Author name code: espagnet
ADS astronomy entries on 2022-09-14
author:"Espagnet, O." 
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Title: Pollux: A weak dynamo-driven dipolar magnetic field and
    implications for its probable planet
Authors: Aurière, M.; Petit, P.; Mathias, P.; Konstantinova-Antova,
   R.; Charbonnel, C.; Donati, J. -F.; Espagnet, O.; Folsom, C. P.;
   Roudier, T.; Wade, G. A.
Bibcode: 2021A&A...646A.130A
Altcode: 2021arXiv210102016A
  Context. Pollux is considered as an archetype of a giant star hosting a
  planet since its radial velocity (RV) presents very stable sinusoidal
  variations with a period of about 590 d. We then discovered a weak
  magnetic field at its surface using spectropolarimetry, questioning the
  planetary hypothesis. <BR /> Aims: We followed up our investigations on
  Pollux to characterize its magnetic field and to infer the effects of
  magnetic activity on the RV variations. <BR /> Methods: We first used
  ESPaDOnS at CFHT and then Narval at TBL to obtain Stokes I and Stokes
  V spectra of Pollux to study their variations for a duration of 4.25
  years, that is, for more than two periods of the RV variations. We
  used the least-squares deconvolution profiles to measure the
  longitudinal magnetic field and to perform a Zeeman Doppler imaging
  (ZDI) investigation. <BR /> Results: The longitudinal magnetic field
  of Pollux is found to vary with a sinusoidal behavior and a period
  similar to that of the RV variations. From the ZDI investigation a
  rotation period of Pollux is determined to be equal to 660 ± 15 days
  and possibly different than the period of variations of the RV. As
  to the magnetic topology, the poloidal component is dominant and
  almost purely dipolar with an inclination of 10.5° of the dipole
  with respect to the rotation axis. The mean strength of the surface
  magnetic field is 0.44 G. Pollux is found approximately as active as
  the Sun observed as a star and this activity could induce moderate
  RV variations. <BR /> Conclusions: As to the origin of the magnetic
  field of Pollux, we favor the hypothesis that it is maintained through
  contemporaneous dynamo action. Pollux appears as the representative
  of a class of slowly rotating and weakly magnetic G-K red giants. To
  explain the sinusoidal RV variations of Pollux, two scenarios are
  proposed. If the RV period is different from the rotation period,
  the observed periodic RV variations are due to the hosted planet and
  the contribution of Pollux magnetic activity is not significantly
  detected. In the peculiar case in which the two periods are equal,
  we cannot discard the possibility that the activity of Pollux could
  explain the total RV variations and that the planet hypothesis would
  appear unnecessary. In any case magnetic activity could contribute
  significantly to RV variations in some intermediate mass G-K red
  giants hosting planets, particularly those with small amplitude RV
  variations. <P />Based on observations obtained at the Télescope
  Bernard Lyot (TBL) at Observatoire du Pic du Midi, CNRS/INSU and
  Université de Toulouse, France, and the Canada-France-Hawaii Telescope
  (CFHT), which is operated by the National Research Council of Canada,
  CNRS/INSU and the University of Hawaii.

Title: Pollux: a stable weak dipolar magnetic field but no planet?
Authors: Aurière, Michel; Konstantinova-Antova, Renada; Espagnet,
   Olivier; Petit, Pascal; Roudier, Thierry; Charbonnel, Corinne; Donati,
   Jean-François; Wade, Gregg A.
Bibcode: 2014IAUS..302..359A
Altcode: 2013arXiv1310.6907A
  Pollux is considered as an archetype of a giant star hosting a planet:
  its radial velocity (RV) presents sinusoidal variations with a period
  of about 590 d, which have been stable for more than 25 years. Using
  ESPaDOnS and Narval we have detected a weak (sub-gauss) magnetic
  field at the surface of Pollux and followed up its variations with
  Narval during 4.25 years, i.e. more than for two periods of the RV
  variations. The longitudinal magnetic field is found to vary with a
  sinusoidal behaviour with a period close to that of the RV variations
  and with a small shift in phase. We then performed a Zeeman Doppler
  imaging (ZDI) investigation from the Stokes V and Stokes I least-squares
  deconvolution (LSD) profiles. A rotational period is determined, which
  is consistent with the period of variations of the RV. The magnetic
  topology is found to be mainly poloidal and this component almost
  purely dipolar. The mean strength of the surface magnetic field is
  about 0.7 G. As an alternative to the scenario in which Pollux hosts a
  close-in exoplanet, we suggest that the magnetic dipole of Pollux can
  be associated with two temperature and macroturbulent velocity spots
  which could be sufficient to produce the RV variations. We finally
  investigate the scenarii of the origin of the magnetic field which
  could explain the observed properties of Pollux.

