Author name code: ludwig ADS astronomy entries on 2022-09-14 author:"Ludwig, Hans-Guenter" ------------------------------------------------------------------------ Title: The Gaia-ESO Public Spectroscopic Survey: Motivation, implementation, GIRAFFE data processing, analysis, and final data products Authors: Gilmore, G.; Randich, S.; Worley, C. C.; Hourihane, A.; Gonneau, A.; Sacco, G. G.; Lewis, J. R.; Magrini, L.; Francois, P.; Jeffries, R. D.; Koposov, S. E.; Bragaglia, A.; Alfaro, E. J.; Allende Prieto, C.; Blomme, R.; Korn, A. J.; Lanzafame, A. C.; Pancino, E.; Recio-Blanco, A.; Smiljanic, R.; Van Eck, S.; Zwitter, T.; Bensby, T.; Flaccomio, E.; Irwin, M. J.; Franciosini, E.; Morbidelli, L.; Damiani, F.; Bonito, R.; Friel, E. D.; Vink, J. S.; Prisinzano, L.; Abbas, U.; Hatzidimitriou, D.; Held, E. V.; Jordi, C.; Paunzen, E.; Spagna, A.; Jackson, R. J.; Maiz Apellaniz, J.; Asplund, M.; Bonifacio, P.; Feltzing, S.; Binney, J.; Drew, J.; Ferguson, A. M. N.; Micela, G.; Negueruela, I.; Prusti, T.; Rix, H. -W.; Vallenari, A.; Bergemann, M.; Casey, A. R.; de Laverny, P.; Frasca, A.; Hill, V.; Lind, K.; Sbordone, L.; Sousa, S. G.; Adibekyan, V.; Caffau, E.; Daflon, S.; Feuillet, D. K.; Gebran, M.; Gonzalez Hernandez, J. I.; Guiglion, G.; Herrero, A.; Lobel, A.; Merle, T.; Mikolaitis, S.; Montes, D.; Morel, T.; Ruchti, G.; Soubiran, C.; Tabernero, H. M.; Tautvaisiene, G.; Traven, G.; Valentini, M.; Van der Swaelmen, M.; Villanova, S.; Viscasillas Vazquez, C.; Bayo, A.; Biazzo, K.; Carraro, G.; Edvardsson, B.; Heiter, U.; Jofre, P.; Marconi, G.; Martayan, C.; Masseron, T.; Monaco, L.; Walton, N. A.; Zaggia, S.; Aguirre Borsen-Koch, V.; Alves, J.; Balaguer-Nunez, L.; Barklem, P. S.; Barrado, D.; Bellazzini, M.; Berlanas, S. R.; Binks, A. S.; Bressan, A.; Capuzzo-Dolcetta, R.; Casagrande, L.; Casamiquela, L.; Collins, R. S.; D'Orazi, V.; Dantas, M. L. L.; Debattista, V. P.; Delgado-Mena, E.; Di Marcantonio, P.; Drazdauskas, A.; Evans, N. W.; Famaey, B.; Franchini, M.; Fremat, Y.; Fu, X.; Geisler, D.; Gerhard, O.; Gonzalez Solares, E. A.; Grebel, E. K.; Gutierrez Albarran, M. L.; Jimenez-Esteban, F.; Jonsson, H.; Khachaturyants, T.; Kordopatis, G.; Kos, J.; Lagarde, N.; Ludwig, H. -G.; Mahy, L.; Mapelli, M.; Marfil, E.; Martell, S. L.; Messina, S.; Miglio, A.; Minchev, I.; Moitinho, A.; Montalban, J.; Monteiro, M. J. P. F. G.; Morossi, C.; Mowlavi, N.; Mucciarelli, A.; Murphy, D. N. A.; Nardetto, N.; Ortolani, S.; Paletou, F.; Palous, J.; Pickering, J. C.; Quirrenbach, A.; Re Fiorentin, P.; Read, J. I.; Romano, D.; Ryde, N.; Sanna, N.; Santos, W.; Seabroke, G. M.; Spina, L.; Steinmetz, M.; Stonkute, E.; Sutorius, E.; Thevenin, F.; Tosi, M.; Tsantaki, M.; Wright, N.; Wyse, R. F. G.; Zoccali, M.; Zorec, J.; Zucker, D. B. Bibcode: 2022arXiv220805432G Altcode: The Gaia-ESO Public Spectroscopic Survey is an ambitious project designed to obtain astrophysical parameters and elemental abundances for 100,000 stars, including large representative samples of the stellar populations in the Galaxy, and a well-defined sample of 60 (plus 20 archive) open clusters. We provide internally consistent results calibrated on benchmark stars and star clusters, extending across a very wide range of abundances and ages. This provides a legacy data set of intrinsic value, and equally a large wide-ranging dataset that is of value for homogenisation of other and future stellar surveys and Gaia's astrophysical parameters. This article provides an overview of the survey methodology, the scientific aims, and the implementation, including a description of the data processing for the GIRAFFE spectra. A companion paper (arXiv:2206.02901) introduces the survey results. Gaia-ESO aspires to quantify both random and systematic contributions to measurement uncertainties. Thus all available spectroscopic analysis techniques are utilised, each spectrum being analysed by up to several different analysis pipelines, with considerable effort being made to homogenise and calibrate the resulting parameters. We describe here the sequence of activities up to delivery of processed data products to the ESO Science Archive Facility for open use. The Gaia-ESO Survey obtained 202,000 spectra of 115,000 stars using 340 allocated VLT nights between December 2011 and January 2018 from GIRAFFE and UVES. The full consistently reduced final data set of spectra was released through the ESO Science Archive Facility in late 2020, with the full astrophysical parameters sets following in 2022. Title: A revisit of the standard composition of the Sun Authors: Bautista, Manuel; Magg, Ekaterina; Bergemann, Maria; Serenelli, Aldo; Plez, Bertrand; Gerber, Geffrey; Ludwig, Hans-Gunter; Basu, Sarbani; Ferguson, Jason; Carvajal Gallego, Helena; Gamrath, Sebaastien; Palmeri, Patrick; Quinet, Pascal Bibcode: 2022AAS...24035011B Altcode: The chemical composition of the Sun is requested in the context of various studies in astrophysics, among them in the calculation of the standard solar models (SSMs), which describe the evolution of the Sun from the pre-main-sequence to its present age.

