Author name code: freytag ADS astronomy entries on 2022-09-14 author:"Freytag, Bernd" ------------------------------------------------------------------------ Title: Probing red supergiant dynamics through photo-center displacements measured by Gaia Authors: Chiavassa, A.; Kudritzki, R.; Davies, B.; Freytag, B.; de Mink, S. E. Bibcode: 2022A&A...661L...1C Altcode: 2022arXiv220505156C Context. Red supergiant (RSGs) are cool massive stars in a late phase of their evolution when the stellar envelope becomes fully convective. They are the brightest stars in the universe at infrared light and can be detected in galaxies far beyond the Local Group, allowing for accurate determination of chemical composition of galaxies. The study of their physical properties is extremely important for various phenomena including the final fate of massive stars as type II supernovae and gravitational wave progenitors.
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 https://www.aanda.org Title: Explaining the winds of AGB stars: Recent progress Authors: Höfner, Susanne; Freytag, Bernd Bibcode: 2022arXiv220409728H Altcode: The winds observed around asymptotic giant branch (AGB) stars are generally attributed to radiation pressure on dust, which is formed in the extended dynamical atmospheres of these pulsating, strongly convective stars. Current radiation-hydrodynamical models can explain many of the observed features, and they are on the brink of delivering a predictive theory of mass loss. This review summarizes recent results and ongoing work on winds of AGB stars, discussing critical ingredients of the driving mechanism, and first results of global 3D RHD star-and-wind-in-a-box simulations. With such models it becomes possible to follow the flow of matter, in full 3D geometry, all the way from the turbulent, pulsating interior of an AGB star, through its atmosphere and dust formation zone into the region where the wind is accelerated by radiation pressure on dust. Advanced instruments, which can resolve the stellar atmospheres, where the winds originate, provide essential data for testing the models. Title: The extended atmosphere and circumstellar environment of the cool evolved star VX Sagittarii as seen by MATISSE Authors: Chiavassa, A.; Kravchenko, K.; Montargès, M.; Millour, F.; Matter, A.; Freytag, B.; Wittkowski, M.; Hocdé, V.; Cruzalèbes, P.; Allouche, F.; Lopez, B.; Lagarde, S.; Petrov, R. G.; Meilland, A.; Robbe-Dubois, S.; Hofmann, K. -H.; Weigelt, G.; Berio, P.; Bendjoya, P.; Bettonvil, F.; Domiciano de Souza, A.; Heininger, M.; Henning, Th.; Isbell, J. W.; Jaffe, W.; Labadie, L.; Lehmitz, M.; Meisenheimer, K.; Soulain, A.; Varga, J.; Augereau, J. -C.; van Boekel, R.; Burtscher, L.; Danchi, W. C.; Dominik, C.; Drevon, J.; Gámez Rosas, V.; Hogerheijde, M. R.; Hron, J.; Klarmann, L.; Kokoulina, E.; Lagadec, E.; Leftley, J.; Mosoni, L.; Nardetto, N.; Paladini, C.; Pantin, E.; Schertl, D.; Stee, P.; Szabados, L.; Waters, R.; Wolf, S.; Yoffe, G. Bibcode: 2022A&A...658A.185C Altcode: 2021arXiv211210695C Context. VX Sgr is a cool, evolved, and luminous red star whose stellar parameters are difficult to determine, which affects its classification.
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 oidb.jmmc.fr Title: Atmosphere of Betelgeuse before and during the Great Dimming event revealed by tomography Authors: Kravchenko, K.; Jorissen, A.; Van Eck, S.; Merle, T.; Chiavassa, A.; Paladini, C.; Freytag, B.; Plez, B.; Montargès, M.; Van Winckel, H. Bibcode: 2021A&A...650L..17K Altcode: 2021arXiv210408105K Context. Despite being the best studied red supergiant star in our Galaxy, the physics behind the photometric variability and mass loss of Betelgeuse is poorly understood. Moreover, recently the star has experienced an unusual fading with its visual magnitude reaching a historical minimum. The nature of this event was investigated by several studies where mechanisms, such as episodic mass loss and the presence of dark spots in the photosphere, were invoked.
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 cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/650/L17 Title: Spatially resolved spectroscopy across stellar surfaces. IV. F, G, and K-stars: Synthetic 3D spectra at hyper-high resolution Authors: Dravins, Dainis; Ludwig, Hans-Günter; Freytag, Bernd Bibcode: 2021A&A...649A..16D Altcode: 2021arXiv210303880D Context. High-precision stellar analyses require hydrodynamic 3D modeling. Such models predict changes across stellar disks of spectral line shapes, asymmetries, and wavelength shifts. For testing models in stars other than the Sun, spatially resolved observations are feasible from differential spectroscopy during exoplanet transits, retrieving spectra of those stellar surface segments that successively become hidden behind the transiting planet, as demonstrated in Papers I, II, and III.
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 >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. Title: Spatially resolved spectroscopy across stellar surfaces. V. Observational prospects: toward Earth-like exoplanet detection Authors: Dravins, Dainis; Ludwig, Hans-Günter; Freytag, Bernd Bibcode: 2021A&A...649A..17D Altcode: 2021arXiv210304996D Context. High-precision stellar analyses require hydrodynamic 3D modeling. Testing such models is feasible by retrieving spectral line shapes across stellar disks, using differential spectroscopy during exoplanet transits. Observations were presented in Papers I, II, and III, while Paper IV explored synthetic data at hyper-high spectral resolution for different classes of stars, identifying characteristic patterns for Fe I and Fe II lines.
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. Title: Horizontal spreading of planetary debris accreted by white dwarfs Authors: Cunningham, Tim; Tremblay, Pier-Emmanuel; Bauer, Evan B.; Toloza, Odette; Cukanovaite, Elena; Koester, Detlev; Farihi, Jay; Freytag, Bernd; Gänsicke, Boris T.; Ludwig, Hans-Günter; Veras, Dimitri Bibcode: 2021MNRAS.503.1646C Altcode: 2021arXiv210209564C; 2021MNRAS.tmp..586C White dwarfs with metal-polluted atmospheres have been studied widely in the context of the accretion of rocky debris from evolved planetary systems. One open question is the geometry of accretion and how material arrives and mixes in the white dwarf surface layers. Using the three-dimensional (3D) radiation hydrodynamics code 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. Title: Investigating mass loss from RSG and AGB stars using the new VLTI-MATISSE imaging instrument Authors: Wittkowski, Markus; Chiavassa, Andrea; Baron, Fabien; Freytag, Bernd; Höfner, Susanne; Paladini, Claudia Bibcode: 2021csss.confE.310W Altcode: It is currently an open question in stellar astrophysics which physical processes initiate the mass loss of red supergiants. Observations of Betelgeuse during its recent great dimming event in 2019/2020 suggested a discrete highly localised mass ejection event, possibly connected to photospheric motion caused by a stochastic occurence of an extreme convection cell and possibly enhanced by pulsation (Dupree et al. 2020, Harper et al. 2020, Montarges et al., submitted), which may also explain the mass-loss history of the red hypergiant VY CMa, and of RSGs in general (Humphreys et al. 2020). There is indication that corundum, metallic iron, or other large transparent grains auch as Ca-Al-rich silicates may be present as close as down to about 2 stellar radii, which may serve as seeds for Mg-Fe-rich silicates at larger radii (Gail 2020). Here, we image the extended molecular layers and inner dust shell of the red supergaint AH Scorpii (AH Sco), which is known to exhibit a strong and narrow classical 9.7 mu silicate feature. We use the newly available infrared beam combiner MATISSE at the Very Large Telescope Interferometer (VLTI) to image AH Sco in the L band with a spectral resolution of 506, focusing on the SiO (2-0) bandhead, as well as in the N band focusing on the silicate dust. Our spatial resolution ranges from 3 mas at 4 mu to 10 mas at 12.5 mu. We also provide an outlook on comparing the levitation of the atmospheres and the dust condensation sequence between RSG and AGB stars, using the same observational setup for the Mira star R Aqr. Title: 3D spectroscopic analysis of helium-line white dwarfs Authors: Cukanovaite, Elena; Tremblay, Pier-Emmanuel; Bergeron, Pierre; Freytag, Bernd; Ludwig, Hans-Günter; Steffen, Matthias Bibcode: 2021MNRAS.501.5274C Altcode: 2020arXiv201112693C; 2020MNRAS.tmp.3465C In this paper, we present corrections to the spectroscopic parameters of DB and DBA white dwarfs with -10.0 ≤ log (H/He) ≤ -2.0, 7.5 ≤ log g ≤ 9.0, and $12\, 000$ ≲ 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. Title: VizieR Online Data Catalog: HERMES spectra of Betelgeuse (Kravchenko+, 2021) Authors: Kravchenko, K.; Jorissen, A.; van Eck, S.; Merle, T.; Chiavassa, A.; Paladini, C.; Freytag, B.; Plez, B.; Montarges, M.; van Winckel, H. Bibcode: 2021yCat..36509017K Altcode: The observations of Betelgeuse were performed with the high-resolution fibre-fed cross-dispersed echelle spectrograph HERMES mounted on the 1.2m Mercator telescope at the Roque de Los Muchachos Observatory, La Palma (Spain). The spectral resolution of HERMES is R=86000, and the wavelength coverage is from 3800 to 9000Å. The spectra were obtained between November 2015 and September 2020.

(2 data files). Title: Tomography of cool giant and supergiant star atmospheres. III. Validation of the method on VLTI/AMBER observations of the Mira star S Ori Authors: Kravchenko, Kateryna; Wittkowski, Markus; Jorissen, Alain; Chiavassa, Andrea; Van Eck, Sophie; Anderson, Richard I.; Freytag, Bernd; Käufl, Ulli Bibcode: 2020A&A...642A.235K Altcode: 2020arXiv200903955K Context. Asymptotic giant branch (AGB) stars are characterized by substantial mass loss, however the mechanism behind it not yet fully understood. The knowledge of the structure and dynamics of AGB-star atmospheres is crucial to better understanding the mass loss. The recently established tomographic method, which relies on the design of spectral masks containing lines that form in given ranges of optical depths in the stellar atmosphere, is an ideal technique for this purpose.
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. Title: Optical interferometry and Gaia measurement uncertainties reveal the physics of asymptotic giant branch stars Authors: Chiavassa, A.; Kravchenko, K.; Millour, F.; Schaefer, G.; Schultheis, M.; Freytag, B.; Creevey, O.; Hocdé, V.; Morand, F.; Ligi, R.; Kraus, S.; Monnier, J. D.; Mourard, D.; Nardetto, N.; Anugu, N.; Le Bouquin, J. -B.; Davies, C. L.; Ennis, J.; Gardner, T.; Labdon, A.; Lanthermann, C.; Setterholm, B. R.; ten Brummelaar, T. Bibcode: 2020A&A...640A..23C Altcode: 2020arXiv200607318C Context. Asymptotic giant branch (AGB) stars are cool luminous evolved stars that are well observable across the Galaxy and populating Gaia data. They have complex stellar surface dynamics, which amplifies the uncertainties on stellar parameters and distances.
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. Title: Focus on Betelgeuse Authors: Dupree, Andrea; Chiavassa, Andrea; Freytag, Bernd; Harper, Graham M.; Kervella, Pierre; Lebre, Agnes; Montarges, Miguel; Ohnaka, Keiichi; Quirrenbach, Andreas; Richards, Anita; Schmitt, Henrique R.; Strassmeier, Klaus G.; Uitenbroek, Han; Wheeler, J. Craig; Wittkowski, Markus; Matthews, Lynn D. Bibcode: 2020hst..prop16216D Altcode: Multiple ultraviolet spectra of the nearby red supergiant, Betelgeuse, using STIS will enable spatially resolved measures of chromospheric structure and mass inflows and outflows. An HST campaign of 3 cycles will be complemented by multi-frequency photometry, spectroscopy, interferometry, and polarimetry at radio, infrared, and optical wavelengths in order to map surface structures and their variability, and the extended outer atmosphere over both the short (400-day) and long secondary (2000-day) periods of this supergiant. These observations, coupled with detailed modeling and simulations, will probe the structure, the dynamics, and the mass loss from Betelgeuse in unprecedented detail and provide crucial insights into the atmospheric physics and wind-driving mechanisms of red supergiants. Title: VLTI-PIONIER imaging of the red supergiant V602 Carinae Authors: Climent, J. B.; Wittkowski, M.; Chiavassa, A.; Baron, F.; Marcaide, J. M.; Guirado, J. C.; Freytag, B.; Höfner, S.; Haubois, X.; Woillez, J. Bibcode: 2020A&A...635A.160C Altcode: 2020arXiv200209243C Context. Red supergiant stars possess surface features and extended molecular atmospheres. Photospheric convection may be a crucial factor of the levitation of the outer atmospheric layers. However, the mechanism responsible is still poorly understood.
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. Title: Using Gaia to measure the atmospheric dynamics in AGB stars Authors: Chiavassa, Andrea; Freytag, Bernd; Schultheis, Mathias Bibcode: 2019IAUS..343..373C Altcode: We use 3D radiative-hydrodynamics simulations of convection with CO5BOLD and the post-processing radiative transfer code Optim3D to compute intensity maps in the Gaia G band [325-1030 nm]. We calculate the intensity-weighted mean of all emitting points tiling the visible stellar surface (i.e., the photo-center) and evaluate its motion as a function of time. We show that the convection-related variability accounts for a substantial part to the Gaia DR2 parallax error of our sample of semiregular variables. Finally, we denote that Gaia parallax variations could be exploited quantitatively to extract stellar parameters using appropriate RHD simulations corresponding to the observed star. Title: Lumpy stars and bumpy winds Authors: Liljegren, Sofie; Höfner, Susanne; Freytag, Bernd; Bladh, Sara Bibcode: 2019IAUS..343..134L Altcode: The wind-driving process of AGB stars is thought to be a two-step process: first matter is levitated by shock waves, and then accelerated outwards by radiation pressure on newly condensed dust grains. When modelling such a wind, spherical symmetry is usually assumed. This is in stark contrast with recent observations, which shows significant non-spherical structures. Giant convection cells cover the surface of the star, and matter is being ejected into the atmosphere where it condenses into lumpy dust clouds. We try to quantify the differences between what is simulated in the 3D star-in-a-box models (CO5BOLD code) and the 1D dynamical atmosphere and wind models (DARWIN code). The impact of having a non-spherical star on the wind properties is also investigated. We find that the inherent non-spherical behaviour of AGB stars might induce a dust-driven weak wind already early on the AGB, and including that the star is anisotropic when simulating the wind leads to large time variations in the density of the outflow. Such variations might be observable as small-scale structures in the circumstellar envelope. Title: Precision Monitoring of Cool Evolved Stars: Constraining Effects of Convection and Pulsation Authors: Wittkowski, M.; Bladh, S.; Chiavassa, A.; de Wit, W. -J.; Eriksson, K.; Freytag, B.; Haubois, X.; Höfner, S.; Kravchenko, K.; Paladini, C.; Paumard, T.; Rau, G.; Wood, P. R. Bibcode: 2019Msngr.178...34W Altcode: Mass loss from cool evolved stars is an important ingredient of the cosmic matter cycle, enriching the Universe with newly formed elements and dust. However, physical processes that are not considered in current models represent uncertainties in our general understanding of mass loss. Time-series of interferometric data provide the strongest tests of dynamical processes in the atmospheres of these stars. Here, we present a pilot study of such measurements obtained with the GRAVITY instrument on the Very Large Telescope Interferometer. Title: TIGvival: High-resolution spectroscopic monitoring of LPV stars Authors: Wolter, Uwe; Engels, Dieter; Aringer, Bernhard; Freytag, Bernd Bibcode: 2019IAUS..343..548W Altcode: TIGvival is a spectroscopic monitoring program of long-period variables (LPV) using our robotic telescope TIGRE. Since 2013, we obtain low-noise, high-resolution spectra (R= 20 000) that cover the optical regime (3800 Å to 8800 Å). We are now continuously monitoring 7 LPVs with different periods and chemical properties. Our 350+ spectra evenly sample the target cycles, as far as ground-based observations allow. Analyzing the TIGvival spectra of Mira as a sample case, our measurements indicate that the strength of the TiO-absorption is phase-shifted with respect to the visual light curve. Title: The atmospheric dynamics of AGB stars revealed by Gaia through numerical simulations Authors: Chiavassa, A.; Freytag, B.; Schultheis, M. Bibcode: 2019sf2a.conf..137C Altcode: A considerable fraction of the detected intrinsically variable stars in Gaia data are Long-Period Variables. These objects have large luminosity amplitudes and variability timescales. They have complex stellar surface dynamics that affect the measurements and amplify the uncertainties on stellar parameters. We explore the impact of the convection-related surface structure in AGBs on the photocentric variability. We quantify these effects to characterise the observed parallax errors and estimate fundamental stellar parameters and dynamical properties. For this purpose, we use state-of-the-art three-dimensional (3D) radiative hydrodynamics simulations of convection with CO5BOLD and the post-processing radiative transfer code OPTIM3D to compute intensity maps in the Gaia G band [325 -- 1030~nm]. Then, we calculate the intensity-weighted mean of all emitting points tiling the visible stellar surface (i.e. the photocentre) and evaluate its motion as a function of time. We show that the convection-related variability accounts for a substantial part of the Gaia DR2 parallax error of our sample of semi-regular variables. We prospect the roadmap to extract quantitatively fundamental properties of AGB stars directly from Gaia errors exploiting appropriate RHD simulations. Title: 3D modelling of AGB stars with CO5BOLD Authors: Freytag, Bernd; Höfner, Susanne; Liljegren, Sofie Bibcode: 2019IAUS..343....9F Altcode: Local three-dimensional radiation-hydrodynamics simulations of patches of the surfaces of solar-type stars, that are governed by small-scale granular convection, have helped analyzing and interpreting observations for decades. These models contributed considerably to the understanding of the atmospheres and indirectly also of the interiors and the active layers above the surface of these stars. Of great help was of course the availability of a close-by prototype of these stars - the sun. Title: Tomography of the red supergiant star μ Cep Authors: Kravchenko, K.; Chiavassa, A.; Van Eck, S.; Jorissen, A.; Merle, T.; Freytag, B. Bibcode: 2019IAUS..343..441K Altcode: A tomographic method, aiming at probing velocity fields at depth in stellar atmospheres, is applied to the red supergiant star μ Cep and to snapshots of 3D radiative-hydrodynamics simulation in order to constrain atmospheric motions and relate them to photometric variability. Title: Constraining convection across the AGB with high-angular-resolution observations Authors: Paladini, Claudia; Baron, Fabien; Jorissen, A.; Le Bouquin, J. -B.; Freytag, B.; Van Eck, S.; Wittkowski, M.; Hron, J.; Chiavassa, A.; Berger, J. -P.; Siopis, C.; Mayer, A.; Sadowski, G.; Kravchenko, K.; Shetye, S.; Kerschbaum, F.; Kluska, J.; Ramstedt, S. Bibcode: 2019IAUS..343...27P Altcode: We present very detailed images of the photosphere of an AGB star obtained with the PIONIER instrument, installed at the Very Large Telescope Interferometer (VLTI). The images show a well defined stellar disc populated by a few convective patterns. Thanks to the high precision of the observations we are able to derive the contrast and granulation horizontal scale of the convective pattern for the first time in a direct way. Such quantities are then compared with scaling relations between granule size, effective temperature, and surface gravity that are predicted by simulations of stellar surface convection. Title: Tomography of cool giant and supergiant star atmospheres. II. Signature of convection in the atmosphere of the red supergiant star μ Cep Authors: Kravchenko, K.; Chiavassa, A.; Van Eck, S.; Jorissen, A.; Merle, T.; Freytag, B.; Plez, B. Bibcode: 2019A&A...632A..28K Altcode: 2019arXiv191004657K Context. Red supergiants are cool massive stars and are the largest and the most luminous stars in the Universe. They are characterized by irregular or semi-regular photometric variations, the physics of which is not clearly understood.