Title: Spatial relation between the 5-minute oscillations and
    granulation patterns.
Authors: Espagnet, O.; Muller, R.; Roudier, T.; Mein, P.; Mein, N.;
   Malherbe, J. M.
Bibcode: 1996A&A...313..297E
Altcode:
  A white light, high resolution movie of the quiet photosphere has been
  used to study the spatial relation between the solar granulation and
  the 5-min oscillations of high degree mode (small spatial scale). The
  granular and oscillatory brightness components have been separated with
  the help of a specific filter, in the k-ω diagram. The surface of the
  Sun appears to be covered with adjacent oscillating cells of apparent
  size 2" (1500km) to 3" (2000km). Some cells are noticeable by their
  amplitude which is much larger than average. These strong oscillations
  usually appear in expanding intergranular spaces. Our observations
  suggest that the 5-min oscillations are stochastically generated by
  turbulent convection. However, the most energetic oscillations occur
  in downflows only, well separated of each other in time and space.

Title: Penetration of the solar granulation into the photosphere:
    height dependence of intensity and velocity fluctuations.
Authors: Espagnet, O.; Muller, R.; Roudier, T.; Mein, N.; Mein, P.
Bibcode: 1995A&AS..109...79E
Altcode:
  A 16-min time series of two-dimensional "Multichanel Subtractive Double
  Pass" spectrograms, recorded in the NaD_2_ 5690 line, has been used to
  study the vertical structure of the photosphere and, in particular,
  the penetration of granules. A statistical and a morphological
  investigations have been made. The 5-min oscillations have been
  carefully filtered in the k-ω space. Intensity fluctuations associated
  to granules disappear in a very short distance, about 60 to 90km above
  the continuum level. The intensity features in the upper photosphere
  are not related to the granules. The velocity fluctuations associated
  to the granulation, cross the whole thickness of the photosphere. The
  photosphere can thus be separated into two distinct layers, but only
  for the intensity features, not for the velocity features which are
  the same from the bottom to the top of the stable atmosphere. In the
  lower photosphere, below 90km, only granules are detected. In the upper
  photosphere, our observations suggest that the weak pattern visible
  there, is turbulent in origin. In addition to this weak pattern,
  bright sinking plumes have been discovered; they are associated with
  strong downflows, often adjacent to strong upflows. The size of most
  plumes is 1" to 2"; however the most outstanding of them may be as
  large as 4". Their origin, which may be photospheric or chromospheric,
  is still unknown.

Title: Peculiar interactions between granules and network bright
    points in the solar photosphere
Authors: Roudier, T.; Espagnet, O.; Muller, R.; Vigneau, J.
Bibcode: 1994A&A...287..982R
Altcode:
  The magnetic elements which form the photospheric network, outside
  active regions, are visible as tiny bright features, smaller than 0.5",
  called network bright points (NBPs). 15 to 20% of them elongate during
  their life, when they are squeezed between two expanding granules,
  or simply pushed by an expanding or a moving granule. Some statistical
  properties are reported. The most interesting property is that their
  size and brightness increases simultaneously indicating a strong
  increase of radiative flux as they grow. They can become as large as
  1000 km x 400 km and as bright as 1.25 the brightness of the average
  photosphere. As theoretical models predict that magnetic flux tubes of
  size larger than 300 km should be darker than the mean photosphere, our
  observations suggest that during the elongation, some mechanical energy
  should contribute in the heating of the tubes. This additional energy
  may heat some features in the upper photosphere like chromospheric
  spicules and transition zone jets or microflares, whose production
  rates and lifetimes are about the same as those of elongation of
  NBPS. Elongated NBPs are shaked by the turbulent granules, similarly
  to the normal NBPs and thus should contribute to the heating of the
  solar corona too.