In this work, we re-analysis of the solar chemical abundances and corresponding SSMs. We employ new high-quality solar observational data collected with the IAG facility, state-of-the-art non-equilibrium modelling, new oscillator strengths, and different atmospheric models, including the MARCS model, but also averages based on Stagger and CO5BOLD 3D radiation-hydrodynamics simulations of stellar convection. We perform new calculations of oscillator strengths for transitions in O I and N I. For O I — the critical element for the interior models — calculations are carried out using several independent methods. We find unprecedented agreement between the new estimates of transition probabilities, thus supporting our revised solar oxygen abundance. We also provide new estimates of the noble gas Ne abundance.

We compare our photospheric abundances with the previous estimates. We discuss the consistency of our measurements with meteoritic values, taking into account systematic and correlated errors. Finally, we provide revised chemical abundances, leading to a new value of the solar photospheric present-day metallicity Z=X = 0:0225, and employ them in the calculations of the SSM. We find that the puzzling mismatch between the helioseismic constraints on the solar interior structure and the model is resolved with the new chemical composition. Title: Interpolation of spectra from 3D model atmospheres Authors: Bertran de Lis, S.; Prieto, Allende; Ludwig, H. -G.; Koesterke, L. Bibcode: 2022A&A...661A..76B Altcode: 2022arXiv220212086B Context. The use of 3D hydrodynamical simulations of stellar surface convection for model atmospheres is computationally expensive. Although these models have been available for quite some time, their use is limited because of the lack of extensive grids of simulations and associated spectra.
Aims: Our goal is to provide a method to interpolate spectra that can be applied to both 1D and 3D models, and implement it in a code available to the community. This tool will enable the routine use of 3D model atmospheres in the analysis of stellar spectra.
Methods: We have developed a code that makes use of radial basis functions to interpolate the spectra included in the CIFIST grid of 84 three-dimensional model atmospheres. Spectral synthesis on the hydrodynamical simulations was previously performed with the code ASSϵT.
Results: We make a tool for the interpolation of 3D spectra available to the community. The code provides interpolated spectra and interpolation errors for a given wavelength interval, and a combination of effective temperature, surface gravity, and metallicity. In addition, it optionally provides graphical representations of the RMS and mean ratio between 1D and 3D spectra, and maps of the errors in the interpolated spectra across the parameter space. Title: Observational constraints on the origin of the elements. IV. Standard composition of the Sun Authors: Magg, Ekaterina; Bergemann, Maria; Serenelli, Aldo; Bautista, Manuel; Plez, Bertrand; Heiter, Ulrike; Gerber, Jeffrey M.; Ludwig, Hans-Günter; Basu, Sarbani; Ferguson, Jason W.; Gallego, Helena Carvajal; Gamrath, Sébastien; Palmeri, Patrick; Quinet, Pascal Bibcode: 2022A&A...661A.140M Altcode: 2022arXiv220302255M Context. The chemical composition of the Sun is required in the context of various studies in astrophysics, among them in the calculation of standard solar models (SSMs) used to describe the evolution of the Sun from the pre-main-sequence to its present age.
Aims: In this work, we provide a critical re-analysis of the solar chemical abundances and corresponding SSMs.
