freytag

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

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

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

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Title: The extended atmosphere and circumstellar environment of the
cool evolved star VX Sagittarii as seen by MATISSE
Authors: Chiavassa, A.; Kravchenko, K.; Montargès, M.; Millour, F.;
Matter, A.; Freytag, B.; Wittkowski, M.; Hocdé, V.; Cruzalèbes, P.;
Allouche, F.; Lopez, B.; Lagarde, S.; Petrov, R. G.; Meilland, A.;
Robbe-Dubois, S.; Hofmann, K. -H.; Weigelt, G.; Berio, P.; Bendjoya,
P.; Bettonvil, F.; Domiciano de Souza, A.; Heininger, M.; Henning, Th.;
Isbell, J. W.; Jaffe, W.; Labadie, L.; Lehmitz, M.; Meisenheimer, K.;
Soulain, A.; Varga, J.; Augereau, J. -C.; van Boekel, R.; Burtscher,
L.; Danchi, W. C.; Dominik, C.; Drevon, J.; Gámez Rosas, V.;
Hogerheijde, M. R.; Hron, J.; Klarmann, L.; Kokoulina, E.; Lagadec,
E.; Leftley, J.; Mosoni, L.; Nardetto, N.; Paladini, C.; Pantin, E.;
Schertl, D.; Stee, P.; Szabados, L.; Waters, R.; Wolf, S.; Yoffe, G.
2022A&A...658A.185C Altcode: 2021arXiv211210695C
Context. VX Sgr is a cool, evolved, and luminous red star whose
stellar parameters are difficult to determine, which affects
its classification. Aims: We aim to spatially resolve the
photospheric extent as well as the circumstellar environment. Methods: We used interferometric observations obtained with
the MATISSE instrument in the L (3-4 μm), M (4.5-5 μm), and N
(8-13 μm) bands. We reconstructed monochromatic images using
the MIRA software. We used 3D radiation-hydrodynamics simulations
carried out with CO5BOLD and a uniform disc model to
estimate the apparent diameter and interpret the stellar surface
structures. Moreover, we employed the radiative transfer codes OPTIM3D
and RADMC3D to compute the spectral energy distribution for the L,
M, and N bands, respectively. Results: MATISSE observations
unveil, for the first time, the morphology of VX Sgr across the L, M,
and N bands. The reconstructed images show a complex morphology with
brighter areas whose characteristics depend on the wavelength probed. We
measured the angular diameter as a function of the wavelength and
showed that the photospheric extent in the L and M bands depends on the
opacity through the atmosphere. In addition to this, we also concluded
that the observed photospheric inhomogeneities can be interpreted
as convection-related surface structures. The comparison in the N
band yielded a qualitative agreement between the N-band spectrum
and simple dust radiative transfer simulations. However, it is not
possible to firmly conclude on the interpretation of the current
data because of the difficulty in constraing the model parameters
using the limited accuracy of our absolute flux calibration. 
Conclusions: MATISSE observations and the derived reconstructed images
unveil the appearance of VX Sgr's stellar surface and circumstellar
environment across a very large spectral domain for the first time. Based on the observations made with VLTI-ESO Paranal, Chile under
the programme IDs 0103.D-0153(D, E, G). The data are available at <A
href="http://oidb.jmmc.fr/index.html">oidb.jmmc.fr</A>

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

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

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

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Title: Horizontal spreading of planetary debris accreted by white
dwarfs
Authors: Cunningham, Tim; Tremblay, Pier-Emmanuel; Bauer, Evan B.;
Toloza, Odette; Cukanovaite, Elena; Koester, Detlev; Farihi, Jay;
Freytag, Bernd; Gänsicke, Boris T.; Ludwig, Hans-Günter; Veras,
Dimitri
2021MNRAS.503.1646C Altcode: 2021arXiv210209564C; 2021MNRAS.tmp..586C
White dwarfs with metal-polluted atmospheres have been studied widely
in the context of the accretion of rocky debris from evolved planetary
systems. One open question is the geometry of accretion and how
material arrives and mixes in the white dwarf surface layers. Using the
three-dimensional (3D) radiation hydrodynamics code CO5BOLD,
we present the first transport coefficients in degenerate star
atmospheres that describe the advection-diffusion of a passive scalar
across the surface plane. We couple newly derived horizontal diffusion
coefficients with previously published vertical diffusion coefficients
to provide theoretical constraints on surface spreading of metals in
white dwarfs. Our grid of 3D simulations probes the vast majority of
the parameter space of convective white dwarfs, with pure-hydrogen
atmospheres in the effective temperature range of 6000-18 000 K and
pure-helium atmospheres in the range of 12 000-34 000 K. Our results
suggest that warm hydrogen-rich atmospheres (DA; ${\gtrsim} 13\, 000$
K) and helium-rich atmospheres (DB and DBA; ${\gtrsim} 30\, 000$ K) are
unable to efficiently spread the accreted metals across their surface,
regardless of the time dependence of accretion. This result may be at
odds with the current non-detection of surface abundance variations in
white dwarfs with debris discs. For cooler hydrogen- and helium-rich
atmospheres, we predict a largely homogeneous distribution of metals
across the surface within a vertical diffusion time-scale. This is
typically less than 0.1 per cent of disc lifetime estimates, a quantity
that is revisited in this paper using the overshoot results. These
results have relevance for studies of the bulk composition of evolved
planetary systems and models of accretion disc physics.