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 http://www.aanda.org

Based on observations made with the Mercator Telescope, operated on the island of La Palma by the Flemish Community, at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrof-sica de Canarias. Title: Calibration of the mixing-length theory for structures of helium-dominated atmosphere white dwarfs Authors: Cukanovaite, E.; Tremblay, P. -E.; Freytag, B.; Ludwig, H. -G.; Fontaine, G.; Brassard, P.; Toloza, O.; Koester, D. Bibcode: 2019MNRAS.490.1010C Altcode: 2019MNRAS.tmp.2282C; 2019arXiv190910532C We perform a calibration of the mixing-length parameter at the bottom boundary of the convection zone for helium-dominated atmospheres of white dwarfs. This calibration is based on a grid of 3D DB (pure-helium) and DBA (helium-dominated with traces of hydrogen) model atmospheres computed with the CO5BOLD radiation-hydrodynamics code, and a grid of 1D DB and DBA envelope structures. The 3D models span a parameter space of hydrogen-to-helium abundances in the range -10.0 ≤ log (H/He) ≤-2.0, surface gravities in the range 7.5 ≤ log g ≤ 9.0, and effective temperatures in the range 12 000 K ≲ 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. Title: Convective overshoot and macroscopic diffusion in pure-hydrogen-atmosphere white dwarfs Authors: Cunningham, Tim; Tremblay, Pier-Emmanuel; Freytag, Bernd; Ludwig, Hans-Günter; Koester, Detlev Bibcode: 2019MNRAS.488.2503C Altcode: 2019MNRAS.tmp.1723C; 2019arXiv190611252C We present a theoretical description of macroscopic diffusion caused by convective overshoot in pure-hydrogen DA white dwarfs using 3D, closed-bottom, radiation hydrodynamics 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. Title: Focus on Betelgeuse Authors: Dupree, Andrea; Chiavassa, Andrea; Freytag, Bernd; Harper, Graham M.; Kervella, Pierre; Lebre, Agnes; Montarges, Miguel; Ohnaka, Keiichi; Quirrenbach, Andreas; Richards, Anita; Schmitt, Henrique R.; Strassmeier, Klaus G.; Uitenbroek, Han; Wheeler, J. Craig; Wittkowski, Markus; Matthews, Lynn D. Bibcode: 2019hst..prop15873D Altcode: Multiple ultraviolet spectra of the nearby red supergiant, Betelgeuse, using STIS will enable spatially resolved measures of chromospheric structure and mass inflows and outflows. An HST campaign of 3 cycles will be complemented by multi-frequency photometry, spectroscopy, interferometry, and polarimetry at radio, infrared, and optical wavelengths in order to map surface structures and their variability, and the extended outer atmosphere over both the short (400-day) and long secondary (2000-day) periods of this supergiant. These observations, coupled with detailed modeling and simulations, will probe the structure, the dynamics, and the mass loss from Betelgeuse in unprecedented detail and provide crucial insights into the atmospheric physics and wind-driving mechanisms of red supergiants. Title: Exploring the origin of clumpy dust clouds around cool giants. A global 3D RHD model of a dust-forming M-type AGB star Authors: Höfner, Susanne; Freytag, Bernd Bibcode: 2019A&A...623A.158H Altcode: 2019arXiv190204074H Context. Dust grains forming in the extended atmospheres of AGB stars are critical for the heavy mass loss of these cool luminous giants, as they provide radiative acceleration for the stellar winds. Characteristic mid-IR spectral features indicate that the grains consist mainly of silicates and corundum. The latter species seems to form in a narrow zone within about 2 stellar radii, preceding the condensation of silicate dust, which triggers the outflow. Recent high-angular-resolution observations show clumpy, variable dust clouds at these distances.
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. Bibcode: 2018MNRAS.481.1522C Altcode: 2018arXiv180900590C; 2018MNRAS.tmp.2259C We present the first grid of 3D simulations for the pure-helium atmospheres of DB white dwarfs. The simulations were computed with the 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 < Teff < 23 000 K for all log g values covered by the 3D grid. These corrections indicate that 1D model atmospheres overpredict log g, reminiscent of the results found from 3D simulations of pure-hydrogen white dwarfs. The next step will be to compute 3D simulations with mixed helium and hydrogen atmospheres to comprehend the full implications for the stellar parameters of DB and DBA white dwarfs. Title: Atmospheres and wind properties of non-spherical AGB stars Authors: Liljegren, S.; Höfner, S.; Freytag, B.; Bladh, S. Bibcode: 2018A&A...619A..47L Altcode: 2018arXiv180805043L Context. The wind-driving mechanism of asymptotic giant branch (AGB) stars is commonly attributed to a two-step process: first, gas in the stellar atmosphere is levitated by shockwaves caused by stellar pulsation, then accelerated outwards by radiative pressure on newly formed dust, inducing a wind. Dynamical modelling of such winds usually assumes a spherically symmetric star.
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. Bibcode: 2018A&A...617L...1C Altcode: 2018arXiv180802548C Context. Asymptotic giant branch (AGB) stars are characterised by complex stellar surface dynamics that affect the measurements and amplify the uncertainties on stellar parameters. The uncertainties in observed absolute magnitudes have been found to originate mainly from uncertainties in the parallaxes. The resulting motion of the stellar photocentre could have adverse effects on the parallax determination with Gaia.
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. Title: Constraining Convection in Evolved Stars with the VLTI Authors: Paladini, C.; Baron, F.; Jorissen, A.; Le Bouquin, J. -B.; Freytag, B.; Van Eck, S.; Wittkowski, M.; Hron, J.; Chiavassa, A.; Berger, J. -P.; Siopis, C.; Mayer, A.; Sadowski, G.; Kravchenko, K.; Shetye, S.; Kerschbaum, F.; Kluska, J.; Ramstedt, S. Bibcode: 2018Msngr.172...24P Altcode: We used the Precision Integrated-Optics Near-infrared Imaging ExpeRiment (PIONIER) at the Very Large Telescope Interferometer (VLTI) to image the stellar surface of the S-type Asymptotic Giant Branch (AGB) star 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. Title: Simulation of the small-scale magnetism in main-sequence stellar atmospheres Authors: Salhab, R. G.; Steiner, O.; Berdyugina, S. V.; Freytag, B.; Rajaguru, S. P.; Steffen, M. Bibcode: 2018A&A...614A..78S Altcode: Context. Observations of the Sun tell us that its granular and subgranular small-scale magnetism has significant consequences for global quantities such as the total solar irradiance or convective blueshift of spectral lines.
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. Title: Spectroscopic Properties of a Two-Dimensional Cepheid Model Authors: Vasilyev, Valeriy; Ludwig, Hans-Günter; Freytag, Bernd; Lemasle, Bertrand; Marconi, Marcella Bibcode: 2018pas6.conf..222V Altcode: 2018pas..conf..222V The chemical composition of Cepheid variables can provide information on the chemo-dynamical evolution of the Galaxy and beyond. The standard method for determining atmospheric parameters and abundances of Cepheids is based on one-dimensional plane-parallel hydrostatic model atmospheres, where convection is treated by Mixing Length Theory. We check the validity of the quasi-static approach against a two-dimensional dynamical Cepheid model computed with CO5BOLD. The spectroscopic investigation of the two-dimensional Cepheid model allowed to derive projection factors and to explain the residual line-of-sight velocity of Galactic Cepheids, long known as the ``K-term'', by line shifts of convective origin. Moreover, hydrostatic 1D model atmospheres can provide unbiased estimates of stellar parameters and abundances of Cepheids for particular phases of their pulsations. Title: Tomography of the Red Supergiant Star MU Cep Authors: Kravchenko, Kateryna; Chiavassa, A.; Van Eck, S.; Jorissen, A.; Merle, T.; Freytag, B. Bibcode: 2018iss..confE..20K Altcode: 2018arXiv180907581K We present a tomographic method allowing to recover the velocity field at different optical depths in a stellar atmosphere. It is based on the computation of the contribution function to identify the depth of formation of spectral lines in order to construct numerical masks probing different optical depths. These masks are cross-correlated with observed spectra to extract information about the average shape of lines forming at a given optical depth and to derive the velocity field projected on the line of sight. We applied this method to series of spectra of the red supergiant star mu Cep and derived velocities in different atmospheric layers. The resulting velocity variations reveal complex atmospheric dynamics and indicate that convective cells are present in the atmosphere of the mu Cep. The mu Cep velocities were compared with those obtained by applying the tomographic masks to series of snapshot spectra from 3D radiative-hydrodynamics CO5BOLD simulations. Title: Using the CIFIST grid of 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. Bibcode: 2018A&A...611A..68B Altcode: 2017arXiv171200024B Context. The atmospheres of cool stars are temporally and spatially inhomogeneous due to the effects of convection. The influence of this inhomogeneity, referred to as granulation, on colours has never been investigated over a large range of effective temperatures and gravities. Aim. We aim to study, in a quantitative way, the impact of granulation on colours.
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 http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/611/A68 Title: Spectroscopic properties of a two-dimensional time-dependent Cepheid model. II. Determination of stellar parameters and abundances Authors: Vasilyev, V.; Ludwig, H. -G.; Freytag, B.; Lemasle, B.; Marconi, M. Bibcode: 2018A&A...611A..19V Altcode: 2017arXiv171100236V Context. Standard spectroscopic analyses of variable stars are based on hydrostatic 1D model atmospheres. This quasi-static approach has not been theoretically validated. Aim. We aim at investigating the validity of the quasi-static approximation for Cepheid variables. We focus on the spectroscopic determination of the effective temperature 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. Title: Tomography of cool giant and supergiant star atmospheres. I. Validation of the method Authors: Kravchenko, K.; Van Eck, S.; Chiavassa, A.; Jorissen, A.; Freytag, B.; Plez, B. Bibcode: 2018A&A...610A..29K Altcode: 2017arXiv171108327K Context. Cool giant and supergiant star atmospheres are characterized by complex velocity fields originating from convection and pulsation processes which are not fully understood yet. The velocity fields impact the formation of spectral lines, which thus contain information on the dynamics of stellar atmospheres. Aim. The tomographic method allows to recover the distribution of the component of the velocity field projected on the line of sight at different optical depths in the stellar atmosphere. The computation of the contribution function to the line depression aims at correctly identifying the depth of formation of spectral lines in order to construct numerical masks probing spectral lines forming at different optical depths.
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. 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. Bibcode: 2018Natur.553..310P Altcode: Convection plays a major part in many astrophysical processes, including energy transport, pulsation, dynamos and winds on evolved stars, in dust clouds and on brown dwarfs. Most of our knowledge about stellar convection has come from studying the Sun: about two million convective cells with typical sizes of around 2,000 kilometres across are present on the surface of the Sun—a phenomenon known as granulation. But on the surfaces of giant and supergiant stars there should be only a few large (several tens of thousands of times larger than those on the Sun) convective cells, owing to low surface gravity. Deriving the characteristic properties of convection (such as granule size and contrast) for the most evolved giant and supergiant stars is challenging because their photospheres are obscured by dust, which partially masks the convective patterns. These properties can be inferred from geometric model fitting, but this indirect method does not provide information about the physical origin of the convective cells. Here we report interferometric images of the surface of the evolved giant star 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. Title: VizieR Online Data Catalog: 3D correction in 5 photometric systems (Bonifacio+, 2018) Authors: Bonifacio, P.; Caffau, E.; Ludwig, H. -G.; Steffen, M.; Castelli, F.; Gallagher, A. J.; Kucinskas, A.; Prakapavicius, D.; Cayrel, R.; Freytag, B.; Plez, B.; Homeier, D. Bibcode: 2018yCat..36110068B Altcode: We have used the CIFIST grid of CO5BOLD models to investigate the effects of granulation on fluxes and colours of stars of spectral type F, G, and K.

We publish tables with 3D corrections that can be applied to colours computed from any 1D model atmosphere. For Teff>=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). Title: Spectroscopic properties of a two-dimensional time-dependent Cepheid model. I. Description and validation of the model Authors: Vasilyev, V.; Ludwig, H. -G.; Freytag, B.; Lemasle, B.; Marconi, M. Bibcode: 2017A&A...606A.140V Altcode: 2017arXiv170903905V Context. Standard spectroscopic analyses of Cepheid variables are based on hydrostatic one-dimensional model atmospheres, with convection treated using various formulations of mixing-length theory.
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. Title: Global 3D radiation-hydrodynamics models of AGB stars. Effects of convection and radial pulsations on atmospheric structures Authors: Freytag, B.; Liljegren, S.; Höfner, S. Bibcode: 2017A&A...600A.137F Altcode: 2017arXiv170205433F Context. Observations of asymptotic giant branch (AGB) stars with increasing spatial resolution reveal new layers of complexity of atmospheric processes on a variety of scales.
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. Title: Convective overshoot and metal accretion onto white dwarfs. Authors: Tremblay, P. -E.; Ludwig, H. -G.; Freytag, B.; Koester, D.; Fontaine, G. Bibcode: 2017MmSAI..88..104T Altcode: A large fraction of white dwarfs host evolved planetary systems and show evidence of accretion from planetary debris. The accretion-diffusion model is the preferred method to understand the metal pollution in these otherwise hydrogen- and helium-rich white dwarf atmospheres. In this scenario, the accreted material first settles on the atmosphere. If the outer stellar layers are unstable to convection, the metals are then rapidly mixed up within the convection zone. In the classical 1D approach, it is generally assumed that the convection zone has a sharp bottom boundary, below which microscopic diffusion is unhampered and slowly removes metals from the visible layers. More realistic 3D radiation-hydrodynamics simulations of white dwarfs with CO5BOLD demonstrate, however, that the bottom of the convection zone does not have a sharp boundary, and that velocities decay exponentially below the unstable convective layers with a velocity scale height of the order of one pressure scale height. This has a potentially dramatic effect on the inferred mass of accreted materiel, hence on the chemical composition and size of planetary debris around white dwarfs. Title: Boundary conditions in CO5BOLD Authors: Freytag, Bernd Bibcode: 2017MmSAI..88...12F Altcode: The declaration of boundary conditions is a crucial step in the setup of a CO5BOLD simulation (and many others) due to the physical nature of the problem, that is reflected in the mathematical description by partial differential equations, discrete versions of which are integrated by the numerical solver(s). While parameters controlling the flux of energy through the computational box are most important for all simulations of convective flows, the detailed specifications describing the behavior of energy, gas and dust densities, velocities, and magnetic fields at or just beyond the boundaries influence the flow, dynamics, and stratification within the box. Recent refinements of the treatment of boundary conditions in CO5BOLD resulted in reliably working implementations of open and closed versions for top, bottom, and ``inner'' boundaries even under conditions with strong velocity fields (waves, shocks, or downdrafts). They are implemented and available in the current version of CO5BOLD - but have to be activated properly with parameters adapted to the type of the star under consideration (by defining for instance the depth of the damping layers for the closed-bottom boundary or by specifying the damping constants for the open-bottom boundary). Title: Using CO5BOLD models to predict the effects of granulation on colours . Authors: Bonifacio, P.; Caffau, E.; Ludwig, H. -G.; Steffen, M.; Castelli, F.; Gallagher, A. J.; Prakapavičius, D.; Kučinskas, A.; Cayrel, R.; Freytag, B.; Plez, B.; Homeier, D. Bibcode: 2017MmSAI..88...90B Altcode: In order to investigate the effects of granulation on fluxes and colours, we computed the emerging fluxes from the models in the CO5BOLD grid with metallicities [M/H]=0.0,-1.0,-2.0 and -3.0. These fluxes have been used to compute colours in different photometric systems. We explain here how our computations have been performed and provide some results. Title: Enhanced methods for computing spectra from CO5BOLD models using Linfor3D. Molecular bands in metal-poor stars Authors: Gallagher, A. J.; Steffen, M.; Caffau, E.; Bonifacio, P.; Ludwig, H. -G.; Freytag, B. Bibcode: 2017MmSAI..88...82G Altcode: 2016arXiv161004427G Molecular features such as the G-band, CN-band and NH-band are important diagnostics for measuring a star's carbon and nitrogen abundances, especially in metal-poor stars where atomic lines are no longer visible in stellar spectra. Unlike atomic transitions, molecular features tend to form in bands, which cover large wavelength regions in a spectrum. While it is a trivial matter to compute carbon and nitrogen molecular bands under the assumption of 1D, it is extremely time consuming in 3D. In this contribution to the 2016 COBOLD workshop we review the improvements made to the 3D spectral synthesis code Linfor3D, and discuss the new challenges found when computing molecular features in 3D. Title: Non-magnetic photospheric bright points in 3D simulations of the solar atmosphere Authors: Calvo, F.; Steiner, O.; Freytag, B. Bibcode: 2016A&A...596A..43C Altcode: 2016arXiv161204278C Context. Small-scale bright features in the photosphere of the Sun, such as faculae or G-band bright points, appear in connection with small-scale magnetic flux concentrations.