Methods: For the photospheric values, we employed new high-quality solar observational data collected with the IAG facility, state-of-the art non-equilibrium modelling, new oscillator strengths, and different atmospheric models, including the MARCS model, along with averages based on Stagger and CO5BOLD 3D radiation-hydrodynamics simulations of stellar convection. We performed new calculations of oscillator strengths for transitions in O I and N I. For O I, which is a critical element with regard to the interior models, calculations were carried out using several independent methods. We investigated our results in comparison with the previous estimates.
Results: We find an unprecedented agreement between the new estimates of transition probabilities, thus supporting our revised solar oxygen abundance value. We also provide new estimates of the noble gas Ne abundance. In addition, we discuss the consistency of our photospheric measurements with meteoritic values, taking into account the systematic and correlated errors. Finally, we provide revised chemical abundances, leading to a new value proposed for the solar photospheric present-day metallicity of Z/X = 0.0225, which we then employed in SSM calculations. We find that the puzzling mismatch between the helioseismic constraints on the solar interior structure and the model can be resolved thanks to this new chemical composition. Title: The Gaia RVS benchmark stars. I. Chemical inventory of the first sample of evolved stars and its Rb NLTE investigation Authors: Caffau, E.; Bonifacio, P.; Korotin, S. A.; François, P.; Lallement, R.; Matas Pinto, A. M.; Di Matteo, P.; Steffen, M.; Mucciarelli, A.; Katz, D.; Haywood, M.; Chemin, L.; Sartoretti, P.; Sbordone, L.; Andrievsky, S. M.; Kovtyukh, V. V.; Spite, M.; Spite, F.; Panuzzo, P.; Royer, F.; Thévenin, F.; Ludwig, H. -G.; Marchal, O.; Plum, G. Bibcode: 2021A&A...651A..20C Altcode: Context. The Radial Velocity Spectrometer (RVS) on board the Gaia satellite is not provided with a wavelength calibration lamp. It uses its observations of stars with known radial velocity to derive the dispersion relation. To derive an accurate radial velocity calibration, a precise knowledge of the line spread function (LSF) of the RVS is necessary. Good-quality ground-based observations in the wavelength range of the RVS are highly desired to determine the LSF.
Aims: Several radial velocity standard stars are available to the Gaia community. The highest possible number of calibrators will surely allow us to improve the accuracy of the radial velocity. Because the LSF may vary across the focal plane of the RVS, a large number of high-quality spectra for the LSF calibration may allow us to better sample the properties of the focal plane.
Methods: We selected a sample of stars to be observed with UVES at the Very Large Telescope, in a setting including the wavelength range of RVS, that are bright enough to allow obtaining high-quality spectra in a short time. We also selected stars that lack chemical investigation in order to increase the sample of bright, close by stars with a complete chemical inventory.
Results: We here present the chemical analysis of the first sample of 80 evolved stars. The quality of the spectra is very good, therefore we were able to derive abundances for 20 elements. The metallicity range spanned by the sample is about 1 dex, from slightly metal-poor to solar metallicity. We derived the Rb abundance for all stars and investigated departures from local thermodynamical equilibrium (NLTE) in the formation of its lines.
Conclusions: The sample of spectra is of good quality, which is useful for a Gaia radial velocity calibration. The Rb NLTE effects in this stellar parameters range are small but sometimes non-negligible, especially for spectra of this good quality.