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

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Title: 3D spectroscopic analysis of helium-line white dwarfs
Authors: Cukanovaite, Elena; Tremblay, Pier-Emmanuel; Bergeron,
Pierre; Freytag, Bernd; Ludwig, Hans-Günter; Steffen, Matthias
2021MNRAS.501.5274C Altcode: 2020arXiv201112693C; 2020MNRAS.tmp.3465C
In this paper, we present corrections to the spectroscopic parameters
of DB and DBA white dwarfs with -10.0 ≤ log (H/He) ≤ -2.0, 7.5
≤ log g ≤ 9.0, and $12\, 000$ ≲ Teff $\lesssim 34\,
000\,\mathrm{ K}$ , based on 282 3D atmospheric models calculated with
the CO5BOLD radiation-hydrodynamics code. These corrections
arise due to a better physical treatment of convective energy transport
in 3D models when compared to the previously available 1D model
atmospheres. By applying the corrections to an existing Sloan Digital
Sky Survey (SDSS) sample of DB and DBA white dwarfs, we find significant
corrections both for effective temperature and surface gravity. The
3D log g corrections are most significant for Teff ≲
18 000 K, reaching up to -0.20 dex at log g = 8.0. However, in this
low effective temperature range, the surface gravity determined from
the spectroscopic technique can also be significantly affected by
the treatment of the neutral van der Waals line broadening of helium
and by non-ideal effects due to the perturbation of helium by neutral
atoms. Thus, by removing uncertainties due to 1D convection, our work
showcases the need for improved description of microphysics for DB
and DBA model atmospheres. Overall, we find that our 3D spectroscopic
parameters for the SDSS sample are generally in agreement with Gaia
Data Release 2 absolute fluxes within 1σ-3σ for individual white
dwarfs. By comparing our results to DA white dwarfs, we determine
that the precision and accuracy of DB/DBA atmospheric models are
similar. For ease of user application of the correction functions,
we provide an example PYTHON code.

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

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Title: Tomography of cool giant and supergiant star
atmospheres. III. Validation of the method on VLTI/AMBER observations
of the Mira star S Ori
Authors: Kravchenko, Kateryna; Wittkowski, Markus; Jorissen, Alain;
Chiavassa, Andrea; Van Eck, Sophie; Anderson, Richard I.; Freytag,
Bernd; Käufl, Ulli
2020A&A...642A.235K Altcode: 2020arXiv200903955K
Context. Asymptotic giant branch (AGB) stars are characterized by
substantial mass loss, however the mechanism behind it not yet fully
understood. The knowledge of the structure and dynamics of AGB-star
atmospheres is crucial to better understanding the mass loss. The
recently established tomographic method, which relies on the design
of spectral masks containing lines that form in given ranges of
optical depths in the stellar atmosphere, is an ideal technique for
this purpose. Aims: We aim to validate the capability of the
tomographic method in probing different geometrical depths in the
stellar atmosphere and recovering the relation between optical and
geometrical depth scales. Methods: We applied the tomographic
method to high-resolution spectro-interferometric VLTI/AMBER
observations of the Mira-type AGB star S Ori. The interferometric
visibilities were extracted at wavelengths contributing to the
tomographic masks and fitted to those computed from a uniform disk
model. This allows us to measure the geometrical extent of the
atmospheric layer probed by the corresponding mask. We then compared
the observed atmospheric extension with others measured from available
1D pulsation CODEX models and 3D radiative-hydrodynamics CO5BOLD
simulations. Results: While the average optical depths probed by
the tomographic masks in S Ori decrease (with ⟨log τ0⟩
= -0.45, - 1.45, and - 2.45 from the innermost to the central and
outermost layers), the angular diameters of these layers increase,
from 10.59 ± 0.09 mas through 11.84 ± 0.17 mas, up to 14.08 ± 0.15
mas. A similar behavior is observed when the tomographic method is
applied to 1D and 3D dynamical models. Conclusions: This study
derives, for the first time, a quantitative relation between optical and
geometrical depth scales when applied to the Mira star S Ori, or to 1D
and 3D dynamical models. In the context of Mira-type stars, knowledge
of the link between the optical and geometrical depths opens the way to
deriving the shock-wave propagation velocity, which cannot be directly
observed in these stars. Based on ESO observing program 084.D-0595.

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

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

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Title: VLTI-PIONIER imaging of the red supergiant V602 Carinae
Authors: Climent, J. B.; Wittkowski, M.; Chiavassa, A.; Baron, F.;
Marcaide, J. M.; Guirado, J. C.; Freytag, B.; Höfner, S.; Haubois,
X.; Woillez, J.
2020A&A...635A.160C Altcode: 2020arXiv200209243C
Context. Red supergiant stars possess surface features and extended
molecular atmospheres. Photospheric convection may be a crucial
factor of the levitation of the outer atmospheric layers. However,
the mechanism responsible is still poorly understood. Aims:
We image the stellar surface of V602 Carinae (V602 Car) to constrain
the morphology and contrast of the surface features and of the
extended atmospheric layers. Methods: We observed V602 Car
with the Very Large Telescope Interferometer PIONIER instrument
(1.53-1.78 μm) between May and July 2016, and April and July
2019 with different telescope configurations. We compared the image
reconstructions with 81 temporal snapshots of 3D radiative-hydrodynamics
(RHD) CO5BOLD simulations in terms of contrast and
morphology, using the Structural Similarity Index. Results:
The interferometric data are compatible with an overall spherical
disk of angular diameter 4.4 ± 0.2 mas, and an extended molecular
layer. In 2016, the reconstructed image reveals a bright arc-like
feature toward the northern rim of the photospheric surface. In 2019,
an arc-like feature is seen at a different orientation and a new
peak of emission is detected on the opposite side. The contrasts of
the reconstructed surface images are 11% ± 2% and 9% ± 2% for 2016
and 2019, respectively. The morphology and contrast of the two images
are consistent with 3D RHD simulations, within our achieved spatial
resolution and dynamic range. The extended molecular layer contributes
10-13% of the total flux with an angular diameter of 6-8 mas. It is
present but not clearly visible in the reconstructed images because it
is close to the limits of the achieved dynamic range. The presence of
the molecular layer is not reproduced by the 3D RHD simulations. Conclusions: 3D RHD simulations predict substructures similar to
the observed surface features of V602 Car at two different epochs. We
interpret the structure on the stellar surface as being related to
instationary convection. This structure is further convolved to larger
observed patches on the stellar surface with our observational spatial
resolution. Even though the simulations reproduce the observed features
on the stellar surface, convection alone may not be the only relevant
process that is levitating the atmosphere. Based on observations
made with the Very Large Telescope Interferometer at the Paranal
Observatory under program IDs 097.D-0286, 60.A-9138 (NAOMI science
verification) and 2103.D-5029.