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 http://www.aanda.org Title: Application of a Theory and Simulation-based Convective Boundary Mixing Model for AGB Star Evolution and Nucleosynthesis Authors: Battino, U.; Pignatari, M.; Ritter, C.; Herwig, F.; Denisenkov, P.; Den Hartogh, J. W.; Trappitsch, R.; Hirschi, R.; Freytag, B.; Thielemann, F.; Paxton, B. Bibcode: 2016ApJ...827...30B Altcode: 2016arXiv160506159B The s-process nucleosynthesis in Asymptotic giant branch (AGB) stars depends on the modeling of convective boundaries. We present models and s-process simulations that adopt a treatment of convective boundaries based on the results of hydrodynamic simulations and on the theory of mixing due to gravity waves in the vicinity of convective boundaries. Hydrodynamics simulations suggest the presence of convective boundary mixing (CBM) at the bottom of the thermal pulse-driven convective zone. Similarly, convection-induced mixing processes are proposed for the mixing below the convective envelope during third dredge-up (TDU), where the {}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. Bibcode: 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. Bibcode: 2015EAS....71..237C Altcode: 2015arXiv151203590C Evolved stars are among the largest and brightest stars and they are ideal targets for the new generation of sensitive, high resolution instrumentation that provides spectrophotometric, interferometric, astrometric, and imaging observables. The interpretation of the complex stellar surface images requires numerical simulations of stellar convection that take into account multi-dimensional time-dependent radiation hydrodynamics with realistic input physics. We show how the evolved star simulations are obtained using the radiative hydrodynamics code CO5BOLD and how the accurate observables are computed with the post-processing radiative transfer code Optim3D. The synergy between observations and theoretical work is supported by a proper and quantitative analysis using these simulations, and by strong constraints from the observational side. Title: On the Evolution of Magnetic White Dwarfs Authors: Tremblay, P. -E.; Fontaine, G.; Freytag, B.; Steiner, O.; Ludwig, H. -G.; Steffen, M.; Wedemeyer, S.; Brassard, P. Bibcode: 2015ApJ...812...19T Altcode: 2015arXiv150905398T We present the first radiation magnetohydrodynamic simulations of the atmosphere of white dwarf stars. We demonstrate that convective energy transfer is seriously impeded by magnetic fields when the plasma-β parameter, the thermal-to-magnetic-pressure ratio, becomes smaller than unity. The critical field strength that inhibits convection in the photosphere of white dwarfs is in the range B = 1-50 kG, which is much smaller than the typical 1-1000 MG field strengths observed in magnetic white dwarfs, implying that these objects have radiative atmospheres. We have employed evolutionary models to study the cooling process of high-field magnetic white dwarfs, where convection is entirely suppressed during the full evolution (B ≳ 10 MG). We find that the inhibition of convection has no effect on cooling rates until the effective temperature (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. Bibcode: 2015ASPC..497...11C Altcode: 2014arXiv1410.3868C Evolved stars are among the largest and brightest stars and they are ideal targets for the new generation of sensitive, high resolution instrumentation that provides spectrophotometric, interferometric, astrometric, and imaging observables. The interpretation of the complex stellar surface images requires numerical simulations of stellar convection that take into account multi-dimensional time-dependent radiation hydrodynamics with realistic input physics. We show how the evolved star simulations are obtained using the radiative hydrodynamics code 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. Bibcode: 2015ApJ...809..148T Altcode: 2015arXiv150701927T We present an extended grid of mean three-dimensional (3D) spectra for low-mass, pure-hydrogen atmosphere DA white dwarfs (WDs). We use CO5BOLD radiation-hydrodynamics 3D simulations covering 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. Bibcode: 2015ASPC..497...91A Altcode: We present recent near-IR interferometric studies of red giant and supergiant stars, which are aimed at obtaining information on the structure of the atmospheric layers and constraining the fundamental parameters of these objects.

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. Bibcode: 2015ASPC..497...23F Altcode: In the Sun, low-amplitude small-scale acoustic waves are just detectable in the photosphere and start to become dynamically relevant only in the lower chromosphere. The generation of these waves by non-stationary convective flows can be studied in detail by local 3-D radiation-hydrodynamics simulations. Using this technique for global models of AGB stars reveals roughly similar phenomena but on a larger scale and with much higher amplitude. Convection cells spanning a significant fraction of the entire surface produce strong waves that cause a network of smaller shocks in the inner photosphere and occasional global shocks, travelling outward in large arcs. Material falling back interacts with the surface convection cells. A new generation of 3-D RHD simulations of these layers with CO5BOLD is presented and analyzed with particular attention given to acoustic waves and shock fronts. Title: Calibration of the Mixing-Length Free Parameter for White Dwarf Structures Authors: Tremblay, P. -E.; Ludwig, H. -G.; Freytag, B.; Fontaine, G.; Steffen, M.; Brassard, P. Bibcode: 2015ASPC..493...89T Altcode: We present a comparison of our grid of 3D radiation-hydrodynamical simulations for 70 pure-hydrogen DA white dwarfs, in the surface gravity range 7.0 ≤log g≤ 9.0, with 1D envelope models based on the mixing-length theory (MLT) for convection. We perform a calibration of the mixing-length parameter for the lower part of the convection zone. The 3D simulations are often restricted to the upper convective layers, and in those cases, we rely on the asymptotic entropy value of the adiabatic 3D upflows to calibrate 1D envelopes. Our results can be applied to 1D structure calculations, and in particular for pulsation and convective mixing studies. We demonstrate that while the 1D MLT only provides a bottom boundary of the convection zone based on the Schwarzschild criterion, the 3D stratifications are more complex. There is a large overshoot region below the convective layers that is likely critical for chemical diffusion applications. Title: What causes the large extensions of red supergiant atmospheres?. Comparisons of interferometric observations with 1D hydrostatic, 3D convection, and 1D pulsating model atmospheres Authors: Arroyo-Torres, B.; Wittkowski, M.; Chiavassa, A.; Scholz, M.; Freytag, B.; Marcaide, J. M.; Hauschildt, P. H.; Wood, P. R.; Abellan, F. J. Bibcode: 2015A&A...575A..50A Altcode: 2015arXiv150101560A
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 http://www.aanda.org Title: Calibration of the Mixing-length Theory for Convective White Dwarf Envelopes Authors: Tremblay, P. -E.; Ludwig, H. -G.; Freytag, B.; Fontaine, G.; Steffen, M.; Brassard, P. Bibcode: 2015ApJ...799..142T Altcode: 2014arXiv1412.1789T A calibration of the mixing-length parameter in the local mixing-length theory (MLT) is presented for the lower part of the convection zone in pure-hydrogen-atmosphere white dwarfs. The parameterization is performed from a comparison of three-dimensional (3D) CO5BOLD simulations with a grid of one-dimensional (1D) envelopes with a varying mixing-length parameter. In many instances, the 3D simulations are restricted to the upper part of the convection zone. The hydrodynamical calculations suggest, in those cases, that the entropy of the upflows does not change significantly from the bottom of the convection zone to regions immediately below the photosphere. We rely on this asymptotic entropy value, characteristic of the deep and adiabatically stratified layers, to calibrate 1D envelopes. The calibration encompasses the convective hydrogen-line (DA) white dwarfs in the effective temperature range 6000 <= T eff (K) <=15, 000 and the surface gravity range 7.0 <= log g <= 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. Bibcode: 2015IAUS..307..280W Altcode: We present near-infrared spectro-interferometric studies of red supergiant (RSG) stars using the VLTI/AMBER instrument, which are compared to previously obtained similar observations of AGB stars. Our observations indicate spatially extended atmospheric molecular layers of water vapor and CO, similar as previously observed for Mira stars. Data of VY~CMa indicate that the molecular layers are asymmetric, possibly clumpy. Thanks to the spectro-interferometric capabilities of the VLTI/AMBER instrument, we can isolate continuum bandpasses, estimate fundamental parameters of our sources, locate them in the HR diagram, and compare their positions to recent evolutionary tracks. For the example of VY CMa, this puts it close to evolutionary tracks of initial mass 25-32 M ⊙. Comparisons of our data to hydrostatic model atmospheres, 3d simulations of convection, and 1d dynamic model atmospheres based on self-excited pulsation models indicate that none of these models can presently explain the observed atmospheric extensions for RSGs. The mechanism that levitates the atmospheres of red supergiant is thus a currently unsolved problem. Title: Properties of small-scale magnetism of stellar atmospheres Authors: Steiner, Oskar; Salhab, René; Freytag, Bernd; Rajaguru, Paul; Schaffenberger, Werner; Steffen, Matthias Bibcode: 2014PASJ...66S...5S Altcode: 2014PASJ..tmp...95S The magnetic field outside of sunspots is concentrated in the intergranular space, where it forms a delicate filigree of bright ribbons and dots as seen on broad band images of the Sun. We expect this small-scale magnetic field to exhibit a similar behavior in stellar atmospheres. In order to find out more about it, we perform numerical simulations of the surface layers of stellar atmospheres. Here, we report on preliminary results from simulations in the range between 4000 K and 6500 K effective temperature with an initial vertical, homogeneous magnetic field of 50 G strength. We find that the field strength of the strongest magnetic flux concentrations increases with decreasing effective temperature at the height level where the average Rosseland optical depth is one. On the other hand, at the same level, the field is less strong than the thermal equipartition value in the coolest model but assumes superequipartition in the models hotter than 5000 K. While the Wilson depression of the strongest field concentrations is about one pressure scale height in the coolest model, it is more than four times the pressure scale height in the hottest one. We also find that the relative contribution of the bright filigree to the bolometric, vertically directed radiative intensity is most significant for the 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. Bibcode: 2014ApJ...788..103T Altcode: We have identified 8 to 10 new cool white dwarfs from the Large Area Survey (LAS) Data Release 9 of the United Kingdom InfraRed Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS). The data set was paired with the Sloan Digital Sky Survey to obtain proper motions and a broad ugrizYJHK wavelength coverage. Optical spectroscopic observations were secured at Gemini Observatory and confirm the degenerate status for eight of our targets. The final sample includes two additional white dwarf candidates with no spectroscopic observations. We rely on improved one-dimensional model atmospheres and new multi-dimensional simulations with CO5BOLD to review the stellar parameters of the published LAS white dwarf sample along with our additional discoveries. Most of the new objects possess very cool atmospheres with effective temperatures below 5000 K, including two pure-hydrogen remnants with a cooling age between 8.5 and 9.0 Gyr, and tangential velocities in the range 40 km s-1 <=v tan <= 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 < T eff (K) < 6000. We review the possible explanations for the spectral evolution from helium-dominated toward hydrogen-rich atmospheres at low temperatures. Title: White Dwarfs In The UKIRT Infrared Deep Sky Survey Data Release 9 Authors: Tremblay, P. -E.; Leggett, S. K.; Lodieu, N.; Freytag, B.; Bergeron, P.; Kalirai, J. S.; Ludwig, H. -G. Bibcode: 2014arXiv1405.0266T Altcode: We have identified eight to ten new cool white dwarfs from the Large Area Survey (LAS) Data Release 9 of the United Kingdom InfraRed Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS). The data set was paired with the Sloan Digital Sky Survey (SDSS) to obtain proper motions and a broad ugrizYJHK wavelength coverage. Optical spectroscopic observations were secured at Gemini Observatory and confirm the degenerate status for eight of our targets. The final sample includes two additional white dwarf candidates with no spectroscopic observations. We rely on improved 1D model atmospheres and new multi-dimensional simulations with CO5BOLD to review the stellar parameters of the published LAS white dwarf sample along with our additional discoveries. Most of the new objects possess very cool atmospheres with effective temperatures below 5000 K, including two pure-hydrogen remnants with a cooling age between 8.5 and 9.0 Gyr, and tangential velocities in the range 40 km/s < vtan < 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 < Teff (K) < 6000. We review the possible explanations for the spectral evolution from helium-dominated towards hydrogen-rich atmospheres at low temperatures. Title: 3D modeling of stellar atmospheres and the impact on the understanding of the reliability of elemental abundances in stars as tracers of galactic chemical evolution Authors: Ludwig, H. -G.; Steffen, M.; Bonifacio, P.; Caffau, E.; Kučinskas, A.; Freytag, B. Bibcode: 2014IAUS..298..343L Altcode: We present a critical review of the construction of 3D model atmospheres with emphasis on modeling challenges. We discuss the basic physical processes which give rise to the effects which set 3D models apart from 1D standard models. We consider elemental abundances derived from molecular features, and the determination of the microturbulence with 3D models. The examples serve as illustration of the limitations inherent to 1D, however, also to 3D modeling. We find that 3D models can provide constraints on the microturbulence parameter, and predict substantial corrections for abundances derived from molecular species. Title: 3D Model Atmospheres of White Dwarfs Authors: Tremblay, Pier-Emmanuel; Ludwig, H.; Steffen, M.; Freytag, B. Bibcode: 2014AAS...22331507T Altcode: We present the first grid of 3D model atmospheres for hydrogen-atmosphere (DA) white dwarfs. These CO5BOLD radiation-hydrodynamics simulations, unlike the previous 1D calculations, do not rely on the mixing-length theory for the treatment of convection. The simulations have been employed to compute model spectra and we compared our improved Balmer line profiles to spectroscopic data from the Sloan Digital Sky Survey and the White Dwarf Catalog. The 3D surface gravities are found to be as much as 0.3 dex lower than the values derived from 1D models. The white dwarfs with a radiative and a convective atmosphere have derived mean masses that are the same within 0.01 Msun with our new models, in much better agreement with our understanding of stellar evolution. Title: Synthetic spectral libraries Authors: Allard, F.; Homeier, D.; Freytag, B. Bibcode: 2014ASInC..11...33A Altcode: Within the next few years, several instruments aiming at imaging extrasolar planets will see first light. In parallel, low mass planets are being searched around red dwarfs which offer more favorable conditions, both for radial velocity detection and transit studies, than solar-type stars. We review recent advancements and issues concerning the construction of synthetic spectral libraries for very low mass stars, brown dwarfs and exoplanets. The revised solar oxygen abundances and cloud models allow to reproduce the photometric and spectroscopic properties of this transition to a degree never achieved before, but problems remain in the important M-L transition characteristic of the Teff range of characterizable exoplanets. Title: Weather on the Nearest Brown Dwarfs: Resolved Simultaneous Multi-wavelength Variability Monitoring of WISE J104915.57-531906.1AB Authors: Biller, Beth A.; Crossfield, Ian J. M.; Mancini, Luigi; Ciceri, Simona; Southworth, John; Kopytova, Taisiya G.; Bonnefoy, Mickaël; Deacon, Niall R.; Schlieder, Joshua E.; Buenzli, Esther; Brandner, Wolfgang; Allard, France; Homeier, Derek; Freytag, Bernd; Bailer-Jones, Coryn A. L.; Greiner, Jochen; Henning, Thomas; Goldman, Bertrand Bibcode: 2013ApJ...778L..10B Altcode: 2013arXiv1310.5144B We present two epochs of MPG/ESO 2.2 m GROND simultaneous six-band (r'i'z' JHK) photometric monitoring of the closest known L/T transition brown dwarf binary WISE J104915.57-531906.1AB. We report here the first resolved variability monitoring of both the T0.5 and L7.5 components. We obtained 4 hr of focused observations on the night of 2013 April 22 (UT), as well as 4 hr of defocused (unresolved) observations on the night of 2013 April 16 (UT). We note a number of robust trends in our light curves. The r' and i' light curves appear to be anti-correlated with z' and H for the T0.5 component and in the unresolved light curve. In the defocused dataset, J appears correlated with z' and H and anti-correlated with r' and i', while in the focused dataset we measure no variability for J at the level of our photometric precision, likely due to evolving weather phenomena. In our focused T0.5 component light curve, the K band light curve displays a significant phase offset relative to both H and z'. We argue that the measured phase offsets are correlated with atmospheric pressure probed at each band, as estimated from one-dimensional atmospheric models. We also report low-amplitude variability in i' and z' intrinsic to the L7.5 component. Title: Spectroscopic analysis of DA white dwarfs with 3D model atmospheres Authors: Tremblay, P. -E.; Ludwig, H. -G.; Steffen, M.; Freytag, B. Bibcode: 2013A&A...559A.104T Altcode: 2013arXiv1309.0886T We present the first grid of mean three-dimensional (3D) spectra for pure-hydrogen (DA) white dwarfs based on 3D model atmospheres. We use 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 < Teff (K) < 15 000 and the surface gravity range of 7 < log g < 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 http://www.aanda.org Title: A Search for Variability in a Young Planet Authors: Biller, Beth; Crossfield, Ian; Deacon, Niall; Buenzli, Esther; Allard, France; Bonnefoy, Mickael; Schlieder, Joshua; Goldman, Bertrand; Homeier, Derek; Freytag, Bernd; Brandner, Wolfgang; Henning, Thomas; Bailer-Jones, Coryn