Tables B.3 and C.1 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/651/A20

Based on observations made with UVES at VLT 104.D.0325. Title: ALMA small-scale features in the quiet Sun and active regions Authors: Brajša, R.; Skokić, I.; Sudar, D.; Benz, A. O.; Krucker, S.; Ludwig, H. -G.; Saar, S. H.; Selhorst, C. L. Bibcode: 2021A&A...651A...6B Altcode: 2021arXiv210503644B
Aims: The main aim of the present analysis is to decipher (i) the small-scale bright features in solar images of the quiet Sun and active regions obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) and (ii) the ALMA correspondence of various known chromospheric structures visible in the Hα images of the Sun.
Methods: Small-scale ALMA bright features in the quiet Sun region were analyzed using single-dish ALMA observations (1.21 mm, 248 GHz) and in an active region using interferometric ALMA measurements (3 mm, 100 GHz). With the single-dish observations, a full-disk solar image is produced, while interferometric measurements enable the high-resolution reconstruction of part of the solar disk, including the active region. The selected quiet Sun and active regions are compared with the Hα (core and wing sum), EUV, and soft X-ray images and with the magnetograms.
Results: In the quiet Sun region, enhanced emission seen in the ALMA is almost always associated with a strong line-of-sight magnetic field. Four coronal bright points were identified, while other small-scale ALMA bright features are most likely associated with magnetic network elements and plages. In the active region, in 14 small-scale ALMA bright features randomly selected and compared with other images, we found five good candidates for coronal bright points, two for plages, and five for fibrils. Two unclear cases remain: a fibril or a jet, and a coronal bright point or a plage. A comparison of the Hα core image and the 3 mm ALMA image of the analyzed active region showed that the sunspot appears dark in both images (with a local ALMA radiation enhancement in sunspot umbra), the four plage areas are bright in both images and dark small Hα filaments are clearly recognized as dark structures of the same shape also in ALMA. Title: TOPoS. VI. The metal-weak tail of the metallicity distribution functions of the Milky Way and the Gaia-Sausage-Enceladus structure Authors: Bonifacio, P.; Monaco, L.; Salvadori, S.; Caffau, E.; Spite, M.; Sbordone, L.; Spite, F.; Ludwig, H. -G.; Di Matteo, P.; Haywood, M.; François, P.; Koch-Hansen, A. J.; Christlieb, N.; Zaggia, S. Bibcode: 2021A&A...651A..79B Altcode: 2021arXiv210508360B Context. The goal of the Turn-Off Primordial Stars survey (TOPoS) project is to find and analyse turn-off (TO) stars of extremely low metallicity. To select the targets for spectroscopic follow-up at high spectral resolution, we relied on low-resolution spectra from the Sloan Digital Sky Survey (SDSS).
Aims: In this paper, we use the metallicity estimates we obtained from our analysis of the SDSS spectra to construct the metallicity distribution function (MDF) of the Milky Way, with special emphasis on its metal-weak tail. The goal is to provide the underlying distribution out of which the TOPoS sample was extracted.
Methods: We made use of SDSS photometry, Gaia photometry, and distance estimates derived from the Gaia parallaxes to derive a metallicity estimate for a large sample of over 24 million TO stars. This sample was used to derive the metallicity bias of the sample for which SDSS spectra are available.
Results: We determined that the spectroscopic sample is strongly biased in favour of metal-poor stars, as intended. A comparison with the unbiased photometric sample allows us to correct for the selection bias. We selected a sub-sample of stars with reliable parallaxes for which we combined the SDSS radial velocities with Gaia proper motions and parallaxes to compute actions and orbital parameters in the Galactic potential. This allowed us to characterise the stars dynamically, and in particular to select a sub-sample that belongs to the Gaia-Sausage-Enceladus (GSE) accretion event. We are thus also able to provide the MDF of GSE.
Conclusions: The metal-weak tail derived in our study is very similar to that derived in the H3 survey and in the Hamburg/ESO Survey. This allows us to average the three MDFs and provide an error bar for each metallicity bin. Inasmuch as the GSE structure is representative of the progenitor galaxy that collided with the Milky Way, that galaxy appears to be strongly deficient in metal-poor stars compared to the Milky Way, suggesting that the metal-weak tail of the latter has been largely formed by accretion of low-mass galaxies rather than massive galaxies, such as the GSE progenitor.

Spectroscopic and photometric metallicities derived and discussed in this paper as well as orbital actions computed and discussed in this paper 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/651/A79 Title: VizieR Online Data Catalog: TO stars metallicity estimate (Bonifacio+, 2021) Authors: Bonifacio, P.; Monaco, L.; Salvadori, S.; Caffau, E.; Spite, M.; Sbordone, L.; Spite, F.; Ludwig, H. -G.; Di Matteo, P.; Haywood, M.; Francois, P.; Koch-Hansen, A. J.; Christlieb, N. C.; Zaggia, S. Bibcode: 2021yCat..36510079B Altcode: We made use of SDSS photometry, Gaia photometry, and distance estimates derived from the Gaia parallaxes to derive a metallicity estimate for a large sample of over 24 million TO stars. This sample was used to derive the metallicity bias of the sample for which SDSS spectra are available.

(3 data files). 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: VizieR Online Data Catalog: Gaia RVS benchmark stars. I. (Caffau+, 2021) Authors: Caffau, E.; Bonifacio, P.; Korotin, S. A.; Francois, P.; Lallement, R.; Matas Pinto, A. M.; Di Matteo, P.; Steffen, M.; Mucciarelli, A.; Katz, D.; Haywood, M.; Chemin, L.; Sartoretti, P.; Sbordone, L.; Andrievsky, S. M.; Kovtyukh, V. V.; Spite, M.; Spite, F.; Panuzzo, P.; Royer, F.; Thevenin, F.; Ludwig, H. -G.; Marchal, O.; Plum, G. Bibcode: 2021yCat..36510020C Altcode: For this project on the UVES spectrograph, we selected the setting 437+760. The choices on the setting were that (i) the 760 range completely covers the RVS range without any gaps, and (ii) the 437 range is the reddest setting that can be coupled with the 760 setting. For metal-rich stars (the majority of our targets), observations in blue settings provide very crowded spectra, and a higher S/N can be achieved in this selected setting than in bluer settings such as the 390 setting.