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

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

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

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

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

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

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

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

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

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

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Title: Convective overshoot and macroscopic diffusion in
pure-hydrogen-atmosphere white dwarfs
Authors: Cunningham, Tim; Tremblay, Pier-Emmanuel; Freytag, Bernd;
Ludwig, Hans-Günter; Koester, Detlev
2019MNRAS.488.2503C Altcode: 2019MNRAS.tmp.1723C; 2019arXiv190611252C
We present a theoretical description of macroscopic diffusion
caused by convective overshoot in pure-hydrogen DA white dwarfs
using 3D, closed-bottom, radiation hydrodynamics CO5BOLD
simulations. We rely on a new grid of deep 3D white dwarf models
in the temperature range 11 400 ≤ T_{eff} ≤ 18 000 K where
tracer particles and a tracer density are used to derive macroscopic
diffusion coefficients driven by convective overshoot. These diffusion
coefficients are compared to microscopic diffusion coefficients from 1D
structures. We find that the mass of the fully mixed region is likely to
increase by up to 2.5 orders of magnitude while inferred accretion rates
increase by a more moderate order of magnitude. We present evidence
that an increase in settling time of up to 2 orders of magnitude is to
be expected, which is of significance for time-variability studies of
polluted white dwarfs. Our grid also provides the most robust constraint
on the onset of convective instabilities in DA white dwarfs to be in
the effective temperature range from 18 000 to 18 250 K.

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

---------------------------------------------------------
Title: Exploring the origin of clumpy dust clouds around cool
giants. A global 3D RHD model of a dust-forming M-type AGB star
Authors: Höfner, Susanne; Freytag, Bernd
2019A&A...623A.158H Altcode: 2019arXiv190204074H
Context. Dust grains forming in the extended atmospheres of AGB
stars are critical for the heavy mass loss of these cool luminous
giants, as they provide radiative acceleration for the stellar
winds. Characteristic mid-IR spectral features indicate that the
grains consist mainly of silicates and corundum. The latter species
seems to form in a narrow zone within about 2 stellar radii,
preceding the condensation of silicate dust, which triggers the
outflow. Recent high-angular-resolution observations show clumpy,
variable dust clouds at these distances. Aims: We explore
possible causes for the formation of inhomogeneous dust layers,
using 3D dynamical simulations. Methods: We modeled the
outer convective envelope and the dust-forming atmosphere of an
M-type AGB star with the CO5BOLD radiation-hydrodynamics code. The
simulations account for frequency-dependent gas opacities, and include
a time-dependent description of grain growth and evaporation for
corundum (Al2O3) and olivine-type silicates
(Mg2SiO4). Results: In the inner,
gravitationally bound, and corundum-dominated layers of the
circumstellar envelope, a patchy distribution of the dust emerges
naturally, due to atmospheric shock waves that are generated by
large-scale convective flows and pulsations. The formation of silicate
dust at somewhat larger distances probably indicates the outer limit of
the gravitationally bound layers. The current models do not describe
wind acceleration, but the cloud formation mechanism should also work
for stars with outflows. Timescales of atmospheric dynamics and grain
growth are similar to observed values. In spherical averages of dust
densities, more easily comparable to unresolved observations and 1D
models, the variable 3D morphology manifests itself as cycle-to-cycle
variations. Conclusions: Grain growth in the wake of large-scale
non-spherical shock waves, generated by convection and pulsations,
is a likely mechanism for producing the observed clumpy dust clouds,
and for explaining their physical and dynamical properties.

---------------------------------------------------------
Title: Pure-helium 3D model atmospheres of white dwarfs
Authors: Cukanovaite, E.; Tremblay, P. -E.; Freytag, B.; Ludwig,
H. -G.; Bergeron, P.
2018MNRAS.481.1522C Altcode: 2018arXiv180900590C; 2018MNRAS.tmp.2259C
We present the first grid of 3D simulations for the pure-helium
atmospheres of DB white dwarfs. The simulations were computed with the
co5bold radiation-hydrodynamics code and cover effective
temperatures and surface gravities between 12 000 K ≲ Teff
≲ 34 000 K and 7.5 ≤ log g (cgs units) ≤ 9.0, respectively. In
this introductory work, synthetic spectra calculated from the 3D
simulations are compared to appropriate 1D model spectra under a
differential approach. This results in the derivation of 3D corrections
for the spectroscopically derived atmospheric parameters of DB stars
with respect to the 1D ML2/α = 1.25 mixing-length parametrization. No
significant Teff corrections are found for the V777 Her
instability strip region, and therefore no 3D revision is expected
for the empirical blue and red edges of the strip. However, large log
g corrections are found in the range 12 000 K &lt; Teff
&lt; 23 000 K for all log g values covered by the 3D grid. These
corrections indicate that 1D model atmospheres overpredict log g,
reminiscent of the results found from 3D simulations of pure-hydrogen
white dwarfs. The next step will be to compute 3D simulations with mixed
helium and hydrogen atmospheres to comprehend the full implications
for the stellar parameters of DB and DBA white dwarfs.

---------------------------------------------------------
Title: Atmospheres and wind properties of non-spherical AGB stars
Authors: Liljegren, S.; Höfner, S.; Freytag, B.; Bladh, S.
2018A&A...619A..47L Altcode: 2018arXiv180805043L
Context. The wind-driving mechanism of asymptotic giant branch
(AGB) stars is commonly attributed to a two-step process: first,
gas in the stellar atmosphere is levitated by shockwaves caused by
stellar pulsation, then accelerated outwards by radiative pressure on
newly formed dust, inducing a wind. Dynamical modelling of such winds
usually assumes a spherically symmetric star. Aims: We explore
the potential consequences of complex stellar surface structures, as
predicted by three-dimensional (3D) star-in-a-box modelling of M-type
AGB stars, on the resulting wind properties with the aim to improve the
current wind models. Methods: Two different modelling approaches
are used; the CO5BOLD 3D star-in-a-box code to simulate
the convective, pulsating interior and lower atmosphere of the star,
and the DARWIN one-dimensional (1D) code to describe the dynamical
atmosphere where the wind is accelerated. The gas dynamics of the
inner atmosphere region at distances of R ∼ 1-2 R⋆,
which both modelling approaches simulate, are compared. Dynamical
properties and luminosity variations derived from CO5BOLD
interior models are used as input for the inner boundary in DARWIN wind
models in order to emulate the effects of giant convection cells and
pulsation, and explore their influence on the dynamical properties. Results: The CO5BOLD models are inherently anisotropic,
with non-uniform shock fronts and varying luminosity amplitudes, in
contrast to the spherically symmetrical DARWIN wind models. DARWIN
wind models with CO5BOLD-derived inner boundary conditions
produced wind velocities and mass-loss rates comparable to the standard
DARWIN models, however the winds show large density variations on
time-scales of 10-20 yr. Conclusions: The method outlined in
this paper derives pulsation properties from the 3D star-in-a-box
CO5BOLD models, to be used in the DARWIN models. If the
current grid of CO5BOLD models is extended, it will be
possible to construct extensive DARWIN grids with inner boundary
conditions derived from 3D interior modelling of convection and
pulsation, and avoid the free parameters of the current approach.

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

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Title: Constraining Convection in Evolved Stars with the VLTI
Authors: Paladini, C.; Baron, F.; Jorissen, A.; Le Bouquin, J. -B.;
Freytag, B.; Van Eck, S.; Wittkowski, M.; Hron, J.; Chiavassa, A.;
Berger, J. -P.; Siopis, C.; Mayer, A.; Sadowski, G.; Kravchenko, K.;
Shetye, S.; Kerschbaum, F.; Kluska, J.; Ramstedt, S.
2018Msngr.172...24P Altcode:
We used the Precision Integrated-Optics Near-infrared Imaging ExpeRiment
(PIONIER) at the Very Large Telescope Interferometer (VLTI) to image
the stellar surface of the S-type Asymptotic Giant Branch (AGB) star
π1 Gruis. The angular resolution of two milliarcseconds
allowed us to observe the surface of this giant star in unprecedented
detail. At the observed wavelength the stellar disc appears circular
and dust-free. Moreover, the disc is characterised by a few bubbles of
a convective nature. We determine the contrast, and the characteristic
horizontal length-scale of the convective granules. The latter is
determined, for the first time, directly from the image, without
involving the usual geometric modelling that has been used in the
literature. The measurements fall along empirical scaling relations
between stellar parameters and convective sizes, which are determined on
the basis of three-dimensional stellar convection models. Our results
open up a new era for the characterisation of stellar convection in
stars other than the Sun.

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Title: Simulation of the small-scale magnetism in main-sequence
stellar atmospheres
Authors: Salhab, R. G.; Steiner, O.; Berdyugina, S. V.; Freytag, B.;
Rajaguru, S. P.; Steffen, M.
2018A&A...614A..78S Altcode:
Context. Observations of the Sun tell us that its granular and
subgranular small-scale magnetism has significant consequences for
global quantities such as the total solar irradiance or convective
blueshift of spectral lines. Aims: In this paper, properties
of the small-scale magnetism of four cool stellar atmospheres,
including the Sun, are investigated, and in particular its effects
on the radiative intensity and flux. Methods: We carried out
three-dimensional radiation magnetohydrodynamic simulations with the
CO5BOLD code in two different settings: with and without
a magnetic field. These are thought to represent states of high and
low small-scale magnetic activity of a stellar magnetic cycle. Results: We find that the presence of small-scale magnetism
increases the bolometric intensity and flux in all investigated
models. The surplus in radiative flux of the magnetic over the magnetic
field-free atmosphere increases with increasing effective temperature,
Teff, from 0.47% for spectral type K8V to 1.05% for the solar
model, but decreases for higher effective temperatures than solar. The
degree of evacuation of the magnetic flux concentrations monotonically
increases with Teff as does their depression of the visible
optical surface, that is the Wilson depression. Nevertheless, the
strength of the field concentrations on this surface stays remarkably
unchanged at ≈1560 G throughout the considered range of spectral
types. With respect to the surrounding gas pressure, the field strength
is close to (thermal) equipartition for the Sun and spectral type F5V
but is clearly sub-equipartition for K2V and more so for K8V. The
magnetic flux concentrations appear most conspicuous for model K2V
owing to their high brightness contrast. Conclusions: For mean
magnetic flux densities of approximately 50 G, we expect the small-scale
magnetism of stars in the spectral range from F5V to K8V to produce a
positive contribution to their bolometric luminosity. The modulation
seems to be most effective for early G-type stars.