Title: La granulation solaire: origine, pénétration dans la
    photosphère et interactions avec les oscillations de 5 minutes
Authors: Espagnet, O.
Bibcode: 1994PhDT........47E
Altcode:
  No abstract at ADS

Title: Turbulent power spectra of solar granulation.
Authors: Espagnet, O.; Muller, R.; Roudier, T.; Mein, N.
Bibcode: 1993A&A...271..589E
Altcode:
  High resolution "Multichannel Subtractive Double Pass" spectrograms
  and white light photographs have been used to compute power spectra
  of the solar granulation. The 5 min oscillations have been carefully
  filtered, and the influence of the variation of the mean size of
  granules at the mesogranulation scale has been taken into account. <P
  />It results that both the power spectra of velocity and intensity
  fluctuations, displayed in a log P - log k scale, are characteristic
  of a turbulent atmosphere: they present a discontinuity at 3" =
  2000 km (which corresponds to the size of a large granules) and an
  energy which decreases according to the Kolmogorov -5/3 power law
  in the granulation range; another discontinuity, at 1".4 = 1000 km
  (which corresponds approximately to a Peclet number Pe = 1, for which
  thermal diffusion and advection are of equal importance) is present in
  the intensity power spectrum, which decreases with a -17/3 power law,
  in the range of sizes smaller than 1".4. <P />These results indicate
  that granules are turbulent eddies. They have some convective characters
  (like the correlation between velocities and brightness) because they
  are formed in a strongly superadiabatic atmosphere.

Title: Turbulence et granulation solaire.
Authors: Espagnet, O.; Muller, R.; Roudier, T.
Bibcode: 1992JAF....43...35E
Altcode:
  No abstract at ADS

Title: Dynamique de la photosphère solaire: granulation,
    mésogranulation, oscillations.
Authors: Espagnet, O.; Muller, R.; Roudier, T.; Mein, N.; Mein, P.
Bibcode: 1992JAF....43...36E
Altcode:
  No abstract at ADS

Title: Dynamics of the solar granulation. II - Statistical analysis:
    Power spectra, coherence, phase
Authors: Roudier, T.; Vigneau, J.; Espagnet, O.; Muller, R.; Mein,
   P.; Malherbe, J. M.
Bibcode: 1991A&A...248..245R
Altcode:
  The solar granulation is analyzed statistically by the computation
  of coherence, phase, and power spectra using data provided by the
  Multichannel Subtractive Double Pass spectrograph of the Pic du Midi
  Observatory. The main result of the analysis is that the velocity
  power spectrum of the granulation changes in shape at 3 arcsec: the raw
  spectrum features a slope discontinuity; the corrected spectra have a
  power maximum at 3 arcsec. Between 3 arcsec and the resolution limit
  (0.8 arcsec), the power decreases almost linearly, with a slope close
  to -5/3, consistent with the Kolmogorov power law for a turbulent
  energy cascade.

Title: Results from high resolution solar images and spectra obtained
    at the Pic du Midi Observatory (1986-1990)
Authors: Roudier, Th.; Muller, R.; Vigneau, J.; Auffret, H.; Espagnet,
   O.; Simon, G. W.; Title, A. M.; Frank, Z.; Shine, R. A.; Tarbell,
   T. D.; Mein, P.; Malherbe, J. M.
Bibcode: 1991AdSpR..11e.205R
Altcode: 1991AdSpR..11..205R
  We present an overview of our recent results about solar granulation and
  mesogranulation, obtained with Pic du Midi observations. These results
  were obtained during 1986-1990 using image and spectrographic analysis
  of high spatial resolution data. The study of the solar granulation,
  with 2 Dim. ``Multichannel Subtractive Double Pass'' (M.S.D.P.) spectra,
  shows a clear change of the dynamical regime at 3'' (⋍ 2200 km)
  of the photospheric velocity field when oscillatory components are
  filtered out. <P />A three hour movie obtained on film at Pic du Midi
  Observatory and analyzed at the Lockheed Research Laboratory and the
  National Solar Observatory (Sacramento Peak) was used to calculate
  the horizontal flow pattern. The mean lifetime of the diverging areas
  related to mesogranulation is estimated at 3 hours; these diverging
  areas are swept by the supergranulation flow towards the supergranule
  boundary with a mean speed of 0.4 km/s.