We chose the highest UVES resolution (slit 0.4" in the blue arm and 0.3" in the red arm). For all observations, the DIC2 437+760 setting was used. For the stars brighter than V magnitude 8.5, an observing block comprises ten observations of 77.5s to avoid detector saturation. For the stars fainter than 8.5, five exposures of 202 s allow avoiding detector saturation. In this program, 90 stars have been observed, 80 of which are evolved stars and are analysed here. The 10 unevolved stars will be analysed with stars of similar stellar parameters that are observed or are scheduled to be observed for the following two ESO periods (P105 and P106).

(2 data files). 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: 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: The signature of granulation in a solar power spectrum as seen with CO5BOLD Authors: Lundkvist, Mia S.; Ludwig, Hans-Günter; Collet, Remo; Straus, Thomas Bibcode: 2021MNRAS.501.2512L Altcode: 2020arXiv201110045L; 2020MNRAS.tmp.3425L The granulation background seen in the power spectrum of a solar-like oscillator poses a serious challenge for extracting precise and detailed information about the stellar oscillations. Using a 3D hydrodynamical simulation of the Sun computed with CO5BOLD, we investigate various background models to infer, using a Bayesian methodology, which one provides the best fit to the background in the simulated power spectrum. We find that the best fit is provided by an expression including the overall power level and two characteristic frequencies, one with an exponent of two and one with a free exponent taking on a value around six. We assess the impact of the 3D hydro-code on this result by repeating the analysis with a simulation from S TAGGER and find that the main conclusion is unchanged. However, the details of the resulting best fits differ slightly between the two codes, but we explain this difference by studying the effect of the spatial resolution and the duration of the simulation on the fit. Additionally, we look into the impact of adding white noise to the simulated time series as a simple way to mimic a real star. We find that, as long as the noise level is not too low, the results are consistent with the no-noise case. Title: Velocity-intensity asymmetry reversal of solar radial p-modes Authors: Philidet, J.; Belkacem, K.; Ludwig, H. -G.; Samadi, R.; Barban, C. Bibcode: 2020A&A...644A.171P Altcode: 2020arXiv201102439P The development of space-borne missions has significantly improved the quality of the measured spectra of solar-like oscillators. Their p-mode line profiles can now be resolved, and the asymmetries inferred for a variety of stars other than the Sun. However, it has been known for a long time that the asymmetries of solar p-modes are reversed between the velocity and the intensity spectra. Understanding the origin of this reversal is necessary in order to use asymmetries as a tool for seismic diagnosis. For stars other than the Sun, only the intensity power spectrum is sufficiently resolved to allow for an estimation of mode asymmetries. We recently developed an approach designed to model and predict these asymmetries in the velocity power spectrum of the Sun and to successfully compare them to their observationally derived counterpart. In this paper we expand our model and predict the asymmetries featured in the intensity power spectrum. We find that the shape of the mode line profiles in intensity is largely dependent on how the oscillation-induced variations of the radiative flux are treated, and that modelling it realistically is crucial to understanding asymmetry reversal. Perturbing a solar-calibrated grey atmosphere model, and adopting the quasi-adiabatic framework as a first step, we reproduce the asymmetries observed in the solar intensity spectrum for low-frequency modes. We conclude that, unlike previously thought, it is not necessary to invoke an additional mechanism (e.g. non-adiabatic effects, coherent non-resonant background signal) to explain asymmetry reversal. This additional mechanism is necessary, however, to explain asymmetry reversal for higher-order modes. Title: The solar gravitational redshift from HARPS-LFC Moon spectra⋆. A test of the general theory of relativity Authors: González Hernández, J. I.; Rebolo, R.; Pasquini, L.; Lo Curto, G.; Molaro, P.; Caffau, E.; Ludwig, H. -G.; Steffen, M.; Esposito, M.; Suárez Mascareño, A.; Toledo-Padrón, B.; Probst, R. A.; Hänsch, T. W.; Holzwarth, R.; Manescau, A.; Steinmetz, T.; Udem, Th.; Wilken, T. Bibcode: 2020A&A...643A.146G Altcode: 2020arXiv200910558G Context. The general theory of relativity predicts the redshift of spectral lines in the solar photosphere as a consequence of the gravitational potential of the Sun. This effect can be measured from a solar disk-integrated flux spectrum of the Sun's reflected light on Solar System bodies.
Aims: The laser frequency comb (LFC) calibration system attached to the HARPS spectrograph offers the possibility of performing an accurate measurement of the solar gravitational redshift (GRS) by observing the Moon or other Solar System bodies. Here, we analyse the line shift observed in Fe absorption lines from five high-quality HARPS-LFC spectra of the Moon.
Methods: We selected an initial sample of 326 photospheric Fe lines in the spectral range between 476-585 nm and measured their line positions and equivalent widths (EWs). Accurate line shifts were derived from the wavelength position of the core of the lines compared with the laboratory wavelengths of Fe lines. We also used a CO5BOLD 3D hydrodynamical model atmosphere of the Sun to compute 3D synthetic line profiles of a subsample of about 200 spectral Fe lines centred at their laboratory wavelengths. We fit the observed relatively weak spectral Fe lines (with EW< 180 mÅ) with the 3D synthetic profiles.
Results: Convective motions in the solar photosphere do not affect the line cores of Fe lines stronger than about ∼150 mÅ. In our sample, only 15 Fe I lines have EWs in the range 150< EW(mÅ) < 550, providing a measurement of the solar GRS at 639 ± 14 m s-1, which is consistent with the expected theoretical value on Earth of ∼633.1 m s-1. A final sample of about 97 weak Fe lines with EW < 180 mÅ allows us to derive a mean global line shift of 638 ± 6 m s-1, which is in agreement with the theoretical solar GRS.
Conclusions: These are the most accurate measurements of the solar GRS obtained thus far. Ultrastable spectrographs calibrated with the LFC over a larger spectral range, such as HARPS or ESPRESSO, together with a further improvement on the laboratory wavelengths, could provide a more robust measurement of the solar GRS and further testing of 3D hydrodynamical models.

Tables A.1 and A.2 are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/643/A146

Based on observations taken with the ESO 3.6 m telescope at La Silla Observatory, Chile. Title: VizieR Online Data Catalog: The solar gravitational redshift (Gonzalez Hernandez+, 2020) Authors: Gonzalez Hernandez, J. I.; Rebolo, R.; Pasquini, L.; Lo Curto, G.; Molaro, P.; Caffau, E.; Ludwig, H. -G.; Steffen, M.; Esposito, M.; Suarez Mascarenno, A.; Toledo-Padron, B.; Probst, R. A.; Hansch, T. W.; Holzwarth, R.; Manescau, A.; Steinmetz, T.; Udem, T.; Wilken, T. Bibcode: 2020yCat..36430146G Altcode: Line data and velocity shifts of the FeI and FeII lines, with laboratory wavelengths, λlab, from Nave et al. (1994ApJS...94..221N, 2013ApJS..204....1N) and excitation potentials, oscillator strengths from the VALD database (Piskunov et al. 1995A&AS..112..525P).

In Table A.1 we provide the mean line core shifts, vcoreobs, measured on the spectral lines from the observed HARPS-LFC spectra of the MOON and computed with respect to the original laboratory wavelengths (Nave et al. 1994ApJS...94..221N, 2013ApJS..204....1N).

We also give the recalibrated wavelengths, lambda_nist, computed from recalibrated wavenumber measurements and Ritz wavelengths, lambda_ritz, computed from recalibrated energy levels, with their corresponding wavelengths uncertainties, extracted from the NIST database (Kramida et al. 2019APS..DMPN09004K).

In Table A.2, we give the line core shifts measured on the observed spectral lines, vcoreobs_n, estimated using the recalibrated wavelengths, lambda_nist, as reference laboratory wavelengths, the 3D profiles, vcore,3D, and the global line shifts, vfit3Dn, from fitting the observed spectral lines using 3D profiles, and corrected using the recalibrated wavelengths lambda_nist as reference laboratory wavelengths.

Wavelengths are given in Angstroms, wavelength uncertainties in miliAngstroems, excitation potentials in eV, equivalent widths (EW) in miliAngstroems, and velocity shifts in m/s.

(2 data files). Title: Chemically Peculiar A and F Stars with Enhanced s-process and Iron-peak Elements: Stellar Radiative Acceleration at Work Authors: Xiang, Mao-Sheng; Rix, Hans-Walter; Ting, Yuan-Sen; Ludwig, Hans-Günter; Coronado, Johanna; Zhang, Meng; Zhang, Hua-Wei; Buder, Sven; Tio, Piero Dal Bibcode: 2020ApJ...898...28X Altcode: 2020arXiv200603329X We present ≳15,000 metal-rich ([Fe/H] > -0.2 dex) A and F stars whose surface abundances deviate strongly from solar abundance ratios and cannot plausibly reflect their birth material composition. These stars are identified by their high [Ba/Fe] abundance ratios ([Ba/Fe] > 1.0 dex) in the LAMOST DR5 spectra analyzed by Xiang et al. They are almost exclusively main-sequence and subgiant stars with Teff ≳ 6300 K. Their distribution in the Kiel diagram (Teff- $\mathrm{log}g$ ) traces a sharp border at low temperatures along a roughly fixed-mass trajectory (around 1.4 M⊙) that corresponds to an upper limit in convective envelope mass fraction of around 10-4. Most of these stars exhibit distinctly enhanced abundances of iron-peak elements (Cr, Mn, Fe, Ni) but depleted abundances of Mg and Ca. Rotational velocity measurements from GALAH DR2 show that the majority of these stars rotate slower than typical stars in an equivalent temperature range. These characteristics suggest that they are related to the so-called Am/Fm stars. Their abundance patterns are qualitatively consistent with the predictions of stellar evolution models that incorporate radiative acceleration, suggesting they are a consequence of stellar internal evolution, particularly involving the competition between gravitational settling and radiative acceleration. These peculiar stars constitute 40% of the whole population of stars with mass above 1.5 M⊙, affirming that "peculiar" photospheric abundances due to stellar evolution effects are a ubiquitous phenomenon for these intermediate-mass stars. This large sample of Ba-enhanced, chemically peculiar A/F stars with individual element abundances provides the statistics to test more stringently the mechanisms that alter the surface abundances in stars with radiative envelopes. Title: Spatially Resolved Stellar Disk Spectra at Hyper-high Resolution: Toward Earth-like Exoplanet Detection Authors: Dravins, D.; Ludwig, H. Bibcode: 2020AAS...23613002D Altcode: High-precision spectroscopy might find 'truly' Earth-like exoplanets. Instrumental precisions are close to being achieved but limitations arise in the complexities of spectral-line formation. Spectral lines become somewhat asymmetric by being formed in dynamic gas flows. Radial-velocity signatures differ between different types of lines, change between stars, vary across stellar disks, and are modulated by magnetic activity. Spectroscopy across spatially resolved stellar disks has become possible by using transiting exoplanets as occulting spatial probes, permitting to test center-to-limb atmospheric hydrodynamics in stars also other than the Sun. Additional suitable target stars will likely be found in exoplanet surveys, and simulated observations are in progress to identify strategies for their near-future observations. From a grid of 3-D hydrodynamic CO5BOLD model atmospheres for solar-type stars, synthetic spectra have been computed at hyper-high spectral resolution (R greater than 1 million), for several center-to-limb locations across stellar disks. (The term 'hyper-high' is used since 'ultra-high' is already taken for lower-resolution data.) Such resolutions are required to fully resolve intrinsic line asymmetries. To segregate those from such arising due to blends, and also to obtain absolute wavelength shifts irrespective of errors in laboratory wavelengths, 3-D spectra are matched against similar data from 1-D models. There, unblended lines appear symmetric at their laboratory wavelength positions, and differences to 3-D profiles isolate effects arising in the dynamic photospheres. Synthetic spectra are surveyed for unblended lines with different strengths, excitation potentials, and ionization levels, each of which contribute characteristic signatures of line asymmetries and apparent Doppler shifts. The hyper-high resolution data are degraded to common spectrometer values to appreciate what signatures may realistically be observed. An adequate understanding of both line formation and of spectrometer performance should enable to disentangle effects from variable stellar atmospheres from those induced by even small Earth-like exoplanets. Title: Modelling the asymmetries of the Sun's radial p-mode line profiles Authors: Philidet, J.; Belkacem, K.; Samadi, R.; Barban, C.; Ludwig, H. -G. Bibcode: 2020A&A...635A..81P Altcode: 2020arXiv200110271P Context. The advent of space-borne missions has substantially increased the number and quality of the measured power spectrum of solar-like oscillators. It now allows for the p-mode line profiles to be resolved and facilitates an estimation of their asymmetry. The fact that this asymmetry can be measured for a variety of stars other than the Sun calls for a revisiting of acoustic mode asymmetry modelling. This asymmetry has been shown to be related to a highly localised source of stochastic driving in layers just beneath the surface. However, existing models assume a very simplified, point-like source of excitation. Furthermore, mode asymmetry could also be impacted by a correlation between the acoustic noise and the oscillating mode. Prior studies have modelled this impact, but only in a parametrised fashion, which deprives them of their predictive power.
Aims: In this paper, we aim to develop a predictive model for solar radial p-mode line profiles in the velocity spectrum. Unlike the approach favoured by prior studies, this model is not described by free parameters and we do not use fitting procedures to match the observations. Instead, we use an analytical turbulence model coupled with constraints extracted from a 3D hydrodynamic simulation of the solar atmosphere. We then compare the resulting asymmetries with their observationally derived counterpart.
Methods: We model the velocity power spectral density by convolving a realistic stochastic source term with the Green's function associated with the radial homogeneous wave equation. We compute the Green's function by numerically integrating the wave equation and we use theoretical considerations to model the source term. We reconstruct the velocity power spectral density and extract the line profile of radial p-modes as well as their asymmetry.
Results: We find that stochastic excitation localised beneath the mode upper turning point generates negative asymmetry for ν < νmax and positive asymmetry for ν > νmax. On the other hand, stochastic excitation localised above this limit generates negative asymmetry throughout the p-mode spectrum. As a result of the spatial extent of the source of excitation, both cases play a role in the total observed asymmetries. By taking this spatial extent into account and using a realistic description of the spectrum of turbulent kinetic energy, both a qualitative and quantitative agreement can be found with solar observations performed by the GONG network. We also find that the impact of the correlation between acoustic noise and oscillation is negligible for mode asymmetry in the velocity spectrum. Title: A high-precision abundance analysis of the nuclear benchmark star HD 20 Authors: Hanke, Michael; Hansen, Camilla Juul; Ludwig, Hans-Günter; Cristallo, Sergio; McWilliam, Andrew; Grebel, Eva K.; Piersanti, Luciano Bibcode: 2020A&A...635A.104H Altcode: 2020arXiv200111038H Metal-poor stars with detailed information available about their chemical inventory pose powerful empirical benchmarks for nuclear astrophysics. Here we present our spectroscopic chemical abundance investigation of the metal-poor ([Fe/H] = -1.60 ± 0.03 dex), r-process-enriched ([Eu/Fe] = 0.73 ± 0.10 dex) halo star HD 20, using novel and archival high-resolution data at outstanding signal-to-noise ratios (up to ∼1000 Å-1). By combining one of the first asteroseismic gravity measurements in the metal-poor regime from a TESS light curve with the spectroscopic analysis of iron lines under non-local thermodynamic equilibrium conditions, we derived a set of highly accurate and precise stellar parameters. These allowed us to delineate a reliable chemical pattern that is comprised of solid detections of 48 elements, including 28 neutron-capture elements. Hence, we establish HD 20 among the few benchmark stars that have nearly complete patterns and low systematic dependencies on the stellar parameters. Our light-element (Z ≤ 30) abundances are representative of other, similarly metal-poor stars in the Galactic halo that exhibit contributions from core-collapse supernovae of type II. In the realm of the neutron-capture elements, our comparison to the scaled solar r-pattern shows that the lighter neutron-capture elements (Z ≲ 60) are poorly matched. In particular, we find imprints of the weak r-process acting at low metallicities. Nonetheless, by comparing our detailed abundances to the observed metal-poor star BD +17 3248, we find a persistent residual pattern involving mainly the elements Sr, Y, Zr, Ba, and La. These are indicative of enrichment contributions from the s-process and we show that mixing with material from predicted yields of massive, rotating AGB stars at low metallicity improves the fit considerably. Based on a solar ratio of heavy- to light-s elements - which is at odds with model predictions for the i-process - and a missing clear residual pattern with respect to other stars with claimed contributions from this process, we refute (strong) contributions from such astrophysical sites providing intermediate neutron densities. Finally, nuclear cosmochronology is used to tie our detection of the radioactive element Th to an age estimate for HD 20 of 11.0 ± 3.8 Gyr.

Full Table C.1 is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/635/A104

This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile. Based in part on data products from observations made with ESO Telescopes under program IDs 090.B-0605(A) (PI: Chanamé) and 60.A-9036(A). Title: VizieR Online Data Catalog: Chemical abundance analysis of HD 20 (Hanke+, 2020) Authors: Hanke, M.; Hansen, C. J.; Ludwig, H. -G.; Cristallo, S.; McWilliam, A.; Grebel, E. K.; Piersanti, L. Bibcode: 2020yCat..36350104H Altcode: Equivalent widths (EWs) measured from HD 20's spectra using EWCODE are presented alongside individual transition parameters and deduced abundances. Profiles for which a standard EW analysis was prohibited were analyzed using spectrum synthesis. NLTE corrections were performed and are listed whenever available.

(1 data file). 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: The 6Li/7Li isotopic ratio in the metal-poor binary CS22876-032 Authors: González Hernández, J. I.; Bonifacio, P.; