Author name code: gehren
ADS astronomy entries on 2022-09-14
author:"Gehren, Thomas" 
------------------------------------------------------------------------  

Title: Statistical Equilibrium of Copper in the Solar Atmosphere
Authors: Shi, J. R.; Gehren, T.; Zeng, J. L.; Mashonkina, L.; Zhao, G.
Bibcode: 2014ApJ...782...80S
Altcode:
  Non-local thermodynamic equilibrium (NLTE) line formation for neutral
  copper in the one-dimensional solar atmospheres is presented for the
  atomic model, including 96 terms of Cu I and the ground state of Cu
  II. The accurate oscillator strengths for all the line transitions in
  model atom and photoionization cross sections were calculated using
  the R-matrix method in the Russell-Saunders coupling scheme. The main
  NLTE mechanism for Cu I is the ultraviolet overionization. We find
  that NLTE leads to systematically depleted total absorption in the Cu
  I lines and, accordingly, positive abundance corrections. Inelastic
  collisions with neutral hydrogen atoms produce minor effects on the
  statistical equilibrium of Cu I in the solar atmosphere. For the
  solar Cu I lines, the departures from LTE are found to be small, the
  mean NLTE abundance correction of ~0.01 dex. It was found that the
  six low-excitation lines, with excitation energy of the lower level E
  <SUB>exc</SUB> &lt;= 1.64 eV, give a 0.14 dex lower mean solar abundance
  compared to that from the six E <SUB>exc</SUB> &gt; 3.7 eV lines,
  when applying experimental gf-values of Kock &amp; Richter. Without
  the two strong resonance transitions, the solar mean NLTE abundance
  from 10 lines of Cu I is log ɛ<SUB>⊙</SUB>(Cu) = 4.19 ± 0.10,
  which is consistent within the error bars with the meteoritic value
  4.25 ± 0.05 of Lodders et al. The discrepancy between E <SUB>exc</SUB>
  = 1.39-1.64 eV and E <SUB>exc</SUB> &gt; 3.7 eV lines can be removed
  when the calculated gf-values are adopted and a mean solar abundance
  of log ɛ<SUB>⊙</SUB>(Cu) = 4.24 ± 0.08 is derived.

Title: Statistical equilibrium of silicon in the atmospheres of
    cool stars
Authors: Shi, J. R.; Gehren, T.; Mashonkina, L.; Zhao, G.
Bibcode: 2014IAUS..298..437S
Altcode:
  The statistical equilibrium of neutral and ionized silicon in the
  atmospheres of cool stars is discussed. Non-local thermodynamic
  equilibrium effects (NLTE) are investigated. It is found that the
  NLTE effects for Si are important, in particular for warm metal-poor
  stars. For warm metal-poor stars, the NLTE abundance correction reaches
  ~0.2 dex relative to standard LTE calculations.

Title: Scandium Abundance in Metal-poor Stars
Authors: Zhang, H. W.; Gehren, T.; Zhao, G.
Bibcode: 2014IAUS..298..453Z
Altcode:
  The Scandium abundances for 85 metal-poor stars are presented. Our
  result shows that NLTE corrections for Sc II lines are small (-0.04
  to +0.06 dex). The abundance trends in stars of different populations
  are discussed.

Title: Statistical equilibrium of silicon in the atmospheres of
    nearby metal-poor stars
Authors: Shi, J. R.; Gehren, T.; Zhao, G.
Bibcode: 2011A&A...534A.103S
Altcode:
  <BR /> Aims: We discuss the statistical equilibrium of neutral and
  ionized silicon in the atmospheres of nearby metal-poor stars. We
  investigated the effects of non-local thermodynamic equilibrium
  (NLTE) and determined the silicon abundances. <BR /> Methods: We
  used high resolution, high signal-to-noise ratio spectra from the
  FOCES spectragraph at the DSAZ telescope. Line-formation calculations
  of Si i and Si ii in the atmospheres of nearby metal-poor stars are
  presented. All abundance results are derived from local thermodynamic
  equilibrium (LTE) and NLTE statistical equilibrium calculations and
  spectrum synthesis methods. <BR /> Results: We find that NLTE effects
  for Si ii optical lines are important for warm stars, and that they
  depend on effective temperature. The Si abundances of thin and thick
  disc stars follow distinct trends, as in the case of Mg. We find
  that [Si/Fe] gradually increases as [Fe/H] decreases in thin disc
  stars, while it remains around at ~+0.30 dex for halo and thick disc
  stars, the halo stars showing larger scatter. <BR /> Conclusions:
  The derived dependence between [Si/Fe] and [Fe/H] is inconsistent
  with the theoretical predictions of published model calculations for
  the chemical evolution of the Galaxy. The nearly constant [Si/Mg]
  ratio with some scatter for halo and thick disc stars suggests that
  the nucleosynthesis of silicon is closely coupled to that of Mg. In
  addition, our results do not support the suggestion that type Ia
  supernove produce significant amounts of silicon. <P />Based on
  observations collected at the German-Spanish Astronomical Center,
  Calar Alto, Spain.

Title: A non-LTE study of neutral and singly-ionized iron line
    spectra in 1D models of the Sun and selected late-type stars
Authors: Mashonkina, L.; Gehren, T.; Shi, J. -R.; Korn, A. J.;
   Grupp, F.
Bibcode: 2011A&A...528A..87M
Altcode: 2011arXiv1101.4570M
  <BR /> Aims: We evaluate non-local thermodynamical equilibrium
  (non-LTE) line formation for the two ions of iron and check the
  ionization equilibrium between Fe i and Fe ii in model atmospheres
  of the cool reference stars based on the best available complete
  model atom for neutral and singly-ionized iron. <BR /> Methods: We
  present a comprehensive model atom for Fe with more than 3000 measured
  and predicted energy levels. As a test and first application of the
  improved model atom, iron abundances are determined for the Sun and
  five stars with well determined stellar parameters and high-quality
  observed spectra. The efficiency of inelastic collisions with
  hydrogen atoms in the statistical equilibrium of iron is empirically
  estimated from inspection of their different influence on the Fe i
  and Fe ii lines in the selected stars. <BR /> Results: Non-LTE leads
  to systematically depleted total absorption in the Fe i lines and to
  positive abundance corrections in agreement with the previous studies,
  however, the magnitude of such corrections is smaller compared to the
  earlier results. These non-LTE corrections do not exceed 0.1 dex for
  the solar metallicity and mildly metal-deficient stars, and they vary
  within 0.21 dex and 0.35 dex in the very metal-poor stars HD 84937
  and HD 122563, respectively, depending on the assumed efficiency of
  collisions with hydrogen atoms. Based on the analysis of the Fe i/Fe
  ii ionization equilibrium in these two stars, we recommend to apply
  the Drawin formalism in non-LTE studies of Fe with a scaling factor
  of 0.1. For the Fe ii lines non-LTE corrections do not exceed 0.01
  dex in absolute value over the whole range of stellar parameters that
  are considered. This study reveals two problems. The first one is that
  gf-values available for the Fe i and Fe ii lines are not accurate enough
  to pursue high-accuracy absolute stellar abundance determinations. For
  the Sun, the mean non-LTE abundance obtained from 54 Fe i lines is 7.56
  ± 0.09 and the mean abundance from 18 Fe ii lines varies between 7.41
  ± 0.11 and 7.56 ± 0.05 depending on the source of the gf-values. The
  second problem is that lines of Fe i give, on average, a 0.1 dex
  lower abundance compared with those of Fe ii lines for HD 61421 and HD
  102870, even when applying a differential line-by-line analysis with
  regard to the Sun. A disparity between neutral atoms and first ions
  points to problems of stellar atmosphere modelling or/and effective
  temperature determination. <P />Based on observations collected at the
  German Spanish Astronomical Center, Calar Alto, Spain and taken from
  the ESO UVES-POP archive.Table 5 is only available in electronic form
  at <A href="http://www.aanda.org">http://www.aanda.org</A>

Title: Fe-peak element abundances in disk and halo stars
Authors: Bergemann, Maria; Gehren, Thomas
Bibcode: 2010IAUS..265..348B
Altcode: 2009arXiv0910.3689B
  At present none of galactic chemical evolution (GCE) models provides
  a self-consistent description of observed trends for all iron-peak
  elements with metallicity simultaneously. The question is whether
  the discrepancy is due to deficiencies of GCE models, such as stellar
  yields, or due to erroneous spectroscopically-determined abundances
  of these elements in metal-poor stars. The present work aims at a
  critical reevaluation of the abundance trends for several odd and
  even-Z Fe-peak elements, which are important for understanding explosive
  nucleosynthesis in supernovae.

Title: Fe I/Fe II ionization equilibrium in cool stars: NLTE
    versus LTE
Authors: Mashonkina, Lyudmila; Gehren, Thomas; Shi, Jianrong; Korn,
   Andreas; Grupp, Frank
Bibcode: 2010IAUS..265..197M
Altcode: 2009arXiv0910.3997M
  Non-local thermodynamic equilibrium (NLTE) line formation for neutral
  and singly-ionized iron is considered through a range of stellar
  parameters characteristic of cool stars. A comprehensive model atom
  for Fe I and Fe II is presented. Our NLTE calculations support the
  earlier conclusions that the statistical equilibrium (SE) of Fe I
  shows an underpopulation of Fe I terms. However, the inclusion of the
  predicted high-excitation levels of Fe I in our model atom leads to a
  substantial decrease in the departures from LTE. As a test and first
  application of the Fe I/II model atom, iron abundances are determined
  for the Sun and four selected stars with well determined stellar
  parameters and high-quality observed spectra. Within the error bars,
  lines of Fe I and Fe II give consistent abundances for the Sun and
  two metal-poor stars when inelastic collisions with hydrogen atoms
  are taken into account in the SE calculations. For the close-to-solar
  metallicity stars Procyon and β Vir, the difference (Fe II - Fe I) is
  about 0.1 dex independent of the line formation model, either NLTE or
  LTE. We evaluate the influence of departures from LTE on Fe abundance
  and surface gravity determination for cool stars.

Title: NLTE analysis of CoI/CoII lines in spectra of cool stars with
    new laboratory hyperfine splitting constants
Authors: Bergemann, Maria; Pickering, Juliet C.; Gehren, Thomas
Bibcode: 2010MNRAS.401.1334B
Altcode: 2009arXiv0909.2178B; 2009MNRAS.tmp.1703B
  The analysis of stellar abundances for odd-Z Fe-peak elements requires
  accurate non-local thermodynamic equilibrium (NLTE) modelling of
  spectral lines fully taking into account the hyperfine structure (HFS)
  splitting of lines. Here, we investigate the statistical equilibrium of
  Co in the atmospheres of cool stars and the influence of NLTE and HFS
  on the formation of Co lines and abundances. Significant departures
  from LTE level populations are found for CoI number densities of
  excited states in CoII also differ from LTE at low metallicity. The
  NLTE level populations are used to determine the abundance of Co in
  solar photosphere, logɛ = 4.95 +/- 0.04dex, which is in agreement
  with that in CI meteorites within the combined uncertainties. The
  spectral lines of CoI were calculated using the results of recent
  measurements of hyperfine interaction constants by UV Fourier transform
  spectrometry. For CoII, the first laboratory measurements of HFS A and
  B factors were performed. These highly accurate A factor measurements
  (errors of the order of 3-7 per cent) allow, for the first time,
  reliable modelling of CoII lines in the solar and stellar spectra
  and, thus, a test of the CoI/CoII ionization equilibrium in stellar
  atmospheres. A differential abundance analysis of Co is carried out
  for 18 stars in the metallicity range -3.12 &lt; [Fe/H] &lt; 0. The
  abundances are derived by the method of spectrum synthesis. At low
  [Fe/H], NLTE abundance corrections for CoI lines are as large as
  +0.6,..., + 0.8dex. Thus, LTE abundances of Co in metal-poor stars are
  severely underestimated. The stellar NLTE abundances determined from
  the single UV line of CoII are lower by ~0.5-0.6dex. The discrepancy
  might be attributed to possible blends that have not been accounted
  for in the solar CoII line and its erroneous oscillator strength. The
  increasing [Co/Fe] trend in metal-poor stars, as calculated from the
  CoI lines under NLTE, can be explained if Co is overproduced relative
  to Fe in massive stars. The models of Galactic chemical evolution are
  wholly inadequate to describe this trend suggesting that the problem
  is in supernova yields. <P />Based on observations collected at the
  European Southern Observatory, Chile, 67.D-0086A, and the Calar Alto
  Observatory, Spain. <P />E-mail: mbergema@mpa-garching.mpg.de (MB);
  j.pickering@imperial.ac.uk (JCP); gehren@usm.lmu.de (TG)

Title: Statistical equilibrium of silicon in the atmospheres of
    metal-poor stars
Authors: Shi, J. R.; Gehren, T.; Mashonkina, L.; Zhao, G.
Bibcode: 2009A&A...503..533S
Altcode: 2009arXiv0907.4928S
  Aims: The statistical equilibrium of neutral and ionized silicon
  in the atmospheres of metal-poor stars is discussed. Non-local
  thermodynamic equilibrium effects (NLTE) are investigated and the
  silicon abundances in metal-poor stars determined. <BR />Methods:
  We have used high resolution, high signal to noise ratio spectra
  from the UVES spectragraph at the ESO VLT telescope. Line formation
  calculations of Si i and Si ii in the atmospheres of metal-poor stars
  are presented for atomic models of silicon including 174 terms and
  1132 line transitions. Recent improved calculations of Si i and Si
  ii photoionization cross-sections are taken into account, and the
  influence of the free-free quasi-molecular absorption in the Lyα
  wing is investigated by comparing theoretical and observed fluxes of
  metal-poor stars. All abundance results are derived from LTE and NLTE
  statistical equilibrium calculations and spectrum synthesis methods. <BR
  />Results: It is found that the extreme ultraviolet radiation is very
  important for metal-poor stars, especially for the high temperature,
  very metal-poor stars. The radiative bound-free cross-sections also
  play a very important role for these stars. <BR />Conclusions: NLTE
  effects for Si are found to be important for metal-poor stars, in
  particular for warm metal-poor stars. It is found that these effects
  depend on the temperature. For warm metal-poor stars, the NLTE abundance
  correction reaches ~0.2 dex relative to standard LTE calculations. Our
  results indicate that Si is overabundant for metal-poor stars. <P
  />Based on observations obtained in the frame of the ESO programme
  ID 165.N-0276(A).

Title: NLTE abundances of Mn in a sample of metal-poor stars
Authors: Bergemann, M.; Gehren, T.
Bibcode: 2008A&A...492..823B
Altcode: 2008arXiv0811.0681B
  Aims: Following our solar work, we perform NLTE calculations of the
  Mn abundance for fourteen stars with [Fe/H] from 0 to -2.5, mainly
  to show how NLTE affects Mn abundances in cool stars of different
  metallicities. <BR />Methods: The spectrum synthesis and Mn abundances
  are based on statistical equilibrium calculations using various
  estimates for the influence of hydrogen collisions. <BR />Results: The
  NLTE abundances of Mn in all studied stars are systematically higher
  than the LTE abundances. At low metallicities, the NLTE abundance
  corrections may run up to 0.5-0.7 dex. Instead of a strong depletion of
  Mn relative to Fe in metal-poor stars as found by the other authors,
  we only find slightly subsolar values of [Mn/Fe] throughout the range
  of metallicities analyzed here. <BR />Conclusions: The [Mn/Fe] trend
  in metal-poor stars is inconsistent with the predictions of galactic
  chemical evolution models, where Mn is less produced than Fe. <P />Based
  on observations collected at the European Southern Observatory, Chile,
  67.D-0086A, and the Calar Alto Observatory, Spain.

