotation by direct numerical
  simulation. For these first experiments, we chose an idealized
  configuration (a scaled-down, fast rotating Sun) whose properties
  resemble those of red supergiants in some respect. We describe the setup
  and time evolution of these models, and discuss the particular problems
  we have encountered. Finally, we derive the resulting differential
  rotation pattern and meridional flow field by temporal and azimuthal
  averaging of the simulation data. We find anti-solar differential
  rotation for all cases studied so far. <P />Movies are available via
  http://www.aip.de/AN/movies

Title: Convective and Non-Convective Mixing in AGB Stars
Authors: Herwig, F.; Freytag, B.; Fuchs, T.; Hansen, J. P.;
   Hueckstaedt, R. M.; Porter, D. H.; Timmes, F. X.; Woodward, P. R.
Bibcode: 2007ASPC..378...43H
Altcode: 2007arXiv0709.0197H
  We review the current state of modeling convective mixing in AGB
  stars. The focus is on results obtained through multi-dimensional
  hydrodynamic simulations of AGB convection, both in the envelope and
  in the unstable He-shell. Using two different codes and a wide range
  of resolutions and modeling assumptions we find that mixing across
  convective boundaries is significant for He-shell flash convection. We
  present a preliminary quantitative analysis of this convectively
  induced extra mixing, based on a sub-set of our simulations. Other
  non-standard mixing will be discussed briefly.

Title: Inter-network regions of the Sun at millimetre wavelengths
Authors: Wedemeyer-Böhm, S.; Ludwig, H. G.; Steffen, M.; Leenaarts,
   J.; Freytag, B.
Bibcode: 2007A&A...471..977W
Altcode: 2007arXiv0705.2304W
  Aims:The continuum intensity at wavelengths around 1 mm provides an
  excellent way to probe the solar chromosphere and thus valuable input
  for the ongoing controversy on the thermal structure and the dynamics of
  this layer. The synthetic continuum intensity maps for near-millimetre
  wavelengths presented here demonstrate the potential of future
  observations of the small-scale structure and dynamics of internetwork
  regions on the Sun. <BR />Methods: The synthetic intensity/brightness
  temperature maps are calculated on basis of three-dimensional
  radiation (magneto-)hydrodynamic (MHD) simulations. The assumption
  of local thermodynamic equilibrium (LTE) is valid for the source
  function. The electron densities are also treated in LTE for most maps
  but also in non-LTE for a representative model snapshot. Quantities
  like intensity contrast, intensity contribution functions, spatial
  and temporal scales are analysed in dependence on wavelength and
  heliocentric angle. <BR />Results: While the millimetre continuum
  at 0.3 mm originates mainly from the upper photosphere, the longer
  wavelengths considered here map the low and middle chromosphere. The
  effective formation height increases generally with wavelength and
  also from disk-centre towards the solar limb. The average intensity
  contribution functions are usually rather broad and in some cases they
  are even double-peaked as there are contributions from hot shock waves
  and cool post-shock regions in the model chromosphere. The resulting
  shock-induced thermal structure translates to filamentary brightenings
  and fainter regions in between. Taking into account the deviations from
  ionisation equilibrium for hydrogen gives a less strong variation of
  the electron density and with it of the optical depth. The result is
  a narrower formation height range although the intensity maps still
  are characterised by a highly complex pattern. The average brightness
  temperature increases with wavelength and towards the limb although
  the wavelength-dependence is reversed for the MHD model and the NLTE
  brightness temperature maps. The relative contrast depends on wavelength
  in the same way as the average intensity but decreases towards the
  limb. The dependence of the brightness temperature distribution on
  wavelength and disk-position can be explained with the differences in
  formation height and the variation of temperature fluctuations with
  height in the model atmospheres. The related spatial and temporal
  scales of the chromospheric pattern should be accessible by future
  instruments. <BR />Conclusions: Future high-resolution millimetre
  arrays, such as the Atacama Large Millimeter Array (ALMA), will
  be capable of directly mapping the thermal structure of the solar
  chromosphere. Simultaneous observations at different wavelengths
  could be exploited for a tomography of the chromosphere, mapping its
  three-dimensional structure, and also for tracking shock waves. The
  new generation of millimetre arrays will be thus of great value for
  understanding the dynamics and structure of the solar atmosphere.

Title: Atmospheric dynamics of red supergiant stars and applications
    to Interferometry
Authors: Chiavassa, A.; Plez, B.; Josselin, E.; Freytag, B.
Bibcode: 2007sf2a.conf..447C
Altcode:
  We have written a 3D radiative transfer code that computes emerging
  spectra and intensity maps. We derive from radiative hydrodynamic
  (RHD) simulations of RSG stars carried out with CO^5BOLD (Freytag
  et al. 2002) observables expected for red supergiant stars (RSG)
  especially for interferometric observations, with emphasis on small
  scale structures. We show that the convection-related surface structures
  are detectable in the H band with today's interferometers and that
  the diameter measurement should not be too dependent on the adopted
  model. The simulations are a great improvement over parametric models
  for the interpretation of interferometric observations.

Title: Formation of convective structures in stellar atmospheres
Authors: Freytag, Bernd; Ludwig, Hans-Günter
Bibcode: 2007sf2a.conf..481F
Altcode:
  Convection is a ubiquitous phenomenon in cool stars. Its interplay
  with radiation leads to the formation of coherent flow structures --
  granular cells -- on the visible surfaces of these stars. We model the
  processes with the 3D radiation-hydrodynamics code CO5BOLD in stars
  of different atmospheric parameters. We find that the granular cell
  size scales with the atmospheric pressure scale height for stars with
  a surface gravity log g&gt;1.0 . However, the scaling breaks down for
  red supergiants having lower surface gravities. This qualitatively
  different behaviour is likely linked to sphericity effects and mainly
  to a larger contribution of radiation to the energy transport in the
  stellar envelope.

Title: 3-D hydrodynamic simulations of convection in A stars
Authors: Kochukhov, O.; Freytag, B.; Piskunov, N.; Steffen, M.
Bibcode: 2007IAUS..239...68K
Altcode: 2006astro.ph.10111K
  Broadening and asymmetry of spectral lines in slowly rotating late
  A-type stars provide evidence for high-amplitude convective motions. The
  properties of turbulence observed in the A-star atmospheres are not
  understood theoretically and contradict results of previous numerical
  simulations of convection. Here we describe an ongoing effort to
  understand the puzzling convection signatures of A stars with the
  help of 3-D hydrodynamic simulations. Our approach combines realistic
  spectrum synthesis and non-grey hydrodynamic models computed with the
  CO5BOLD code. We discuss these theoretical predictions and confront
  them with high-resolution spectra of A stars. Our models have, for
  the first time, succeeded in reproducing the observed profiles of weak
  spectral lines without introducing fudge broadening parameters.

Title: First local helioseismic experiments with CO<SUP>5</SUP>BOLD
Authors: Steiner, O.; Vigeesh, G.; Krieger, L.; Wedemeyer-Böhm, S.;
   Schaffenberger, W.; Freytag, B.
Bibcode: 2007AN....328..323S
Altcode: 2007astro.ph..1029S
  With numerical experiments we explore the feasibility of using high
  frequency waves for probing the magnetic fields in the photosphere and
  the chromosphere of the Sun. We track a plane-parallel, monochromatic
  wave that propagates through a non-stationary, realistic atmosphere,
  from the convection-zone through the photosphere into the magnetically
  dominated chromosphere, where it gets refracted and reflected. We
  compare the wave travel time between two fixed geometrical height levels
  in the atmosphere (representing the formation height of two spectral
  lines) with the topography of the surface of equal magnetic and thermal
  energy density (the magnetic canopy or β=1 contour) and find good
  correspondence between the two. We conclude that high frequency waves
  indeed bear information on the topography of the `magnetic canopy'.

Title: Multi-dimensional Simulations of Helium Shell Flash Convection
Authors: Hueckstaedt, Robert M.; Freytag, B.; Herwig, F.; Timmes, F.
Bibcode: 2006AAS...20910107H
Altcode: 2006BAAS...38.1046H
  The Asymptotic Giant Branch (AGB) phase is the most productive
  evolutionary phase in terms of nucleosynthesis for low and intermediate
  mass stars. Nucleosythesis in these stars is aided by the mixing and
  heating triggered by recurrent He-shell flashes. Extreme nuclear
  energies (corresponding to 10^8L_sun) are generated during these
  thermonuclear flashes. This results in efficient shell convection with
  multiple implications for nucleosynthesis and further evolution. The
  evolution of AGB stars, including these He-flashes, has in the past
  been studied exclusively with one-dimensional stellar evolution codes
  that have to adopt simplifying assumptions on mixing, especially at
  convective boundaries. Here, we report on efforts to augment these
  studies with 2-d and 3-d hydrodynamic models of convective mixing in
  the AGB He-shell. We characterize the dominant morphology of He-shell
  flash convection. As opposed to the shallow surface convection in A-type
  stars studied by Freytag et al. (1996), coherently moving convective
  cells do not cross the convective boundary significantly. In other
  words, penetration is minimal for this convection zone. We find that
  convective motions induce a rich spectrum of internal gravity waves
  in the neighboring stable layers. Interactions of these (mainly
  horizontal) oscillations with the convective boundary, as well as
  oscillations with convective characteristics within the stable layers,
  do cause a finite amount of mixing across the convective boundary. Our
  preliminary analysis of this mixing is consistent with semi-analytical
  results obtained from observations and 1D-stellar evolution simulations.

Title: A First Three-Dimensional Model for the Carbon Monoxide
    Concentration in the Solar Atmosphere
Authors: Wedemeyer-Böhm, S.; Kamp, I.; Freytag, B.; Bruls, J.;
   Steffen, M.
Bibcode: 2006ASPC..354..301W
Altcode:
  The time-dependent and self-consistent treatment of carbon monoxide
  (CO) has been added to the radiation chemo-hydrodynamics code
  CO5BOLD. It includes the solution of a chemical reaction network and
  the advection of the resulting particle densities with the hydrodynamic
  flow field. Here we present a first 3D simulation of the non-magnetic
  solar photosphere and low chromosphere, calculated with the upgraded
  code. In the resulting model, the highest amount of CO is located in
  the cool regions of the reversed granulation pattern in the middle
  photosphere. A large fraction of carbon is bound by CO throughout
  the chromosphere with exception of hot shock waves where the CO
  concentration is strongly reduced. The distribution of carbon monoxide
  is very inhomogeneous due to co-existing regions of hot and cool gas
  caused by the hydrodynamic flow. High-resolution observations of CO
  could thus provide important constraints for the thermal structure of
  the solar photosphere and chromosphere.

Title: Holistic MHD-Simulation from the Convection Zone to the
    Chromosphere
Authors: Schaffenberger, W.; Wedemeyer-Böhm, S.; Steiner, O.;
   Freytag, B.
Bibcode: 2006ASPC..354..345S
Altcode:
  A three-dimensional magnetohydrodynamic simulation of the integral
  layers from the convection zone to the chromosphere has been
  carried out. The simulation represents magnetoconvection in a quiet
  network-cell interior. The following preliminary new results are
  obtained: The chromospheric magnetic field is very dynamic with a
  continuous rearrangement of magnetic flux on a time scale of less than
  one~minute. Rapidly moving magnetic filaments (rarely exceeding 40~G)
  form in the compression zone downstream and along propagating shock
  fronts that are present throughout the chromosphere. The magnetic
  filaments rapidly move, form, and dissolve with the shock waves. Flux
  concentrations strongly expand through the photosphere into a more
  homogeneous, space filling chromospheric field. ``Canopy fields''
  form on a granular scale above largely field-free granule centers
  leading to a mesh-work of current sheets in a height range between
  approximately 400 and 900~km.

Title: Line formation in 3D radiation hydrodynamics simulations of
    red supergiants
Authors: Chiavassa, A.; Plez, B.; Josselin, E.; Freytag, B.
Bibcode: 2006sf2a.conf..455C
Altcode:
  We developed a 3D radiative transfer code which computes emerging
  spectra and monochromatic maps from radiative-hydrodynamic (RHD)
  simulations of red supergiant stars. Computed emerging spectra show
  that our RHD models qualitatively reproduce the velocity amplitude
  and line asymmetries in observations. However, they cannot reproduce
  strong and weak lines simultaneously. This is explained by the shallow
  thermal gradient which weakens the contrast between strong and weak
  lines. The non-grey treatment of opacity in RHD models is planned in
  the near future to solve this problem. Moreover, we are now studying
  the possibility to detect and measure the granulation pattern with
  interferometers such as VLTI/AMBER, using our monochromatic intensity
  maps and visibility calculations.

Title: Hydrodynamic Simulations of He Shell Flash Convection
Authors: Herwig, Falk; Freytag, Bernd; Hueckstaedt, Robert M.; Timmes,
   Francis X.
Bibcode: 2006ApJ...642.1057H
Altcode: 2006astro.ph..1164H
  We present the first hydrodynamic, multidimensional simulations of
  He shell flash convection. We investigate the properties of shell
  convection immediately before the He luminosity peak during the 15th
  thermal pulse of a stellar evolution track with initially 2 solar masses
  and metallicity Z=0.01. This choice is a representative example of a
  low-mass asymptotic giant branch thermal pulse. We construct the initial
  vertical stratification with a set of polytropes to resemble the stellar
  evolution structure. Convection is driven by a constant volume heating
  in a thin layer at the bottom of the unstable layer. We calculate a grid
  of two-dimensional simulations with different resolutions and heating
  rates, plus one low-resolution three-dimensional run. The flow field is
  dominated by large convective cells that are centered in the lower half
  of the convection zone. It generates a rich spectrum of gravity waves
  in the stable layers both above and beneath the convective shell. The
  magnitude of the convective velocities from our one-dimensional
  mixing-length theory model and the rms-averaged vertical velocities from
  the hydrodynamic model are consistent within a factor of a few. However,
  the velocity profile in the hydrodynamic simulation is more asymmetric
  and decays exponentially inside the convection zone. Both g-modes and
  convective motions cross the formal convective boundaries, which leads
  to mixing across the boundaries. Our resolution study shows consistent
  flow structures among the higher resolution runs, and we see indications
  for convergence of the vertical velocity profile inside the convection
  zone for the highest resolution simulations. Many of the convective
  properties, in particular the exponential decay of the velocities,
  depend only weakly on the heating rate. However, the amplitudes of the
  gravity waves increase with both the heating rate and the resolution.

Title: Convection in giant stars
Authors: Freytag, B.
Bibcode: 2006EAS....21..325F
Altcode:
  The observed brightness fluctuations and large-scale structures on
  the surface of the Red Supergiant Betelgeuse make the star a prime
  targed for future interferometric measurements. At the same time, they
  open the possibility to resolve these structures in numerical radiation
  hydrodynamics simulations of the entire star. After some general remarks
  about the possibility and difficulties of 3D simulations of stars in
  general results of such calcuations of the outer convective envelope
  and the atmosphere of a Red Supergiant and a star on the Asmyptotic
  Giant Branch are presented. These show that numerous short-lived
  small-scale surface granules coexist with a few long-lived large-scale
  envelope convection cells. Pressure fluctuations deform the star
  and influence the surface convection. The convective “granulation"
  pattern differs from the solar one. Convection and pulsations produce
  large-scale high-contrast brightness fluctuations that might explain
  the observed luminosity variations and surface “spots”. Shock
  waves and supersonic atmospheric velocities manifest themselves in
  broad line profiles.

Title: Radiative transfer in snapshots of 3D radiative hydrodynamic
    models of red supergiants
Authors: Chiavassa, A.; Plez, B.; Josselin, E.; Freytag, B.
Bibcode: 2006EAS....18..177C
Altcode:
  Red Supergiant (RSG) represent a key-phase in the evolution of massive
  stars. These stars are characterized by strong mass loss of unknown
  origin. Observations show strong line profile fluctuations in depth,
  width and velocity, suggesting giant convective cells which could
  explain mass loss. Recently, radiative hydrodynamics (RHD) simulations
  of these stars show a peculiar convection pattern with giant cells. We
  performed 3D pure LTE radiative transfer calculations in snapshots of
  3D hydrodynamical simulations taking into account the Doppler shifts
  caused by the convective motions. Computed spectra from RHD models
  qualitatively reproduce observations.

Title: The Evolution of Central Stars of Planetary Nebulae
Authors: Herwig, Falk; Freytag, Bernd; Werner, Klaus
Bibcode: 2006IAUS..234..103H
Altcode: 2006astro.ph..6603H
  The evolution of central stars of planetary nebulae can proceed in
  several distinct ways, either leading to H-deficiency or to H-normal
  surface composition. Several new simulations of the evolution channels
  that lead to H-deficiency are now available, mainly the born-again
  scenarios that are triggered by a He-shell flash during the hot
  pre-white dwarf evolution phase. A realistic AGB progenitor evolution is
  important for correct HRD tracks, that allow mass determinations. New
  hydrodynamic simulations of He-shell flash convection including cases
  with H-ingestion are now performed, and allow a determination of the
  convective extra-mixing efficiency. This has direct consequences for
  the intershell abundance distribution of AGB stars that can be observed
  in the H-deficient CSPN.

Title: Simulations of Magnetohydrodynamics and CO Formation from
    the Convection Zone to the Chromosphere
Authors: Wedemeyer-Böhm, S.; Schaffenberger, W.; Steiner, O.; Steffen,
   M.; Freytag, B.; Kamp, I.
Bibcode: 2005ESASP.596E..16W
Altcode: 2005ccmf.confE..16W
  No abstract at ADS

Title: Magnetohydrodynamic Simulation from the Convection Zone to
    the Chromosphere
Authors: Schaffenberger, W.; Wedemeyer-Böhm, S.; Steiner, O.;
   Freytag, B.
Bibcode: 2005ESASP.596E..65S
Altcode: 2005ccmf.confE..65S
  No abstract at ADS

Title: Carbon monoxide in the solar atmosphere. I. Numerical method
    and two-dimensional models
Authors: Wedemeyer-Böhm, S.; Kamp, I.; Bruls, J.; Freytag, B.
Bibcode: 2005A&A...438.1043W
Altcode: 2005astro.ph..3496W
  The radiation hydrodynamic code CO5BOLD has been supplemented with
  the time-dependent treatment of chemical reaction networks. Advection
  of particle densities due to the hydrodynamic flow field is also
  included. The radiative transfer is treated frequency-independently,
  i.e. grey, so far. The upgraded code has been applied to two-dimensional
  simulations of carbon monoxide (CO) in the non-magnetic solar
  photosphere and low chromosphere. For this purpose a reaction network
  has been constructed, taking into account the reactions that are most
  important for the formation and dissociation of CO under the physical
  conditions of the solar atmosphere. The network has been strongly
  reduced to 27 reactions, involving the chemical species H, H2, C, O,
  CO, CH, OH and a representative metal. The resulting CO number density
  is highest in the cool regions of the reversed granulation pattern
  at mid-photospheric heights and decreases strongly above. There, the
  CO abundance stays close to a value of 8.3 on the usual logarithmic
  abundance scale with [H] = 12 but is reduced in hot shock waves which
  are a ubiquitous phenomenon of the model atmosphere. For comparison, the
  corresponding equilibrium densities have been calculated, based on the
  reaction network but also under the assumption of instantaneous chemical
  equilibrium by applying the Rybicki &amp; Hummer (RH) code. Owing to the
  short chemical timescales, the assumption holds for a large fraction
  of the atmosphere, in particular the photosphere. In contrast, the CO
  number density deviates strongly from the corresponding equilibrium
  value in the vicinity of chromospheric shock waves. Simulations with
  altered reaction networks clearly show that the formation channel via
  hydroxide (OH) is the most important one under the conditions of the
  solar atmosphere.

Title: The shock-patterned solar chromosphere in the light of ALMA
Authors: Wedemeyer-Böhm, S.; Ludwig, H. -G.; Steffen, M.; Freytag,
   B.; Holweger, H.
Bibcode: 2005ESASP.560.1035W
Altcode: 2005csss...13.1035W; 2005astro.ph..9747W
  Recent three-dimensional radiation hydrodynamic simulations by Wedemeyer
  et al. (2004) suggest that the solar chromosphere is highly structured
  in space and time on scales of only 1000 km and 20-25 sec, resp.. The
  resulting pattern consists of a network of hot gas and enclosed cool
  regions which are due to the propagation and interaction of shock
  fronts. In contrast to many other diagnostics, the radio continuum at
  millimeter wavelengths is formed in LTE, and provides a rather direct
  measure of the thermal structure. It thus facilitates the comparison
  between numerical model and observation. While the involved time
  and length scales are not accessible with todays equipment for that
  wavelength range, the next generation of instruments, such as the
  Atacama Large Millimeter Array (ALMA), will provide a big step towards
  the required resolution. Here we present results of radiative transfer
  calculations at mm and sub-mm wavelengths with emphasis on spatial
  and temporal resolution which are crucial for the ongoing discussion
  about the chromospheric temperature structure.

Title: 3D simulation of convection and spectral line formation in
    A-type stars
Authors: Steffen, M.; Freytag, B.; Ludwig, H. -G.
Bibcode: 2005ESASP.560..985S
Altcode: 2005astro.ph..9464S; 2005csss...13..985S
  We present first realistic numerical simulations of 3D radiative
  convection in the surface layers of main sequence A-type stars with Teff
  = 8000 K and 8500 K, log g = 4.4 and 4.0, recently performed with the
  CO5BOLD radiation hydrodynamics code. The resulting models are used to
  investigate the structure of the H+HeI and the HeII convection zones
  in comparison with the predictions of local and non-local convection
  theories, and to determine the amount of "overshoot" into the stable
  layers below the HeII convection zone. The simulations also predict
  how the topology of the photospheric granulation pattern changes from
  solar to A-type star convection. The influence of the photospheric
  temperature fluctuations and velocity fields on the shape of spectral
  lines is demonstrated by computing synthetic line profiles and line
  bisectors for some representative examples, allowing us to confront
  the 3D model results with observations.

Title: Convection, atmospheres and winds of red supergiant stars
Authors: Josselin, E.; Plez, B.; Freytag, B.
Bibcode: 2005ESASP.560..689J
Altcode: 2005csss...13..689J
  No abstract at ADS

Title: Dynamic Model Atmospheres of Cool Giants
Authors: Höfner, Susanne; Gautschy-Loidl, Rita; Aringer, Bernhard;
   Nowotny, Walter; Hron, Josef; Freytag, Bernd
Bibcode: 2005hris.conf..269H
Altcode: 2004astro.ph..3573H
  Cool giant stars are highly dynamical objects, and complex
  micro-physical processes play an important role in their extended
  atmospheres and winds. The interpretation of observations, and
  in particular of high-resolution IR spectra, requires realistic
  self-consistent model atmospheres. Current dynamical models include
  rather detailed micro-physics, and the resulting synthetic spectra
  compare reasonably well with observations. A transition from qualitative
  to quantitative modelling is taking place at present. We give an
  overview of existing dynamical model atmospheres for AGB stars,
  discussing recent advances and current trends in modelling. When
  comparing synthetic spectra and other observable properties resulting
  from dynamical models with observations we focus on the near- and
  mid-IR wavelength range.