Bibcode: 2013sptz.prop10061B Altcode: Variability attributed to cloud structure already appears to be a persistent feature for L and T type field brown dwarfs. Directly imaged planets occupy the same temperature regime as L and T type brown dwarfs and are likely to be equally variable. We propose mid-IR variability monitoring for the young free-floating planet CFHTWIR-Oph33 (<0.5 Myr, ~6 MJup). Such variability is predicted due to rotationally-modulated cloud features. These observations will probe weather conditions on a directly imaged exoplanet for the first time, providing benchmark data for ongoing development of multidimensional cloud modeling approaches. These variability monitoring observations are only possible with Spitzer and are a pilot study for future variability monitoring with JWST-MIRI of directly imaged exoplanets around young stars. Title: Granulation properties of giants, dwarfs, and white dwarfs from the CIFIST 3D model atmosphere grid Authors: Tremblay, P. -E.; Ludwig, H. -G.; Freytag, B.; Steffen, M.; Caffau, E. Bibcode: 2013A&A...557A...7T Altcode: 2013arXiv1307.2810T Three-dimensional model atmospheres for giants, dwarfs, and white dwarfs, computed with the CO5BOLD code and part of the CIFIST grid, have been used for spectroscopic and asteroseismic studies. Unlike existing plane-parallel 1D structures, these simulations predict the spatially and temporally resolved emergent intensity so that granulation can be analysed, which provides insights on how convective energy transfer operates in stars. The wide range of atmospheric parameters of the CIFIST 3D simulations (3600 < Teff (K) < 13 000 and 1 < log g < 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 http://www.aanda.org Title: 3D hydrodynamical simulations to interpret observations of stellar surfaces of red supergiant stars Authors: Chiavassa, A.; Freytag, B.; Plez, B. Bibcode: 2013EAS....60..145C Altcode: As red supergiants are the largest and brightest stars, they are ideal targets for the new generation of sensitive, high resolution instrumentation that provides spectrophotometric, interferometric, astrometric, and imaging observables. The interpretation of the complex stellar surface images requires numerical simulations of stellar convection that take into account multi-dimensional time-dependent radiation hydrodynamics with realistic input physics. We show the results obtained with the synergy between the radiative hydrodynamics code CO5BOLD and the post-processing radiative transfer code Optim3D. Such simulations support a proper and quantitative analysis of these observations, and the observations provide the theoretical work with strong constraints. Title: Global radiation-hydrodynamics simulations of red supergiant stars Authors: Freytag, B.; Chiavassa, A. Bibcode: 2013EAS....60..137F Altcode: The small-scale surface granulation on cool main-sequence stars and white dwarfs influences the overall appearance of these objects only weakly. And it is only indirectly observable by analyzing e.g. line-shapes or temporal fluctuations - except for the Sun. The large-scale and high-contrast convective surface cells and accompanying sound waves on supergiants and low-gravity AGB stars on the other hand have a strong impact on the outer atmospheric layers and are directly detectable by interferometric observations. Necessary to interpret modern observations with their high resolution in frequency, time, and/or space are detailed numerical multi-dimensional time-dependent radiation-hydrodynamical simulations. Local simulations of small patches of convective surface layers and the atmosphere of main-sequence stars have matured over three decades and have reached an impressive level of agreement with observations and also between different computational codes. However, global simulations of the entire convective surface and atmosphere of a red supergiants are considerably more demanding - and limited - and have become available only for about one decade. Still, they show how the surface is shaped by the interaction of small surface granules, that sit on top of large envelope convection cells, and waves, that can travel as shocks into the outer atmosphere. The route to more complete future models will be discussed, that comprise the outer atmosphere of the stars and that could explain some of the little-understood phenomena like chromosphere, molsphere, or wind-formation. Title: Pure-hydrogen 3D model atmospheres of cool white dwarfs Authors: Tremblay, P. -E.; Ludwig, H. -G.; Steffen, M.; Freytag, B. Bibcode: 2013A&A...552A..13T Altcode: 2013arXiv1302.2013T A sequence of pure-hydrogen CO5BOLD 3D model atmospheres of DA white dwarfs is presented for a surface gravity of log g = 8 and effective temperatures from 6000 to 13 000 K. We show that convective properties, such as flow velocities, characteristic granulation size and intensity contrast of the granulation patterns, change significantly over this range. We demonstrate that these 3D simulations are not sensitive to numerical parameters unlike the 1D structures that considerably depend on the mixing-length parameters. We conclude that 3D spectra can be used directly in the spectroscopic analyses of DA white dwarfs. We confirm the result of an earlier preliminary study that 3D model spectra provide a much better characterization of the mass distribution of white dwarfs and that shortcomings of the 1D mixing-length theory are responsible for the spurious high-log g determinations of cool white dwarfs. In particular, the 1D theory is unable to account for the cooling effect of the convective overshoot in the upper atmospheres. Title: Convective line shifts for the Gaia RVS from the CIFIST 3D model atmosphere grid Authors: Allende Prieto, C.; Koesterke, L.; Ludwig, H. -G.; Freytag, B.; Caffau, E. Bibcode: 2013A&A...550A.103A Altcode: 2013arXiv1301.3703A Context. To derive space velocities of stars along the line of sight from wavelength shifts in stellar spectra requires accounting for a number of second-order effects. For most stars, gravitational redshifts, convective blueshifts, and transverse stellar motion are the dominant contributors.
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 http://www.aanda.orgModel spectra from the 1D and 3D calculations are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr130.79.128.5 or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/550/A103 Title: Radiation hydrodynamics simulations of brown dwarf atmospheres with CO5BOLD Authors: Freytag, B.; Allard, F.; Homeier, D. Bibcode: 2013MmSAI..84.1070F Altcode: The interplay of radiative and hydrodynamic processes governs the stratified atmospheres of cool stars and substellar objects, with the underlying convection zone playing an important role. At sufficiently low temperatures, matter can condense into dust grains, whose formation, growths, destruction, and gravitational settling critically depend on the thermal structure of the atmosphere. Equally important are horizontal and vertical mixing and transport of material, that can compensate the rain-out of dust, induced on small scales by convective motions, overshoot, gravity waves, and turbulence, but also on larger scales by a global wind system strongly influenced by the rapid rotation of a typical brown dwarf. Local and first simple global radiation-hydrodynamics simulations of cool objects with the CO5BOLD code are presented. Title: First steps with CO5BOLD using HLLMHD and PP reconstruction . Authors: Steiner, O.; Rajaguru, S. P.; Vigeesh, G.; Steffen, M.; Schaffenberger, W.; Freytag, B. Bibcode: 2013MSAIS..24..100S Altcode: We report on first experiences with real-life applications using the MHD-module of CO5BOLD together with the piecewise parabolic reconstruction scheme and present preliminary results of stellar magnetic models with 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. Bibcode: 2013MSAIS..24...61T Altcode: Time-dependent 3D simulations of pure-hydrogen DA white dwarf atmospheres have been computed in recent years. Synthetic Balmer lines spectra drawn from these radiation-hydrodynamics (RHD) simulations have been shown to predict surface gravities significantly lower than the standard 1D models, in much better agreement with the expectation that white dwarfs cool at constant mass. We have now computed a grid of CO5BOLD pure-hydrogen 3D model atmospheres for surface gravities from log g = 7 to log g = 8.5 and effective temperatures from 6000 to 13,000 K. Over this range, we observe a significant variation of the intensity contrast of the surface granulation patterns, which indicates the strength of the 3D effects. Furthermore, the size and appearance of granules are also varying considerably. An explanation of these behaviours can lead to a better understanding of the physical processes responsible for the energy transfer in white dwarf atmospheres. Title: Progress in modeling very low mass stars, brown dwarfs, and planetary mass objects. Authors: Allard, F.; Homeier, D.; Freytag, B.; Schaffenberger, W.; Rajpurohit, A. S. Bibcode: 2013MSAIS..24..128A Altcode: 2013arXiv1302.6559A We review recent advancements in modeling the stellar to substellar transition. The revised molecular opacities, solar oxygen abundances and cloud models allow to reproduce the photometric and spectroscopic properties of this transition to a degree never achieved before, but problems remain in the important M-L transition characteristic of the effective temperature range of characterizable exoplanets. We discuss of the validity of these classical models. We also present new preliminary global Radiation HydroDynamical M dwarfs simulations. Title: Atmospheres from very low-mass stars to extrasolar planets. Authors: Allard, F.; Homeier, D.; Freytag, B. Bibcode: 2013MmSAI..84.1053A Altcode: Within the next few years, several instruments aiming at imaging extrasolar planets will see first light. In parallel, low mass planets are being searched around red dwarfs which offer more favorable conditions, both for radial velocity detection and transit studies, than solar-type stars. We review recent advancements in modeling the stellar to substellar transition. The revised solar oxygen abundances and cloud models allow to reproduce the photometric and spectroscopic properties of this transition to a degree never achieved before, but problems remain in the important M-L transition characteristic of the T_{eff } range of characterisable exoplanets. Title: Advances in the hydrodynamics solver of CO5BOLD Authors: Freytag, Bernd Bibcode: 2013MSAIS..24...26F Altcode: Many features of the Roe solver used in the hydrodynamics module of CO5BOLD have recently been added or overhauled, including the reconstruction methods (by adding the new second-order ``Frankenstein's method''), the treatment of transversal velocities, energy-flux averaging and entropy-wave treatment at small Mach numbers, the CTU scheme to combine the one-dimensional fluxes, and additional safety measures. All this results in a significantly better behavior at low Mach number flows, and an improved stability at larger Mach numbers requiring less (or no) additional tensor viscosity, which then leads to a noticeable increase in effective resolution. Title: Oxygen spectral line synthesis: 3D non-LTE with CO5BOLD hydrodynamical model atmospheres. Authors: Prakapavičius, D.; Steffen, M.; Kučinskas, A.; Ludwig, H. -G.; Freytag, B.; Caffau, E.; Cayrel, R. Bibcode: 2013MSAIS..24..111P Altcode: 2013arXiv1303.2016P In this work we present first results of our current project aimed at combining the 3D hydrodynamical stellar atmosphere approach with non-LTE (NLTE) spectral line synthesis for a number of key chemical species. We carried out a full 3D-NLTE spectrum synthesis of the oxygen IR 777 nm triplet, using a modified and improved version of our NLTE3D package to calculate departure coefficients for the atomic levels of oxygen in a 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. Title: 3D Model Atmospheres of DA White Dwarfs Authors: Tremblay, P. -E.; Ludwig, H. -G.; Steffen, M.; Freytag, B. Bibcode: 2013ASPC..469..155T Altcode: The spectroscopically determined surface gravities of cool hydrogen-atmosphere DA white dwarfs are significantly higher than the mean value of log g ∼ 8 found for hotter objects with radiative atmospheres (Teff > 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. Title: VizieR Online Data Catalog: Model 1D (LHD) and 3D (CO5BOLD) spectra (Allende Prieto+, 2013) Authors: Allende Prieto, C.; Koesterke, L. Ludwig H. -G.; Freytag, B.; Caffau, E. Bibcode: 2012yCat..35500103A Altcode: 2012yCat..35509103A Model spectral fluxes for late-type stars computed from 3D hydrodynamical simulations of surface convection performed with the CO5BOLD code. Their 1D hydrostatic counterparts are included, based on the LHD code, sharing the same microphysics as the CO5BOLD models. The fluxes for both the 3D and 1D models are calculated with the same opacities and radiative transfer code (ASSET).

(6 data files). Title: Atmospheres From Very Low-Mass Stars to Extrasolar Planets Authors: Allard, F.; Homeier, D.; Freytag, B.; Sharp, C. M. Bibcode: 2012EAS....57....3A Altcode: 2012arXiv1206.1021A Within the next few years, several instruments aiming at imaging extrasolar planets will see first light. In parallel, low mass planets are being searched around red dwarfs which offer more favorable conditions - both for radial velocity detection and transit studies - than solar-type stars. We review recent advancements in modeling the stellar to substellar transition. The revised solar oxygen abundances and cloud models allow to reproduce the photometric and spectroscopic properties of this transition to a degree never achieved before, but problems remain at the stellar-brown dwarf transition typical of the Teff range of characterizable exoplanets. Title: Models of very-low-mass stars, brown dwarfs and exoplanets Authors: Allard, F.; Homeier, D.; Freytag, B. Bibcode: 2012RSPTA.370.2765A Altcode: 2011arXiv1112.3591A Within the next few years, GAIA and several instruments aiming at imag- ing extrasolar planets will see first light. In parallel, low mass planets are being searched around red dwarfs which offer more favourable conditions, both for radial velocity de- tection and transit studies, than solar-type stars. Authors of the model atmosphere code which has allowed the detection of water vapour in the atmosphere of Hot Jupiters re- view recent advancement in modelling the stellar to substellar transition. The revised solar oxygen abundances and cloud model allow for the first time to reproduce the pho- tometric and spectroscopic properties of this transition. Also presented are highlight results of a model atmosphere grid for stars, brown dwarfs and extrasolar planets. Title: Imaging the heart of astrophysical objects with optical long-baseline interferometry Authors: Berger, J. -P.; Malbet, F.; Baron, F.; Chiavassa, A.; Duvert, G.; Elitzur, M.; Freytag, B.; Gueth, F.; Hönig, S.; Hron, J.; Jang-Condell, H.; Le Bouquin, J. -B.; Monin, J. -L.; Monnier, J. D.; Perrin, G.; Plez, B.; Ratzka, T.; Renard, S.; Stefl, S.; Thiébaut, E.; Tristram, K. R. W.; Verhoelst, T.; Wolf, S.; Young, J. Bibcode: 2012A&ARv..20...53B Altcode: 2012arXiv1204.4363B The number of publications of aperture-synthesis images based on optical long-baseline interferometry measurements has recently increased due to easier access to visible and infrared interferometers. The interferometry technique has now reached a technical maturity level that opens new avenues for numerous astrophysical topics requiring milli-arcsecond model-independent imaging. In writing this paper our motivation was twofold: (1) review and publicize emblematic excerpts of the impressive corpus accumulated in the field of optical interferometry image reconstruction; (2) discuss future prospects for this technique by selecting four representative astrophysical science cases in order to review the potential benefits of using optical long-baseline interferometers. For this second goal we have simulated interferometric data from those selected astrophysical environments and used state-of-the-art codes to provide the reconstructed images that are reachable with current or soon-to-be facilities. The image-reconstruction process was "blind" in the sense that reconstructors had no knowledge of the input brightness distributions. We discuss the impact of optical interferometry in those four astrophysical fields. We show that image-reconstruction software successfully provides accurate morphological information on a variety of astrophysical topics and review the current strengths and weaknesses of such reconstructions. We investigate how to improve image reconstruction and the quality of the image possibly by upgrading the current facilities. We finally argue that optical interferometers and their corresponding instrumentation, existing or to come, with six to ten telescopes, should be well suited to provide images of complex sceneries. Title: Stellar to Substellar Model Atmospheres Authors: Allard, France; Homeier, Derek; Freytag, Bernd Bibcode: 2012IAUS..282..235A Altcode: The spectral transition from Very Low Mass stars (VLMs) to brown dwarfs (BDs) and planetary mass objects (Planemos) requires model atmospheres that can treat line, molecule, and dust-cloud formation with completeness and accuracy. One of the essential problems is the determination of the surface velocity field throughout the main sequence down to the BD and planemo mass regimes. We present local 2D and 3D radiation hydrodynamic simulations using the CO5BOLD code with binned Phoenix gas opacities, forsterite dust formation (and opacities) and rotation. The resulting velocity field vs depth and Teff has been used in the general purpose model atmosphere code Phoenix, adapted in static 1D spherical symmetry for these cool atmospheres. The result is a better understanding of the spectral transition from the stellar to substellar regimes. However, problems remain in reproducing the colors of the dustiest brown dwarfs. The global properties of rotation can change the averaged spectral properties of these objects. Our project for the period 2011-2015 is therefore to develop scaled down global 3D simulations of convection, cloud formation and rotation thanks to funding by the Agence Nationale de la Recherche in France. Title: Simulations of the solar near-surface layers with the CO5BOLD, MURaM, and Stagger codes Authors: Beeck, B.; Collet, R.; Steffen, M.; Asplund, M.; Cameron, R. H.; Freytag, B.; Hayek, W.; Ludwig, H. -G.; Schüssler, M. Bibcode: 2012A&A...539A.121B Altcode: 2012arXiv1201.1103B Context. Radiative hydrodynamic simulations of solar and stellar surface convection have become an important tool for exploring the structure and gas dynamics in the envelopes and atmospheres of late-type stars and for improving our understanding of the formation of stellar spectra.
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. Title: Simulations of stellar convection with CO5BOLD Authors: Freytag, B.; Steffen, M.; Ludwig, H. -G.; Wedemeyer-Böhm, S.; Schaffenberger, W.; Steiner, O. Bibcode: 2012JCoPh.231..919F Altcode: 2011arXiv1110.6844F High-resolution images of the solar surface show a granulation pattern of hot rising and cooler downward-sinking material - the top of the deep-reaching solar convection zone. Convection plays a role for the thermal structure of the solar interior and the dynamo acting there, for the stratification of the photosphere, where most of the visible light is emitted, as well as for the energy budget of the spectacular processes in the chromosphere and corona. Convective stellar atmospheres can be modeled by numerically solving the coupled equations of (magneto)hydrodynamics and non-local radiation transport in the presence of a gravity field. The CO5BOLD code described in this article is designed for so-called "realistic" simulations that take into account the detailed microphysics under the conditions in solar or stellar surface layers (equation-of-state and optical properties of the matter). These simulations indeed deserve the label "realistic" because they reproduce the various observables very well - with only minor differences between different implementations. The agreement with observations has improved over time and the simulations are now well-established and have been performed for a number of stars. Still, severe challenges are encountered when it comes to extending these simulations to include ideally the entire star or substellar object: the strong stratification leads to completely different conditions in the interior, the photosphere, and the corona. Simulations have to cover spatial scales from the sub-granular level to the stellar diameter and time scales from photospheric wave travel times to stellar rotation or dynamo cycle periods. Various non-equilibrium processes have to be taken into account. Last but not least, realistic simulations are based on detailed microphysics and depend on the quality of the input data, which can be the actual accuracy limiter. This article provides an overview of the physical problem and the numerical solution and the capabilities of CO5BOLD, illustrated with a number of applications. Title: Model Atmospheres From Very Low Mass Stars to Brown Dwarfs Authors: Allard, F.; Homeier, D.; Freytag, B. Bibcode: 2011ASPC..448...91A Altcode: 2011csss...16...91A; 2010arXiv1011.5405A Since the discovery of brown dwarfs in 1994, and the discovery of dust cloud formation in the latest Very Low Mass Stars (VLMs) and Brown Dwarfs (BDs) in 1996, the most important challenge in modeling their atmospheres as become the understanding of cloud formation and advective mixing. For this purpose, we have developed radiation hydrodynamic 2D model atmosphere simulations to study the formation of forsterite dust in presence of advection, condensation, and sedimentation across the M-L-T VLMs to BDs sequence (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 > Teff > 400 K, 5.5 > logg > -0.5, and [M/H]= +0.5 to -1.5, and the reference solar abundances of Asplund et al. (2009). We found that the new solar abundances allow an improved (close to perfect) reproduction of the photometric and spectroscopic VLMs properties, and, for the first time, a smooth transition between stellar and substellar regimes -- unlike the transition between the NextGen models from Hauschildt et al. 1999a,b, and the AMES-Dusty models from Allard et al. 2001. In the BDs regime, the BT-Settl models propose an improved explanation for the M-L-T spectral transition. In this paper, we therefore present the new BT-Settl model atmosphere grid, which explains the entire transition from the stellar to planetary mass regimes. Title: Radiation-Hydrodynamics Simulations of Cool Stellar and Substellar Atmospheres Authors: Freytag, B.; Allard, F.; Ludwig, H. -G.; Homeier, D.; Steffen, M. Bibcode: 2011ASPC..448..855F Altcode: 2011csss...16..855F In the atmospheres of brown dwarfs, not only molecules but much larger and heavier "dust" particles can form. The latter should sink under the influence of gravity into deeper layers and vanish from the atmosphere, clearing it from condensable material. However, observed spectra can only be reproduced by models assuming the presence of dust and its resulting greenhouse effect in the visible layers. Apparently, hydrodynamical mixing can counteract the gravitational settling. We present new 2D and 3D radiation-hydrodynamics simulations with CO5BOLD of the upper part of the convection zone and the atmosphere of cool stars and brown dwarfs in a range of temperatures and gravities that enable the formation of dust clouds in the visible layers. We find that the differences between 2D and 3D models are remarkably small. Lowering the gravity has a somewhat similar effect on the surface intensity contrast as increasing the effective temperature. The biggest uncertainties of the simulations come from approximations made in the description of the dust chemistry. Global circulation and rotation likely play an important role. Title: Radiation Hydrodynamics Simulations of Dust Clouds in the Atmospheres of Substellar Objects Authors: Freytag, B.; Allard, F.; Homeier, D.; Ludwig, H.; Steffen, M. Bibcode: 2011ASPC..450..125F Altcode: The temperature structure and the motions in the atmospheres of cool stars are affected by the underlying convection zone. The radiation hydrodynamics code CO5BOLD has been developed to simulate (small patches of the) convective surface layers of these stars. Updated opacity tables based on PHOENIX data and a description for the formation, destruction, advective transport, and settling of dust have made the code fit to handle the conditions in brown dwarf atmospheres. Currently, objects from 8500 K down to about 900 K have been simulated. Recently, incident radiation has been included, allowing simulations with conditions found on hot planets. In non-irradiated brown dwarf models we encounter mixing by gravity waves and in the cooler models convection within the clouds. The qualitative effects of incident radiation are surprisingly small, as long as the effective temperature of the object stays well below the dust condensation temperature. Beyond that point, there are no layers where dust could form, anymore. Title: Radiative hydrodynamics simulations of red supergiant stars. IV. Gray versus non-gray opacities Authors: Chiavassa, A.; Freytag, B.; Masseron, T.; Plez, B. Bibcode: 2011A&A...535A..22C Altcode: 2011arXiv1109.3619C Context. Red supergiants are massive evolved stars that contribute extensively to the chemical enrichment of our Galaxy. It has been shown that convection in those stars produces large granules that cause surface inhomogeneities and shock waves in the photosphere. The understanding of their dynamics is crucial for unveiling the unknown mass-loss mechanism, their chemical composition, and their stellar parameters.
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. Title: Photocentric and Photometric Variability of Red Supergiant Stars Authors: Chiavassa, A.; Pasquato, E.; Jorissen, A.; Sacuto, S.; Babusiaux, C.; Freytag, B.; Ludwig, H. -G.; Cruzalèbes, P.; Rabbia, Y.; Spang, A.; Chesneau, O. Bibcode: 2011ASPC..445..169C Altcode: Red supergiant stars are characterized by large convection-related surface structures that cause surface inhomogeneities and shock waves. We explore the impact of granulation on photocentric and photometric variability using 3D simulations of convection with Co5BOLD and the post-processing radiative transfer code OPTIM 3D to compute spectra and intensity maps in the Gaia G band (325 - 1030 nm). Title: Solution to the problem of the surface gravity distribution of cool DA white dwarfs from improved 3D model atmospheres Authors: Tremblay, P. -E.; Ludwig, H. -G.; Steffen, M.; Bergeron, P.; Freytag, B. Bibcode: 2011A&A...531L..19T Altcode: 2011arXiv1106.6007T The surface gravities of cool (Teff < 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 < Teff < 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 Title: Radiative hydrodynamic simulations of red supergiant stars. III. Spectro-photocentric variability, photometric variability, and consequences on Gaia measurements Authors: Chiavassa, A.; Pasquato, E.; Jorissen, A.; Sacuto, S.; Babusiaux, C.; Freytag, B.; Ludwig, H. -G.; Cruzalèbes, P.; Rabbia, Y.; Spang, A.; Chesneau, O. Bibcode: 2011A&A...528A.120C Altcode: 2010arXiv1012.5234C Context. It has been shown that convection in red supergiant stars (RSG) gives rise to large granules that cause surface inhomogeneities and shock waves in the photosphere. The resulting motion of the photocentre (on time scales ranging from months to years) could possibly have adverse effects on the parallax determination with Gaia.
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. Title: Solar Chemical Abundances Determined with a CO5BOLD 3D Model Atmosphere Authors: Caffau, E.; Ludwig, H. -G.; Steffen, M.; Freytag, B.; Bonifacio, P. Bibcode: 2011SoPh..268..255C Altcode: 2010SoPh..tmp...66C; 2010arXiv1003.1190C In the last decade, the photospheric solar metallicity as determined from spectroscopy experienced a remarkable downward revision. Part of this effect can be attributed to an improvement of atomic data and the inclusion of NLTE computations, but also the use of hydrodynamical model atmospheres seemed to play a role. This "decrease" with time of the metallicity of the solar photosphere increased the disagreement with the results from helioseismology. With a 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. Title: Photocentric variability of red supergiant stars and consequences on Gaia measurements Authors: Chiavassa, A.; Pasquato, E.; Jorissen, A.; Sacuto, S.; Babusiaux, C.; Freytag, B.; Ludwig, H. -G.; Cruzalèbes, P.; Rabbia, Y.; Spang, A.; Chesneau, O. Bibcode: 2010sf2a.conf..339C Altcode: Red supergiant stars are characterized by large convection-related surface structures that cause surface inhomogeneities and shock waves. We explore the impact of granulation on the photocentric motion using 3D simulations of convection with CO5BOLD and post-processing radiative transfer code Optim3D to compute spectra and intensity maps in the Gaia G band [325 -- 1030~nm]. We found that the Gaia parallax for Betelgeuse-like supergiants are characterized by a systematic error of a few percents. Title: The metal-poor end of the Spite plateau. I. Stellar parameters, metallicities, and lithium abundances Authors: Sbordone, L.; Bonifacio, P.; Caffau, E.; Ludwig, H. -G.; Behara, N. T.; González Hernández, J. I.; Steffen, M.; Cayrel, R.; Freytag, B.; van't Veer, C.; Molaro, P.; Plez, B.; Sivarani, T.; Spite, M.; Spite, F.; Beers, T. C.; Christlieb, N.; François, P.; Hill, V. Bibcode: 2010A&A...522A..26S Altcode: 2010arXiv1003.4510S Context. The primordial nature of the Spite plateau is at odds with the WMAP satellite measurements, implying a primordial Li production at least three times higher than observed. It has also been suggested that A(Li) might exhibit a positive correlation with metallicity below [Fe/H] ~ -2.5. Previous samples studied comprised few stars below [Fe/H] = -3.
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] <-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 < 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 <A(Li)3D, NLTE> = 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 http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/522/A26IDL code (appendix) is only available in electronic form at http://www.aanda.org Title: Brown Dwarf Model Atmospheres Based on Multi-Dimensional Radiation Hydrodynamics Authors: Allard, France; Freytag, Bernd Bibcode: 2010HiA....15..756A Altcode: The atmospheres of Brown Dwarfs (BDs) are the site of molecular opacities and cloud formation, and control their cooling rate, radius and brightness evolution. Brown dwarfs evolve from stellar-like properties (magnetic activity, spots, flares, mass loss) to planet-like properties (electron degeneracy of the interior, cloud formation, dynamical molecular transport) while retaining, due to their fully convective interior, larger rotational velocities (≤ 30 km/s i.e. P < 4 hrs versus 11 hrs for Jupiter). Model atmospheres treating all this complexity are therefore essential to understand the evolution properties, and to interpret the observations of these objects. While the pure gas-phase based NextGen model atmospheres (Allard et al. 1997, Hauschildt et al. 1999) have allowed the understanding of the several populations of Very Low Mass Stars (VLMs), the AMES-Dusty models (Allard et al. 2001) based on equilibrium chemistry have reproduced some near-IR photometric properties of M and L-type brown dwarfs, and played a key role in the determination of the mass of brown dwarfs and Planetary Mass Objects (PMOs) in the eld and in young stellar clusters. In this paper, we present a new model atmosphere grid for VLMs, BDs, PMOs named BT-Settl, which includes a cloud model and dynamical molecular transport based on mixing information from 2D Radiation Hydrodynamic (RHD) simulations (Freytag et al. 2009). We also present the status of our 3D RHD simulations including rotation (Coriolis forces) of a cube on the surface of a brown dwarf. The BT-Settl model atmosphere grid will be available shortly via the Phoenix web simulator (http://phoenix.ens-lyon.fr/simulator/). Title: CO5BOLD: COnservative COde for the COmputation of COmpressible COnvection in a BOx of L Dimensions with l=2,3 Authors: Freytag, Bernd; Steffen, Matthias; Wedemeyer-Böhm, Sven; Ludwig, Hans-Günter; Leenaarts, Jorrit; Schaffenberger, Werner; Allard, France; Chiavassa, Andrea; Höfner, Susanne; Kamp, Inga; Steiner, Oskar Bibcode: 2010ascl.soft11014F Altcode: CO5BOLD - nickname COBOLD - is the short form of "COnservative COde for the COmputation of COmpressible COnvection in a BOx of L Dimensions with l=2,3".

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 & 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. Bibcode: 2010ASPC..429..167V Altcode: 2010arXiv1003.5555V We report on the development of an implicit multi-D hydrodynamic code for stellar evolution. We present two test-cases relevant for the first scientific goal of the code: the simulation of convection in pulsating stars. First results on a realistic stellar model are also presented. Title: Galactic evolution of oxygen. OH lines in 3D hydrodynamical model atmospheres Authors: González Hernández, J. I.; Bonifacio, P.; Ludwig, H. -G.; Caffau, E.; Behara, N. T.; Freytag, B. Bibcode: 2010A&A...519A..46G Altcode: 2010arXiv1005.3754G Context. Oxygen is the third most common element in the Universe. The measurement of oxygen lines in metal-poor unevolved stars, in particular near-UV OH lines, can provide invaluable information about the properties of the Early Galaxy.
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 < [Fe/H] < 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. Bibcode: 2010yCat..35220026S Altcode: 2010yCat..35229026S Line-by-line abundances for FeI and FeII lines used to estimate metallicity and gravity for the program stars. The first column lists the star name, then the ion (FeI or FeII) The the wavelength in nm, the loggf, the measured EW (pm) and the derived abundance assuming the four stellar parameter sets used in the article, respectively 3D, BA, ALI and IRFM.

(3 data files). Title: Radiative hydrodynamics simulations of red supergiant stars. II. Simulations of convection on Betelgeuse match interferometric observations Authors: Chiavassa, A.; Haubois, X.; Young, J. S.; Plez, B.; Josselin, E.; Perrin, G.; Freytag, B. Bibcode: 2010A&A...515A..12C Altcode: 2010arXiv1003.1407C Context. The red supergiant (RSG) Betelgeuse is an irregular variable star. Convection may play an important role in understanding this variability. Interferometric observations can be interpreted using sophisticated simulations of stellar convection.
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. Bibcode: 2010A&A...514A..92C Altcode: 2010arXiv1002.2628C Context. The analysis of the solar spectra using hydrodynamical simulations, with a specific selection of lines, atomic data, and method for computing deviations from local thermodynamical equilibrium, has led to a downward revision of the solar metallicity, Z. We are using the latest simulations computed with the CO5BOLD code to reassess the solar chemical composition. Our previous analyses of the key elements, oxygen and nitrogen, have not confirmed any extreme downward revision of Z, as derived in other works based on hydrodynamical models.
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. Bibcode: 2010A&A...513A..19F Altcode: 2010arXiv1002.3437F Context. Observationally, spectra of brown dwarfs indicate the presence of dust in their atmospheres while theoretically it is not clear what prevents the dust from settling and disappearing from the regions of spectrum formation. Consequently, standard models have to rely on ad hoc assumptions about the mechanism that keeps dust grains aloft in the atmosphere.
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. Bibcode: 2010IAUS..268..355S Altcode: We recently presented (Sbordone et al., 2009a) the largest sample to date of lithium abundances in extremely metal-poor (EMP) Halo dwarf and Turn-Off (TO) stars. One of the most crucial aspects in estimating Li abundances is the 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 Bibcode: 2010IAUS..265..201L Altcode: 2009arXiv0911.4248L We present solar photospheric abundances for 12 elements from optical and near-infrared spectroscopy. The abundance analysis was conducted employing 3D hydrodynamical (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. Bibcode: 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 <A(Li)3D, NLTE> = 2.199 ± 0.086. If the primordial A(Li) is the one derived from standard Big Bang Nucleosynthesis (BBN), it appears difficult to envision a single depletion phenomenon producing a thin, metallicity independent plateau above [Fe/H] = -2.8, and a highly scattered, metallicity dependent distribution below. Title: VLTI/AMBER spectro-interferometric imaging of VX Sagittarii's inhomogenous outer atmosphere Authors: Chiavassa, A.; Lacour, S.; Millour, F.; Driebe, T.; Wittkowski, M.; Plez, B.; Thiébaut, E.; Josselin, E.; Freytag, B.; Scholz, M.; Haubois, X. Bibcode: 2010A&A...511A..51C Altcode: 2009arXiv0911.4422C
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. Bibcode: 2009A&A...506.1351C Altcode: 2009arXiv0907.1860C Context: It has been proposed that convection in red supergiant (RSG) stars produces large-scale granules causing observable surface inhomogeneities. This convection is also extremely vigorous and is suspected to be one of the main causes of mass-loss in RSGs. It should thus be understood in detail. Evidence has accumulated of asymmetries in the photospheres of RSGs, but detailed studies of granulation are still lacking. Interferometric observations provide an innovative way of addressing this question, but they are still often interpreted using smooth symmetrical limb-darkened intensity distributions, or simple, spotted, ad hoc models.
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 Bibcode: 2009AIPC.1094..489F Altcode: 2009csss...15..489F Observed spectra of brown dwarfs demonstrate that their atmospheres are influenced by dust. To investigate the mechanism that controls the formation and gravitational settling of dust grains as well as the mixing of fresh condensable material into the atmosphere, we performed 2D radiation-hydrodynamics simulations with CO5BOLD. The models comprise the upper part of the convection zone and the atmosphere with the dust cloud layers. We find that direct convective overshoot does not play a major role. Instead, the mixing in the clouds is controlled by gravity waves. Title: The CIFIST 3D model atmosphere grid. Authors: Ludwig, H. -G.; Caffau, E.; Steffen, M.; Freytag, B.; Bonifacio, P.; Kučinskas, A. Bibcode: 2009MmSAI..80..711L Altcode: 2009arXiv0908.4496L Grids of stellar atmosphere models and associated synthetic spectra are numerical products which have a large impact in astronomy due to their ubiquitous application in the interpretation of radiation from individual stars and stellar populations. 3D model atmospheres are now on the verge of becoming generally available for a wide range of stellar atmospheric parameters. We report on efforts to develop a grid of 3D model atmospheres for late-type stars within the CIFIST Team at Paris Observatory. The substantial demands in computational and human labor for the model production and post-processing render this apparently mundane task a challenging logistic exercise. At the moment the CIFIST grid comprises 77 3D model atmospheres with emphasis on dwarfs of solar and sub-solar metallicities. While the model production is still ongoing, first applications are already worked upon by the CIFIST Team and collaborators. Title: Simulations of dust clouds in the atmospheres of substellar objects. Theory toddles after observations Authors: Freytag, B.; Allard, F.; Ludwig, H. -G.; Homeier, D.; Steffen, M. Bibcode: 2009MmSAI..80..670F Altcode: The atmospheres of brown dwarfs allow the formation of dust grains and their rain-out into deeper, invisible layers. However, observed spectra of L dwarfs can only be reproduced when static 1D models account for dust formation and its resulting greenhouse effect in the visible layers. Time-dependent hydrodynamical processes can mix up the material giving rise to complex unsteady weather phenomena on these objects. We performed radiation hydrodynamics simulations in two and three dimensions of the atmospheres of brown dwarfs with CO5BOLD, including a treatment of dust particles. We find that exponential overshoot (close to the gas convection zone), gravity waves (weak omni-present mixing), and convection within dust layers (in the thick clouds in cooler models) contribute to the atmospheric mixing, which is far from being a stationary process. The presence of dust in the atmospheres is accompanied by large temporal and spatial intensity fluctuations. Title: Models of surface convection and dust clouds in brown dwarfs Authors: Freytag, B.; Allard, F.; Ludwig, H. -G.; Homeier, D.; Steffen, M. Bibcode: 2008PhST..133a4005F Altcode: The influence of dust grains on the atmospheres of brown dwarfs is visible in observed spectra. To investigate what prevents the dust grains from falling down, or how fresh condensable material is mixed up in the atmosphere to allow new grains to form, we performed 2D radiation-hydrodynamics simulations with CO5BOLD of the upper part of the convection zone and the atmosphere containing the dust cloud layers. We find that unlike in models of Cepheids, the convective overshoot does not play a major role. Instead, the mixing in the dust clouds is controlled by gravity waves. Title: he models comprise the upper part of the convection zone and the atmosphere with %the dust cloud layers. We find that direct convective overshoot does not play a major role. Instead, the mixing in the clouds is controlled by gravity waves. Authors: Freytag, B.; Allard, F.; Ludwig, H. -G.; Homeier, D.; Steffen, M.; Sharp, C. Bibcode: 2008sf2a.conf..469F Altcode: To investigate the mechanism that controls the formation and gravitational settling of dust grains as well as the mixing of fresh condensable material into the atmosphere of brown dwarfs, we performed 2D radiation-hydrodynamics simulations with CO5BOLD. Title: The photospheric solar oxygen project. I. Abundance analysis of atomic lines and influence of atmospheric models Authors: Caffau, E.; Ludwig, H. -G.; Steffen, M.; Ayres, T. R.; Bonifacio, P.; Cayrel, R.; Freytag, B.; Plez, B. Bibcode: 2008A&A...488.1031C Altcode: 2008arXiv0805.4398C Context: The solar oxygen abundance has undergone a major downward revision in the past decade, the most noticeable one being the update including 3D hydrodynamical simulations to model the solar photosphere. Up to now, such an analysis has only been carried out by one group using one radiation-hydrodynamics code.
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 Bibcode: 2008asd..soft...36F Altcode: The code is used to perform radiation hydrodynamics simulations of the convective surface layers and the photosphere of cool stars. Title: The solar photospheric abundance of europium. Results from CO5BOLD 3D hydrodynamical model atmospheres Authors: Mucciarelli, A.; Caffau, E.; Freytag, B.; Ludwig, H. -G.; Bonifacio, P. Bibcode: 2008A&A...484..841M Altcode: 2008arXiv0803.0863M Context: Europium is an almost pure r-process element, which may be useful as a reference in nucleocosmochronology.
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. Bibcode: 2008A&A...483..571F Altcode: Context: Winds of asymptotic giant branch stars are assumed to be driven by radiation pressure on dust. Previously, this process has been modeled with detailed time-dependent simulations of atmospheres and winds assuming spherically symmetric flows. In such models kinetic energy is injected by a variable inner boundary (“piston”) simulating the effects of stellar pulsation. However, the dynamical processes in these atmospheres - convection and pulsations - are actually three-dimensional.
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. Bibcode: 2008arXiv0802.1403C Altcode: We have written a 3D radiative transfer code that computes emerging spectra and intensity maps. We derive from radiative hydrodynamic (RHD) simulations of RSG stars carried out with CO5BOLD (Freytag et al. 2002) observables expected for red supergiant stars (RSG) especially for interferometric observations, with emphasis on small scale structures. We show that the convection-related surface structures are detectable in the H band with today's interferometers and that the diameter measurement should not be too dependent on the adopted model. The simulations are a great improvement over parametric models for the interpretation of interferometric observations. Title: Numerical Simulations of Stellar Surface Convection and Related Phenomena Authors: Freytag, B. Bibcode: 2008EAS....28....9F Altcode: Sophisticated radiative transfer methods have been used for decades to model one-dimensional static stellar atmospheres. They predict an outward decrease of the atmospheric temperature that is now observable with simple one-baseline interferometers via measurements of limb darkening. However, the surface layers of many stars are affected by convection which requires a treatment by time-dependent multi-dimensional radiation hydrodynamics simulations. Solar granulation is directly observable with “ordinary” telescopes. The simulated granule pattern and evolution compares well with the observed ones. The upcoming radio interferometer ALMA could be used to probe the convection induced shock-pattern in the chromosphere that is predicted by simulations and that is not easily observable otherwise. The typical granular scale on other near main-sequence stars is too small to be accessible by interferometers. However, scaling arguments and recent numerical simulations predict very large structures on cool supergiants and AGB stars. These stars were and are candidates for optical/near-infrared interferometry. The complexity of the predicted surface phenomena requires good (or at least some) spatial resolution in conjunction with temporal and frequency resolution. To fully exploit and interpret these data the simulations have to be improved in terms of treatment of microphysics (especially opacities in the radiative transfer step) and spatial resolution. Title: Rotating `star-in-a-box' experiments Authors: Steffen, M.; Freytag, B. Bibcode: 2007AN....328.1054S Altcode: Using the radiation hydrodynamics code 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. Bibcode: 2007ASPC..378...43H Altcode: 2007arXiv0709.0197H We review the current state of modeling convective mixing in AGB stars. The focus is on results obtained through multi-dimensional hydrodynamic simulations of AGB convection, both in the envelope and in the unstable He-shell. Using two different codes and a wide range of resolutions and modeling assumptions we find that mixing across convective boundaries is significant for He-shell flash convection. We present a preliminary quantitative analysis of this convectively induced extra mixing, based on a sub-set of our simulations. Other non-standard mixing will be discussed briefly. Title: Inter-network regions of the Sun at millimetre wavelengths Authors: Wedemeyer-Böhm, S.; Ludwig, H. G.; Steffen, M.; Leenaarts, J.; Freytag, B. Bibcode: 2007A&A...471..977W Altcode: 2007arXiv0705.2304W Aims:The continuum intensity at wavelengths around 1 mm provides an excellent way to probe the solar chromosphere and thus valuable input for the ongoing controversy on the thermal structure and the dynamics of this layer. The synthetic continuum intensity maps for near-millimetre wavelengths presented here demonstrate the potential of future observations of the small-scale structure and dynamics of internetwork regions on the Sun.
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. Bibcode: 2007sf2a.conf..447C Altcode: We have written a 3D radiative transfer code that computes emerging spectra and intensity maps. We derive from radiative hydrodynamic (RHD) simulations of RSG stars carried out with CO^5BOLD (Freytag et al. 2002) observables expected for red supergiant stars (RSG) especially for interferometric observations, with emphasis on small scale structures. We show that the convection-related surface structures are detectable in the H band with today's interferometers and that the diameter measurement should not be too dependent on the adopted model. The simulations are a great improvement over parametric models for the interpretation of interferometric observations. Title: Formation of convective structures in stellar atmospheres Authors: Freytag, Bernd; Ludwig, Hans-Günter Bibcode: 2007sf2a.conf..481F Altcode: Convection is a ubiquitous phenomenon in cool stars. Its interplay with radiation leads to the formation of coherent flow structures -- granular cells -- on the visible surfaces of these stars. We model the processes with the 3D radiation-hydrodynamics code CO5BOLD in stars of different atmospheric parameters. We find that the granular cell size scales with the atmospheric pressure scale height for stars with a surface gravity log g>1.0 . However, the scaling breaks down for red supergiants having lower surface gravities. This qualitatively different behaviour is likely linked to sphericity effects and mainly to a larger contribution of radiation to the energy transport in the stellar envelope. Title: 3-D hydrodynamic simulations of convection in A stars Authors: Kochukhov, O.; Freytag, B.; Piskunov, N.; Steffen, M. Bibcode: 2007IAUS..239...68K Altcode: 2006astro.ph.10111K Broadening and asymmetry of spectral lines in slowly rotating late A-type stars provide evidence for high-amplitude convective motions. The properties of turbulence observed in the A-star atmospheres are not understood theoretically and contradict results of previous numerical simulations of convection. Here we describe an ongoing effort to understand the puzzling convection signatures of A stars with the help of 3-D hydrodynamic simulations. Our approach combines realistic spectrum synthesis and non-grey hydrodynamic models computed with the CO5BOLD code. We discuss these theoretical predictions and confront them with high-resolution spectra of A stars. Our models have, for the first time, succeeded in reproducing the observed profiles of weak spectral lines without introducing fudge broadening parameters. Title: First local helioseismic experiments with CO5BOLD Authors: Steiner, O.; Vigeesh, G.; Krieger, L.; Wedemeyer-Böhm, S.; Schaffenberger, W.; Freytag, B. Bibcode: 2007AN....328..323S Altcode: 2007astro.ph..1029S With numerical experiments we explore the feasibility of using high frequency waves for probing the magnetic fields in the photosphere and the chromosphere of the Sun. We track a plane-parallel, monochromatic wave that propagates through a non-stationary, realistic atmosphere, from the convection-zone through the photosphere into the magnetically dominated chromosphere, where it gets refracted and reflected. We compare the wave travel time between two fixed geometrical height levels in the atmosphere (representing the formation height of two spectral lines) with the topography of the surface of equal magnetic and thermal energy density (the magnetic canopy or β=1 contour) and find good correspondence between the two. We conclude that high frequency waves indeed bear information on the topography of the `magnetic canopy'. Title: Multi-dimensional Simulations of Helium Shell Flash Convection Authors: Hueckstaedt, Robert M.; Freytag, B.; Herwig, F.; Timmes, F. Bibcode: 2006AAS...20910107H Altcode: 2006BAAS...38.1046H The Asymptotic Giant Branch (AGB) phase is the most productive evolutionary phase in terms of nucleosynthesis for low and intermediate mass stars. Nucleosythesis in these stars is aided by the mixing and heating triggered by recurrent He-shell flashes. Extreme nuclear energies (corresponding to 10^8L_sun) are generated during these thermonuclear flashes. This results in efficient shell convection with multiple implications for nucleosynthesis and further evolution. The evolution of AGB stars, including these He-flashes, has in the past been studied exclusively with one-dimensional stellar evolution codes that have to adopt simplifying assumptions on mixing, especially at convective boundaries. Here, we report on efforts to augment these studies with 2-d and 3-d hydrodynamic models of convective mixing in the AGB He-shell. We characterize the dominant morphology of He-shell flash convection. As opposed to the shallow surface convection in A-type stars studied by Freytag et al. (1996), coherently moving convective cells do not cross the convective boundary significantly. In other words, penetration is minimal for this convection zone. We find that convective motions induce a rich spectrum of internal gravity waves in the neighboring stable layers. Interactions of these (mainly horizontal) oscillations with the convective boundary, as well as oscillations with convective characteristics within the stable layers, do cause a finite amount of mixing across the convective boundary. Our preliminary analysis of this mixing is consistent with semi-analytical results obtained from observations and 1D-stellar evolution simulations. Title: A First Three-Dimensional Model for the Carbon Monoxide Concentration in the Solar Atmosphere Authors: Wedemeyer-Böhm, S.; Kamp, I.; Freytag, B.; Bruls, J.; Steffen, M. Bibcode: 2006ASPC..354..301W Altcode: The time-dependent and self-consistent treatment of carbon monoxide (CO) has been added to the radiation chemo-hydrodynamics code CO5BOLD. It includes the solution of a chemical reaction network and the advection of the resulting particle densities with the hydrodynamic flow field. Here we present a first 3D simulation of the non-magnetic solar photosphere and low chromosphere, calculated with the upgraded code. In the resulting model, the highest amount of CO is located in the cool regions of the reversed granulation pattern in the middle photosphere. A large fraction of carbon is bound by CO throughout the chromosphere with exception of hot shock waves where the CO concentration is strongly reduced. The distribution of carbon monoxide is very inhomogeneous due to co-existing regions of hot and cool gas caused by the hydrodynamic flow. High-resolution observations of CO could thus provide important constraints for the thermal structure of the solar photosphere and chromosphere. Title: Holistic MHD-Simulation from the Convection Zone to the Chromosphere Authors: Schaffenberger, W.; Wedemeyer-Böhm, S.; Steiner, O.; Freytag, B. Bibcode: 2006ASPC..354..345S Altcode: A three-dimensional magnetohydrodynamic simulation of the integral layers from the convection zone to the chromosphere has been carried out. The simulation represents magnetoconvection in a quiet network-cell interior. The following preliminary new results are obtained: The chromospheric magnetic field is very dynamic with a continuous rearrangement of magnetic flux on a time scale of less than one~minute. Rapidly moving magnetic filaments (rarely exceeding 40~G) form in the compression zone downstream and along propagating shock fronts that are present throughout the chromosphere. The magnetic filaments rapidly move, form, and dissolve with the shock waves. Flux concentrations strongly expand through the photosphere into a more homogeneous, space filling chromospheric field. ``Canopy fields'' form on a granular scale above largely field-free granule centers leading to a mesh-work of current sheets in a height range between approximately 400 and 900~km. Title: Line formation in 3D radiation hydrodynamics simulations of red supergiants Authors: Chiavassa, A.; Plez, B.; Josselin, E.; Freytag, B. Bibcode: 2006sf2a.conf..455C Altcode: We developed a 3D radiative transfer code which computes emerging spectra and monochromatic maps from radiative-hydrodynamic (RHD) simulations of red supergiant stars. Computed emerging spectra show that our RHD models qualitatively reproduce the velocity amplitude and line asymmetries in observations. However, they cannot reproduce strong and weak lines simultaneously. This is explained by the shallow thermal gradient which weakens the contrast between strong and weak lines. The non-grey treatment of opacity in RHD models is planned in the near future to solve this problem. Moreover, we are now studying the possibility to detect and measure the granulation pattern with interferometers such as VLTI/AMBER, using our monochromatic intensity maps and visibility calculations. Title: Hydrodynamic Simulations of He Shell Flash Convection Authors: Herwig, Falk; Freytag, Bernd; Hueckstaedt, Robert M.; Timmes, Francis X. Bibcode: 2006ApJ...642.1057H Altcode: 2006astro.ph..1164H We present the first hydrodynamic, multidimensional simulations of He shell flash convection. We investigate the properties of shell convection immediately before the He luminosity peak during the 15th thermal pulse of a stellar evolution track with initially 2 solar masses and metallicity Z=0.01. This choice is a representative example of a low-mass asymptotic giant branch thermal pulse. We construct the initial vertical stratification with a set of polytropes to resemble the stellar evolution structure. Convection is driven by a constant volume heating in a thin layer at the bottom of the unstable layer. We calculate a grid of two-dimensional simulations with different resolutions and heating rates, plus one low-resolution three-dimensional run. The flow field is dominated by large convective cells that are centered in the lower half of the convection zone. It generates a rich spectrum of gravity waves in the stable layers both above and beneath the convective shell. The magnitude of the convective velocities from our one-dimensional mixing-length theory model and the rms-averaged vertical velocities from the hydrodynamic model are consistent within a factor of a few. However, the velocity profile in the hydrodynamic simulation is more asymmetric and decays exponentially inside the convection zone. Both g-modes and convective motions cross the formal convective boundaries, which leads to mixing across the boundaries. Our resolution study shows consistent flow structures among the higher resolution runs, and we see indications for convergence of the vertical velocity profile inside the convection zone for the highest resolution simulations. Many of the convective properties, in particular the exponential decay of the velocities, depend only weakly on the heating rate. However, the amplitudes of the gravity waves increase with both the heating rate and the resolution. Title: Convection in giant stars Authors: Freytag, B. Bibcode: 2006EAS....21..325F Altcode: The observed brightness fluctuations and large-scale structures on the surface of the Red Supergiant Betelgeuse make the star a prime targed for future interferometric measurements. At the same time, they open the possibility to resolve these structures in numerical radiation hydrodynamics simulations of the entire star. After some general remarks about the possibility and difficulties of 3D simulations of stars in general results of such calcuations of the outer convective envelope and the atmosphere of a Red Supergiant and a star on the Asmyptotic Giant Branch are presented. These show that numerous short-lived small-scale surface granules coexist with a few long-lived large-scale envelope convection cells. Pressure fluctuations deform the star and influence the surface convection. The convective “granulation" pattern differs from the solar one. Convection and pulsations produce large-scale high-contrast brightness fluctuations that might explain the observed luminosity variations and surface “spots”. Shock waves and supersonic atmospheric velocities manifest themselves in broad line profiles. Title: Radiative transfer in snapshots of 3D radiative hydrodynamic models of red supergiants Authors: Chiavassa, A.; Plez, B.; Josselin, E.; Freytag, B. Bibcode: 2006EAS....18..177C Altcode: Red Supergiant (RSG) represent a key-phase in the evolution of massive stars. These stars are characterized by strong mass loss of unknown origin. Observations show strong line profile fluctuations in depth, width and velocity, suggesting giant convective cells which could explain mass loss. Recently, radiative hydrodynamics (RHD) simulations of these stars show a peculiar convection pattern with giant cells. We performed 3D pure LTE radiative transfer calculations in snapshots of 3D hydrodynamical simulations taking into account the Doppler shifts caused by the convective motions. Computed spectra from RHD models qualitatively reproduce observations. Title: The Evolution of Central Stars of Planetary Nebulae Authors: Herwig, Falk; Freytag, Bernd; Werner, Klaus Bibcode: 2006IAUS..234..103H Altcode: 2006astro.ph..6603H The evolution of central stars of planetary nebulae can proceed in several distinct ways, either leading to H-deficiency or to H-normal surface composition. Several new simulations of the evolution channels that lead to H-deficiency are now available, mainly the born-again scenarios that are triggered by a He-shell flash during the hot pre-white dwarf evolution phase. A realistic AGB progenitor evolution is important for correct HRD tracks, that allow mass determinations. New hydrodynamic simulations of He-shell flash convection including cases with H-ingestion are now performed, and allow a determination of the convective extra-mixing efficiency. This has direct consequences for the intershell abundance distribution of AGB stars that can be observed in the H-deficient CSPN. Title: Simulations of Magnetohydrodynamics and CO Formation from the Convection Zone to the Chromosphere Authors: Wedemeyer-Böhm, S.; Schaffenberger, W.; Steiner, O.; Steffen, M.; Freytag, B.; Kamp, I. Bibcode: 2005ESASP.596E..16W Altcode: 2005ccmf.confE..16W No abstract at ADS Title: Magnetohydrodynamic Simulation from the Convection Zone to the Chromosphere Authors: Schaffenberger, W.; Wedemeyer-Böhm, S.; Steiner, O.; Freytag, B. Bibcode: 2005ESASP.596E..65S Altcode: 2005ccmf.confE..65S No abstract at ADS Title: Carbon monoxide in the solar atmosphere. I. Numerical method and two-dimensional models Authors: Wedemeyer-Böhm, S.; Kamp, I.; Bruls, J.; Freytag, B. Bibcode: 2005A&A...438.1043W Altcode: 2005astro.ph..3496W The radiation hydrodynamic code CO5BOLD has been supplemented with the time-dependent treatment of chemical reaction networks. Advection of particle densities due to the hydrodynamic flow field is also included. The radiative transfer is treated frequency-independently, i.e. grey, so far. The upgraded code has been applied to two-dimensional simulations of carbon monoxide (CO) in the non-magnetic solar photosphere and low chromosphere. For this purpose a reaction network has been constructed, taking into account the reactions that are most important for the formation and dissociation of CO under the physical conditions of the solar atmosphere. The network has been strongly reduced to 27 reactions, involving the chemical species H, H2, C, O, CO, CH, OH and a representative metal. The resulting CO number density is highest in the cool regions of the reversed granulation pattern at mid-photospheric heights and decreases strongly above. There, the CO abundance stays close to a value of 8.3 on the usual logarithmic abundance scale with [H] = 12 but is reduced in hot shock waves which are a ubiquitous phenomenon of the model atmosphere. For comparison, the corresponding equilibrium densities have been calculated, based on the reaction network but also under the assumption of instantaneous chemical equilibrium by applying the Rybicki & Hummer (RH) code. Owing to the short chemical timescales, the assumption holds for a large fraction of the atmosphere, in particular the photosphere. In contrast, the CO number density deviates strongly from the corresponding equilibrium value in the vicinity of chromospheric shock waves. Simulations with altered reaction networks clearly show that the formation channel via hydroxide (OH) is the most important one under the conditions of the solar atmosphere. Title: The shock-patterned solar chromosphere in the light of ALMA Authors: Wedemeyer-Böhm, S.; Ludwig, H. -G.; Steffen, M.; Freytag, B.; Holweger, H. Bibcode: 2005ESASP.560.1035W Altcode: 2005csss...13.1035W; 2005astro.ph..9747W Recent three-dimensional radiation hydrodynamic simulations by Wedemeyer et al. (2004) suggest that the solar chromosphere is highly structured in space and time on scales of only 1000 km and 20-25 sec, resp.. The resulting pattern consists of a network of hot gas and enclosed cool regions which are due to the propagation and interaction of shock fronts. In contrast to many other diagnostics, the radio continuum at millimeter wavelengths is formed in LTE, and provides a rather direct measure of the thermal structure. It thus facilitates the comparison between numerical model and observation. While the involved time and length scales are not accessible with todays equipment for that wavelength range, the next generation of instruments, such as the Atacama Large Millimeter Array (ALMA), will provide a big step towards the required resolution. Here we present results of radiative transfer calculations at mm and sub-mm wavelengths with emphasis on spatial and temporal resolution which are crucial for the ongoing discussion about the chromospheric temperature structure. Title: 3D simulation of convection and spectral line formation in A-type stars Authors: Steffen, M.; Freytag, B.; Ludwig, H. -G. Bibcode: 2005ESASP.560..985S Altcode: 2005astro.ph..9464S; 2005csss...13..985S We present first realistic numerical simulations of 3D radiative convection in the surface layers of main sequence A-type stars with Teff = 8000 K and 8500 K, log g = 4.4 and 4.0, recently performed with the CO5BOLD radiation hydrodynamics code. The resulting models are used to investigate the structure of the H+HeI and the HeII convection zones in comparison with the predictions of local and non-local convection theories, and to determine the amount of "overshoot" into the stable layers below the HeII convection zone. The simulations also predict how the topology of the photospheric granulation pattern changes from solar to A-type star convection. The influence of the photospheric temperature fluctuations and velocity fields on the shape of spectral lines is demonstrated by computing synthetic line profiles and line bisectors for some representative examples, allowing us to confront the 3D model results with observations. Title: Convection, atmospheres and winds of red supergiant stars Authors: Josselin, E.; Plez, B.; Freytag, B. Bibcode: 2005ESASP.560..689J Altcode: 2005csss...13..689J No abstract at ADS Title: Dynamic Model Atmospheres of Cool Giants Authors: Höfner, Susanne; Gautschy-Loidl, Rita; Aringer, Bernhard; Nowotny, Walter; Hron, Josef; Freytag, Bernd Bibcode: 2005hris.conf..269H Altcode: 2004astro.ph..3573H Cool giant stars are highly dynamical objects, and complex micro-physical processes play an important role in their extended atmospheres and winds. The interpretation of observations, and in particular of high-resolution IR spectra, requires realistic self-consistent model atmospheres. Current dynamical models include rather detailed micro-physics, and the resulting synthetic spectra compare reasonably well with observations. A transition from qualitative to quantitative modelling is taking place at present. We give an overview of existing dynamical model atmospheres for AGB stars, discussing recent advances and current trends in modelling. When comparing synthetic spectra and other observable properties resulting from dynamical models with observations we focus on the near- and mid-IR wavelength range. Title: Numerical simulations of convection in A-stars Authors: Freytag, Bernd; Steffen, Matthias Bibcode: 2004IAUS..224..139F Altcode: Radiation hydrodynamics simulations have been used to produce numerical models of the convective surface layers of a number of stars, including the Sun and other stars on or above the main-sequence, white dwarfs of type DA, and red supergiants. Title: Numerical simulation of the three-dimensional structure and dynamics of the non-magnetic solar chromosphere Authors: Wedemeyer, S.; Freytag, B.; Steffen, M.; Ludwig, H. -G.; Holweger, H. Bibcode: 2004A&A...414.1121W Altcode: 2003astro.ph.11273W Three-dimensional numerical simulations with 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. Bibcode: 2003csss...12.1024F Altcode: The spatial inhomogeneities on the surface of Betelgeuse revealed by modern interferometric observations are typically modelled as zero to three hot spots on a circular disk. While the surface structure appears to be generally consistent with an explanation as large-scale granular intensity fluctuations, the nature of the spots remains unclear.

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 Bibcode: 2003ANS...324..174S Altcode: 2003ANS...324..P96S No abstract at ADS Title: Three-dimensional Model of the Atmosphere of an AGB Star Authors: Freytag, Bernd; Höfner, Susanne Bibcode: 2003ANS...324..173F Altcode: 2003ANS...324..P95F No abstract at ADS Title: Modelling the Chromospheric Background Pattern of the Non-magnetic Sun Authors: Wedemeyer, Sven; Freytag, Bernd; Steffen, Matthias; Ludwig, Hans-Günter; Holweger, Hartmut Bibcode: 2003ANS...324R..66W Altcode: 2003ANS...324..I07W No abstract at ADS Title: Betelgeuse - Improved Numerical Simulations of an Entire Supergiant Authors: Freytag, Bernd Bibcode: 2003ANS...324...67F Altcode: 2003ANS...324..P38F; 2003ANS...324b..67F No abstract at ADS Title: Alpha Ori imaging science Authors: Freytag, Bernd Bibcode: 2003SPIE.4838..348F Altcode: Three-dimensional radiation hydrodynamics simulation of the convective envelope and the atmosphere of a red supergiant (e.g. α Ori) have been performed, including ionization effects and realistic (grey) opacities. Only a handful of giant convection cells with lifetimes up to a few years are found in the envelope. The stellar surface itself is covered with smaller short-lived cells related to the well-known solar granulation but differing in many ways. Pressure fluctuations, acoustic waves, and shocks play an important role affecting the surface energy fluxes and accordingly the emitted intensity. The interaction with convection and waves generates observable large-scale high-contrast surface features. The complexity of the surface structures in the numerical models which will probably be even higher in nature calls for interferometric observations with high spatial and some temporal resolution. The comparison with the results of numerical simulations will allow to deduce information about the nature of the surface phenomena and fundamental stellar parameters. Title: Acoustic Waves in the Solar Chromosphere - Numerical Simulations with COBOLD Authors: Wedemeyer, S.; Freytag, B.; Steffen, M.; Ludwig, H. -G.; Holweger, H. Bibcode: 2003IAUS..210P..C1W Altcode: No abstract at ADS Title: Modelling the Entire Atmosphere of Betelgeuse with 3D Simulations Authors: Freytag, B.; Mizuno-Wiedner, M. Bibcode: 2003IAUS..210P..C4F Altcode: No abstract at ADS Title: 3-D hydrodynamic simulations of the solar chromosphere Authors: Wedemeyer, S.; Freytag, B.; Steffen, M.; Ludwig, H. -G.; Holweger, H. Bibcode: 2003AN....324..410W Altcode: We present first results of three-dimensional numerical simulations of the non-magnetic solar chromosphere, computed with the radiation hydrodynamics code 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. Bibcode: 2003IAUS..210P.A12D Altcode: 2002astro.ph..8523D Recent numerical simulations by Freytag et al. of the outer convection envelope of the cool super-giant Betelgeuse, have shown that the fluctuations in the star's apparent luminosity may be caused by giant cell convection. These simulations bring forth the possibility of addressing another question; namely whether stars such as Betelgeuse may harbor magnetic activity. Taking the detailed numerical simulations of the star at face value, we have applied a kinematic dynamo analysis, to study whether the flow field of the super-giant may be able to amplify a weak seed magnetic field. We do indeed find a positive exponential growth rate of the magnetic energy. The possible Betelgeusian dynamo may be characterized as belonging to the class of so-called ``local small-scale dynamos'' (i.e. dynamos where rotation is not dominant), but this is a less meaningful designation in the case of Betelgeuse, since the field is both global and large-scale. Title: Typical Scales of Structures in Numerical Models of Betelgeuse Authors: Freytag, B.; Finnsson, S. Bibcode: 2003IAUS..210P.C12F Altcode: No abstract at ADS Title: Spots on the surface of Betelgeuse -- Results from new 3D stellar convection models Authors: Freytag, B.; Steffen, M.; Dorch, B. Bibcode: 2002AN....323..213F Altcode: The observed irregular brightness fluctuations of the well-known red supergiant Betelgeuse (alpha Ori, M2 Iab) have been attributed by M. Schwarzschild (1975) to the changing granulation pattern formed by only a few giant convection cells covering the surface of this giant star. The surface structure revealed by modern interferometric methods appears to be generally consistent with the explanation as large-scale granular intensity fluctuations. The interferometric data can be modeled equally well by assuming the presence of a few (up to 3) unresolved hot or cool spots on a limb-darkened disk. In an effort to improve our theoretical understanding of the Betelgeuse phenomena, we have applied a new radiation hydrodynamics code (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 Bibcode: 2002HiA....12..298F Altcode: Numerical radiation hydrodynamics simulations of stellar surface convection have gained a high level of reliability and perform very well in the confrontation with observations. Similar simulations of convection zones in stellar interiors are much more difficult to set up because of the huge range of time-scales to cover. Therefore, simplified models have to be used, where the results have to be carefully extrapolated to apply them to stellar interiors. Simulations of shallow surface convection zones show that the overshooting velocity fields extend far beyond the region with significant convective energy flux. Title: Betelgeuse - improved numerical simulations of an entire supergiant. Authors: Freytag, B. Bibcode: 2002AGAb...19Q..90F Altcode: No abstract at ADS Title: Acoustic Energy Generated by Convection: 3-D Numerical Simulations for the Sun Authors: Wedemeyer, Sven; Freytag, Bernd; Holweger, Hartmut; Ludwig, Hans-Günter; Steffen, Matthias Bibcode: 2001AGM....18..P01W Altcode: Dissipation of acoustic waves may be an efficient heating mechanism for the lower and middle chromosphere of the quiet Sun. The basic idea is that turbulent motions at the top of the solar convection zone generate acoustic waves which propagate upwards and dissipate in the lower and middle chromosphere, transporting energy into the higher layers. But still the question remains if this amount of energy is sufficient to explain the observed temperature increase without invoking magnetic fields. With a new version of the COBOLD radiation hydrodynamics code we are able to compute 3-D models extending all the way from the upper convection zone to the middle chromosphere. First 3-D simulations reveal a complex, inhomogenous and highly dynamical structure of the lower and middle chromosphere which evolves on rather short timescales. On small spatial dimensions very cool regions are present next to a "network" of hotter matter. The code is being developed further to provide a more detailed analysis and comparison with observations. Title: Betelgeuse - Numerical Simulations of an Entire Supergiant Authors: Freytag, Bernd Bibcode: 2001AGM....18..P18F Altcode: 2001AGAb...18Q.144F The red supergiant Betelgeuse varies in visual brightness on time-scales of weeks, months, and years. In 1975 Martin Schwarzschild attributed these fluctuations to huge convection cells, each of them covering a significant fraction of the stellar surface, so that the individual brightness changes result in a non-vanishing variation of the total luminosity. Starting about 11 years ago, interferometric observations in the visible wavelength regime revealed the existence of large-scale inhomogeneities on the surface of Betelgeuse, typically described as 0 to 3 unresolved ``hot spots'' on a cooler circular stellar disk, varying with time in number, intensity, and position. Nevertheless, the observations still have a poor resolution, resulting in surface ``images'' with only a handful of pixels. And there has been some debate about the nature of the detected surface features: Are they of convective origin due to the action of granules or supergranules? And what is the role of shocks, stellar rotation, or magnetic fields? To improve the theoretical understanding of the surface phenomena of Betelgeuse, a new radiation hydrodynamics code (``COBOLD'') has been written with the aim to include the entire star in the computational box. It employs special inner (for the stellar core) and outer boundaries appropriate for this particular geometry. The simulated star shows indeed huge surface convection cells and high photospheric velocities. But the cells look different from solar granulation and produce features which might be interpreted as ``hot spots'' in low-resolution observations. Title: Stellar Surface Convection from White Dwarfs to Red Supergiants (CD-ROM Directory: contribs/freytag) Authors: Freytag, B. Bibcode: 2001ASPC..223..785F Altcode: 2001csss...11..785F No abstract at ADS Title: Radiation Hydrodynamics Simulations of the Solar Chromosphere Authors: Wedemeyer, Sven; Freytag, Bernd; Steffen, Matthias; Holweger, Hartmut Bibcode: 2000AGM....17..P01W Altcode: While heating of the solar corona is commonly attributed to reconnection of magnetic field lines, the mechanism responsible for heating the chromosphere of the quiet Sun, away from active regions, is still under 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 Bibcode: 2000IAUJD...5E..13F Altcode: Numerical radiation hydrodynamics simulations of shallow stellar surface convection zones allow the investigation of the mixing properties of the adjacent overshoot layers. Typical time scales of diffusion processes which lead to element separation (e.g. by gravitational settling or radiation pressure) are orders of magnitudes longer than convective turnover times. Thus, inside a convection zone the mixing can be considered as occurring instantaneously. On the other hand, even small traces of the convective flow that extend beyond the classical Schwarzschild boundaries of the unstable layers can be able to counteract element separation. Numerical simulations display a fairly complicated structure of the boundaries of a convection zone. At some distance above the surface granulation the photospheric overshoot layers are dominated by shocks and large amplitude sound waves generated in the upper-most region of the convection zone. In the undershoot layers the signature of convection with the longest range of influence is a field of velocities and pressure fluctuations with exponentially declining amplitudes. These relatively small velocities still contribute to the mixing of material without any significant energy transport. Title: Betelgeuse -- Towards Numerical Simulations of an Entire Supergiant Authors: Freytag, Bernd Bibcode: 2000AGM....17..P20F Altcode: 2000AGAb...17Q..59F The brightness of the well-known red supergiant Betelgeuse varies on time-scales of weeks, months, and years. For a dedicated observer these fluctuations are even visible with the naked eye. In 1975 Martin Schwarzschild attributed them to huge convection cells, each of them covering a significant fraction of the stellar surface, so that the individual fluctuations result in an overall non-vanishing variation of the stars luminosity. Starting about 10 years ago, numerous interferometric observations in the visible wavelength regime revealed the existence of large-scale brightness inhomogeneities on the surface of Betelgeuse, typically described as 0 to 3 unresolved ``hot spots'' on a cooler circular stellar disk, varying with time in number, intensity, and position. HST demonstrated deviations of the stellar UV image from spherical symmetry. Nevertheless, the observations have still a poor resolution, resulting in surface ``images'' with only a handful of pixels. And there has been some debate about the nature of the detected surface features: Are they of convective origin due to the action of granules or supergranules? And what is the role of shocks, stellar rotation, or magnetic fields? To improve the theoretical understanding of the surface phenomena of Betelgeuse, a new radiation hydrodynamics code has been written with the aim to include the entire star in the computational box. It employs special inner (for the stellar core) and outer boundaries appropriate for this particular geometry. Starting with down-scaled toy models and approaching the parameter regime appropriate for Betelgeuse, some results about the dynamics of large convective cells on a spherical stellar surface will be presented. Title: The Solar p-Mode Background: Observations and Hydrodynamical Models Authors: Straus, Th.; Steffen, M.; Severino, G.; Freytag, B. Bibcode: 1999ESASP.448..203S Altcode: 1999mfsp.conf..203S; 1999ESPM....9..203S No abstract at ADS Title: A calibration of the mixing-length for solar-type stars based on hydrodynamical simulations. I. Methodical aspects and results for solar metallicity Authors: Ludwig, Hans-Günter; Freytag, Bernd; Steffen, Matthias Bibcode: 1999A&A...346..111L Altcode: 1998astro.ph.11179L Based on detailed 2D numerical radiation hydrodynamics (RHD) calculations of time-dependent compressible convection, we have studied the dynamics and thermal structure of the convective surface layers of solar-type stars. The RHD models provide information about the convective efficiency in the superadiabatic region at the top of convective envelopes and predict the asymptotic value of the entropy of the deep, adiabatically stratified layers (Fig. \ref{f:sstarhd}). This information is translated into an effective mixing-length parameter \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}<=Teff<= 7100pun {K} and gravities 2.54<={log g}<= 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 &\ Mazzitelli (1991, 1992; CMT) gives a different temperature dependence but a similar variation of the free parameter (Fig. \ref{f:mlpcm}). For the first time, values for the gravity-darkening exponent beta are derived independently of mixing-length theory: beta = 0.07... 0.10. We show that our findings are consistent with constraints from stellar stability considerations and provide compact fitting formulae for the calibrations. Title: Stellar Envelope Convection Calibrated by Radiation Hydrodynamics Simulations: Influence on Globular Cluster Isochrones Authors: Freytag, Bernd; Salaris, Maurizio Bibcode: 1999ApJ...513L..49F Altcode: 1999astro.ph..1074F One of the largest sources of uncertainty in the computation of globular cluster isochrones and hence in the age determination of globular clusters is the lack of a rigorous description of convection. Therefore, we calibrated the superadiabatic temperature gradient in the envelope of metal-poor low-mass stars according to the results from a new grid of two-dimensional hydrodynamical models, which cover the main sequence and the lower red giant branch of globular cluster stars. In practice, we still use for computing the evolutionary stellar models the traditional mixing-length formalism, but we fix the mixing-length parameter α in order to reproduce the run of the entropy of the deeper adiabatic region of the stellar envelopes with effective temperature and gravity as obtained from the hydrodynamical models. The detailed behavior of the calibrated α depends in a nontrivial way on the effective temperature, gravity, and metallicity of the star. Nevertheless, the resulting isochrones for the relevant age range of Galactic globular clusters have only small differences with respect to isochrones computed adopting a constant solar calibrated value of the mixing length. Accordingly, the age of globular clusters is reduced by 0.2 Gyr at most. Title: Treatment of the Superadiabatic Convection in Low-Mass Metal-Poor Stars from Realistic Hydrodynamics Simulations: Application to Globular Clusters Isochrones Authors: Freytag, B.; Salaris, M.; Ludwig, H. -G. Bibcode: 1999ASPC..173..201F Altcode: 1999sstt.conf..201F No abstract at ADS Title: A Calibration of the Mixing-Length for Solar-Type Stars Based on Hydrodynamical Models of Stellar Surface Convection Authors: Freytag, B.; Ludwig, H. -G.; Steffen, M. Bibcode: 1999ASPC..173..225F Altcode: 1999sstt.conf..225F No abstract at ADS Title: Stellar Surface Convection in Stars of Various Radii Authors: Freytag, Bernd Bibcode: 1999AGAb...15...99F Altcode: 1999AGM....15..P24F Based on detailed numerical radiation hydrodynamics calculations of time-dependent compressible convection, the dynamics, spatial scales, and thermal structure of the convective surface layers of different stars with a large range of diameters have been studied. While solar convection is efficient in carrying energy and the convection zone comprises a significant part of the Sun, a typical granule is tiny compared to the solar diameter. Models of solar-type convection from F to K type stars from the main-sequence to the base of the Red Giant Branch with solar and reduced metallicities show a qualitatively similar behavior: The horizontal extent of the granules scales approximately with the pressure scale height. Main-sequence A type stars have shallow radiation dominated convection zones with inefficient convective energy transport. The simulations show that in these stars the strong overshoot mixes much more matter than is actually included in the convectively unstable region. The granules have only a slightly larger relative size than the solar ones. A sequence of models of hydrogen-rich White Dwarfs in the neighborhood of the ZZ Ceti instability strip demonstrates the transition from inefficient to efficient convection. The granules have a typical size of 1 km (compared to 1000 km in the Sun) according to the larger gravity and are again small in comparison to the diameter of a White Dwarf. This is completely different in Red Supergiants where the low surface gravity allows only a couple of granules on their surface. Their random brightness fluctuations might be responsible for the observed luminosity fluctuations of e.g. Betelgeuse. First 3D radiation hydrodynamics models of a Red Supergiant will be presented and and the properties of its convective envelope will be contrasted to those of other stars. Title: Lithium Depletion in the Sun: A Study of Mixing Based on Hydrodynamical Simulations Authors: Blöcker, T.; Holweger, H.; Freytag, B.; Herwig, F.; Ludwig, H. -G.; Steffen, M. Bibcode: 1998SSRv...85..105B Altcode: 1998astro.ph..6310B Based on radiation hydrodynamics modeling of stellar convection zones, a diffusion scheme has been devised describing the downward penetration of convective motions beyond the Schwarzschild boundary (overshoot) into the radiative interior. This scheme of exponential diffusive overshoot has already been successfully applied to AGB stars. Here we present an application to the Sun in order to determine the time scale and depth extent of this additional mixing, i.e. diffusive overshoot at the base of the convective envelope. We calculated the associated destruction of lithium during the evolution towards and on the main-sequence. We found that the slow-mixing processes induced by the diffusive overshoot may lead to a substantial depletion of lithium during the Sun's main-sequence evolution. Title: Lithium Depletion in the Sun: A Study of Mixing Based on Hydrodynamical Simulations Authors: Blöcker, T.; Holweger, H.; Freytag, B.; Herwig, F.; Ludwig, H. -G.; Steffen, M. Bibcode: 1998sce..conf..105B Altcode: No abstract at ADS Title: Numerical simulations of convection in low-mass metal-poor stars Authors: Freytag, Bernd Bibcode: 1998AGAb...14..113F Altcode: 1998AGM....14..P28F Based on detailed 2D numerical radiation hydrodynamics (RHD) calculations of time-dependent compressible convection, we have studied the dynamics and thermal structure of the convective surface layers of stars in the range of effective temperatures, gravities, and metallicities between 4300K <= Teff <= 7100K, 2.54 <= log g <= 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 &\ Mazzitelli in an analogous manner. The size of the variations of the free parameter of both theories is compared. Title: An improved calibration of the mixing-length based on simulations of solar-type convection Authors: Ludwig, H. -G.; Freytag, B.; Steffen, M. Bibcode: 1998IAUS..185..115L Altcode: Based on detailed 2D numerical radiation hydrodynamics (RHD) calculations of time-dependent compressible convection, we have studied the dynamics and thermal structure of the convective surface layers of stars in the range of effective temperatures and gravities between 4500 pun{K} <= Teff <= 7100 pun{K} and 2.54 <= logg <= 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 & Mazzitelli extended by including a variable amount of overshoot does not lead to a smaller variation of the controlling parameter. We discuss the consistency of our results with findings derived in the context of the tentative detection of solar-like oscillations in eta Bootis. Title: A calibration of mixing length theory based on RHD simulations of solar-type convection Authors: Ludwig, H. -G.; Freytag, B.; Steffen, M. Bibcode: 1997ASSL..225...59L Altcode: 1997scor.proc...59L Radiation hydrodynamics (RHD) models provide detailed information about the dynamics, thermal structure, and convective efficiency of the superadiabatic region at the top of solar-type convection zones, and allow an extrapolation of the entropy (s*) in their deep, adiabatic layers. For the Sun we find a close agreement between s* inferred from our RHD models and an empirical determination of s* from helioseismology. In the framework of mixing length theory (MLT), s* is translated to an effective mixing-length parameter (alpha c) appropriate to construct global stellar models. The calibration based on our present set of 2D RHD models shows a moderate variation of alpha c across the domain of the HRD investigated so far. Title: "Discovery" of a β Pictoris-like circumstellar disk in the Internet. Authors: Hempel, M.; Holweger, H.; Rentzsch-Holm, I.; Freytag, B. Bibcode: 1997AGAb...13..201H Altcode: No abstract at ADS Title: Numerical simulations of stellar surface convection. Authors: Freytag, B.; Steffen, M. Bibcode: 1997AGAb...13..176F Altcode: No abstract at ADS Title: On the Scale of Photospheric Convection Authors: Freytag, B.; Holweger, H.; Steffen, M.; Ludwig, H. -G. Bibcode: 1997svlt.work..316F Altcode: No abstract at ADS Title: Hydrodynamical models of stellar convection. The role of overshoot in DA white dwarfs, A-type stars, and the Sun. Authors: Freytag, B.; Ludwig, H. -G.; Steffen, M. Bibcode: 1996A&A...313..497F Altcode: Based on two-dimensional numerical radiation hydrodynamics simulations of time-dependent compressible convection, we have studied the structure and dynamics of a variety of shallow stellar surface convection zones. Our present grid of models includes detailed simulations of surface convection in solar-type stars, main-sequence A-type stars and cool DA white dwarfs, as well as numerical experiments to study convection and overshoot at the base of the solar convection zone. Taking into account a realistic equation of state (including the effects of ionization) and adopting an elaborate treatment of non-local radiative transfer (with appropriate grey or frequency-dependent opacities), our simulations are designed to represent specific stellar objects characterized by T_eff_, logg, and chemical composition. Contrary to solar-type stars, the A-type stars and cool DA white dwarfs investigated here have shallow convection zones which fit into the computational domain together with thick stable buffer layers on top and below, thus permitting a study of convective overshoot under genuine conditions. We find that convective motions extend well beyond the boundary of the convectively unstable region, with vertical velocities decaying exponentially with depth in the deeper parts of the lower overshoot region, as expected for linear g^-^-modes. Even though convective velocities are reduced by orders of magnitude, they are still able to counteract molecular diffusion. For a quantitative description of convective mixing in the far overshoot layers we have derived a depth dependent diffusion coefficient from the numerical simulations. In combination with otherwise independent 1D diffusion calculations for a trace element, this allows the determination of the "effective depth" of the overshoot region. For a typical main-sequence A-type star (T_eff_=7943#1, logg=4.34) the mass in the overshoot region exceeds the mass in the unstable region by approximately a factor 10. The amount of overshoot in cool DA white dwarfs (around T_eff_=12200#1) is even larger: the convectively mixed mass is increased by roughly a factor 100. Title: Overtures to the pulsational instability of ZZ Ceti variables. Authors: Gautschy, A.; Ludwig, H. -G.; Freytag, B. Bibcode: 1996A&A...311..493G Altcode: 1995astro.ph..8002G Results of nonradial, nonadiabatic pulsation calculations on hydrogen-rich white dwarf models are presented. In contrast to earlier attempts, the modeling builds on hydrodynamically simulated convective surface layers supplemented with standard interior models. Based on our stellar models and despite of various simple attempts to couple convection and pulsation we could not reproduce theoretically the presently adopted location of the observed blue edge of the ZZ Ceti variables. When the convective efficiency is high enough we found a sensitive dependence of the stability properties of the g modes on the pulsational treatment of shear within the convection zone. Title: Problems in Modeling Photospheric Convective Overshooting Authors: Freytag, B. Bibcode: 1996ASPC..108...93F Altcode: 1996mass.conf...93F No abstract at ADS Title: Lyapunov exponents for solar surface convection. Authors: Steffen, M.; Freytag, B. Bibcode: 1995CSF.....5.1965S Altcode: The authors have carried out detailed 2D numerical radiation hydrodynamics calculations, specifically designed to model time-dependent, compressible convection in the surface layers of the Sun. These simulations, which take into account a realistic equation of state and use an elaborate scheme to describe multi-dimensional, non-local, frequency-dependent radiative transfer, allow a direct comparison with observed photometric and spectroscopic properties of solar granulation. Their purpose is to enhance the understanding of the dynamics and thermal structure of convective stellar atmospheres, and to investigate the generation of acoustic energy by turbulent convection. The authors briefly present some of the main properties of their solar convection models. In particular, the authors demonstrate the chaotic behaviour of solar surface convection, estimating the magnitude of the two largest Lyapunov exponents, 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. Title: Synthetic spectra computed from hydrodynamical model atmospheres of DA white dwarfs. Authors: Steffen, M.; Ludwig, H. -G.; Freytag, B. Bibcode: 1995A&A...300..473S Altcode: From detailed 2-dimensional numerical radiation hydrodynamics calculations of time-dependent compressible convection we have obtained the thermal structure of the convective surface layers of DA white dwarfs with effective temperatures near the blue edge of the ZZ Ceti instability strip. Synthetic line profiles of 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. Title: The Mixing-Length Parameter for Solar-Type Convection Zones Inferred from Hydrodynamical Models of the Surface Layers Authors: Ludwig, H. -G.; Freytag, B.; Steffen, M.; Wagenhuber, J. Bibcode: 1995LIACo..32..213L Altcode: 1995sews.book..213L No abstract at ADS Title: Numerical Simulations of Convection and Overshoot in the Envelope of DA White Dwarfs Authors: Freytag, Bernd; Steffen, Matthias; Ludwig, Hans-Günter Bibcode: 1995LNP...443...88F Altcode: 1995whdw.conf...88F We present results of realistic 21) numerical radiation hydrodynamics calculations, simulating the surface convection zones of DA white dwarfs in the range of effective temperatures from 14 200 K down to 11400 K. Comparison with mixing length theory (MLT) yields a conflicting picture: The dynamics of convection is not governed by up- and downflowing bubbles which dissolve after travelling some characteristic distance - but by the formation, advection, merging, and disruption of fast narrow downdrafts in a slowly upstreaming surrounding. MLT tremendously underestimates the depth of the region where material is mixed. Nevertheless, it turns out that a mixing length model with α = 1.5 gives a good fit of the photospheric temperature structure (T 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. Title: Numerical simulations of surface convection in solar-type stars Authors: Freytag, B.; Steffen, M. Bibcode: 1995IAUS..176P.111F Altcode: No abstract at ADS Title: Shocks in the solar photosphere and their spectroscopic signature Authors: Steffen, M.; Freytag, B.; Holweger, H. Bibcode: 1994smf..conf..298S Altcode: No abstract at ADS Title: Hydrodynamics of the Solar Photosphere: Model Calculations and Spectroscopic Observations. Authors: Steffen, M.; Freytag, B. Bibcode: 1991RvMA....4...43S Altcode: No abstract at ADS