Title: Solar scandium abundance
Authors: Zhang, H. W.; Gehren, T.; Zhao, G.
Bibcode: 2008IAUS..252..127Z
Altcode:
  We investigate the formation of neutral and singly ionized scandium
  lines in the solar photospheres. Extensive statistical equilibrium
  calculations were carried out for a model atom, which comprises 92 terms
  for Sc I and 79 for Sc II. Synthetic line profiles calculated from
  the level populations according to the NLTE departure coefficients
  were compared with the observed solar spectral atlas. Abundance
  determinations using the ODF model lead to a solar Sc abundance of
  between log γ<SUB>⊙</SUB> = 3.07 and 3.13, depending on the choice
  of f values.

Title: Statistical equilibrium of silicon in the solar atmosphere
Authors: Shi, J. R.; Gehren, T.; Butler, K.; Mashonkina, L. I.;
   Zhao, G.
Bibcode: 2008A&A...486..303S
Altcode: 2008arXiv0805.3564S
  Aims: The statistical equilibrium of neutral and ionised silicon in the
  solar photosphere is investigated. Line formation is discussed and the
  solar silicon abundance determined. <BR />Methods: High-resolution
  solar spectra were used to determine solar log gf\varepsilon_Si
  values by comparison with Si line synthesis based on LTE and NLTE
  level populations. The results will be used in a forthcoming paper
  for differential abundance analyses of metal-poor stars. A detailed
  analysis of silicon line spectra leads to setting up realistic model
  atoms, which are exposed to interactions in plane-parallel solar
  atmospheric models. The resulting departure coefficients are entered
  into a line-by-line analysis of the visible and near-infrared solar
  silicon spectrum. <BR />Results: The statistical equilibrium of Si
  I turns out to depend marginally on bound-free interaction processes,
  both radiative and collisional. Bound-bound interaction processes do not
  play a significant role either, except for hydrogen collisions, which
  have to be chosen adequately for fitting the cores of the near-infrared
  lines. Except for some near-infrared lines, the NLTE influence on the
  abundances is weak. <BR />Conclusions: Taking the deviations from LTE
  in silicon into account, it is possible to calculate the ionisation
  equilibrium from neutral and ionised lines. The solar abundance based
  on the experimental f-values of Garz corrected for the Becker et al.'s
  measurement is 7.52 ± 0.05. Combined with an extended line sample
  with selected NIST f-values, the solar abundance is 7.52 ± 0.06, with
  a nearly perfect ionisation equilibrium of Δlog\varepsilon_⊙(Si
  II/Si I) = -0.01. <P />Table 1 is only available in electronic form
  at http://www.aanda.org

Title: A non-local thermodynamic equilibrum study of scandium in
    the Sun
Authors: Zhang, H. W.; Gehren, T.; Zhao, G.
Bibcode: 2008A&A...481..489Z
Altcode:
  Aims: We investigate the formation of neutral and singly ionized
  scandium lines in the solar photospheres. The research is aimed at
  derive solar log gf\varepsilon<SUB>⊙</SUB>(Sc) values for scandium
  lines, which will later be used in differential abundance analyses of
  metal-poor stars. <BR />Methods: Extensive statistical equilibrium
  calculations were carried out for a model atom, which comprises 92
  terms for Sc I and 79 for Sc II. Photoionization cross-sections are
  assumed to be hydrogenic. Synthetic line profiles calculated from the
  level populations according to the NLTE departure coefficients were
  compared with the observed solar spectral atlas. Hyperfine structure
  (HFS) broadening is taken into account. <BR />Results: The statistical
  equilibrium of scandium is dominated by a strong underpopulation of Sc I
  caused by missing strong lines. It is nearly unaffected by the variation
  in interaction parameters and only marginally sensitive to the choice
  of the solar atmospheric model. Abundance determinations using the
  ODF model lead to a solar Sc abundance of between log\varepsilon_⊙
  = 3.07 and 3.13, depending on the choice of f values. The long known
  difference between photospheric and meteoritic scandium abundances is
  confirmed for the experimental f-values.

Title: NLTE study of scandium in the Sun
Authors: Zhang, H. W.; Gehren, T.; Zhao, G.
Bibcode: 2008arXiv0802.2609Z
Altcode:
  We investigate the formation of neutral and singly ionized scandium
  lines in the solar photospheres. The research is aimed derive solar
  $\log gf\epsilon_{\odot}$(Sc) values for scandium lines, which
  will later be used in differential abundance analyses of metal-poor
  stars. Extensive statistical equilibrium calculations were carried
  out for a model atom, which comprises 92 terms for \ion{Sc}{i}
  and 79 for \ion{Sc}{ii}. Photoionization cross-sections are assumed
  to be hydrogenic. Synthetic line profiles calculated from the level
  populations according to the NLTE departure coefficients were compared
  with the observed solar spectral atlas. Hyperfine structure (HFS)
  broadening is taken into account. The statistical equilibrium of
  scandium is dominated by a strong underpopulation of \ion{Sc}{i} caused
  by missing strong lines. It is nearly unaffected by the variation in
  interaction parameters and only marginally sensitive to the choice of
  the solar atmospheric model. Abundance determinations using the ODF
  model lead to a solar Sc abundance of between $\log\epsilon_\odot =
  3.07$ and 3.13, depending on the choice of $f$ values. The long known
  difference between photospheric and meteoritic scandium abundances is
  confirmed for the experimental $f$-values.

Title: Non-LTE line formation for heavy elements in four very
    metal-poor stars
Authors: Mashonkina, L.; Zhao, G.; Gehren, T.; Aoki, W.; Bergemann,
   M.; Noguchi, K.; Shi, J. R.; Takada-Hidai, M.; Zhang, H. W.
Bibcode: 2008A&A...478..529M
Altcode: 2007arXiv0711.4454M
  Aims:Stellar parameters and abundances of Na, Mg, Al, K, Ca, Sr, Ba,
  and Eu are determined for four very metal-poor (VMP) stars (-2.15
  ≥ [Fe/H] ≥ -2.66). For two of them, HD 84937 and HD 122563, the
  fraction of the odd isotopes of Ba derived for the first time. <BR
  />Methods: Determination of an effective temperature, surface gravity,
  and element abundances was based on non-local thermodynamic equilibrium
  (non-LTE) line formation and analysis of high-resolution (R ∼60 000
  and 90 000) high signal-to-noise ({S/N} ≥ 200) observed spectra. A
  model atom for H I is presented. An effective temperature was obtained
  from the Balmer Hα and Hβ line wing fits. The surface gravity was
  calculated from the Hipparcos parallax if available and the non-LTE
  ionization balance between Ca I and Ca II. Based on the hyperfine
  structure affecting the Ba II resonance line λ 4554, the fractional
  abundance of the odd isotopes of Ba was derived from a requirement
  that Ba abundances from the resonance line and subordinate lines of
  Ba II must be equal. <BR />Results: For each star, non-LTE leads
  to a consistency of T_eff from two Balmer lines and to a higher
  temperature compared to the LTE case, by up to 60 K. Non-LTE effects
  are important in spectroscopic determination of surface gravity from
  the ionization balance between Ca I and Ca II. For each star with
  a known trigonometric surface gravity, non-LTE abundances from the
  lines of two ionization stages, Ca I and Ca II, agree within the error
  bars, while a difference in the LTE abundances consists of 0.23 dex
  to 0.40 dex for different stars. Departures from LTE are found to be
  significant for all investigated atoms, and they strongly depend on
  stellar parameters. For HD 84937, the Eu/Ba ratio is consistent with
  the relative solar system r-process abundances, and the fraction of the
  odd isotopes of Ba, f_odd, equals 0.43±0.14. The latter can serve as
  an observational constraint on r-process models. The lower Eu/Ba ratio
  and f_odd = 0.22±0.15 found for HD 122563 suggest that the s-process
  or the unknown process has contributed significantly to the Ba abundance
  in this star. <P />Based on observations collected at Subaru Telescope,
  which is operated by the National Astronomical Observatory of Japan.

Title: Formation of Mn I lines in the solar atmosphere
Authors: Bergemann, M.; Gehren, T.
Bibcode: 2007A&A...473..291B
Altcode: 2007arXiv0707.4131B
  Context: We present a detailed NLTE analysis of 39 Mn I lines in the
  solar spectrum. The influence of NLTE effects on the line formation
  and element abundance is investigated. <BR />Aims: Our goal is the
  derivation of solar log gf\varepsilon values for manganese lines, which
  will later be used in differential abundance analyses of metal-poor
  stars. <BR />Methods: The method of spectrum synthesis is employed,
  which is based on a solar model atmosphere with initially specified
  element abundances. A manganese abundance of log\varepsilon<SUB>Mn,
  ⊙</SUB>= 5.47 dex is used with the theoretical line-blanketed model
  atmosphere. Statistical equilibrium calculations are carried out for
  the model atom, which comprises 245 and 213 levels for Mn I and Mn II,
  respectively. Photoionization cross-sections are assumed hydrogenic. <BR
  />Results: For line synthesis van der Waals broadening is calculated
  according to Anstee &amp; O'Mara's formalism. It is shown that hyperfine
  structure of the Mn lines also has strong broadening effects, and that
  manganese is prone to NLTE effects in the solar atmosphere. The nature
  of the NLTE effects and the validity of the LTE approach are discussed
  in detail. The role of photoionization and collisional interaction
  is investigated. <BR />Conclusions: Maximum NLTE corrections of +0.1
  dex with respect to LTE profiles are found. We propose an absolute
  solar abundance of 5.36 ± 0.1 dex. The main source of errors in
  the abundance calculations is uncertain oscillator strengths. <P
  />Research supported by the International Max Planck Research School
  (IMPRS), Munich, Germany. Figure 8 and Table 5 are only available in
  electronic form at http://www.aanda.org

Title: Lithium abundances in metal-poor stars
Authors: Shi, J. R.; Gehren, T.; Zhang, H. W.; Zeng, J. L.; Zhao, G.
Bibcode: 2007A&A...465..587S
Altcode:
  Aims:Lithium abundances for 19 metal-poor stars are determined using
  high-resolution spectroscopy. The abundances of stars on the lithium
  plateau are discussed. <BR />Methods: All abundance results are derived
  from NLTE statistical equilibrium calculations and spectrum synthesis
  methods. <BR />Results: In agreement with previous analyses it is
  found that excitation and de-excitation due to hydrogen collisions are
  negligible for the lithium line formation process, while charge transfer
  reactions are an important source of thermalization. However, the
  resulting NLTE effects on the determination of lithium abundances for
  metal-poor stars are negligible (&lt;0.06 dex). <BR />Conclusions: .The
  mean lithium abundance for stars on the lithium plateau determined from
  NLTE analyses is A(Li) ~ 2.26, while it is 2.21 dex when charge transfer
  reactions are included. The latter result enhances the discrepancy
  between the observed lithium abundances and the primordial lithium
  abundance as inferred by the WMAP analysis of the cosmic microwave
  background. This discrepancy may be explained by metal diffusion. <P
  />Based on observations collected at the Germany-Spanish Astronomical
  Center, Calar Alto, Spain.

Title: Potassium abundances in nearby metal-poor stars
Authors: Zhang, H. W.; Gehren, T.; Butler, K.; Shi, J. R.; Zhao, G.
Bibcode: 2006A&A...457..645Z
Altcode:
  Aims.The potassium abundances for 58 metal-poor stars are determined
  using high-resolution spectroscopy. The abundance trends in stars
  of different population are discussed.<BR /> Methods: .All abundance
  results have been derived from NLTE statistical equilibrium calculations
  and spectrum synthesis methods.<BR /> Results: .The NLTE corrections
  are significant (-0.20 to -0.55 dex) and they depend on the effective
  temperatures and surface gravities. The potassium abundances of
  thin disk, thick disk and halo stars show distinct trends, such as
  in the case of the α-elements. [K/Fe] gradually increases with a
  decrease in [Fe/H] for thin disk stars, [K/Fe] of thick disk stars
  is nearly constant at [K/Fe] ~ +0.30 dex; halo stars also have
  nearly constant values of [K/Fe] ~ +0.20 dex. <BR /> Conclusions:
  .The derived dependence between [K/Fe] and [Fe/H] is in agreement
  with the theoretical prediction of published model calculations of
  the chemical evolution of the Galaxy. The nearly constant [K/Mg] ratio
  with small scatter suggests that the nucleosynthesis of potassium is
  closely coupled to the α-elements.

Title: NLTE analysis of the solar potassium abundance
Authors: Zhang, H. W.; Butler, K.; Gehren, T.; Shi, J. R.; Zhao, G.
Bibcode: 2006A&A...453..723Z
Altcode:
  We investigate the formation of neutral potassium lines in the solar
  photosphere based on extensive statistical equilibrium calculations to
  determine the solar potassium abundance. The computations are based on
  a 68-level potassium atomic model. Hydrogen collisions were supplied
  with an enhancement factor S<SUB>H</SUB> = 0.05 to the classical Drawin
  formula. Synthetic line profiles calculated from the level populations
  according to the NLTE departure coefficients were compared with the
  observed solar spectral atlas. The solar potassium abundance based
  on theoretical model atmospheres is log \varepsilon⊙ (K) = 5.12 ±
  0.03, which agrees with the meteoritic value of 5.09 ± 0.05 compiled
  by Lodders.

Title: Na, Mg and Al abundances as a population discriminant for
    nearby metal-poor stars
Authors: Gehren, T.; Shi, J. R.; Zhang, H. W.; Zhao, G.; Korn, A. J.
Bibcode: 2006A&A...451.1065G
Altcode:
  Aims.Parameters for 55 nearby metal-poor stars are determined using
  high-resolution spectroscopy. Together with similar data taken from
  a recent analysis, they are used to show trends of their Galactic
  evolution with stellar [Fe/H] or [Mg/H] abundances. The separation
  of abundance ratios between disk and halo stars is used as a basic
  criterion for population membership. <BR />Methods.After careful
  selection of a clean subsample free of suspected or known binaries
  and peculiar stars, abundances of Mg, Na and Al are based on NLTE
  kinetic equilibrium calculations applied to spectrum synthesis
  methods. <BR />Results.The relation between [Na/Mg] and [Fe/H] is a
  continuous enrichment through all three Galactic populations spanning
  a range of values between a metal-poor plateau at [ Na/Mg] = -0.7
  and solar values. [Al/Mg] displays a step-like difference between
  stars of the Galactic halo with overline[Al/Mg] ∼ -0.45 and the
  two disk populations with overline[Al/Mg] ∼ +0.10. [Al/Mg] ratios,
  together with the [Mg/Fe] ratios, asymmetric drift velocities V, and
  stellar evolutionary ages, make possible the individual discrimination
  between stars of the thick disk and the halo. At present, this evidence
  is limited by the small number of stars, and by the theoretical and
  empirical uncertainties of stellar age determinations, but it achieves
  a high significance. <BR />Conclusions.While the stellar sample is
  not complete with respect to space volume, the resulting abundances
  indicate the necessity to revise current models of chemical evolution
  to allow for an adequate production of Al in early stellar generations.

Title: VizieR Online Data Catalog: Sodium abundances in nearby disk
    stars (Shi+, 2004)
Authors: Shi, J. R.; Gehren, T.; Zhao, G.
Bibcode: 2005yCat..34230683S
Altcode:
  The spectra of our samples were obtained through the years 1995 to
  2000 by Klaus Fuhrmann with the fiber-coupled Cassegrain echelle
  spectrograph FOCES mounted at the 2.2m telescope of the Calar Alto
  Observatory. <P />(1 data file).

Title: Sodium abundances in nearby disk stars
Authors: Shi, J. R.; Gehren, T.; Zhao, G.
Bibcode: 2004A&A...423..683S
Altcode: 2004astro.ph..5535S
  We present sodium abundances for a sample of nearby stars. All results
  have been derived from NLTE statistical equilibrium calculations. The
  influence of collisional interactions with electrons and hydrogen
  atoms is evaluated by comparison of the solar spectrum with very
  precise fits to the Na I line cores. The NLTE effects are more
  pronounced in metal-poor stars since the statistical equilibrium is
  dominated by collisions of which at least the electronic component is
  substantially reduced. The resulting influence on the determination of
  sodium abundances is in a direction opposite to that found previously
  for Mg and Al. The NLTE corrections are about -0.1 in thick-disk stars
  with [Fe/H] ∼-0.6. Our [Na/Fe] abundance ratios are about solar for
  thick- and thin-disk stars. The increase in [Na/Fe] as a function of
  [Fe/H] for metal-rich stars found by Edvardsson et al. (\cite{EAG93})
  is confirmed. Our results suggest that sodium yields increase with
  the metallicity, and quite large amounts of sodium may be produced by
  AGB stars. We find that [Na/Fe] ratios, together with either [Mg/Fe]
  ratio, kinematic data or stellar evolutionary ages, make possible the
  individual discrimination between thin- and thick-disk membership. <P
  />Based on observations collected at the Germany-Spanish Astronomical
  Center, Calar Alto, Spain. <P />Tables \ref{table2} and \ref{table3}
  are only available in electronic form at http://www.edpsciences.org

Title: Abundances of Na, Mg and Al in nearby metal-poor stars
Authors: Gehren, T.; Liang, Y. C.; Shi, J. R.; Zhang, H. W.; Zhao, G.
Bibcode: 2004A&A...413.1045G
Altcode:
  To determine the population membership of nearby stars we explore
  abundance results obtained for the light neutron-rich elements
  <SUP>23</SUP>Na and <SUP>27</SUP> Al in a small sample of moderately
  metal-poor stars. Spectroscopic observations are limited to the solar
  neighbourhood so that gravities can be determined from HIPPARCOS
  parallaxes, and the results are confronted with those for a separate
  sample of more metal-poor typical halo stars. Following earlier
  investigations, the abundances of Na, Mg and Al have been derived
  from NLTE statistical equilibrium calculations used as input to
  line profile synthesis. Compared with LTE the abundances require
  systematic corrections, with typical values of +0.05 for [Mg/Fe],
  -0.1 for [Na/Fe] and +0.2 for [Al/Fe] in thick disk stars where [Fe/H]
  ∼ -0.6. In more metal-poor halo stars these values reach +0.1, -0.4,
  and +0.5, respectively, differences that can no longer be ignored. <P
  />After careful selection of a clean subsample free from suspected or
  known binaries and peculiar stars, we find that [Na/Mg] and [Al/Mg],
  in combination with [Mg/Fe], space velocities and stellar evolutionary
  ages, make possible an individual discrimination between thick disk and
  halo stars. At present, this evidence is limited by the small number of
  stars analyzed. We identify a gap at [Al/Mg] ∼ -0.15 and [Fe/H] ∼
  -1.0 that isolates stars of the thick disk from those in the halo. A
  similar separation occurs at [Na/Mg] ∼ -0.4. We do not confirm the
  age gap between thin and thick disk found by Fuhrmann. Instead we find
  an age boundary between halo and thick disk stars, however, with an
  absolute value of 14 Gyr that must be considered as preliminary. While
  the stellar sample is by no means complete, the resulting abundances
  indicate the necessity to revise current models of chemical evolution
  and/or stellar nucleosynthesis to allow for an adequate production
  of neutron-rich species in early stellar generations. <P />Based
  on observations collected at the German-Spanish Astronomical Center,
  Calar Alto (CAHA H01-2.2-002) and at the European Southern Observatory,
  Chile (ESO 67.D-0086).

Title: Kinetic equilibrium of iron in the atmospheres of cool
    stars. III. The ionization equilibrium of selected reference stars
Authors: Korn, A. J.; Shi, J.; Gehren, T.
Bibcode: 2003A&A...407..691K
Altcode: 2003astro.ph..6337K
  Non-LTE line formation calculations of Fe I are performed for a small
  number of reference stars to investigate and quantify the efficiency of
  neutral hydrogen collisions. Using the atomic model that was described
  in previous publications, the final discrimination with respect
  to hydrogen collisions is based on the condition that the surface
  gravities as determined by the Fe I/Fe Ii ionization equilibria are in
  agreement with their astrometric counterparts obtained from HIPPARCOS
  parallaxes.\ High signal-to-noise, high-resolution échelle spectra
  are analysed to determine individual profile fits and differential
  abundances of iron lines. Depending on the choice of the hydrogen
  collision scaling factor S<SUB>H</SUB>, we find deviations from
  LTE in Fe I ranging from 0.00 (S<SUB>H</SUB>,= infty ) to 0.46 dex
  (S<SUB>H</SUB>,= 0 for HD 140283) in the logarithmic abundances while
  Fe Ii follows LTE. With the exception of Procyon, for which a mild
  temperature correction is needed to fulfil the ionization balance,
  excellent consistency is obtained for the metal-poor reference stars if
  Balmer profile temperatures are combined with S<SUB>H</SUB>,= 3. This
  value is much higher than what is found for simple atoms like Li or Ca,
  both from laboratory measurements and inference of stellar analyses.\
  The correct choice of collisional damping parameters (``van-der-Waals''
  constants) is found to be generally more important for these little
  evolved metal-poor stars than considering departures from LTE. For
  the Sun the calibrated value for S<SUB>H</SUB> leads to average Fe I
  non-LTE corrections of 0.02 dex and a mean abundance from Fe I lines of
  log varepsilon (Fe) = 7.49 +/- 0.08.\ We confront the deduced stellar
  parameters with comparable spectroscopic analyses by other authors which
  also rely on the iron ionization equilibrium as a gravity indicator. On
  the basis of the HIPPARCOS astrometry our results are shown to be
  an order of magnitude more precise than published data sets, both in
  terms of offset and star-to-star scatter. <P />Based on observations
  collected at the German-Spanish Astronomical Centre, Calar Alto, Spain.

Title: NLTE Heavy Element Abundances in Cool Dwarfs: an Implication
    for the Galaxy Chemical Evolution
Authors: Mashonkina, L.; Gehren, T.
Bibcode: 2003IAUS..210P.E39M
Altcode:
  No abstract at ADS

Title: Iron in Non-LTE - Pitfalls and Prospects
Authors: Korn, A. J.; Gehren, T.
Bibcode: 2003IAUS..210P.B12K
Altcode:
  No abstract at ADS

Title: Mg, Ba and Eu abundances in thick disk and halo stars
Authors: Mashonkina, L.; Gehren, T.; Travaglio, C.; Borkova, T.
Bibcode: 2003A&A...397..275M
Altcode: 2002astro.ph.10366M
  Our sample of cool dwarf stars from previous papers (Mashonkina &amp;
  Gehren \cite{euba, eubasr}) is extended in this study including 15
  moderately metal-deficient stars. The samples of halo and thick disk
  stars have overlapping metallicities with [Fe/H] in the region from -0.9
  to -1.5, and we compare chemical properties of these two kinematically
  different stellar populations independent of their metallicity. We
  present barium, europium and magnesium abundances for the new sample
  of stars. The results are based on NLTE line formation obtained in
  differential model atmosphere analyses of high resolution spectra
  observed mainly using the UVES spectrograph at the VLT of the European
  Southern Observatory. We confirm the overabundance of Eu relative to Mg
  in halo stars as reported in our previous papers. Eight halo stars show
  [Eu/Mg] values between 0.23 and 0.41, whereas stars in the thick and
  thin disk display a solar europium to magnesium ratio. The [Eu/Ba]
  values found in the thick disk stars to lie between 0.35 and 0.57
  suggest that during thick disk formation evolved low-mass stars started
  to enrich the interstellar gas by s-nuclei of Ba, and the s-process
  contribution to barium thus varies from 30% to 50%. Based on these
  results, and using the chemical evolution calculations by Travaglio et
  al. (\cite{eu99}), we estimate that the thick disk stellar population
  formed on a timescale between 1.1 to 1.6 Gyr from the beginning of the
  protogalactic collapse. In the halo stars the [Eu/Ba] values are found
  mostly between 0.40 and 0.67, which suggests a duration of the halo
  formation of about 1.5 Gyr. For the whole sample of stars we present the
  even-to-odd Ba isotope ratios as determined from hyperfine structure
  seen in the Ba Ii resonance line lambda 4554. As expected, the solar
  ratio 82:18 (Cameron \cite{cam}) adjusts to observations of the Ba
  Ii lines in the thin disk stars. In our halo stars the even-to-odd Ba
  isotope ratios are close to the pure r-process ratio 54:46 (Arlandini
  et al. \cite{rs99}), and in the thick disk stars the isotope ratio is
  around 65:35 (+/-10%). Based on these data we deduce for thick disk
  stars the ratio of the s/r-process contribution to barium as 30:70
  (+/-30%), in agreement with the results obtained from the [Eu/Ba]
  values. Based on observations collected at the European Southern
  Observatory, Chile, 67.D-0086A, and the German Spanish Astronomical
  Center, Calar Alto, Spain.

Title: Kinetic equilibrium of iron in the atmospheres of cool dwarf
    stars. II. Weak Fe I lines in the solar spectrum
Authors: Gehren, T.; Korn, A. J.; Shi, J.
Bibcode: 2001A&A...380..645G
Altcode: 2001astro.ph.10605G
  NLTE line formation calculations of FeI in the solar atmosphere
  are extended to include weak lines in the visual spectrum of the
  Sun. Previously established atomic models are used to discriminate
  between different ways of treating collisional interaction
  processes. As indicated by the analysis of strong FeI lines,
  the influence of deviations from LTE in the solar atmosphere on
  the Fe abundance is small for all lines. To derive a common solar
  FeI abundance from both strong and weak lines fine-tuning of the
  microturbulence velocity parameter and the van der Waals damping
  constants is required. The solar FeI abundances based on all available
  f-values are dominated by the large scatter already found for the
  stronger lines. In particular the bulk of the data from the work of
  May et al. and O'Brian et al. is not adequate for accurate abundance
  work. Based on f-values measured by the Hannover and Oxford groups
  alone, the FeI LTE abundances are \log\varepsilon<SUB>FeI,solar</SUB>
  = 7.57 for the empirical and \log\varepsilon<SUB>FeI,solar</SUB> =
  7.48 \ldots 7.51 for the line-blanketed solar model. The solar Fe
  ionization equilibrium obtained for different atomic and atmospheric
  models rules out NLTE atomic models with a low efficiency of hydrogen
  collisions. At variance with Paper I, it is now in better agreement with
  laboratory FeII f-values for all types of line-blanketed models. Our
  final model assumptions consistent with a single unique solar Fe
  abundance \log\varepsilon<SUB>FeI,solar</SUB> ~ 7.48 \ldots 7.51
  calculated from NLTE line formation are (a) a line-blanketed solar
  model atmosphere, (b) an iron model atom with hydrogen collision rates
  $0.5 &lt; S<SUB>H</SUB> &lt; 5 times the standard value to compensate
  for the large photoionization cross-sections, (c) a microturbulence
  velocity ξ<SUB>t</SUB> = 1.0 km s<SUP>-1</SUP>, (d) van der Waals
  damping parameters decreased by Δ\log C<SUB>6</SUB> = -0.10 \ldots
  -0.15 as compared to Anstee &amp; O'Mara's calculations, depending on
  S<SUB>H</SUB>, (e) FeII f-values as published by Schnabel et al., and
  (f) FeI f-values published by the Hannover and Oxford groups.

Title: Heavy element abundances in cool dwarf stars: An implication
    for the evolution of the Galaxy
Authors: Mashonkina, L.; Gehren, T.
Bibcode: 2001A&A...376..232M
Altcode:
  We present revised strontium, barium and europium abundances for 63
  cool stars with metallicities [Fe/H] ranging from -2.20 to 0.25. The
  stellar sample has been extracted from Fuhrmann's lists (\cite{Fuhr3,
  Fuhr5}). It is confined to main-sequence and turnoff stars. The
  results are based on NLTE line formation obtained in differential model
  atmosphere analyses of spectra that have a typical S/N of 200 and a
  resolution of 40 000 to 60 000. The element abundance ratios reveal a
  distinct chemical history of the halo and thick disk compared with that
  of the thin disk. Europium is overabundant relative to iron and barium
  in halo and thick disk stars suggesting that during the formation of
  these galactic populations high-mass stars exploding as SNe II dominated
  nucleosynthesis on a short time scale of the order of 1 Gyr. We note the
  importance of [Eu/Mg] determinations for halo stars. Our analysis leads
  to the preliminary conclusion that Eu/Mg ratios found in halo stars
  do not support current theoretical models of the r-process based on
  low-mass SNe; instead they seem to point at a halo formation time much
  shorter than 1 Gyr. A steep decline of [Eu/Fe] and a slight decline of
  [Eu/Ba] with increasing metallicity have been first obtained for thick
  disk stars. This indicates the start of nucleosynthesis in the lower
  mass stars, in SN I and AGB stars, which enriched the interstellar gas
  with iron and the most abundant s-process elements. From a decrease of
  the Eu/Ba ratio by ~ 0.10 ... 0.15 dex the time interval corresponding
  to the thick disk formation phase can be estimated. The step-like
  change of element abundance ratios at the thick to thin disk transition
  found in our previous analysis (Mashonkina &amp; Gehren \cite{euba})
  is confirmed in this study: [Eu/Ba] and [Eu/Fe] are reduced by ~ 0.25
  dex and ~ 0.15 dex, respectively; [Ba/Fe] increases by ~ 0.1 dex. This
  is indicative of an intermediate phase before the early stage of the
  thin disk developed, during which only evolved middle and low mass
  (&lt;8 M_sun) stars contributed to nucleosynthesis. Our data provide
  an independent method to calculate the duration of this phase. The
  main s-process becomes dominant in the production of heavy elements
  beyond the iron group during the thin disk evolution. We find that
  in the thin disk stars Ba/Fe ratios increase with time from [Ba/Fe]
  = -0.06 in stars older than 8 Gyr to [Ba/Fe] = 0.06 in stars that are
  between 2 and 4 Gyr old. Based on observations collected at the German
  Spanish Astronomical Center, Calar Alto, Spain.

Title: Kinetic equilibrium of iron in the atmospheres of cool dwarf
    stars. I. The solar strong line spectrum
Authors: Gehren, T.; Butler, K.; Mashonkina, L.; Reetz, J.; Shi, J.
Bibcode: 2001A&A...366..981G
Altcode:
  Line formation calculations of Fe I and Fe II in the solar atmosphere
  are presented for atomic models of iron including all observed terms and
  line transitions with available f-values. Recent improved calculations
  of Fe I photoionization cross-sections are taken into account, and
  the influence of collision processes is investigated by comparing
  synthesized and observed solar line flux profiles. The background
  is represented by the opacity of all important non-iron elements
  with iron lines added. Using a representative sample of sufficiently
  unblended strong Fe I and Fe II line profiles, it is evident that line
  formation is affected by (a) velocity fields and (b) deviations from
  local thermodynamic equilibrium (NLTE). The calculations are extended
  to a systematic analysis demonstrating that the ionization equilibrium
  of iron is recovered for solar parameters (T_eff = 5780 K, log g =
  4.44) either using the empirical atmospheric model of Holweger &amp;
  Müller (\cite{HM74}) and assuming LTE for both Fe I and Fe II or
  a line-blanketed theoretical atmospheric model with NLTE iron line
  formation. In the latter case the kinetic equilibrium of Fe I shows a
  substantial underpopulation of Fe I terms which depends sensitively
  on both the improved photoionization calculations and the choice of
  hydrogen collision rates while the Fe II ion is well approximated by
  LTE. Although the source functions of most of the Fe I lines are nearly
  thermal, their formation is shifted deeper into the photosphere. NLTE
  wings of strong Fe I lines are therefore shallower than under the
  LTE assumption, whereas the cores of the strongest lines display
  the usual chromospheric contributions. Based on both calculated and
  laboratory f-values the abundances of 37 Fe II lines range between eps
  {ion {Fe}iI,sun} = 7.50 and 7.56, depending on atomic and atmospheric
  models, and those of 117 Fe I lines between eps {ion {Fe}iI,sun} = 7.47
  and 7.56, both with a relatively large scatter of 0.08 ... 0.12. The
  collisional coupling of Fe I levels is investigated. Electron collisions
  seem to play only a minor role. Hydrogen collisions are very important
  between terms of low excitation, and they efficiently thermalize
  the line source functions but not necessarily the populations of the
  lower levels that determine the line optical depth. Thermalization
  of those low-excitation terms that are responsible for most of the
  lines analyzed is achieved only if the collisional coupling among
  highly excited Fe I terms and their Fe II parent terms is increased
  by large factors compared with standard collision rates. Solar flux
  profiles are reproduced under the assumption of both LTE or NLTE,
  with nearly all types of atomic and atmospheric models, because the
  Fe ionization equilibrium depends on the corresponding sets of f-values.

Title: Stellar Abundances of the Galactic Thick Disk
Authors: Pettinger, M. M.; Bernkopf, J.; Fuhrmann, K.; Korn, A. J.;
   Gehren, T.
Bibcode: 2001AGM....18.P166P
Altcode: 2001AGAb...18Q.218P
  We present the results from model atmosphere analyses of two G dwarfs
  of the Galactic thick disk, 72 Her and HD 64606. High resolution,
  high signal-to-noise échelle spectra were obtained with the FOCES
  spectrograph on the 2.2m telescope of the Calar Alto observatory,
  Spain. Due to the well-defined blaze function of FOCES the determination
  of the continuum within an order and from order to order in the Hα,
  Hβ (for T<SUB>eff</SUB>) and Mg Ib triplett (for log g) region is
  very precise and leads to very accurately determined spectroscopic
  stellar parameters. The aim of our analysis is to study the chemical
  behaviour of the thick disk in particular with respect to the α-,
  r- and s-process elements. The principal results are as follows:
  both stars show significant enhancement in all analysed α-elements,
  in the r-process element Eu as well as in Al and Zn. Mn and the
  s-process element Ba are underabundant relative to iron while the
  other iron-peak elements exhibit a slight enhancement. N, Na, Ce and
  the r-process element Sr also show a weak overabundance. Based on the
  very accurate HIPPARCOS astrometry the stellar ages were determined to
  be 13 Gyrs. This allows us to identify both stars as members of the
  thick disk which is also in accord with their kinematics. The high
  Eu/Ba ratios are consistent with the ratio expected for stars older
  than 12 Gyr under the assumption of r-process dominated enrichment in
  the early phase of Galactic chemical evolution. We argue that the high
  [Al/Fe] and [Zn/Fe] ratios potentially allow to spectroscopically
  distinguish between the halo and thick-disk populations.

Title: New Gravities for Old Stars
Authors: Korn, A. J.; Gehren, T.
Bibcode: 2001ASPC..245..337K
Altcode: 2001aats.conf..337K
  No abstract at ADS

Title: HIPPARCOS and the Distance Scale to Local Halo Stars
Authors: Korn, A. J.; Gehren, T.
Bibcode: 2001ASPC..228..494K
Altcode: 2001dscm.conf..494K
  No abstract at ADS

Title: Barium and europium abundances in cool dwarf stars and
    nucleosynthesis of heavy elements
Authors: Mashonkina, L.; Gehren, T.
Bibcode: 2000A&A...364..249M
Altcode:
  We revise barium abundances in 29 cool stars with metallicities
  [Fe/H] ranging from -2.20 to 0.07 and europium abundances in 15 stars
  with [Fe/H] from -1.52 to 0.07. The sample has been extracted from
  Fuhrmann's lists (\cite{Fuhr3, Fuhr4}) and confined to main-sequence
  and turnoff stars with only one subgiant added. The results are based
  on differential NLTE model atmosphere analyses of spectra that have a
  typical S/N of 200 and a resolution of 40000 or 60000. The statistical
  equilibrium of Eu Ii is first investigated with a model atom containing
  32 levels of Eu Ii plus the ground state of Eu Iii. NLTE effects
  decrease the equivalent widths of the Eu Ii lines compared with LTE
  resulting in positive NLTE abundance corrections which are below
  0.08 dex for all the stars investigated. The solar barium abundance
  eps {Ba,sun} = 2.21 and the europium abundance eps {Eu,sun} = 0.53
  are found from the Ba Ii and Eu Ii solar flux line profile fitting,
  and they coincide within error bars with meteoritic abundances of
  Grevesse et al. (\cite{met96}). Here the usual scale with eps {H} =
  12 is used. The isotopic ratio \iso{151}{Eu}: \iso{153}{Eu} = 55: 45
  is obtained from solar disk center intensity profile fitting of the
  Eu Ii lambda 4129 Åline. We report here for the first time that the
  elemental ratios [Ba/Fe], [Eu/Fe] and [Eu/Ba] show a different behaviour
  for stars of different Galactic populations. For the halo stars the
  [Ba/Fe] ratios are approximately solar, europium is overabundant
  relative to iron and barium with the mean values [Eu/Fe] = 0.62 and
  [Eu/Ba] = 0.64. For thick disk stars it is found that a) barium is
  slightly underabundant relative to iron by about 0.1 dex; b) europium is
  overabundant relative to iron with the [Eu/Fe] ratios between 0.30 and
  0.44; and c) europium is overabundant relative to barium with a mean
  value of [Eu/Ba] = 0.49 +/- 0.03. A step-like change in the [Eu/Ba]
  and [Ba/Fe] ratios occurs at the thick to thin disk transition; so,
  nearly solar elemental ratios [Ba/Fe], [Eu/Fe] and [Eu/Ba] are found
  for the thin disk stars. These data suggest that a) the halo and thick
  disk stellar population formed quickly during an interval comparable
  with the evolution time of an AGB progenitor of 3 to 4 M_sun, and the
  r-process dominated heavy element production at that epoch; b) there
  was a hiatus in star formation before the early stage of the thin disk
  developed. The even-to-odd Ba isotope ratios estimated from hyperfine
  structure (HFS) affecting the Ba Ii resonance line in the halo and
  thick disk stars favour a significant contribution of \iso{138}{Ba}
  to barium for a pure r-process, and this is supported by the recent
  data of Arlandini et al. (\cite{rs99}). Based on observations at the
  German Spanish Astronomical Center, Calar Alto, Spain

Title: Non-LTE analysis of neutral magnesium in cool stars
Authors: Zhao, G.; Gehren, T.
Bibcode: 2000A&A...362.1077Z
Altcode: 2000astro.ph.11289Z
  Calculations of the statistical equilibrium of magnesium in the solar
  photosphere have shown that NLTE populations hardly affect Mg line
  formation in the Sun. However, in metal-poor dwarfs and giants the
  influence of electron collisions is reduced, and the ultraviolet
  radiation field, enhanced due to reduced background line opacity,
  results in more pronounced NLTE effects. In the photosphere of a cool
  star excitation and ionization due to collisions with neutral hydrogen
  can outweigh electron collisions. Analyses based on NLTE populations
  lead to significantly higher Mg abundances than those calculated from
  LTE. We calculate magnesium abundances in 10 cool dwarfs and subgiants
  with metallicities from -2.29 to 0.0. The results are based on spectra
  of high-resolution and high signal-to-noise ratio. Stellar effective
  temperatures are derived from Balmer line profiles, surface gravities
  from Hipparcos parallaxes and the wings of the Mg Ib triplet, and metal
  abundances and microturbulence velocities are obtained from LTE analyses
  of Fe Ii line profiles. For stars with metallicities between -2.0 &lt;
  [Fe/H] &lt; -1.0 abundance corrections Delta [Mg/H]<SUB>NLTE-LTE</SUB>
  ~ 0.05-0.11 are found. As expected the corrections increase with
  decreasing metal abundance, and they increase slightly with decreasing
  surface gravity. We also calculate the statistical equilibrium of
  magnesium for series of model atmospheres with different stellar
  parameters and find that Delta [Mg/H]<SUB>NLTE-LTE</SUB> increases with
  effective temperature between 5200 and 6500 K. For extremely metal-poor
  stars the abundance corrections approach Delta [Mg/H]<SUB>NLTE-LTE</SUB>
  ~ 0.23 at [Fe/H] ~ -3.0. Based on observations collected at the
  German-Spanish Astronomical Center, Calar Alto, Spain and Beijing
  Astronomical Observatory, Xinglong, China

Title: Finding the First Stars: The Hamburg/ESO Objective Prism Survey
Authors: Christlieb, Norbert; Reimers, Dieter; Wisotzki, Lutz; Reetz,
   Johannes; Gehren, Thomas; Beers, Timothy C.
Bibcode: 2000fist.conf...49C
Altcode: 1999astro.ph.11016C
  The Hamburg/ESO survey (HES, [4]) is an objective-prism survey for
  bright quasars based on IIIa-J plates taken with the ESO Schmidt
  telescope and its 4° prism. It covers the total southern extragalactic
  sky (δ &lt; +2° \vert b\vert ;≳ 30°). All 380 Schmidt plates
  (effective area ∼ 7000 square degrees) have been taken, and have
  been digitized and reduced in Hamburg. The spectral range of the HES
  plates is 3200 Å &lt; λ &lt; 5300 Å, with a seeing-limited spectral
  resolution of 15 Å at Hγ. This makes it possible to efficiently
  exploit the stellar content of the survey.

Title: Gravities of Metal-Poor Halo Dwarfs and the Age of the Universe
Authors: Korn, Andreas J.; Gehren, Thomas
Bibcode: 2000fist.conf...75K
Altcode:
  We report on the methodology of an ongoing project to determine accurate
  stellar parameters (T<SUB> eff </SUB>, log g, [Fe/H], [α/Fe]) for a
  number of metal-poor halo stars located at the turnoff (TO). With the
  aid of shift-free unified stellar evolutionary models we envision to
  derive absolute ages for these stars. Their ages will set stringent
  lower limits to the age of the Galaxy and the Universe.

Title: HIPPARCOS and the Spectroscopic Distance to Local Halo Stars
Authors: Korn, A. J.; Gehren, T.
Bibcode: 2000AGM....16..P58K
Altcode:
  We are currently extending the study of "Nearby stars of the Galactic
  disk and halo I &amp; II" (Fuhrmann 1998 &amp; 2000) to a sample of
  metal-poor halo stars out to about 500 pc. Our ultimate aim is to
  derive spectroscopic ages for these stars. The basis of our analysis
  are spectra with R between 40,000 and 60,000 and signal-to-noise ratios
  (after co-addition) above 200 at Hα. These spectra were acquired
  using FOCES on the 2.2 m telescope at Calar Alto in 1999. Some of the
  brighter halo stars have significant Hipparcos parallaxes against
  which we can cross-check our method of gravity determination. For
  halo stars down to [m/H] ~-2 (to which the strong line method is
  applicable) the resulting gravities are practically free of systematics
  when compared to Hipparcos. From 150 pc outward, the spectroscopic
  uncertainty in distance is smaller than the astrometric one. The
  question of whether or not metal-poor stars require a temperature
  label and/or temperature-depth structure different from that of the
  Sun and solar-metallicity stars may be addressed in the context of
  non-LTE ionization equilibria of e. g. iron or calcium.

Title: NLTE Analysis of magenisum in cool stars
Authors: Zhao, G.; Gehren, T.
Bibcode: 2000LIACo..35..277Z
Altcode: 2000ghgc.conf..277Z
  No abstract at ADS

Title: Photospheric metal abundances of AR Lacertae
Authors: Gehren, Thomas; Ottmann, Renate; Reetz, Johannes
Bibcode: 1999A&A...344..221G
Altcode:
  High-resolution spectra of the RS CVn system AR Lac taken during total
  eclipse of the primary component lead to a precise determination of the
  stellar effective temperature of the secondary, T<SUB>eff</SUB> = 5100
  +/- 100 K, and a surface gravity log g = 3.65 +/- 0.1. The small-scale
  motions are well represented by a microturbulence velocity of v_t =
  1.6 +/- 0.3 km s(-1) . Examination of a number of spectral windows
  by means of spectrum synthesis based on solar spectrum reference fits
  produces a pattern of photospheric metal abundances that is essentially
  represented by solar or slightly enhanced values. This is at variance
  with the subsolar metal abundances reported from X-ray observations
  with the ROSAT and ASCA satellites. The effective temperature determined
  here restricts the surface fraction of cool matter such as confined in
  star spots during the present observations to values around 0.3. For
  a close binary system with an active chromosphere the photospheric
  spectra are normal. No spectrum variations on a half hour time scale
  are detected. Based on observations collected at the German-Spanish
  Astronomical Center, Calar Alto, Spain

Title: Barium abundances in cool dwarf stars as a constraint to s-
    and r-process nucleosynthesis
Authors: Mashonkina, L.; Gehren, T.; Bikmaev, I.
Bibcode: 1999A&A...343..519M
Altcode:
  We revise barium abundances in 11 cool stars with metallicities ranging
  from -2.65 to 0.05. The results are based on differential NLTE model
  atmosphere analyses of spectra that have a typical S/N of 200 and
  a resolution of 40000 or 60000. To minimize systematic errors of
  theoretical modeling and to be sure that elemental surface abundances
  are not contaminated by thermonuclear reaction products from the
  stellar interior the sample is confined to main-sequence and turnoff
  stars with only two subgiants added. Stellar fundamental parameters
  are derived from either (V-K) colours or Balmer line profiles for
  the effective temperature, from HIPPARCOS parallaxes for the surface
  gravity and from the LTE analyses of the Fe II line profiles for metal
  abundance and microturbulence values. The statistical equilibrium
  of Ba II is investigated with a model atom containing 41 levels of
  Ba II plus the ground state of Ba III. NLTE effects depend on the
  metallicity of a star: they increase the equivalent widths compared
  with LTE for [Fe/H] &gt; -2, and they show the opposite behaviour at
  lower metallicities. Empirical evidence for the necessity to include H
  atom collisions in the statistical equilibrium of Ba II is found from
  comparison of Ba abundances in the metal-poor stars derived from the
  different spectral lines. The formula of Drawin with a scaling factor
  of 1/3 gives quite sufficient results. [Ba/Fe] abundance ratios are
  approximately solar above [Fe/H] ~ - 2.2 where they decrease rapidly by
  0.5-0.6 dex. The direct method based on the hyperfine structure (HFS)
  of the resonance line of the odd isotopes is suggested to estimate the
  contribution of the s- and r- process to Ba synthesis. Its application
  requires the knowledge of the total Ba abundance that can be deduced
  from the subordinate lines free of HFS. Thus, we cannot estimate the
  ratio of the s- and r- processes for the two most metal-deficient
  stars of our sample. Our conclusion is that the s-process dominated
  Ba production, at least, for the metal-poor stars with [Fe/H] &gt;
  -2.2. Based on observations at the German Spanish Astronomical Center,
  Calar Alto, Spain

Title: An Automated Search for Metal-Poor Halo Stars in the
    Hamburg/ESO Objective-Prism Survey
Authors: Christlieb, Norbert; Wisotzki, Lutz; Reimers, Dieter; Gehren,
   Thomas; Reetz, Johannes; Beers, Timothy C.
Bibcode: 1999ASPC..165..259C
Altcode: 1998astro.ph.10183C; 1999gaha.conf..259C
  An automated search for metal-poor stars is carried out in the course
  of the Hamburg/ESO objective-prism survey (HES), which covers the full
  southern sky at Galactic latitudes |b| &gt;~ 30 deg. As the HES reaches
  ~1 magnitude deeper and covers areas of the sky which have not been
  touched by the HK Survey of Beers and collaborators, the total survey
  volume of the HES represents an increase by a factor of 4.5 compared
  to the HK Survey. Because of the limited availability of telescope time
  for spectroscopic follow-up observations, we will focus on a search for
  the most metal-poor, unevolved stars. We present a simulation study of
  the HES selection function P(B_J, T_eff, [Fe/H], log g) for metal-poor
  stars, and results from a first follow-up campaign at the ESO-NTT.

Title: Analyzing metal-poor halo stars using FOCES
Authors: Korn, A. J.; Gehren, T.
Bibcode: 1999AGAb...15..102K
Altcode: 1999AGM....15..P31F
  We present first results of an ongoing project to derive fundamental
  stellar parameters, atmospheric abundances and absolute ages of some of
  the most metal-poor field stars known. High resolution (R ~ 40 000),
  high S/N (&gt; 150) spectra obtained using FOCES at the Calar Alto
  2.2m telescope serve this purpose very well. While T_eff is reliably
  deduced from fitting Balmer profiles, log g cannot be determined easily
  due to the extraordinary metal deficiency of these objects ([m/H]~-3)
  and the corresponding weakness/absence of log g-sensitive spectroscopic
  features. Since using the strong line method for deriving log g has been
  shown to be inapplicable below [m/H]~-2.4, we concentrate our efforts
  on ionization equilibria, in particular Fe i/ii. However, iron is known
  to yield results discrepant with Hipparcos parallaxes for these stars
  thus demanding a full non-LTE approach. The determination of log g will
  enable us to derive accurate abundances and absolute ages, the latter
  profitting from unified stellar evolutionary models with a superior
  treatment of convection and diffusion (Bernkopf 1998). The ages of
  these stars will set stringent lower limits to the age of the Universe.

Title: Testing SN IA progenitor scenarios: SNR 1006
Authors: Wellstein, S.; Langer, N.; Gehren, T.; Burleigh, M.; Heber, U.
Bibcode: 1999AGAb...15R..24W
Altcode:
  For Type Ia supernovae several progenitor scenarios have been proposed:
  1) Roche lobe overflow from main sequence stars onto white dwarfs, 2)
  Roche lobe overflow from subgiants onto white dwarfs, 3) Wind accretion
  from a giant companion 4) Merging white dwarf pairs. So far, none of
  these scenarios have been clearly demonstrated to occur in nature,
  and only for scenario 1 we know potential observed counterparts,
  the supersoft X-ray sources. We have computed detailed evolutionary
  model grids for scenarios 1 and 2, and relied on literature data for
  scenario 3, in order to determine the properties of the white dwarf
  companion star at the time of the supernova explosion. E.g., we found
  that the companions in scenario 1 stem from 1.7 dots 2.3 M_odot main
  sequence stars and are underluminous stars of more than 1 M_odot, out
  of thermal equilibrium, with luminosities in the range 1 dots 10 L_odot,
  devoid of Li, Be, and B, and partly enriched in carbon. Scenarios 2 and
  3 are found to leave low mass O or B subdwarfs. From Scenarios 1 and 2,
  we expect large space velocities for the companions after the white
  dwarf explosion (100 dots 1000 km/s). The remnant of the historical
  Type Ia supernova SN 1006 is close enough to unambiguously identify
  the former companion star of the white dwarf and, according to its
  properties, distinguish between scenarios 1, 2 and 3. We propose a
  detailed observational strategy to find the companion star. The sdO
  star near the center of SN 1006 has already been investigated in part. A
  failure of finding any suitable star in SNR 1006 would support scenario
  4. In any case, a distinctive test of Type Ia supernova progenitor
  scenarios appears to be possible with SNR 1006.

Title: Sodium in the Sun and in metal-poor stars
Authors: Baumueller, D.; Butler, K.; Gehren, T.
Bibcode: 1998A&A...338..637B
Altcode:
  Systematic effects in the statistical equilibrium of sodium in cool
  metal-poor stars are analyzed using full NLTE line formation. To
  determine the important influence of collision processes and of the
  atomic model, NLTE effects of neutral sodium are first evaluated
  in the solar photosphere where the statistical equilibrium of Na I
  can be followed by examination of a relatively large number of line
  transitions. In agreement with previous analyses it is found that
  even very simple atomic models are sufficient to describe the most
  important interactions. In the solar atmosphere the inner cores of the
  lines are most affected by deviations from LTE, but the corresponding
  abundance corrections due to NLTE populations are small. The influence
  of collisional interactions with electrons and hydrogen atoms is
  evaluated by comparison of the solar spectrum with very precise fits
  to the Na I line cores. The profile analysis depends sensitively on
  the appropriate choice of velocity amplitudes and its variation with
  depth. The resulting solar sodium abundance is obtained with small
  scatter, log varepsilon_Na ,sun = 6.30 +/- 0.03. In metal-poor stars
  NLTE effects are more pronounced since the statistical equilibrium is
  dominated by collisions in which at least the electronic component is
  substantially reduced. The resulting influence on the determination
  of Na abundances is in a direction opposite to that found previously
  for Al. Stars determined in LTE analyses to have a solar [Na/Fe]
  ratio reveal a lower [Na/Fe] when NLTE line formation is taken into
  account. As for aluminium, the extremely metal-poor and the hotter
  subdwarfs are affected most strongly by Na abundance corrections
  reaching -0.5 dex for the D lines. The resulting Galactic evolution
  of the Na/Fe and Na/Mg ratios is not adequately described by current
  chemical evolution scenarios. Based on observations collected at
  the European Southern Observatory, La Silla, Chile, and at the
  German-Spanish Astronomical Center, Calar Alto, Spain

Title: Photospheric metal abundances in active stellar atmospheres
Authors: Ottmann, R.; Pfeiffer, M. J.; Gehren, T.
Bibcode: 1998A&A...338..661O
Altcode:
  With a sophisticated analysis we determine the photospheric metal
  abundances of active stars to investigate the abundance stratification
  in active stellar atmospheres. For selected single stars and RS
  CVn binaries we have taken high-resolution, high-SN and partially
  phase-resolved spectra of the entire visible range. With a purely
  spectroscopic, self-consistent method based on line profile synthesis
  and an equivalent width analysis, we derive the effective temperature
  T<SUB>eff</SUB>, surface gravity log g, microturbulence xi_t and
  the abundances of Fe, Mg and Si with high internal accuracy (Delta
  T<SUB>eff</SUB> =~ 70 K, Delta log g =~ 0.1 dex, Delta xi_t =~ 0.1
  km/s, Delta [Fe/H], Delta [Mg/H], Delta [Si/H] =~ 0.05 dex). Stellar
  parameters and metal abundances are obtained for the single stars beta
  Cet, kappa Cet and pi (1) UMa as well as for the RS CVn binaries AY Cet,
  VY Ari, EI Eri, IM Peg, lambda And and II Peg. The effect of stellar
  activity features (spots and plages) on the derived parameters is
  investigated by phase-resolved spectroscopy; we find that one out of
  four systems shows a significant variation of the Mg and Si abundances
  with rotational phase. The results help solving the puzzle about the
  photospheric Fe abundances of RS CVn binaries; specifically, we obtain
  [Fe/H] &gt;= -0.4 for all systems, indicating that they definitely
  are not metal-poor. In II Peg and lambda And, the metals are strongly
  depleted in the corona relative to the photosphere; in beta Cet and
  pi (1) UMa, Fe and Si are weakly depleted, but Mg is enhanced (with
  1sigma significance). The abundance stratifications are discussed in
  terms of the FIP-effect and the hydrostatic equilibrium stratification.

Title: FOCES - a fibre optics Cassegrain Echelle spectrograph
Authors: Pfeiffer, M. J.; Frank, C.; Baumueller, D.; Fuhrmann, K.;
   Gehren, T.
Bibcode: 1998A&AS..130..381P
Altcode:
  We have designed and built the echelle spectrograph FOCES fed by 100
  mu m optical fibres to be mounted at the Cassegrain focus of either
  the 2.2 m or the 3.5 m telescope at the Calar Alto Observatory. The
  spectrograph itself follows a white-pupil design collimated with two
  off-axis parabolic mirrors. The 15 cm beam leaving the 31.6 lines/mm R2
  echelle is refocussed in the vicinity of a small folding mirror which
  allows efficient removal of scattered light. The cross-dispersion is
  achieved with a tandem prism mounting, and the beam imaged with an f/3
  transmission camera onto a field centered on a 1024(2) thinned Tektronix
  CCD with 24 mu m pixel diameter. The echelle image covers the visible
  spectral region from 380 to 750 nm displayed in 70 spectral orders with
  full spectral coverage. Spectral orders are separated by 20 pixels in
  the blue and by 10 pixels in the red. The maximum spectral resolution is
  R = lambda / Delta lambda \ = 40600 with a 2 pixel resolution element;
  unvignetted resolution as defined by the fibre alone would be obtained
  at R = 18000. Replacing the CCD by a 2048(2) chip with 15 mu m pixel
  diameter and taking into account light losses from a reduced entrance
  slit width a full 2 pixel resolution of R = 65000 is obtained. The
  above concept has made FOCES an extremely well-defined instrument. A
  number of successful test installations at the Cassegrain foci of the
  Wendelstein 80 cm telescope, the Calar Alto 2.2 m and 3.5 m telescopes
  has produced spectra of high quality for up to 60 min exposures. The
  limiting magnitude for a 1 hr exposure with an S/N ratio of 100
  scales to V = 12 for a 3.5 m telescope which is only slightly less
  than expected from laboratory tests. In an alternative mode FOCES
  offers a second fibre carrying the sky background signal to correct
  extremely faint object spectra. This mode obtains the required higher
  cross-dispersion from an additional grism resulting in a correspondingly
  reduced spectral coverage. Based in part on observations collected at
  the German-Spanish Astronomical Center, Calar Alto, Spain.

Title: Non-LTE analysis of neutral magnesium in the solar atmosphere
Authors: Zhao, G.; Butler, K.; Gehren, T.
Bibcode: 1998A&A...333..219Z
Altcode:
  We investigate the formation of neutral magnesium lines in the
  solar photosphere with an atomic model containing 83 levels plus
  the ground state of Mg ii connected via radiative and collisional
  interactions. Synthetic line flux and intensity profiles are compared
  with the solar spectrum to study the relevant physical processes and
  their influence on the level populations and line profiles. For neutral
  magnesium with the photoionization edges of its three lowest states
  at lambda lambda 1620, 2514 and 3757 Angstroms the reduction of the
  ultraviolet radiation field due to metallic line absorption has been
  taken into account using Kurucz' (\cite{KURUCZ92}) ODF opacities. In the
  photosphere of a cool star excitation and ionization due to collisions
  with neutral hydrogen can outweigh electron collisions. Therefore the
  influence of different types of collisional interactions with electrons
  and neutral hydrogen atoms is examined. General agreement with solar
  line profiles in the visible and infrared is found for an atomic model
  with both electron collisions and strongly reduced but significantly
  large neutral hydrogen collision rates. Our investigation thus extends
  previous results to lines of all different excitation energies. The
  atomic model found from the analysis of the solar spectrum will
  serve as a reference for the investigation of cool metal-poor stars
  in which both the reduced electron collision rates and the enhanced
  UV intensities lead us to expect more pronounced deviations from LTE.

Title: Real-time spectroscopy of gravitational microlensing events -
    probing the evolution of the Galactic bulge.
Authors: Lennon, D. J.; Mao, S.; Reetz, J.; Gehren, T.; Yan, L.;
   Renzini, A.
Bibcode: 1997Msngr..90...30L
Altcode: 1997astro.ph.11147L
  Over several observing seasons, and with the help of gravitational
  microlensing surveys, the authors aim is to perform a systematic
  spectroscopic investigation of bulge sources. They expect that the
  results from this campaign will provide a fundamental insight into
  the formation and evolution of the bulge of the Galaxy. In the rest
  of this article the authors describe their first steps on this road,
  and summarise the current status of the project.

Title: Aluminium in metal-poor stars.
Authors: Baumueller, D.; Gehren, T.
Bibcode: 1997A&A...325.1088B
Altcode:
  Previous calculations of the statistical equilibrium of aluminium
  in the solar photosphere have shown that NLTE populations hardly
  affect Al line formation in the Sun; however, in metal-poor stars
  the influence of electron collisions is reduced, and a UV radiation
  field enhanced due to smaller background line opacity results in more
  pronounced NLTE effects. Thus analyses based on NLTE populations lead to
  significantly higher Al abundances than those calculated from LTE. For
  stars of intermediate metallicity between -1.0&lt;[Fe/H]&lt;-0.5
  some overabundance relative to iron is found. For more metal-poor
  stars the overabundance disappears and approaches the solar ratio,
  [Al/Fe]=0. Only a weak overabundance in the [Al/Mg] ratio is detected
  for stars with intermediate metallicity and a small underabundance of
  -0.2 to -0.3dex for the metal-poor stars. From investigation of both
  solar and stellar Al spectra the influence of hydrogen collisions could
  be better estimated. The previously defined atomic model thus had to be
  slightly modified to fit both metal-rich and metal-poor stars. Compared
  with LTE analyses the present results completely change the chemical
  enrichment scenario with [Al/Fe] now following the trend of primary
  elements for all metal-poor stars. The hump of enhanced Al/Fe values for
  stars between -1.0&lt;[Fe/H]&lt;-0.5 does not seem to be an artefact. It
  nearly vanishes for the [Mg/Fe] abundance ratios. It may not necessarily
  have to be explained in terms of stellar nucleosynthesis because it
  could result from our reference to LTE abundances for Mg and Fe.

Title: The surface gravities of cool dwarf stars revisited.
Authors: Fuhrmann, K.; Pfeiffer, M.; Frank, C.; Reetz, J.; Gehren, T.
Bibcode: 1997A&A...323..909F
Altcode:
  On the base of high-resolution spectra and standard model atmosphere
  analyses we propose to employ the pressure-broadened Mg Ib lines
  to derive the gravity parameter for F and G stars. These lines are
  advocated to be a much more robust and reliable tracer compared to the
  ionization equilibrium of, say, Fe I/Fe II, which is susceptible to
  overionization effects and uncertainties in the temperature structure
  of the model atmosphere. It is demonstrated that the strong line method
  circumvents the long-standing discrepancy ({DELTA}logg~0.5dex) of the
  standard F star Procyon, the surface gravity of which is precisely known
  due to its nearness and binary nature. We also discuss similar effects
  on other, predominantly metal-poor stars. In fact, many of the F and
  hotter G stars deviate in the LTE metal abundances of neutral and
  ionized species by up to 0.2dex. However, once the surface gravity
  parameter is fixed, very reliable iron abundances from Fe II can
  be derived as well. As a consequence a number of stars considered
  to be standards will require revised stellar parameters in future
  analyses. This will have some impact on stellar distances, ages,
  and galactic evolution in particular.

Title: The First Spectroscopic Observations of Caustic Crossing in
    a Binary Microlensing Event
Authors: Lennon, Daniel J.; Mao, Shude; Fuhrmann, K.; Gehren, T.
Bibcode: 1996ApJ...471L..23L
Altcode:
  We present the first spectroscopic observations of a binary
  microlensing event when it was undergoing a caustic crossing with a
  high magnification of A ~ 25. The event 96-BLG-3 was identified in
  real time by the MACHO collaboration, in the Baade's window field
  toward the Galactic bulge. Three spectra were taken consecutively,
  spanning the light-curve peak of the caustic crossing, each integration
  lasting 30 minutes. The spectrograms covered the wavelength range
  3985--6665 A and are almost identical, the third one differing only
  in having an amplitude ~6% lower than the others. By comparison with
  reference star spectra and by using spectrum synthesis techniques,
  we infer that the source star is a G0 IV--V star, with an effective
  temperature of Teff +/- 150 K, a metallicity in the range [M/H] =
  +0.3 to +0.6, and a logarithmic surface gravity of log g = 4.25 +/-
  0.25. Using theoretical evolutionary tracks, we derive a radius of
  ~1.4 ^{-0.4}_{+0.6} Rsolar and hence a distance of 6.9 ^{-1.7}_{+3.1}
  kpc, consistent with the source residing in the Galactic bulge. We
  also determine its heliocentric radial velocity to be vr +/- 3 km
  s-1. Caustic-crossing microlensing events such as 96-BLG-3, if they
  are observed with 8--10 m class telescopes, can resolve the stellar
  surface of distant sources with a resolution of 1010 cm or better. This
  permits a detailed study of the center-to-limb variation of the stellar
  surface and the intrinsic properties of the lensed source.

Title: Line formation of neutral aluminium in the Sun.
Authors: Baumueller, D.; Gehren, T.
Bibcode: 1996A&A...307..961B
Altcode:
  We investigate the formation of neutral aluminium lines in the
  solar photosphere using an atomic model containing 58 levels plus
  the ground state of Al II connected via radiative and collisional
  interaction. Synthetic line flux and intensity profiles are compared
  with the solar spectrum to study the relevant kinetic processes and
  their influence on level populations and line profiles. For neutral
  aluminium with its extremely large ground state photoionization
  cross-section near 2071 A the reduction of the ultraviolet radiation
  field due to metallic line absorption has to be taken into account
  using Kurucz' (1992) ODF opacities. In the photosphere of a cool star
  excitation and ionization due to collisions with neutral hydrogen can
  outweight electronic collisions. The influence of different types
  of collisional interactions with electrons and neutral hydrogen is
  therefore examined. As expected, the non-LTE effects in most of the
  solar Al I lines are small, irrespective of the details of the atomic
  model. The cores and innermost parts of the wings of the resonance
  lines at 3944 and 3961A are affected by only small deviations from the
  observed profiles, part of which is due to the uncertainty connected
  with the proper choice of the continuum flux in the region of the
  Ca II H+K lines. Since the first excited state, 4s^2^S , is slightly
  overpopulated with respect to both the 3p^2^P^o^ ground state and to
  4p^2^P^o^ the strongest evidence for non-thermal excitation is found
  in the infrared lines at 1.3 and particularly at 12μm. Empirical
  evidence for the necessity to include neutral particle collisions in
  the kinetic equilibrium of aluminium arises from comparison of these
  lines with observations. General agreement with solar line profiles
  in the infrared and in the visible is found for an atomic model with
  both electronic collisions and a strongly reduced amount of neutral
  particle collisions. The solar model will serve as a reference for
  the investigation of cool metal-poor stars in which both the reduced
  electronic collision rates and the enhanced UV intensities lead us to
  expect more pronounced deviations from LTE.

Title: Spectroscopic analyses of metal-poor stars. III. Magnesium
    abundances.
Authors: Fuhrmann, K.; Axer, M.; Gehren, T.
Bibcode: 1995A&A...301..492F
Altcode:
  We present magnesium abundances and Mg/Fe ratios for 56 metal-poor
  dwarfs and subgiants based on differential LTE model atmosphere
  analyses of spectra that have a typical S/N of 50-100 and a resolution
  of about 20000-30000. The distribution of Mg/Fe abundance ratios with
  Fe abundances is characterized by a steep increase of [Mg/Fe] near
  [Fe/H]=-0.6. At that iron abundance we recognize an abrupt change
  from a solar value for stars with [Fe/H]&gt;=-0.6 to an upper limit of
  +0.4dex for the metal-poor stars. Our data suggest that this step in
  the Mg/Fe abundance ratio at [Fe/H]=-0.6 is the result of the onset of
  Galactic disk formation, whereas [Mg/Fe]=+0.4 apparently is an upper
  limit arising from the constraints of nucleosynthesis in massive SN
  II events during the first stellar generations, the yields of which
  we observe in the most metal-poor halo stars.

Title: Spectroscopic analyses of metal-poor stars. II. The
    evolutionary stage of subdwarfs.
Authors: Axer, M.; Fuhrmann, K.; Gehren, T.
Bibcode: 1995A&A...300..751A
Altcode:
  Models of post-main sequence stellar evolution of VandenBerg &amp; Bell
  (???) have been applied to determine spectroscopic masses and distances
  for metal-poor stars. Careful consideration of the most important error
  sources published in more recent papers such as VandenBerg (???) for
  the first time allow us to draw firm statistical conclusions. It is
  shown that the evolutionary calculations qualitatively fit to the
  observed stellar parameters whereas quantitatively they predict too
  high ages for metal-poor stars. As an important result we confirm that
  evolutionary sequences need to be calibrated with respect to their metal
  abundance in order to use their absolute predictions of temperature and
  luminosity. It turns out that this can be achieved by a simple shift of
  the evolutionary tracks and isochrones in effective temperature with
  values {DELTA}log T_eff_&lt;~0.03 which accounts for possible changes
  of the mixing-length and the O/Fe ratio with metallicity. The stellar
  luminosities and surface gravities obtained from evolutionary models
  are much more reliable than their effective temperatures. Therefore
  we conclude that the accuracy of the corresponding spectroscopic
  stellar gravities is systematically affected by deviations from LTE,
  in particular along the subgiant sequence where systematic errors less
  than {DELTA}log g =~0.3 must be ascribed to the non-LTE ionization
  equilibrium of Fe II/Fe I. In our spectroscopic analyses the strong
  dependence between surface gravity and abundances determined from
  Fe I lines restricts the accuracy of Fe abundances in subgiants to
  0.1 dex at best. The most remarkable result of our evolutionary and
  kinematic investigations of halo stars refers to the large fraction
  of slightly evolved subgiants among the so-called subdwarfs. Since
  conventional photometric approaches often assume that the great majority
  of metal-poor stars are dwarfs this results in distances that are
  systematically too low for their samples. Consequently, significant
  differences are found when comparing evolutionary and kinematic
  parameters obtained from either photometric or spectroscopic data. We
  demonstrate this by comparing the space velocities of the stars. It
  appears that stars with particularly high space velocities derived from
  spectroscopic distances show very often much lower velocities based on
  their main sequence parallaxes. We find that results refering to main
  sequence parallaxes are doubtful and can be used only with greatest
  care. An advantageous side-effect of the application of spectroscopic
  data to evolutionary calculations is the possibility to identify binary
  systems that are either standing out from the Toomre diagram with
  their unusually high space velocities, or from a log g - log T_eff_
  diagram with apparently contradictory luminosities.

Title: VizieR Online Data Catalog: Metal-poor stars spectroscopy. II
    (Axer+, 1995)
Authors: Axer, M.; Fuhrmann, K.; Gehren, T.
Bibcode: 1995yCat..33000751A
Altcode:
  Models of post-main sequence stellar evolution of VandenBerg &amp;
  Bell have been applied to determine spectroscopic masses and distances
  for metal-poor stars. Careful consideration of the most important
  error sources published in more recent papers such as VandenBerg for
  the first time allow us to draw firm statistical conclusions. It is
  shown that the evolutionary calculations qualitatively fit to the
  observed stellar parameters whereas quantitatively they predict too
  high ages for metal-poor stars. As an important result we confirm that
  evolutionary sequences need to be calibrated with respect to their metal
  abundance in order to use their absolute predictions of temperature and
  luminosity. In our spectroscopic analyses the strong dependence between
  surface gravity and abundances determined from Fe I lines restricts
  the accuracy of Fe abundances in subgiants to 0.1 dex at best. The most
  remarkable result of our evolutionary and kinematic investigations of
  halo stars refers to the large fraction of slightly evolved subgiants
  among the so-called subdwarfs. Since conventional photometric approaches
  often assume that the great majority of metal-poor stars are dwarfs
  this results in distances that are systematically too low for their
  samples. (4 data files).

Title: Constraints from Element Abundances in the Galaxy
Authors: Gehren, T.
Bibcode: 1995LNP...463..190G
Altcode: 1995gyu..conf..190G; 1996LNP...463..190G
  No abstract at ADS

Title: The galactic distribution of chemical elements as derived
    from B-stars in open clusters II. NGC6611, S285, and S289.
Authors: Kilian-Montenbruck, J.; Gehren, T.; Nissen, P. E.
Bibcode: 1994A&A...291..757K
Altcode:
  The chemical composition of the open cluster NGC6611 and the Sharpless
  regions S285 and S289 have been analysed, and stellar parameters
  (effective temperature, gravity, microturbulence) and chemical
  abundances of 4 unevolved B-stars in NGC6611, 2 B-stars in S285, and 1
  B-star in S289 are given. Line blanketed LTE model atmospheres and NLTE
  line formation calculations were used for the abundance determination
  of C, N, O, Mg, Al, and Si; LTE line formation calculations were used
  for the abundance determination of Ne, S, and Fe. In agreement with
  results obtained from B-type stars in the solar neighbourhood and the
  young open cluster NGC6231 presented in earlier papers, all elements
  except neon show an average underabundance of 0.1-0.4dex with respect
  to the solar value. Among the cluster stars abundance differences of
  0.2-0.7dex may be observed. First results for the galactic metallicity
  gradient show an insignificant value for most observed elements, except
  for neon and sulfur, which is in contrast to results of analyses of
  HII regions by Shaver et al. (1983).

Title: Spectroscopic analyses of metal-poor stars. I. Basic data
    and stellar parameters.
Authors: Axer, M.; Fuhrmann, K.; Gehren, T.
Bibcode: 1994A&A...291..895A
Altcode:
  Accurate stellar parameters have been obtained from the analyses of
  more than thousand spectra of 115 metal-poor dwarfs and subgiants
  with visual magnitudes brighter than V=~12. The stellar sample was
  selected mainly by high proper motion, with additional restrictions
  from B-V colours and U-B excesses. The effective temperatures cover
  a range from 5000 to 6500K while metal abundances are found between
  [M/H]=-0.1 and -3.0dex. For the first time a representative number of
  unevolved cool metal-poor stars has been analysed individually using
  purely spectroscopic methods. Based on homogeneous ODF blanketed
  model atmospheres in LTE and working differentially with respect to
  the Sun we derive a consistent set of stellar parameters, effective
  temperature, surface gravity, metal abundance and microturbulence
  velocity. Individual profile synthesis is applied to a number of
  spectral lines for each star, which has led as a rule to accuracies
  in T_eff_ of better than 100K, in logg of better than 0.15, and in
  [Fe/H] of better than 0.1dex. Because of the consistent treatment
  with only one type of model atmosphere, this sample provides an
  opportunity to examine the individual parameters statistically and
  investigate in detail their relation to the formation and evolution of
  the Galaxy. One aspect of this analysis is a general shift to higher
  iron abundances for the most metal-poor stars. Along with the recently
  preferred meteoritic solar iron abundance and effective temperatures
  from consistent Balmer line profile fits that tend to be 100-200K
  hotter than found from photometric calibrations, discrepancies of up
  to 0.5dex in [Fe/H] can be explained in comparison with other abundance
  analyses. The most important results refer to the evolutionary status of
  the bona fide subdwarf sample. Irrespective of the different effective
  temperatures found here, there exists a severe problem when comparing
  post main sequence evolutionary models of cool stars with our observed
  parameters. Even more interesting is the fact, demonstrated by the
  results of a consistent analysis of the iron ionization equilibrium,
  that roughly half of the subdwarfs are subgiants, some of them having
  nearly reached the bottom of the giant branch.

Title: Hans-Günter Groth (7. Februar 1927 - 20. Oktober 1993).
Authors: Gehren, Thomas
Bibcode: 1994MitAG..77....9G
Altcode:
  No abstract at ADS

Title: Balmer lines in cool dwarf stars II. Effective temperatures
    and calibration of colour indices
Authors: Fuhrmann, K.; Axer, M.; Gehren, T.
Bibcode: 1994A&A...285..585F
Altcode:
  Effective temperatures obtained from synthesis of the extended
  profile wings of the first four Balmer lines are presented for more
  than 100 dwarfs and subgiants of different metal abundances and
  surface gravities in the temperature range from 5000 to 6500 K. Line
  formation is based on homogeneous ODF blanketed model atmospheres in
  LTE. The resulting temperatures of the more metal-rich stars differ
  systematically from those determined by reference to synthetic broad-
  or intermediate-band colours such as B-V , b-y , R-I or V-K . While the
  Balmer line temperatures give room to only very small individual errors
  and result in a convincingly small mean error for all four lines, the
  scatter against temperatures determined from broad-band colours is by
  far outside the internal errors claimed in recent applications. This may
  be attributed to either (a) observational errors, (b) dependence on the
  relative mixture of metal abundances, (c) unknown line blocking in most
  of the visible and near-infrared spectrum or (d) the inhomogeneity found
  in the granular patterns of stellar surfaces. Our results suggest that
  broad-band colours are insufficient individual temperature indicators,
  reliable only in a statistical sense.

Title: FOCES: the fibre-optic-coupled Cassegrain ecelle spectrograph.
Authors: Pfeiffer, M. J.; Frank, C.; Fuhrmann, K.; Gehren, T.
Bibcode: 1994AGAb...10..107P
Altcode:
  No abstract at ADS

Title: Balmer lines in cool dwarf stars. I. Basic influence of
    atmospheric models.
Authors: Fuhrmann, K.; Axer, M.; Gehren, T.
Bibcode: 1993A&A...271..451F
Altcode:
  Based on horizontally homogeneous model atmospheres the analysis
  of the first members of the hydrogen Balmer line series leads to a
  revision of the convective efficiency near the surfaces of cool dwarf
  stars. The synthesis of the Balmer lines includes Stark and resonance
  broadening. It fits the observed profiles of Hα through Hδ in such
  different stars as the Sun, Procyon, and the extremely metal-poor
  subdwarfs G41-41 and HD 140283. It turns out that the existence of the
  two different but equally important line-broadening mechanisms requires
  a temperature stratification near continuum optical depth unity which
  cannot be reconciled with a mixing-length 1 ≃ 1... 2 H<SUB>p</SUB>
  as is usually derived from stellar interior calculations. With such
  high values of 1 the synthesis of the Balmer lines leads to effective
  temperatures differing by as much as 400 K. Comparison of Hα and
  Hβ profiles instead results in a much smaller mixing-length, 1 =
  (0.5±0.3) H<SUB>p</SUB>, which seems to fit for all FG-type dwarfs
  even in the metal-poor limit. Thus, for the first time, spectroscopic
  observations of stars lead directly to a consistent determination of
  the temperature stratification in the lower photosphere.

Title: The Texas/Pennstate/Stanford/Göttingen/Munich Spectroscopic
    Survey Telescope.
Authors: Kudritzki, R. -P.; Gehren, T.; Fricke, K. J.; Beuermann,
   K.; Nicklas, H.
Bibcode: 1993AGAb....9....9K
Altcode:
  No abstract at ADS

Title: The galactic abundance gradient: chemical abundances in early
    B-type stars.
Authors: Kilian, J.; Gehren, T.; Nissen, P. E.
Bibcode: 1992AGAb....7...17K
Altcode:
  No abstract at ADS

Title: Chemical abundances in early B-type stars
Authors: Kilian, J.; Becker, S. R.; Gehren, T.; Nissen, P. E.
Bibcode: 1992LNP...401...30K
Altcode: 1992aets.conf...30K
  No abstract at ADS

Title: The efficiency of convection in cool dwarf stars.
Authors: Axer, M.; Fuhrmann, K.; Gehren, T.
Bibcode: 1992AGAb....7..128A
Altcode:
  No abstract at ADS

Title: Effective temperatures of F and G dwarfs: photometric
    vs. spectroscopic methods.
Authors: Axer, M.; Fuhrmann, K.; Gehren, T.
Bibcode: 1992AGAb....7..126A
Altcode:
  No abstract at ADS

Title: Chemical abundances in early-type stars. III. NLTE temperature
    and gravity determination.
Authors: Kilian, J.; Becker, S. R.; Gehren, T.; Nissen, P. E.
Bibcode: 1991A&A...244..419K
Altcode:
  Effective temperatures Teff and gravities log g of l9 unevolved B stars
  in the local field and nearby associations are given. Determinations
  of Teff due to non-LTE analyses of silicon line formation and non-LTE
  analyses of helium line formation are compared, and a reasonable
  agreement is obtained. Small differences were observed for four B
  stars only. The temperature calibration of some important photometric
  observables is discussed, with particular emphasis on a comparison
  with synthetic spectral indices. Temperatures obtained from the
  nonthermodynamic ionization equilibria of helium and silicon were
  found to be systematically higher by 2000 K than those calculated from
  synthetic photometry and LTE assumptions.

Title: Numerical simulation of photospheric convection in solar-type
    stars. I - Hydrodynamical test calculations
Authors: Reile, C.; Gehren, T.
Bibcode: 1991A&A...242..142R
Altcode:
  A computerized model of photospheric convection occurring in granules
  or similar structures in cool stars has been implemented to investigate
  the dependence of the granular flow and the structure of cool stellar
  atmospheres on radiative transfer and various element abundances. The
  numerical properties of the hydrodynamical method used are examined,
  emphasizing the treatment of advective terms. The method is applied to
  a number of idealized but important test problems such as shock waves,
  the Sedov problem in different dimensions, and standing gravoacoustic
  waves. It is shown that the combination of monotonic transport and
  operator-splitting is remarkably stable against error propagation.

Title: The ROSAT/XUV data centre: pre-processing, distribution and
    archiving of ROSAT/XUV pointed phase data for German PIs.
Authors: Brunner, H.; Kreysing, H. -C.; Staubert, R.; Herold, H.;
   Friedrich, S.; Knödler, M.; Gehren, T.; Kunze, D.
Bibcode: 1991AGAb....6..116B
Altcode:
  No abstract at ADS

Title: Spectroscopic Tests of Late Type Model Atmospheres of Dwarf
    Stars
Authors: Gehren, T.; Reile, C.; Steenbock, W.
Bibcode: 1991ASIC..341..387G
Altcode: 1991sabc.conf..387G
  No abstract at ADS

Title: Numerical Simulation of Photospheric Convection -
    Hydrodynamical Test Calculations
Authors: Reile, C.; Gehren, T.
Bibcode: 1991ASIC..341..303R
Altcode: 1991sabc.conf..303R
  No abstract at ADS

Title: Dolidze 25 : a metal-deficient galactic open cluster.
Authors: Lennon, D. J.; Dufton, P. L.; Fitzsimmons, A.; Gehren, T.;
   Nissen, P. E.
Bibcode: 1990A&A...240..349L
Altcode:
  High resolution spectra of three OB-type stars in the distant (5-6 kpc)
  anticenter galactic cluster Dolidze 25 have been analyzed using non-LTE
  and LTE model atmosphere techniques to derive atmospheric parameters
  and chemical compositions, respectively. The cluster appears to be
  significantly deficient in metals, with one star having underabundances
  for individual elements ranging from -0.55 to -0.93 dex with respect
  to the spectroscopic standard Tau Scorpii. This in conjunction with
  previous work on galactic clusters (Fitzsimmons et al. 1990, Gehren et
  al. 1985) implies that the variation of abundance with galactocentric
  distance is not linear and indeed may not be unique.

Title: NLTE temperature and gravity determination of early B-type
    stars.
Authors: Kilian, J.; Becker, S. R.; Gehren, T.; Nissen, P. E.
Bibcode: 1990AGAb....5...61K
Altcode:
  No abstract at ADS

Title: Model atmosphere analyses of halo dwarfs.
Authors: Axer, M.; Fuhrmann, K.; Gehren, T.; Reetz, J.
Bibcode: 1990AGAb....5...70A
Altcode:
  No abstract at ADS

Title: Astronomical Information Obtained from Echelle Spectra
Authors: Gehren, T.
Bibcode: 1990ESOC...34..103G
Altcode: 1990daan.work..103G
  No abstract at ADS

Title: The initial mass function in early stages of galactic evolution
Authors: Gehren, T.
Bibcode: 1990nuas.symp..137G
Altcode:
  No abstract at ADS

Title: The initial mass function in early stages of Galactic
    evolution.
Authors: Gehren, T.
Bibcode: 1989nuas.conf..137G
Altcode:
  Contents: 1. Introduction. 2. The initial mass function of
  globular cluster stars. 3. Spectroscopic parallaxes of halo field
  stars. 4. Masses of halo subdwarfs. 5. Conclusions.

Title: Numerical Simulation of Photospheric Convection
Authors: Reile, C.; Gehren, T.; Steenbock, W.
Bibcode: 1989AGAb....3...50R
Altcode:
  No abstract at ADS

Title: Two-dimensional N-body simulations for the Evolution of
    Elliptical Galaxies
Authors: Zeilinger, W.; Spies, W.; Burkert, A.; Gehren, T.; Hensler, G.
Bibcode: 1989AGAb....3..110Z
Altcode:
  No abstract at ADS

Title: Metal-poor subdwarfs and early galactic nucleosynthesis.
Authors: Hartmann, K.; Gehren, T.
Bibcode: 1988A&A...199..269H
Altcode:
  Observations of more than 20 metal-poor subdwarfs are presented,
  discussing the spectroscopic abundance analyses based on high-reolution
  spectra and scaled solar model atmospheres. Many of the stars are shown
  to be more evolved than can be reconciled with reasonable time scales
  based on standard theory of stellar evolution, suggesting that the
  post main sequence evolution of cool metal-poor stars is influenced by
  an enhanced O/Fe ratio compared with a standard solar mixture. Radial
  velocities are given for 60 metal-poor stars, 25 percent of which are
  suspected to be velocity variable. The kinematic properties of several
  specific stars are discussed. The abundance ratios Mg/Fe, Ca/Fe, Ti/Fe,
  Al/Fe, and Mn/Fe as a function of the Fe abundance itself indicate that
  these elements probably have not been produced in purely explosive
  nucleosynthesis. It is suggested that a considerable fraction of
  the isotopes must have been synthesized during preceding hydrostatic
  burning phases.

Title: Chemical Abundances in Stars.
Authors: Gehren, T.
Bibcode: 1988RvMA....1...52G
Altcode:
  Contents: 1. Introduction. 2. Methods of abundance
  determination. 3. Chemical elements in the solar
  system. 4. Abundances in unevolved disk stars. 5. Peculiar signs
  of stellar evolution. 6. Stellar abundances and galactic chemical
  evolution. 7. Summary.

Title: Proper Motions of Herbig-Haro Objects. VII. The Region of
    NGC 2068
Authors: Jones, B. F.; Cohen, Martin; Wehinger, Peter A.; Gehren,
   Thomas
Bibcode: 1987AJ.....94.1260J
Altcode:
  The authors have determined proper motions and have obtained long-slit
  low-resolution spectroscopy for many of the Herbig-Haro (HH) objects in
  the region of NGC 2068. The HH 24 complex alone contains two dynamically
  distinct bipolar "jets". The authors believe there are three exciting
  stars in this region, two within the bounds of HH 24: one (SSV 63)
  is responsible for HH 24 knots CEA and the objects HH 19 and HH 20 to
  the northwest of HH 24, and possibly for HH 27 to the south; a second
  for HH 24 knots G1 - G3 and probably for HH 23. A third star (probably
  SSV 59) is responsible for HH 25 and HH 26, which also define a bipolar
  flow. The proper motions and radial velocities suggest that objects
  have undergone both acceleration in the vicinity of the exciting stars,
  and deceleration a parsec away.

Title: Echelle background correction.
Authors: Gehren, T.; Ponz, D.
Bibcode: 1986A&A...168..386G
Altcode:
  An improved method for the scattered light background correction of
  two-dimensional echelle spectra is described, and the results are
  compared with more approximate methods, such as the widely used
  interorder minimum subtraction. The new correction is applied to
  a CCD echelle spectrum, and the accuracy of the continuum level is
  demonstrated to be better than 1 percent.

Title: Non-Lte Analysis of Massive Stars in the Magellanic Clouds
Authors: Gehren, T.; Husfeld, D.; Kudritzki, R. P.; Conti, P. S.;
   Hummer, D. G.
Bibcode: 1986IAUS..116..413G
Altcode:
  The massive stars of the Magellanic Clouds are of considerable current
  interest with regard to questions of initial mass function, star
  formation mechanisms, stellar evolution with mass loss and the chemical
  evolution of galaxies. The effective temperatures, surface gravities and
  helium abundances of 6 main sequence O-type stars, obtained by fitting
  non-LTE model atmospheres to high quality spectra, are presented.

Title: Die Rotverschiebung der Quasare.
Authors: Gehren, T.
Bibcode: 1985S&W....24..647G
Altcode:
  Theories on the nature of the quasar redshift are discussed. The
  arguments for the redshift being due to the relativistic Doppler
  effect, the gravitational fields of compact masses, and the expansion
  of space are summarized. The surface brightness theorem is addressed,
  and the theoretical importance of observations of quasar galaxies is
  examined. The evolution of these galaxies is discussed.

Title: Abundance gradients in the galactic disk from young B-type
stars in clusters: first results.
Authors: Gehren, T.; Nissen, P. E.; Kudritzki, R. P.; Butler, K.
Bibcode: 1985ESOC...21..171G
Altcode: 1985pdcn.conf..171G; 1985pdce.work..171G
  First results from detailed spectroscopic analyses of early B-type main
  sequence stars in galactic clusters and associations are presented,
  covering a range in galactocentric distances from about 8 to 18
  kpc. Nitrogen and oxygen abundances for a subsample of 11 sharp-lined
  stars do not reveal a significant galactic abundance gradient, at
  variance with the results obtained from H II-region observations. Since
  the subsample spans only a very restricted stellar temperature range,
  the analyses have followed a strictly differential approach leading to
  very small error limits. Thus any explanation of the resulting abundance
  scatter in a single cluster or at a given galactocentric distance in
  terms of observational or systematic errors can probably be ruled out.

Title: Photometry of Quasar Host Galaxies and Cosmological
    Implications
Authors: Gehren, T.
Bibcode: 1985LNP...232..227G
Altcode: 1985nagp.meet..227G
  Surface photometry of sky-limited red photographic and CCD observations,
  corrected for galactic extinction, the K term and seeing image
  degradation, reveals the decomposition of low-redshift quasar images
  into a central point source and an extended underlying nebulosity. The
  investigation of the statistical properties of a well-resolved subsample
  of these nebulosities shows that (1) the underlying nebulosities are
  in fact the host galaxies of quasar nuclei, (2) quasar host galaxies
  show strong evidence for luminosity evolution, (3) quasar host galaxies
  in many cases appear to be heavily distorted by interaction with faint
  companion galaxies.

Title: Host galaxies of quasars and their association with galaxy
    clusters.
Authors: Gehren, T.; Fried, J.; Wehinger, P. A.; Wyckoff, S.
Bibcode: 1984ApJ...278...11G
Altcode:
  Seventeen quasars with redshifts ranging from 0.044 to 0.828 have been
  observed through an r filter using a CCD detector. Using a point-spread
  function defined by field stars exposed on the same CCD frame as the
  quasar, point-by-point subtraction of the quasar nucleus from the image
  reveals some of the structure in the nebulosity underlying essentially
  all quasars with redshifts ⪉0.5. Under the assumption that the quasar
  nuclei and the underlying nebulosities have the same cosmological
  redshifts, absolute magnitudes and metric diameters of the resolved
  structures again support the interpretation that quasars are central
  events in distant galaxies. The morphologies of the resolved galaxies
  underlying the quasars PHL 909, 3Cr 48, and 0241+622 are suggestive
  of tidal interactions with nearby companion galaxies. Counts of faint
  galaxies in the vicinity of the quasar images indicate that quasars are
  usually situated in groups or clusters of galaxies which are dominated
  by the luminosity contribution of the quasar host galaxy.

Title: Host galaxies of quasars
Authors: Gehren, T.; Fried, J.; Wehinger, P. A.; Wyckoff, S.
Bibcode: 1983LIACo..24..489G
Altcode: 1983qgl..conf..489G
  By analogy with other active extragalactic phenomena, a quasar is
  defined to consist of at least two components: a central point source
  and an extended underlying nebulosity, referred to as the host galaxy
  of a quasar. Here, some integrated properties of the host galaxies
  are derived from CCD surface photometry obtained for 17 low-redshift
  quasars at the Cassegrain focus of a 2.2-m telescope. The luminosities,
  diameters, and intensity profiles derived from the CCD observations
  are consistent with the assumption that radio quasars reside in
  host galaxies at cosmological distances comparable with first-ranked
  elliptical cluster galaxies. A less luminous type of galaxy may be
  associated with radio-quiet quasars.

Title: Statistical properties of quasar galaxies
Authors: Wyckoff, S.; Wehinger, P. A.; Gehren, T.; Fried, J.; Spinrad,
   H.; Tapia, S.
Bibcode: 1983LIACo..24..483W
Altcode: 1983qgl..conf..483W
  The authors present data on a statistically significant sample of
  quasars covering a wide range in redshifts. The results support the view
  that all quasars sit in galaxies. The correlations presented indicate
  that the nebulosities surrounding both radio-loud and radio-quiet
  quasars have metric diameters and intrinsic optical luminosities
  corresponding to galaxies at the quasar redshifts. Also the authors
  present evidence for significant differences in the diameters and
  absolute magnitudes of radio-loud and radio-quiet quasars in the sense
  that radio-loud quasars sit in more luminous, larger galaxies.

Title: Quasar Galaxies - Two-Dimensional Image Deconvolutions
Authors: Wehinger, P. A.; Wyckoff, S.; Gehren, T.; Spinrad, H.
Bibcode: 1983IAUS..104...49W
Altcode:
  No abstract at ADS

Title: Quasargalaxien
Authors: Fried, J. W.; Gehren, T.
Bibcode: 1983MitAG..60..450F
Altcode:
  No abstract at ADS

Title: Properties of Quasar Nebulosities
Authors: Wyckoff, S.; Wehinger, P. A.; Gehren, T.; Spinrad, H.
Bibcode: 1982BAAS...14Q.908W
Altcode:
  No abstract at ADS

Title: Models of stellar evolution and their use in calibrating
    distances and element abundances of stars
Authors: Gehren, T.
Bibcode: 1982A&A...109..187G
Altcode:
  Recently published grids of stellar interior models fail to account for
  the temperature and luminosity of the Sun with reasonable values of
  mass, age and chemical composition. It is shown how this discrepancy
  may affect the determination of element abundances and distances of
  field stars and clusters.

Title: Extremely Metal-Poor Subdwarfs
Authors: Gehren, T.
Bibcode: 1982Msngr..27...22G
Altcode:
  The formation of our Galaxy and its evolution from an extended
  spherical halo to a highly flattened spiral disk can be convincingly
  documented by observing cool dwarf stars that have remained essentially
  unevolved since they formed billions of years ago. Model predictions
  of nucleosynthesis in stars, starting from a zerometal primordial
  composition, combined with a theoretical outline of the galactic
  collapse, have led to a coarse description of the history of our
  Galaxy, in which the oldest stars are extremely metal-poor and have
  highly eccentric galactic orbits with relatively small orbital angular
  momenta as compared with young disk stars.

Title: Characteristics of Nebulosity Associated with Parkes Quasars
Authors: Wehinger, P.; Wyckoff, S.; Gehren, T.
Bibcode: 1982IAUS...97..375W
Altcode:
  No abstract at ADS

Title: Resolution of quasar images.
Authors: Wyckoff, S.; Wehinger, P. A.; Gehren, T.
Bibcode: 1981ApJ...247..750W
Altcode:
  Thirteen low redshift (z approximately less than 0.6) quasars have
  been resolved on large-scale, sky-limited photographs, and the
  images were analyzed by digitally removing the plate background and
  the point-spread function defined by images of nearby field stars,
  having magnitudes comparable to those of the quasars. The nebulosities
  around the quasars were found to have an average metric diameter of
  approximately 90 + or - 30 kpc and an integrated absolute magnitude
  of approximately -21.8 + or - 0.8. An expression is presented for the
  average intensity profile of the underlying nebulosity. Correlations
  (between the isophotal diameters of the resolved nebulosity and quasar
  redshifts; and the integrated apparent magnitudes of the underlying
  nebulosity and isophotal diameters) indicate roughly constant diameters
  and surface brightnesses for the nebulosities associated with the
  quasars. The physical and statistical properties of the nebulosities
  surrounding the quasars support the hypothesis that quasars are the
  luminous nuclei of distant galaxies.

Title: The temperature scale of solar-type stars.
Authors: Gehren, T.
Bibcode: 1981A&A...100...97G
Altcode:
  Effective temperatures for the atmospheres of solar-type stars have
  been determined by comparison of observed hydrogen lines with profiles
  computed from scaled solar model atmospheres. The analysis is based
  on more than one hundred high-dispersion spectra of 20 main sequence
  stars with spectral types F6 V to G5 V and calibrated directly with
  spectra of the sun, moon, and daylight sky. For early G-type stars
  the resulting spectroscopic temperature scale deviates considerably
  from color-temperature relations derived from synthesis of theoretical
  color indices using model atmospheres with statistical line distribution
  functions. The spectroscopic calibration yields effective temperatures
  for G-type stars that are about 200-300 K higher than those predicted
  from comparison of observed and synthesized color indices.

Title: Properties of Quasar Galaxies
Authors: Wehinger, P. A.; Wyckoff, S.; Gehren, T.
Bibcode: 1981BAAS...13..806W
Altcode:
  No abstract at ADS

Title: Räumliche Auflösung von Quasaren
Authors: Wehinger, P. A.; Gehren, T.; Wyckoff, S.; Boksenberg, A.
Bibcode: 1981MitAG..52R.157W
Altcode: 1981MitAG..52..157W
  No abstract at ADS

Title: Metallarme Subdwarfs der Spektraltypen F und G
Authors: Gehren, T.; Hippelein, H.; Münch, G.
Bibcode: 1981MitAG..52...68G
Altcode:
  No abstract at ADS

Title: Discovery of nebulosity associated with the quasar 3C 273.
Authors: Wyckoff, S.; Gehren, T.; Morton, D. C.; Albrecht, R.;
   Wehinger, P. A.; Boksenberg, A.
Bibcode: 1980ApJ...242L..59W
Altcode:
  The discovery of a diffuse nebulosity physically associated with the
  quasar 3C 273 (excluding its optical jet) is reported. The nebulosity
  was detected as a diffuse asymmetrical extension in the isophotes
  of a photographic image of the quasar obtained at the prime focus
  of a 3.6-m telescope which is resolved with a radial size scale of
  approximately 15 arcsec at a surface brightness level of 25 mag/sq
  arcsec. Spectra of the extended structure exhibit narrow emission
  lines at the redshifted positions of the forbidden O II line at 3727
  A and Ne III line at 3869 A, indicating the presence of low-density
  hot gas at a projected distance of approximately 4.5 arcsec from the
  quasar. Comparison of photometric and spectroscopic data indicates the
  dominant emission to be continuum radiation, which may originate from
  an underlying galaxy associated with 3C 273.

Title: Nebulosity Associated with Quasars
Authors: Wyckoff, S.; Wehinger, P. A.; Gehren, T.
Bibcode: 1980BAAS...12..808W
Altcode:
  No abstract at ADS

Title: IPCS Spectroscopy of the Quasar PKS 0812+020 and Its Associated
    Nebulosity
Authors: Wehinger, P. A.; Gehren, T.; Wyckoff, S.; Boksenberg, A.
Bibcode: 1980BAAS...12R.807W
Altcode:
  No abstract at ADS

Title: The Radio/Optical Structure of the QSO 0812+020
Authors: Wyckoff, S.; Johnston, K.; Ghigo, F.; Rudnick, L.; Wehinger,
   P.; Gehren, T.; Boksenberg, A.
Bibcode: 1980BAAS...12..497W
Altcode:
  No abstract at ADS

Title: Quasars Resolved
Authors: Wehinger, P. A.; Gehren, T.; Wyckoff, S.
Bibcode: 1980Msngr..20R...1W
Altcode:
  While observers have obtained spectra of more than 1,400 quasars since
  they were discovered in 1963, fewer than one per cent have been studied
  by direct imaging techniques at significantly faint surface brightness
  levels and high angular resolution to detect anything more than a
  bright point-like source. La Silla's very dark sky, excellent seeing,
  plus the superb optics of the 3.6-m telescope have been combined with
  digital analysis of sky-limited photographs to produce two-dimensional
  intensity contour maps and image profiles of quasars. The contour maps
  reveal extended structure on ascale of 5-10 are sec for 80 per cent of
  a sample of 20 low redshift (z = 0.1-0.5) quasars. In a significant
  number of cases the data also show the presence of galaxies near
  the QSO's, some of which have measured redshifts nearly equal to
  the QSO redshifts. This programme is the joint effort of Drs. Peter
  Wehinger and Thomas Gehren of the Max-Planck Institute for Astronomy
  in Heidelberg and Professor Susan Wyckoff of Arizona State University
  in Tempe, Arizona.

Title: Deep Imagery of Quasar Fields - Techniques
Authors: Wehinger, P. A.; Gehren, T.; Wyckoff, S.
Bibcode: 1980tdp..conf..401W
Altcode:
  No abstract at ADS

Title: Deep Imaging of Quasar Fields
Authors: Wehinger, P. A.; Wyckoff, S.; Gehren, T.
Bibcode: 1980IAUS...92..133W
Altcode:
  No abstract at ADS

Title: Turbulence in main sequence stars
Authors: Gehren, T.
Bibcode: 1980LNP...114..103G
Altcode: 1980sttu.coll..103G; 1980IAUCo..51..103G
  Turbulence in main sequence stars is discussed, considering micro-
  and macroturbulence, the horizontally homogeneous atmosphere approach,
  and small scale velocity fields. With regard to microturbulence and
  basic stellar parameters, sources of systematic errors are described,
  concerning abundance analyses based on absolute oscillator strengths,
  the problem of absolute f-values, departures from LTE, and the damping
  constants. Also discussed are microturbulence values from narrow band
  photometry, curve-of-growth analyses, model atmosphere analyses, and
  Fourier transform analysis. It is noted that a correlation between
  microturbulence and convective velocities exists, and the question of
  the nature of the transport mechanism is addressed.

Title: BL LAC Objects: Direct Imaging
Authors: Wehinger, P. A.; Wyckoff, S.; Tapia, S.; Gehren, T.
Bibcode: 1979BAAS...11..693W
Altcode:
  No abstract at ADS

Title: Metal abundances and microturbulence in seven solar-type
    stars. II. Model atmosphere analysis.
Authors: Gehren, T.
Bibcode: 1979A&A....75...73G
Altcode:
  High-dispersion spectra of seven near main sequence stars of MK
  spectral types F6 to G0 were investigated with differential model
  atmosphere analyses relative to the sun in order to obtain metal
  abundances and microturbulence velocities. It is shown that three of
  the stars hitherto assumed to be metal-rich from photometric narrow-band
  observations (9 Com, 14 Boo, and o Aql) are only marginally overabundant
  with respect to the sun, provided their microturbulence is properly
  taken into account. It is found that all the metal-overabundant stars
  examined are younger than the sun. Apart from a general metal over or
  under-abundance, the details of the abundance patterns are the same
  as in the sun.

Title: On the chemical composition and age of beta Vir.
Authors: Gehren, T.
Bibcode: 1978A&A....65..427G
Altcode:
  The F8V star Beta Vir is reanalyzed by means of a differential model
  atmosphere analysis. Taking into account the revision of Holweger's
  solar model and improved solar equivalent widths as measured from the
  Liege atlas, the final atmospheric parameters are given. A solar He/H
  ratio has been assumed. Evolutionary isochrones suggest a stellar age
  of approximately 3 billion years, in contrast to earlier investigations.

Title: Metal abundances and microturbulence in seven solar-type stars:
    spectroscopic data
Authors: Gehren, T.; Reimers, D.; Berthold, L.; Berthold, J.;
   Hennig, R.
Bibcode: 1978A&AS...31..297G
Altcode:
  Basic spectroscopic data for the analyses of seven solar-type stars
  are presented, including the computational results for individual
  spectral lines in the wavelength region from 4400 to 6800 A. Line
  strengths are compared with previous observations by other authors.

Title: Model atmosphere analysis of HR 8799.
Authors: Gehren, T.
Bibcode: 1977A&A....59..303G
Altcode:
  Eight high-dispersion spectrograms of the metal-deficient star HR
  8799, covering the wavelength range from 3700 to 6000 A, are analyzed
  using a model-atmosphere program that assumes LTE but neglects line
  blanketing. A 'spectroscopic' model atmosphere is obtained, but a
  discrepancy is found between the model-atmosphere computations and
  photometric observations. Unsuccessful attempts are made to fit
  the star's mass-luminosity ratio to various evolutionary tracks;
  the results indicate that the observations cannot be accounted
  for by a single stellar object and that the present evolutionary
  stage of HR 8799 requires the existence of an efficient mass-transfer
  process. Therefore, a binary model is proposed in which the primary had
  a mass of approximately 0.85 solar mass at the start of core hydrogen
  burning, the secondary (HR 8799) had a mass of about 0.80 solar mass
  at the same stage, and the primary transferred about 0.45 solar mass
  to HR 8799 after reaching the giant stage.

Title: HR 8799-ein metallarmer Doppelstern?
Authors: Gehren, T.
Bibcode: 1977MitAG..42..139G
Altcode:
  No abstract at ADS

Title: Über die Bahnen der regulären Satelliten im Planetensystem
Authors: Unsöld, A.; Gehren, T.
Bibcode: 1975NW.....62...95U
Altcode:
  No abstract at ADS

Title: Entstehung von Na I-Linien in der Sonnenatmosphäre
Authors: Gehren, T.
Bibcode: 1975MitAG..36..141G
Altcode:
  No abstract at ADS

Title: Kinetic equilibrium and line formation of Na I in the solar
    atmosphere.
Authors: Gehren, T.
Bibcode: 1975A&A....38..289G
Altcode:
  Summary. The influence of deviations from local thermodynamic
  equilibrium (LTE) on the formation of Na I lines under solar conditions
  is investigated. Based on different prescribed solar model atmospheres
  and model atoms which include up to 9 energy levels and 14 line
  transitions, the line transfer problem is solved using the integral
  equation approach, assuming a stationary, plane-parallel and homogeneous
  atmosphere, and complete redistribution of line photons. The kinetic
  ionization equilibrium turns out to be determined through radiative
  interaction between highly excited levels and the continuum. The kinetic
  excitation equilibrium between the high-lying levels deviates from
  LTE already in the photosphere as a result of an outwardly increasing
  radiation loss in the line transitions. The excitation of the Na D
  lines follows the LTE predictions out to T = I0 2, where the onset
  of photon escape from the resonance line radiation field yields
  a stronger population of the ground state. Under the assumption
  of anisotropic microturbulence with h &gt;= and d /dr &lt;0 for r
  &lt; I0 , the computed D line cores closely fit observation. Since
  the formation of the Na D lines is nearly independent of the solar
  chromospheric temperature, such a fit is obtained for three different
  solar models. There appears to be ne need for the high chromospheric
  electron densities suggested in previous investigations. Attempts to
  fit observation to the computed cores of higher excited lines have
  failed. A further investigation of the influence of different model
  parameters reveals that the kinetic ionization equilibrium and the
  excitation equilibrium between highly excited levels depend on the
  number of energy levels included in our model atom, as well as on
  the collision cross-sections. The equivalent widths of the Na ilines
  are affected by deviations from LTE only a few per cent. Key words:
  non-LTE kinetic equilibrium Na I lines solar atmosphere

Title: Kinetic equilibrium and line formation of Na I in the solar
    atmosphere
Authors: Gehren, Thomas
Bibcode: 1974PhDT.......153G
Altcode:
  No abstract at ADS

Title: An Interpolation Method for the Solution of Non-LTE Line
    Transfer Problems
Authors: Gehren, T.
Bibcode: 1973A&A....27..291G
Altcode:
  Summary: Iterative procedures solving non-LTE line transfer problems
  through the integral equation approach require the calculation of
  monochromatic lambda operators. We outline a simple interpolation method
  that avoids the repeated evaluation of exponential integrals. Based on
  polynomial approximation of the source function, gaussian integration
  weights 0cm and divisions tm have to be calculated only once from the
  integral representation for a set of r-points. While saving more than
  3/4 of computing time, first solutions of multilevel line transfer
  problems indicate that the accuracy is comparable to other methods. Key
  words: non-LTE - line transfer - interpolation method (t) E,(t) dt =
  M mZ=i 0cmf(tm)