Title: Numerical simulations of convection in A-stars
Authors: Freytag, Bernd; Steffen, Matthias
Bibcode: 2004IAUS..224..139F
Altcode:
  Radiation hydrodynamics simulations have been used to produce numerical
  models of the convective surface layers of a number of stars, including
  the Sun and other stars on or above the main-sequence, white dwarfs
  of type DA, and red supergiants.

Title: Numerical simulation of the three-dimensional structure and
    dynamics of the non-magnetic solar chromosphere
Authors: Wedemeyer, S.; Freytag, B.; Steffen, M.; Ludwig, H. -G.;
   Holweger, H.
Bibcode: 2004A&A...414.1121W
Altcode: 2003astro.ph.11273W
  Three-dimensional numerical simulations with CO<SUP>5</SUP>, a
  new radiation hydrodynamics code, result in a dynamic, thermally
  bifurcated model of the non-magnetic chromosphere of the quiet Sun. The
  3D model includes the middle and low chromosphere, the photosphere,
  and the top of the convection zone, where acoustic waves are excited
  by convective motions. While the waves propagate upwards, they steepen
  into shocks, dissipate, and deposit their mechanienergy as heat in
  the chromosphere. Our numerical simulations show for the first time
  a complex 3D structure of the chromospheric layers, formed by the
  interaction of shock waves. Horizontal temperature cross-sections of
  the model chromosphere exhibit a network of hot filaments and enclosed
  cool regions. The horizontal pattern evolves on short time-scales of
  the order of typically 20-25 s, and has spatial scales comparable to
  those of the underlying granulation. The resulting thermal bifurcation,
  i.e., the co-existence of cold and hot regions, provides temperatures
  high enough to produce the observed chromospheric UV emission and -
  at the same time - temperatures cold enough to allow the formation
  of molecules (e.g., carbon monoxide). Our 3D model corroborates the
  finding by \citet{carlsson94} that the chromospheric temperature rise
  of semi-empirical models does not necessarily imply an increase in
  the average gas temperature but can be explained by the presence of
  substantial spatial and temporal temperature inhomogeneities.

Title: Hot Spots in Numerical Simulations of Betelgeuse
Authors: Freytag, B.
Bibcode: 2003csss...12.1024F
Altcode:
  The spatial inhomogeneities on the surface of Betelgeuse revealed by
  modern interferometric observations are typically modelled as zero
  to three hot spots on a circular disk. While the surface structure
  appears to be generally consistent with an explanation as large-scale
  granular intensity fluctuations, the nature of the spots remains
  unclear. <P />The newly developed code COBOLD is now able to produce
  the first 3D radiation hydrodynamics simulations of a red supergiant,
  including realistic but simplified microphysics and non-local radiation
  transport. <P />The resulting models show giant convection cells,
  high spatial intensity fluctuations, and a surface pattern rather
  dissimilar to solar granulation. Some bright features over downdrafts
  might be interpreted as hot spots in low-resolution observations.

Title: 3D Simulation of the Solar Granulation: A Comparison of two
    Different Hydrodynamics Codes
Authors: Steffen, Matthias; Ludwig, Hans-Günter; Freytag, Bernd
Bibcode: 2003ANS...324..174S
Altcode: 2003ANS...324..P96S
  No abstract at ADS

Title: Three-dimensional Model of the Atmosphere of an AGB Star
Authors: Freytag, Bernd; Höfner, Susanne
Bibcode: 2003ANS...324..173F
Altcode: 2003ANS...324..P95F
  No abstract at ADS

Title: Modelling the Chromospheric Background Pattern of the
    Non-magnetic Sun
Authors: Wedemeyer, Sven; Freytag, Bernd; Steffen, Matthias; Ludwig,
   Hans-Günter; Holweger, Hartmut
Bibcode: 2003ANS...324R..66W
Altcode: 2003ANS...324..I07W
  No abstract at ADS

Title: Betelgeuse - Improved Numerical Simulations of an Entire
    Supergiant
Authors: Freytag, Bernd
Bibcode: 2003ANS...324...67F
Altcode: 2003ANS...324..P38F; 2003ANS...324b..67F
  No abstract at ADS

Title: Alpha Ori imaging science
Authors: Freytag, Bernd
Bibcode: 2003SPIE.4838..348F
Altcode:
  Three-dimensional radiation hydrodynamics simulation of the convective
  envelope and the atmosphere of a red supergiant (e.g. α Ori) have
  been performed, including ionization effects and realistic (grey)
  opacities. Only a handful of giant convection cells with lifetimes up
  to a few years are found in the envelope. The stellar surface itself is
  covered with smaller short-lived cells related to the well-known solar
  granulation but differing in many ways. Pressure fluctuations, acoustic
  waves, and shocks play an important role affecting the surface energy
  fluxes and accordingly the emitted intensity. The interaction with
  convection and waves generates observable large-scale high-contrast
  surface features. The complexity of the surface structures in the
  numerical models which will probably be even higher in nature calls
  for interferometric observations with high spatial and some temporal
  resolution. The comparison with the results of numerical simulations
  will allow to deduce information about the nature of the surface
  phenomena and fundamental stellar parameters.

Title: Acoustic Waves in the Solar Chromosphere - Numerical
    Simulations with COBOLD
Authors: Wedemeyer, S.; Freytag, B.; Steffen, M.; Ludwig, H. -G.;
   Holweger, H.
Bibcode: 2003IAUS..210P..C1W
Altcode:
  No abstract at ADS

Title: Modelling the Entire Atmosphere of Betelgeuse with 3D
    Simulations
Authors: Freytag, B.; Mizuno-Wiedner, M.
Bibcode: 2003IAUS..210P..C4F
Altcode:
  No abstract at ADS

Title: 3-D hydrodynamic simulations of the solar chromosphere
Authors: Wedemeyer, S.; Freytag, B.; Steffen, M.; Ludwig, H. -G.;
   Holweger, H.
Bibcode: 2003AN....324..410W
Altcode:
  We present first results of three-dimensional numerical simulations
  of the non-magnetic solar chromosphere, computed with the radiation
  hydrodynamics code CO<SUP>5</SUP>BOLD. Acoustic waves which are
  excited at the top of the convection zone propagate upwards into the
  chromosphere where the waves steepen into shocks. The interaction of
  the waves leads to the formation of complex structures which evolve
  on short time scales. Consequently, the model chromosphere is highly
  dynamical, inhomogeneous, and thermally bifurcated.

Title: Does Betelgeuse Have a Magnetic Field?
Authors: Dorch, S. B. F.; Freytag, B.
Bibcode: 2003IAUS..210P.A12D
Altcode: 2002astro.ph..8523D
  Recent numerical simulations by Freytag et al. of the outer convection
  envelope of the cool super-giant Betelgeuse, have shown that the
  fluctuations in the star's apparent luminosity may be caused by giant
  cell convection. These simulations bring forth the possibility of
  addressing another question; namely whether stars such as Betelgeuse may
  harbor magnetic activity. Taking the detailed numerical simulations of
  the star at face value, we have applied a kinematic dynamo analysis, to
  study whether the flow field of the super-giant may be able to amplify
  a weak seed magnetic field. We do indeed find a positive exponential
  growth rate of the magnetic energy. The possible Betelgeusian dynamo
  may be characterized as belonging to the class of so-called ``local
  small-scale dynamos'' (i.e. dynamos where rotation is not dominant),
  but this is a less meaningful designation in the case of Betelgeuse,
  since the field is both global and large-scale.

Title: Typical Scales of Structures in Numerical Models of Betelgeuse
Authors: Freytag, B.; Finnsson, S.
Bibcode: 2003IAUS..210P.C12F
Altcode:
  No abstract at ADS

Title: Spots on the surface of Betelgeuse --  Results from new 3D
    stellar convection models
Authors: Freytag, B.; Steffen, M.; Dorch, B.
Bibcode: 2002AN....323..213F
Altcode:
  The observed irregular brightness fluctuations of the well-known red
  supergiant Betelgeuse (alpha Ori, M2 Iab) have been attributed by
  M. Schwarzschild (1975) to the changing granulation pattern formed
  by only a few giant convection cells covering the surface of this
  giant star. The surface structure revealed by modern interferometric
  methods appears to be generally consistent with the explanation as
  large-scale granular intensity fluctuations. The interferometric
  data can be modeled equally well by assuming the presence of a few
  (up to 3) unresolved hot or cool spots on a limb-darkened disk. In an
  effort to improve our theoretical understanding of the Betelgeuse
  phenomena, we have applied a new radiation hydrodynamics code
  (CO<SUP>5</SUP>BOLD) to the problem of global convection in giant
  stars. For this purpose, the "local box" setup usually employed for the
  simulation of solar-type surface convection cannot be used. Rather, we
  have chosen a radically different approach: the whole star is enclosed
  in a cube ("star-in-a-box" setup). The properties of the stellar model
  are defined by the prescribed gravitational central potential and by
  a special inner boundary condition which replaces the unresolved core,
  including the source of nuclear energy production. We present current
  results obtained from this novel generation of 3D stellar convection
  simulations, proceeding from a toy model ("Mini-Sun") towards the
  numerically more demanding supergiant regime. We discuss the basic
  observational properties of Betelgeuse in the light of our best model
  obtained so far (T_eff = 3300 K, log g = -0.4). Finally, we describe a
  first attempt to investigate the interaction of the global convective
  flows with magnetic fields based on the kinematic approximation.

Title: Hydrodynamical models of mixing beyond a convection zone
Authors: Freytag, Bernd
Bibcode: 2002HiA....12..298F
Altcode:
  Numerical radiation hydrodynamics simulations of stellar surface
  convection have gained a high level of reliability and perform very
  well in the confrontation with observations. Similar simulations of
  convection zones in stellar interiors are much more difficult to set up
  because of the huge range of time-scales to cover. Therefore, simplified
  models have to be used, where the results have to be carefully
  extrapolated to apply them to stellar interiors. Simulations of shallow
  surface convection zones show that the overshooting velocity fields
  extend far beyond the region with significant convective energy flux.

Title: Betelgeuse - improved numerical simulations of an entire
    supergiant.
Authors: Freytag, B.
Bibcode: 2002AGAb...19Q..90F
Altcode:
  No abstract at ADS

Title: Acoustic Energy Generated by Convection: 3-D Numerical
    Simulations for the Sun
Authors: Wedemeyer, Sven; Freytag, Bernd; Holweger, Hartmut; Ludwig,
   Hans-Günter; Steffen, Matthias
Bibcode: 2001AGM....18..P01W
Altcode:
  Dissipation of acoustic waves may be an efficient heating mechanism for
  the lower and middle chromosphere of the quiet Sun. The basic idea is
  that turbulent motions at the top of the solar convection zone generate
  acoustic waves which propagate upwards and dissipate in the lower and
  middle chromosphere, transporting energy into the higher layers. But
  still the question remains if this amount of energy is sufficient to
  explain the observed temperature increase without invoking magnetic
  fields. With a new version of the COBOLD radiation hydrodynamics code
  we are able to compute 3-D models extending all the way from the upper
  convection zone to the middle chromosphere. First 3-D simulations reveal
  a complex, inhomogenous and highly dynamical structure of the lower and
  middle chromosphere which evolves on rather short timescales. On small
  spatial dimensions very cool regions are present next to a "network"
  of hotter matter. The code is being developed further to provide a
  more detailed analysis and comparison with observations.

Title: Betelgeuse - Numerical Simulations of an Entire Supergiant
Authors: Freytag, Bernd
Bibcode: 2001AGM....18..P18F
Altcode: 2001AGAb...18Q.144F
  The red supergiant Betelgeuse varies in visual brightness on time-scales
  of weeks, months, and years. In 1975 Martin Schwarzschild attributed
  these fluctuations to huge convection cells, each of them covering a
  significant fraction of the stellar surface, so that the individual
  brightness changes result in a non-vanishing variation of the total
  luminosity. Starting about 11 years ago, interferometric observations
  in the visible wavelength regime revealed the existence of large-scale
  inhomogeneities on the surface of Betelgeuse, typically described as
  0 to 3 unresolved ``hot spots'' on a cooler circular stellar disk,
  varying with time in number, intensity, and position. Nevertheless,
  the observations still have a poor resolution, resulting in surface
  ``images'' with only a handful of pixels. And there has been some
  debate about the nature of the detected surface features: Are they of
  convective origin due to the action of granules or supergranules? And
  what is the role of shocks, stellar rotation, or magnetic fields? To
  improve the theoretical understanding of the surface phenomena
  of Betelgeuse, a new radiation hydrodynamics code (``COBOLD'')
  has been written with the aim to include the entire star in the
  computational box. It employs special inner (for the stellar core)
  and outer boundaries appropriate for this particular geometry. The
  simulated star shows indeed huge surface convection cells and high
  photospheric velocities. But the cells look different from solar
  granulation and produce features which might be interpreted as ``hot
  spots'' in low-resolution observations.

Title: Stellar Surface Convection from White Dwarfs to Red Supergiants
(CD-ROM Directory: contribs/freytag)
Authors: Freytag, B.
Bibcode: 2001ASPC..223..785F
Altcode: 2001csss...11..785F
  No abstract at ADS

Title: Radiation Hydrodynamics Simulations of the Solar Chromosphere
Authors: Wedemeyer, Sven; Freytag, Bernd; Steffen, Matthias; Holweger,
   Hartmut
Bibcode: 2000AGM....17..P01W
Altcode:
  While heating of the solar corona is commonly attributed to
  reconnection of magnetic field lines, the mechanism responsible for
  heating the chromosphere of the quiet Sun, away from active regions,
  is still under debate<SUP>1,2</SUP>. The basic question which we will
  address in this contribution i s: Can generation of acoustic waves by
  turbulent convection in photospheric and subphotospheric layers explain
  the chromospheric emission of the quiet Sun? With a new 3D radiation
  hydrodynamics code<SUP>3</SUP> we are able to compute models ex tending
  from the upper convection zone to the middle chromosphere. The code
  can handle shocks with a minimum of numerical dissipation. Therefore
  generation and propagation of acoustic waves can be investigated,
  permitting the evaluation of wave dissipation in the chromosphere in
  a physically consistent manner. We present first results and discuss
  the principal problems and future prospects.

Title: Hydrodynamical Models of Mixing beyond a Convection Zone
Authors: Freytag, Bernd
Bibcode: 2000IAUJD...5E..13F
Altcode:
  Numerical radiation hydrodynamics simulations of shallow stellar surface
  convection zones allow the investigation of the mixing properties
  of the adjacent overshoot layers. Typical time scales of diffusion
  processes which lead to element separation (e.g. by gravitational
  settling or radiation pressure) are orders of magnitudes longer
  than convective turnover times. Thus, inside a convection zone the
  mixing can be considered as occurring instantaneously. On the other
  hand, even small traces of the convective flow that extend beyond the
  classical Schwarzschild boundaries of the unstable layers can be able to
  counteract element separation. Numerical simulations display a fairly
  complicated structure of the boundaries of a convection zone. At some
  distance above the surface granulation the photospheric overshoot layers
  are dominated by shocks and large amplitude sound waves generated in the
  upper-most region of the convection zone. In the undershoot layers the
  signature of convection with the longest range of influence is a field
  of velocities and pressure fluctuations with exponentially declining
  amplitudes. These relatively small velocities still contribute to the
  mixing of material without any significant energy transport.

Title: Betelgeuse -- Towards Numerical Simulations of an Entire
    Supergiant
Authors: Freytag, Bernd
Bibcode: 2000AGM....17..P20F
Altcode: 2000AGAb...17Q..59F
  The brightness of the well-known red supergiant Betelgeuse varies on
  time-scales of weeks, months, and years. For a dedicated observer
  these fluctuations are even visible with the naked eye. In 1975
  Martin Schwarzschild attributed them to huge convection cells, each
  of them covering a significant fraction of the stellar surface, so
  that the individual fluctuations result in an overall non-vanishing
  variation of the stars luminosity. Starting about 10 years ago, numerous
  interferometric observations in the visible wavelength regime revealed
  the existence of large-scale brightness inhomogeneities on the surface
  of Betelgeuse, typically described as 0 to 3 unresolved ``hot spots'' on
  a cooler circular stellar disk, varying with time in number, intensity,
  and position. HST demonstrated deviations of the stellar UV image from
  spherical symmetry. Nevertheless, the observations have still a poor
  resolution, resulting in surface ``images'' with only a handful of
  pixels. And there has been some debate about the nature of the detected
  surface features: Are they of convective origin due to the action of
  granules or supergranules? And what is the role of shocks, stellar
  rotation, or magnetic fields? To improve the theoretical understanding
  of the surface phenomena of Betelgeuse, a new radiation hydrodynamics
  code has been written with the aim to include the entire star in the
  computational box. It employs special inner (for the stellar core) and
  outer boundaries appropriate for this particular geometry. Starting
  with down-scaled toy models and approaching the parameter regime
  appropriate for Betelgeuse, some results about the dynamics of large
  convective cells on a spherical stellar surface will be presented.

Title: The Solar p-Mode Background: Observations and Hydrodynamical
    Models
Authors: Straus, Th.; Steffen, M.; Severino, G.; Freytag, B.
Bibcode: 1999ESASP.448..203S
Altcode: 1999mfsp.conf..203S; 1999ESPM....9..203S
  No abstract at ADS

Title: A calibration of the mixing-length for solar-type stars based
    on hydrodynamical simulations. I. Methodical aspects and results
    for solar metallicity
Authors: Ludwig, Hans-Günter; Freytag, Bernd; Steffen, Matthias
Bibcode: 1999A&A...346..111L
Altcode: 1998astro.ph.11179L
  Based on detailed 2D numerical radiation hydrodynamics (RHD)
  calculations of time-dependent compressible convection, we have
  studied the dynamics and thermal structure of the convective surface
  layers of solar-type stars. The RHD models provide information about
  the convective efficiency in the superadiabatic region at the top of
  convective envelopes and predict the asymptotic value of the entropy of
  the deep, adiabatically stratified layers (Fig. \ref{f:sstarhd}). This
  information is translated into an effective mixing-length parameter
  \alpha<SUB>MLT</SUB> suitable to construct standard stellar
  structure models. We validate the approach by a detailed comparison
  to helioseismic data. The grid of RHD models for solar metallicity
  comprises 58 simulation runs with a helium abundance of Y=0.28 in the
  range of effective temperatures 4300pun {K}&lt;=T<SUB>eff</SUB>&lt;=
  7100pun {K} and gravities 2.54&lt;={log g}&lt;= 4.74. We find a
  moderate, nevertheless significant variation of \alpha<SUB>MLT</SUB>
  between about 1.3 for F-dwarfs and 1.75 for K-subgiants with a
  dominant dependence on T<SUB>eff</SUB> (Fig. \ref{f:mlp}). In the close
  neighbourhood of the Sun we find a plateau where \alpha<SUB>MLT</SUB>
  remains almost constant. The internal accuracy of the calibration
  of \alpha<SUB>MLT</SUB> is estimated to be +/- 0.05 with a possible
  systematic bias towards lower values. An analogous calibration of
  the convection theory of Canuto &amp;\ Mazzitelli (1991, 1992; CMT)
  gives a different temperature dependence but a similar variation of
  the free parameter (Fig. \ref{f:mlpcm}). For the first time, values
  for the gravity-darkening exponent beta are derived independently of
  mixing-length theory: beta = 0.07... 0.10. We show that our findings
  are consistent with constraints from stellar stability considerations
  and provide compact fitting formulae for the calibrations.

Title: Stellar Envelope Convection Calibrated by Radiation
Hydrodynamics Simulations: Influence on Globular Cluster Isochrones
Authors: Freytag, Bernd; Salaris, Maurizio
Bibcode: 1999ApJ...513L..49F
Altcode: 1999astro.ph..1074F
  One of the largest sources of uncertainty in the computation of globular
  cluster isochrones and hence in the age determination of globular
  clusters is the lack of a rigorous description of convection. Therefore,
  we calibrated the superadiabatic temperature gradient in the envelope
  of metal-poor low-mass stars according to the results from a new grid of
  two-dimensional hydrodynamical models, which cover the main sequence and
  the lower red giant branch of globular cluster stars. In practice, we
  still use for computing the evolutionary stellar models the traditional
  mixing-length formalism, but we fix the mixing-length parameter α
  in order to reproduce the run of the entropy of the deeper adiabatic
  region of the stellar envelopes with effective temperature and gravity
  as obtained from the hydrodynamical models. The detailed behavior of the
  calibrated α depends in a nontrivial way on the effective temperature,
  gravity, and metallicity of the star. Nevertheless, the resulting
  isochrones for the relevant age range of Galactic globular clusters have
  only small differences with respect to isochrones computed adopting
  a constant solar calibrated value of the mixing length. Accordingly,
  the age of globular clusters is reduced by 0.2 Gyr at most.

Title: Treatment of the Superadiabatic Convection in Low-Mass
Metal-Poor Stars from Realistic Hydrodynamics Simulations: Application
    to Globular Clusters Isochrones
Authors: Freytag, B.; Salaris, M.; Ludwig, H. -G.
Bibcode: 1999ASPC..173..201F
Altcode: 1999sstt.conf..201F
  No abstract at ADS

Title: A Calibration of the Mixing-Length for Solar-Type Stars Based
    on Hydrodynamical Models of Stellar Surface Convection
Authors: Freytag, B.; Ludwig, H. -G.; Steffen, M.
Bibcode: 1999ASPC..173..225F
Altcode: 1999sstt.conf..225F
  No abstract at ADS

Title: Stellar Surface Convection in Stars of Various Radii
Authors: Freytag, Bernd
Bibcode: 1999AGAb...15...99F
Altcode: 1999AGM....15..P24F
  Based on detailed numerical radiation hydrodynamics calculations of
  time-dependent compressible convection, the dynamics, spatial scales,
  and thermal structure of the convective surface layers of different
  stars with a large range of diameters have been studied. While solar
  convection is efficient in carrying energy and the convection zone
  comprises a significant part of the Sun, a typical granule is tiny
  compared to the solar diameter. Models of solar-type convection from
  F to K type stars from the main-sequence to the base of the Red Giant
  Branch with solar and reduced metallicities show a qualitatively similar
  behavior: The horizontal extent of the granules scales approximately
  with the pressure scale height. Main-sequence A type stars have shallow
  radiation dominated convection zones with inefficient convective energy
  transport. The simulations show that in these stars the strong overshoot
  mixes much more matter than is actually included in the convectively
  unstable region. The granules have only a slightly larger relative size
  than the solar ones. A sequence of models of hydrogen-rich White Dwarfs
  in the neighborhood of the ZZ Ceti instability strip demonstrates the
  transition from inefficient to efficient convection. The granules have
  a typical size of 1 km (compared to 1000 km in the Sun) according to
  the larger gravity and are again small in comparison to the diameter of
  a White Dwarf. This is completely different in Red Supergiants where
  the low surface gravity allows only a couple of granules on their
  surface. Their random brightness fluctuations might be responsible
  for the observed luminosity fluctuations of e.g. Betelgeuse. First 3D
  radiation hydrodynamics models of a Red Supergiant will be presented
  and and the properties of its convective envelope will be contrasted
  to those of other stars.

Title: Lithium Depletion in the Sun: A Study of Mixing Based on
    Hydrodynamical Simulations
Authors: Blöcker, T.; Holweger, H.; Freytag, B.; Herwig, F.; Ludwig,
   H. -G.; Steffen, M.
Bibcode: 1998SSRv...85..105B
Altcode: 1998astro.ph..6310B
  Based on radiation hydrodynamics modeling of stellar convection zones,
  a diffusion scheme has been devised describing the downward penetration
  of convective motions beyond the Schwarzschild boundary (overshoot)
  into the radiative interior. This scheme of exponential diffusive
  overshoot has already been successfully applied to AGB stars. Here
  we present an application to the Sun in order to determine the time
  scale and depth extent of this additional mixing, i.e. diffusive
  overshoot at the base of the convective envelope. We calculated the
  associated destruction of lithium during the evolution towards and on
  the main-sequence. We found that the slow-mixing processes induced by
  the diffusive overshoot may lead to a substantial depletion of lithium
  during the Sun's main-sequence evolution.

Title: Lithium Depletion in the Sun: A Study of Mixing Based on
    Hydrodynamical Simulations
Authors: Blöcker, T.; Holweger, H.; Freytag, B.; Herwig, F.; Ludwig,
   H. -G.; Steffen, M.
Bibcode: 1998sce..conf..105B
Altcode:
  No abstract at ADS

Title: Numerical simulations of convection in low-mass metal-poor
    stars
Authors: Freytag, Bernd
Bibcode: 1998AGAb...14..113F
Altcode: 1998AGM....14..P28F
  Based on detailed 2D numerical radiation hydrodynamics (RHD)
  calculations of time-dependent compressible convection, we have
  studied the dynamics and thermal structure of the convective
  surface layers of stars in the range of effective temperatures,
  gravities, and metallicities between 4300K &lt;= T<SUB>eff</SUB> &lt;=
  7100K, 2.54 &lt;= log g &lt;= 4.74, and M/H=0.0, -0.5, -1.0, -2.0,
  respectively. Although our models describe only the shallow, strongly
  superadiabatic layers at the top of the convective stellar envelope,
  they provide information about the value of the entropy of the deeper,
  adiabatically stratified regions. This quantity can be translated
  into an effective mixing-length parameter alpha<SUB>MLT</SUB> for
  every single RHD model. A fit for all models with the same metallicity
  results in a function alpha=alpha<SUB>fit</SUB>(T<SUB>eff</SUB>, log g)
  for each of the four metallicities. With the help of these functions,
  evolutionary tracks and isochrones with constant and calibrated
  alpha values have been computed and compared to investigate the
  significance of the variations in alpha<SUB>fit</SUB>. The behavior
  of alpha<SUB>fit</SUB> in the HRD region corresponding to Turn-Off,
  Subgiant Branch and base of the Red Giant Branch of globular clusters
  is of particular interest; a variable alpha may change the color
  (and even, at a lesser extent, the luminosity) of the Turn-Off
  and the color difference between Turn-Off and Red Giant Branch,
  as derived from theoretical isochrones, with respect to the case of
  constant alpha. This, in turn, might affect absolute and relative
  age determinations of globular clusters. Instead of the mixing-length
  parameter of the MLT it is possible to calibrate the free overshoot
  parameter of the turbulence theory of Canuto &amp;\ Mazzitelli in an
  analogous manner. The size of the variations of the free parameter of
  both theories is compared.

Title: An improved calibration of the mixing-length based on
    simulations of solar-type convection
Authors: Ludwig, H. -G.; Freytag, B.; Steffen, M.
Bibcode: 1998IAUS..185..115L
Altcode:
  Based on detailed 2D numerical radiation hydrodynamics (RHD)
  calculations of time-dependent compressible convection, we have studied
  the dynamics and thermal structure of the convective surface layers
  of stars in the range of effective temperatures and gravities between
  4500 pun{K} &lt;= Teff &lt;= 7100 pun{K} and 2.54 &lt;= logg &lt;=
  4.74. Although our hydrodynamical models describe only the shallow,
  strongly superadiabatic layers at the top of the convective stellar
  envelope, we demonstrate that they provide information about the value
  of the entropy of the deeper, adiabatically stratified regions. This
  quantity can be translated into an effective mixing-length parameter
  suitable for constructing standard stellar structure models. We
  show that a hydrodynamically calibrated envelope model for the Sun
  closely matches the known adiabat and corresponding depth of the solar
  convection zone. We determined the dependence of the mixing-length
  parameter on Teff, log g, and chemical composition obtaining a moderate
  variation over the range studied. We note that the recent description
  of convection by Canuto &amp; Mazzitelli extended by including a
  variable amount of overshoot does not lead to a smaller variation of
  the controlling parameter. We discuss the consistency of our results
  with findings derived in the context of the tentative detection of
  solar-like oscillations in eta Bootis.

Title: A calibration of mixing length theory based on RHD simulations
    of solar-type convection
Authors: Ludwig, H. -G.; Freytag, B.; Steffen, M.
Bibcode: 1997ASSL..225...59L
Altcode: 1997scor.proc...59L
  Radiation hydrodynamics (RHD) models provide detailed information
  about the dynamics, thermal structure, and convective efficiency
  of the superadiabatic region at the top of solar-type convection
  zones, and allow an extrapolation of the entropy (s*) in their deep,
  adiabatic layers. For the Sun we find a close agreement between s*
  inferred from our RHD models and an empirical determination of s*
  from helioseismology. In the framework of mixing length theory (MLT),
  s* is translated to an effective mixing-length parameter (alpha c)
  appropriate to construct global stellar models. The calibration based
  on our present set of 2D RHD models shows a moderate variation of
  alpha c across the domain of the HRD investigated so far.

Title: "Discovery" of a β Pictoris-like circumstellar disk in
    the Internet.
Authors: Hempel, M.; Holweger, H.; Rentzsch-Holm, I.; Freytag, B.
Bibcode: 1997AGAb...13..201H
Altcode:
  No abstract at ADS

Title: Numerical simulations of stellar surface convection.
Authors: Freytag, B.; Steffen, M.
Bibcode: 1997AGAb...13..176F
Altcode:
  No abstract at ADS

Title: On the Scale of Photospheric Convection
Authors: Freytag, B.; Holweger, H.; Steffen, M.; Ludwig, H. -G.
Bibcode: 1997svlt.work..316F
Altcode:
  No abstract at ADS

Title: Hydrodynamical models of stellar convection. The role of
    overshoot in DA white dwarfs, A-type stars, and the Sun.
Authors: Freytag, B.; Ludwig, H. -G.; Steffen, M.
Bibcode: 1996A&A...313..497F
Altcode:
  Based on two-dimensional numerical radiation hydrodynamics
  simulations of time-dependent compressible convection, we have
  studied the structure and dynamics of a variety of shallow stellar
  surface convection zones. Our present grid of models includes
  detailed simulations of surface convection in solar-type stars,
  main-sequence A-type stars and cool DA white dwarfs, as well as
  numerical experiments to study convection and overshoot at the base of
  the solar convection zone. Taking into account a realistic equation of
  state (including the effects of ionization) and adopting an elaborate
  treatment of non-local radiative transfer (with appropriate grey
  or frequency-dependent opacities), our simulations are designed to
  represent specific stellar objects characterized by T_eff_, logg, and
  chemical composition. Contrary to solar-type stars, the A-type stars and
  cool DA white dwarfs investigated here have shallow convection zones
  which fit into the computational domain together with thick stable
  buffer layers on top and below, thus permitting a study of convective
  overshoot under genuine conditions. We find that convective motions
  extend well beyond the boundary of the convectively unstable region,
  with vertical velocities decaying exponentially with depth in the
  deeper parts of the lower overshoot region, as expected for linear
  g^-^-modes. Even though convective velocities are reduced by orders of
  magnitude, they are still able to counteract molecular diffusion. For
  a quantitative description of convective mixing in the far overshoot
  layers we have derived a depth dependent diffusion coefficient from
  the numerical simulations. In combination with otherwise independent
  1D diffusion calculations for a trace element, this allows the
  determination of the "effective depth" of the overshoot region. For
  a typical main-sequence A-type star (T_eff_=7943#1, logg=4.34) the
  mass in the overshoot region exceeds the mass in the unstable region
  by approximately a factor 10. The amount of overshoot in cool DA white
  dwarfs (around T_eff_=12200#1) is even larger: the convectively mixed
  mass is increased by roughly a factor 100.

Title: Overtures to the pulsational instability of ZZ Ceti variables.
Authors: Gautschy, A.; Ludwig, H. -G.; Freytag, B.
Bibcode: 1996A&A...311..493G
Altcode: 1995astro.ph..8002G
  Results of nonradial, nonadiabatic pulsation calculations on
  hydrogen-rich white dwarf models are presented. In contrast to earlier
  attempts, the modeling builds on hydrodynamically simulated convective
  surface layers supplemented with standard interior models. Based on
  our stellar models and despite of various simple attempts to couple
  convection and pulsation we could not reproduce theoretically the
  presently adopted location of the observed blue edge of the ZZ Ceti
  variables. When the convective efficiency is high enough we found a
  sensitive dependence of the stability properties of the g modes on
  the pulsational treatment of shear within the convection zone.

Title: Problems in Modeling Photospheric Convective Overshooting
Authors: Freytag, B.
Bibcode: 1996ASPC..108...93F
Altcode: 1996mass.conf...93F
  No abstract at ADS

Title: Lyapunov exponents for solar surface convection.
Authors: Steffen, M.; Freytag, B.
Bibcode: 1995CSF.....5.1965S
Altcode:
  The authors have carried out detailed 2D numerical radiation
  hydrodynamics calculations, specifically designed to model
  time-dependent, compressible convection in the surface layers of the
  Sun. These simulations, which take into account a realistic equation
  of state and use an elaborate scheme to describe multi-dimensional,
  non-local, frequency-dependent radiative transfer, allow a direct
  comparison with observed photometric and spectroscopic properties of
  solar granulation. Their purpose is to enhance the understanding of
  the dynamics and thermal structure of convective stellar atmospheres,
  and to investigate the generation of acoustic energy by turbulent
  convection. The authors briefly present some of the main properties of
  their solar convection models. In particular, the authors demonstrate
  the chaotic behaviour of solar surface convection, estimating the
  magnitude of the two largest Lyapunov exponents, λ<SUB>1</SUB> and
  λ<SUB>2</SUB>, by analysing the time evolution of three simulations
  with slightly different initial conditions. The authors find that
  both λ<SUB>1</SUB> and λ<SUB>2</SUB> are positive and of similar
  magnitude as expected for a chaotic system of high dimension. The
  corresponding characteristic time scale λ<SUB>1</SUB><SUP>-1</SUP>
  of approximately 320 s is comparable to the convective turnover time.

Title: Synthetic spectra computed from hydrodynamical model
    atmospheres of DA white dwarfs.
Authors: Steffen, M.; Ludwig, H. -G.; Freytag, B.
Bibcode: 1995A&A...300..473S
Altcode:
  From detailed 2-dimensional numerical radiation hydrodynamics
  calculations of time-dependent compressible convection we have obtained
  the thermal structure of the convective surface layers of DA white
  dwarfs with effective temperatures near the blue edge of the ZZ Ceti
  instability strip. Synthetic line profiles of H<SUB>beta</SUB>_ and the
  red wing of L<SUB>alpha</SUB>_ (including the very temperature sensitive
  satellite absorption features) computed from two representative
  inhomogeneous hydrodynamical models (T_eff_=12200K, 12600K; log g=8.0)
  are compared with the spectra resulting from different plane-parallel
  model atmospheres. We find that it is possible to represent a given
  inhomogeneous atmosphere by a spectroscopically equivalent 1D model,
  constructed to have the same frequency-integrated radiative flux as
  the respective 2D hydrodynamical model at all depths. Synthetic spectra
  computed from this representative 1D model are almost indistinguishable
  from the horizontally averaged 2D synthetic spectra of the corresponding
  inhomogeneous model. We conclude that in the investigated range of
  effective temperature (probably even for the whole range of convective
  DAs), spectroscopic analysis based on appropriate 1D atmospheres is
  almost unaffected by systematic errors associated with non-linear flux
  variations due to the substantial thermal inhomogeneities generated
  by photospheric convection in these stars. This work provides the
  basis for a well defined comparison between 2D or 3D hydrodynamical
  convection models and 1D standard mixing length models.

Title: The Mixing-Length Parameter for Solar-Type Convection Zones
    Inferred from Hydrodynamical Models of the Surface Layers
Authors: Ludwig, H. -G.; Freytag, B.; Steffen, M.; Wagenhuber, J.
Bibcode: 1995LIACo..32..213L
Altcode: 1995sews.book..213L
  No abstract at ADS

Title: Numerical Simulations of Convection and Overshoot in the
    Envelope of DA White Dwarfs
Authors: Freytag, Bernd; Steffen, Matthias; Ludwig, Hans-Günter
Bibcode: 1995LNP...443...88F
Altcode: 1995whdw.conf...88F
  We present results of realistic 21) numerical radiation hydrodynamics
  calculations, simulating the surface convection zones of DA white
  dwarfs in the range of effective temperatures from 14 200 K down
  to 11400 K. Comparison with mixing length theory (MLT) yields a
  conflicting picture: The dynamics of convection is not governed by
  up- and downflowing bubbles which dissolve after travelling some
  characteristic distance - but by the formation, advection, merging,
  and disruption of fast narrow downdrafts in a slowly upstreaming
  surrounding. MLT tremendously underestimates the depth of the region
  where material is mixed. Nevertheless, it turns out that a mixing
  length model with α = 1.5 gives a good fit of the photospheric
  temperature structure (T <SUB>eff</SUB> = 12 600 K) and that a 1D
  temperature stratification suffices to reproduce the mean spectrum
  of the 2D simulations, indicating that the photospheric temperature
  inhomogeneities are negligible for spectroscopic analysis. In deeper
  layers the temperature stratification of our hydrodynamical models
  corresponds to larger values of α. Introducing our envelope models into
  nonadiabatic pulsation calculations results in a blue edge of the ZZ
  Ceti instability strip near T <SUB>eff</SUB> = 12 400 K at log g = 8.0.

Title: Numerical simulations of surface convection in solar-type stars
Authors: Freytag, B.; Steffen, M.
Bibcode: 1995IAUS..176P.111F
Altcode:
  No abstract at ADS

Title: Shocks in the solar photosphere and their spectroscopic
    signature
Authors: Steffen, M.; Freytag, B.; Holweger, H.
Bibcode: 1994smf..conf..298S
Altcode:
  No abstract at ADS

Title: Hydrodynamics of the Solar Photosphere: Model Calculations
    and Spectroscopic Observations.
Authors: Steffen, M.; Freytag, B.
Bibcode: 1991RvMA....4...43S
Altcode:
  No abstract at ADS