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

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

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

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

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

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Title: Large granulation cells on the surface of the giant star
π1 Gruis
Authors: Paladini, C.; Baron, F.; Jorissen, A.; Le Bouquin, J. -B.;
Freytag, B.; van Eck, S.; Wittkowski, M.; Hron, J.; Chiavassa, A.;
Berger, J. -P.; Siopis, C.; Mayer, A.; Sadowski, G.; Kravchenko, K.;
Shetye, S.; Kerschbaum, F.; Kluska, J.; Ramstedt, S.
2018Natur.553..310P Altcode:
Convection plays a major part in many astrophysical processes,
including energy transport, pulsation, dynamos and winds on evolved
stars, in dust clouds and on brown dwarfs. Most of our knowledge
about stellar convection has come from studying the Sun: about two
million convective cells with typical sizes of around 2,000 kilometres
across are present on the surface of the Sun—a phenomenon known
as granulation. But on the surfaces of giant and supergiant stars
there should be only a few large (several tens of thousands of times
larger than those on the Sun) convective cells, owing to low surface
gravity. Deriving the characteristic properties of convection (such as
granule size and contrast) for the most evolved giant and supergiant
stars is challenging because their photospheres are obscured by dust,
which partially masks the convective patterns. These properties can be
inferred from geometric model fitting, but this indirect method does
not provide information about the physical origin of the convective
cells. Here we report interferometric images of the surface of the
evolved giant star π1 Gruis, of spectral type S5,7. Our
images show a nearly circular, dust-free atmosphere, which is very
compact and only weakly affected by molecular opacity. We find that
the stellar surface has a complex convective pattern with an average
intensity contrast of 12 per cent, which increases towards shorter
wavelengths. We derive a characteristic horizontal granule size of about
1.2 × 1011 metres, which corresponds to 27 per cent
of the diameter of the star. Our measurements fall along the scaling
relations between granule size, effective temperature and surface
gravity that are predicted by simulations of stellar surface convection.

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

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

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

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

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

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

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

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

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

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

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

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

---------------------------------------------------------
Title: 3D Hydrodynamical Simulations of Evolved Stars and Observations
of Stellar Surfaces
Authors: Chiavassa, A.; Freytag, B.
2015ASPC..497...11C Altcode: 2014arXiv1410.3868C
Evolved stars are among the largest and brightest stars and they are
ideal targets for the new generation of sensitive, high resolution
instrumentation that provides spectrophotometric, interferometric,
astrometric, and imaging observables. The interpretation of the complex
stellar surface images requires numerical simulations of stellar
convection that take into account multi-dimensional time-dependent
radiation hydrodynamics with realistic input physics. We show how the
evolved star simulations are obtained using the radiative hydrodynamics
code CO5BOLD and how the accurate observables are computed
with the post-processing radiative transfer code OPTIM3D. The synergy
between observations and theoretical work is supported by a proper
and quantitative analysis using these simulations, and by strong
constraints from the observational side.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

---------------------------------------------------------
Title: Spectroscopic analysis of DA white dwarfs with 3D model
atmospheres
Authors: Tremblay, P. -E.; Ludwig, H. -G.; Steffen, M.; Freytag, B.
2013A&A...559A.104T Altcode: 2013arXiv1309.0886T
We present the first grid of mean three-dimensional (3D) spectra for
pure-hydrogen (DA) white dwarfs based on 3D model atmospheres. We use
CO5BOLD radiation-hydrodynamics 3D simulations instead of the
mixing-length theory for the treatment of convection. The simulations
cover the effective temperature range of 6000 &lt; Teff
(K) &lt; 15 000 and the surface gravity range of 7 &lt; log g &lt;
9 where the large majority of DAs with a convective atmosphere
are located. We rely on horizontally averaged 3D structures (over
constant Rosseland optical depth) to compute ⟨3D⟩ spectra. It
is demonstrated that our ⟨3D⟩ spectra can be smoothly connected
to their 1D counterparts at higher and lower Teff where
the 3D effects are small. Analytical functions are provided in order
to convert spectroscopically determined 1D effective temperatures
and surface gravities to 3D atmospheric parameters. We apply our
improved models to well studied spectroscopic data sets from the
Sloan Digital Sky Survey and the White Dwarf Catalog. We confirm
that the so-called high-log g problem is not present when employing
⟨3D⟩ spectra and that the issue was caused by inaccuracies in
the 1D mixing-length approach. The white dwarfs with a radiative
and a convective atmosphere have derived mean masses that are
the same within ~0.01 M⊙, in much better agreement
with our understanding of stellar evolution. Furthermore, the 3D
atmospheric parameters are in better agreement with independent
Teff and log g values from photometric and parallax
measurements. Appendices are only available in electronic form
at <A href="http://www.aanda.org">http://www.aanda.org</A>

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

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

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

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

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

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

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

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

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

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

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

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

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Title: Oxygen spectral line synthesis: 3D non-LTE with
CO5BOLD hydrodynamical model atmospheres.
Authors: Prakapavičius, D.; Steffen, M.; Kučinskas, A.; Ludwig,
H. -G.; Freytag, B.; Caffau, E.; Cayrel, R.
2013MSAIS..24..111P Altcode: 2013arXiv1303.2016P
In this work we present first results of our current project aimed at
combining the 3D hydrodynamical stellar atmosphere approach with non-LTE
(NLTE) spectral line synthesis for a number of key chemical species. We
carried out a full 3D-NLTE spectrum synthesis of the oxygen IR 777 nm
triplet, using a modified and improved version of our NLTE3D package to
calculate departure coefficients for the atomic levels of oxygen in a
CO5BOLD 3D hydrodynamical solar model atmosphere. Spectral
line synthesis was subsequently performed with the Linfor3D code. In
agreement with previous studies, we find that the lines of the
oxygen triplet produce deeper cores under NLTE conditions, due to
the diminished line source function in the line forming region. This
means that the solar oxygen IR 777 nm lines should be stronger in NLTE,
leading to negative 3D NLTE-LTE abundance corrections. Qualitatively
this result would support previous claims for a relatively low solar
oxygen abundance. Finally, we outline several further steps that need
to be taken in order to improve the physical realism and numerical
accuracy of our current 3D-NLTE calculations.

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Title: 3D Model Atmospheres of DA White Dwarfs
Authors: Tremblay, P. -E.; Ludwig, H. -G.; Steffen, M.; Freytag, B.
2013ASPC..469..155T Altcode:
The spectroscopically determined surface gravities of cool
hydrogen-atmosphere DA white dwarfs are significantly higher than
the mean value of log g ∼ 8 found for hotter objects with radiative
atmospheres (Teff &gt; 13000 K). It was recently suggested
that a problem with the treatment of convective energy transport in
the 1D mixing-length theory was the explanation for this high-log
g problem. We have now computed a grid of pure-hydrogen 3D model
atmospheres with the CO5BOLD code for surface gravities
from log g = 7 to 8.5 and effective temperatures from 6000 to 13000
K. Over this range, the intensity contrast of the surface granulation
pattern, which describes the strength of the 3D effects, is varying
significantly. We confirm the result of an earlier investigation that
3D model spectra provide a much better characterization of the mass
distribution of cool white dwarfs.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

---------------------------------------------------------
Title: Galactic evolution of oxygen. OH lines in 3D hydrodynamical
model atmospheres
Authors: González Hernández, J. I.; Bonifacio, P.; Ludwig, H. -G.;
Caffau, E.; Behara, N. T.; Freytag, B.
2010A&A...519A..46G Altcode: 2010arXiv1005.3754G
Context. Oxygen is the third most common element in the Universe. The
measurement of oxygen lines in metal-poor unevolved stars, in
particular near-UV OH lines, can provide invaluable information
about the properties of the Early Galaxy. Aims: Near-UV OH
lines constitute an important tool to derive oxygen abundances in
metal-poor dwarf stars. Therefore, it is important to correctly model
the line formation of OH lines, especially in metal-poor stars, where
3D hydrodynamical models commonly predict cooler temperatures than
plane-parallel hydrostatic models in the upper photosphere. 
Methods: We have made use of a grid of 52 3D hydrodynamical model
atmospheres for dwarf stars computed with the code CO5BOLD,
extracted from the more extended CIFIST grid. The 52 models cover
the effective temperature range 5000-6500 K, the surface gravity
range 3.5-4.5 and the metallicity range -3 &lt; [Fe/H] &lt; 0. 
Results: We determine 3D-LTE abundance corrections in all 52 3D models
for several OH lines and ion{Fe}{i} lines of different excitation
potentials. These 3D-LTE corrections are generally negative and reach
values of roughly -1 dex (for the OH 3167 with excitation potential
of approximately 1 eV) for the higher temperatures and surface
gravities. Conclusions: We apply these 3D-LTE corrections
to the individual O abundances derived from OH lines of a sample
the metal-poor dwarf stars reported in Israelian et al. (1998, ApJ,
507, 805), Israelian et al. (2001, ApJ, 551, 833) and Boesgaard et
al. (1999, AJ, 117, 492) by interpolating the stellar parameters of the
dwarfs in the grid of 3D-LTE corrections. The new 3D-LTE [O/Fe] ratio
still keeps a similar trend as the 1D-LTE, i.e., increasing towards
lower [Fe/H] values. We applied 1D-NLTE corrections to 3D ion{Fe}{i}
abundances and still see an increasing [O/Fe] ratio towards lower
metallicites. However, the Galactic [O/Fe] ratio must be revisited
once 3D-NLTE corrections become available for OH and Fe lines for a
grid of 3D hydrodynamical model atmospheres.

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

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Title: Radiative hydrodynamics simulations of red supergiant
stars. II. Simulations of convection on Betelgeuse match
interferometric observations
Authors: Chiavassa, A.; Haubois, X.; Young, J. S.; Plez, B.; Josselin,
E.; Perrin, G.; Freytag, B.
2010A&A...515A..12C Altcode: 2010arXiv1003.1407C
Context. The red supergiant (RSG) Betelgeuse is an irregular variable
star. Convection may play an important role in understanding this
variability. Interferometric observations can be interpreted using
sophisticated simulations of stellar convection. Aims: We compare
the visibility curves and closure phases obtained from our 3D simulation
of RSG convection with CO5BOLD to various interferometric observations
of Betelgeuse from the optical to the H band to characterize and
measure the convection pattern on this star. Methods: We use a
3D radiative-hydrodynamics (RHD) simulation to compute intensity maps
in different filters and thus derive interferometric observables using
the post-processing radiative transfer code OPTIM3D. The synthetic
visibility curves and closure phases are compared to observations. Results: We provide a robust detection of the granulation pattern on
the surface of Betelgeuse in both the optical and the H band based on
excellent fits to the observed visibility points and closure phases. We
determine that the Betelgeuse surface in the H band is covered by
small to medium scale (5-15 mas) convection-related surface structures
and a large (≈30 mas) convective cell. In this spectral region,
H2O molecules are the main absorbers and contribute to
both the small structures and the position of the first null of the
visibility curve (i.e., the apparent stellar radius).

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

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

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

---------------------------------------------------------
Title: Solar abundances and 3D model atmospheres
Authors: Ludwig, Hans-Günter; Caffau, Elisabetta; Steffen, Matthias;
Bonifacio, Piercarlo; Freytag, Bernd; Cayrel, Roger
2010IAUS..265..201L Altcode: 2009arXiv0911.4248L
We present solar photospheric abundances for 12 elements from optical
and near-infrared spectroscopy. The abundance analysis was conducted
employing 3D hydrodynamical (CO5BOLD) as well as standard
1D hydrostatic model atmospheres. We compare our results to others
with emphasis on discrepancies and still lingering problems, in
particular exemplified by the pivotal abundance of oxygen. We argue
that the thermal structure of the lower solar photosphere is very
well represented by our 3D model. We obtain an excellent match of
the observed center-to-limb variation of the line-blanketed continuum
intensity, also at wavelengths shortward of the Balmer jump.

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

---------------------------------------------------------
Title: VLTI/AMBER spectro-interferometric imaging of VX Sagittarii's
inhomogenous outer atmosphere
Authors: Chiavassa, A.; Lacour, S.; Millour, F.; Driebe, T.;
Wittkowski, M.; Plez, B.; Thiébaut, E.; Josselin, E.; Freytag, B.;
Scholz, M.; Haubois, X.
2010A&A...511A..51C Altcode: 2009arXiv0911.4422C
 Aims: We aim to explore the photosphere of the very cool
late-type star VX Sgr and in particular the characterization of
molecular layers above the continuum forming photosphere. 
Methods: We obtained interferometric observations with the VLTI/AMBER
interferometer using the fringe tracker FINITO in the spectral domain
1.45-2.50 μm with a spectral resolution of ≈35 and baselines ranging
from 15 to 88 m. We performed independent image reconstruction for
different wavelength bins and fit the interferometric data with a
geometrical toy model. We also compared the data to 1D dynamical
models of Miras atmosphere and to 3D hydrodynamical simulations of
red supergiant (RSG) and asymptotic giant branch (AGB) stars. 
Results: Reconstructed images and visibilities show a strong wavelength
dependence. The H-band images display two bright spots whose positions
are confirmed by the geometrical toy model. The inhomogeneities are
qualitatively predicted by 3D simulations. At ≈2.00 μm and in the
region 2.35-2.50 μm, the photosphere appears extended and the radius
is larger than in the H band. In this spectral region, the geometrical
toy model locates a third bright spot outside the photosphere that
can be a feature of the molecular layers. The wavelength dependence of
the visibility can be qualitatively explained by 1D dynamical models
of Mira atmospheres. The best-fitting photospheric models show a good
match with the observed visibilities and give a photospheric diameter
of Theta=8.82 ± 0.50 mas. The H2O molecule seems to be the
dominant absorber in the molecular layers. Conclusions: We show
that the atmosphere of VX Sgr seems to resemble Mira/AGB star model
atmospheres more closely than do RSG model atmospheres. In particular,
we see molecular (water) layers that are typical of Mira stars. Based on the observations made with VLTI-ESO Paranal, Chile under
the programs IDs 081.D-0005(A, B, C, D, E, F, G, H).

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

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

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

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

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

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

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

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

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

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

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

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

---------------------------------------------------------
Title: Rotating `star-in-a-box' experiments
Authors: Steffen, M.; Freytag, B.
2007AN....328.1054S Altcode:
Using the radiation hydrodynamics code CO5BOLD in its
`star-in-a-box' setup, we have performed exploratory simulations
of global convection in a rotating reference frame. The goal is to
study the interaction of convection and rotation by direct numerical
simulation. For these first experiments, we chose an idealized
configuration (a scaled-down, fast rotating Sun) whose properties
resemble those of red supergiants in some respect. We describe the setup
and time evolution of these models, and discuss the particular problems
we have encountered. Finally, we derive the resulting differential
rotation pattern and meridional flow field by temporal and azimuthal
averaging of the simulation data. We find anti-solar differential
rotation for all cases studied so far. Movies are available via
http://www.aip.de/AN/movies

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

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

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

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

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

---------------------------------------------------------
Title: First local helioseismic experiments with CO5BOLD
Authors: Steiner, O.; Vigeesh, G.; Krieger, L.; Wedemeyer-Böhm, S.;
Schaffenberger, W.; Freytag, B.
2007AN....328..323S Altcode: 2007astro.ph..1029S
With numerical experiments we explore the feasibility of using high
frequency waves for probing the magnetic fields in the photosphere and
the chromosphere of the Sun. We track a plane-parallel, monochromatic
wave that propagates through a non-stationary, realistic atmosphere,
from the convection-zone through the photosphere into the magnetically
dominated chromosphere, where it gets refracted and reflected. We
compare the wave travel time between two fixed geometrical height levels
in the atmosphere (representing the formation height of two spectral
lines) with the topography of the surface of equal magnetic and thermal
energy density (the magnetic canopy or β=1 contour) and find good
correspondence between the two. We conclude that high frequency waves
indeed bear information on the topography of the `magnetic canopy'.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

---------------------------------------------------------
Title: Numerical simulation of the three-dimensional structure and
dynamics of the non-magnetic solar chromosphere
Authors: Wedemeyer, S.; Freytag, B.; Steffen, M.; Ludwig, H. -G.;
Holweger, H.
2004A&A...414.1121W Altcode: 2003astro.ph.11273W
Three-dimensional numerical simulations with CO5, a
new radiation hydrodynamics code, result in a dynamic, thermally
bifurcated model of the non-magnetic chromosphere of the quiet Sun. The
3D model includes the middle and low chromosphere, the photosphere,
and the top of the convection zone, where acoustic waves are excited
by convective motions. While the waves propagate upwards, they steepen
into shocks, dissipate, and deposit their mechanienergy as heat in
the chromosphere. Our numerical simulations show for the first time
a complex 3D structure of the chromospheric layers, formed by the
interaction of shock waves. Horizontal temperature cross-sections of
the model chromosphere exhibit a network of hot filaments and enclosed
cool regions. The horizontal pattern evolves on short time-scales of
the order of typically 20-25 s, and has spatial scales comparable to
those of the underlying granulation. The resulting thermal bifurcation,
i.e., the co-existence of cold and hot regions, provides temperatures
high enough to produce the observed chromospheric UV emission and -
at the same time - temperatures cold enough to allow the formation
of molecules (e.g., carbon monoxide). Our 3D model corroborates the
finding by \citet{carlsson94} that the chromospheric temperature rise
of semi-empirical models does not necessarily imply an increase in
the average gas temperature but can be explained by the presence of
substantial spatial and temporal temperature inhomogeneities.

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

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

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

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

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

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

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

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

---------------------------------------------------------
Title: 3-D hydrodynamic simulations of the solar chromosphere
Authors: Wedemeyer, S.; Freytag, B.; Steffen, M.; Ludwig, H. -G.;
Holweger, H.
2003AN....324..410W Altcode:
We present first results of three-dimensional numerical simulations
of the non-magnetic solar chromosphere, computed with the radiation
hydrodynamics code CO5BOLD. Acoustic waves which are
excited at the top of the convection zone propagate upwards into the
chromosphere where the waves steepen into shocks. The interaction of
the waves leads to the formation of complex structures which evolve
on short time scales. Consequently, the model chromosphere is highly
dynamical, inhomogeneous, and thermally bifurcated.

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

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

---------------------------------------------------------
Title: Spots on the surface of Betelgeuse -- Results from new 3D
stellar convection models
Authors: Freytag, B.; Steffen, M.; Dorch, B.
2002AN....323..213F Altcode:
The observed irregular brightness fluctuations of the well-known red
supergiant Betelgeuse (alpha Ori, M2 Iab) have been attributed by
M. Schwarzschild (1975) to the changing granulation pattern formed
by only a few giant convection cells covering the surface of this
giant star. The surface structure revealed by modern interferometric
methods appears to be generally consistent with the explanation as
large-scale granular intensity fluctuations. The interferometric
data can be modeled equally well by assuming the presence of a few
(up to 3) unresolved hot or cool spots on a limb-darkened disk. In an
effort to improve our theoretical understanding of the Betelgeuse
phenomena, we have applied a new radiation hydrodynamics code
(CO5BOLD) to the problem of global convection in giant
stars. For this purpose, the "local box" setup usually employed for the
simulation of solar-type surface convection cannot be used. Rather, we
have chosen a radically different approach: the whole star is enclosed
in a cube ("star-in-a-box" setup). The properties of the stellar model
are defined by the prescribed gravitational central potential and by
a special inner boundary condition which replaces the unresolved core,
including the source of nuclear energy production. We present current
results obtained from this novel generation of 3D stellar convection
simulations, proceeding from a toy model ("Mini-Sun") towards the
numerically more demanding supergiant regime. We discuss the basic
observational properties of Betelgeuse in the light of our best model
obtained so far (T_eff = 3300 K, log g = -0.4). Finally, we describe a
first attempt to investigate the interaction of the global convective
flows with magnetic fields based on the kinematic approximation.

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

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

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

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

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

---------------------------------------------------------
Title: Radiation Hydrodynamics Simulations of the Solar Chromosphere
Authors: Wedemeyer, Sven; Freytag, Bernd; Steffen, Matthias; Holweger,
Hartmut
2000AGM....17..P01W Altcode:
While heating of the solar corona is commonly attributed to
reconnection of magnetic field lines, the mechanism responsible for
heating the chromosphere of the quiet Sun, away from active regions,
is still under debate1,2. The basic question which we will
address in this contribution i s: Can generation of acoustic waves by
turbulent convection in photospheric and subphotospheric layers explain
the chromospheric emission of the quiet Sun? With a new 3D radiation
hydrodynamics code3 we are able to compute models ex tending
from the upper convection zone to the middle chromosphere. The code
can handle shocks with a minimum of numerical dissipation. Therefore
generation and propagation of acoustic waves can be investigated,
permitting the evaluation of wave dissipation in the chromosphere in
a physically consistent manner. We present first results and discuss
the principal problems and future prospects.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

---------------------------------------------------------
Title: Lyapunov exponents for solar surface convection.
Authors: Steffen, M.; Freytag, B.
1995CSF.....5.1965S Altcode:
The authors have carried out detailed 2D numerical radiation
hydrodynamics calculations, specifically designed to model
time-dependent, compressible convection in the surface layers of the
Sun. These simulations, which take into account a realistic equation
of state and use an elaborate scheme to describe multi-dimensional,
non-local, frequency-dependent radiative transfer, allow a direct
comparison with observed photometric and spectroscopic properties of
solar granulation. Their purpose is to enhance the understanding of
the dynamics and thermal structure of convective stellar atmospheres,
and to investigate the generation of acoustic energy by turbulent
convection. The authors briefly present some of the main properties of
their solar convection models. In particular, the authors demonstrate
the chaotic behaviour of solar surface convection, estimating the
magnitude of the two largest Lyapunov exponents, λ1 and
λ2, by analysing the time evolution of three simulations
with slightly different initial conditions. The authors find that
both λ1 and λ2 are positive and of similar
magnitude as expected for a chaotic system of high dimension. The
corresponding characteristic time scale λ1-1
of approximately 320 s is comparable to the convective turnover time.

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

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

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

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

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

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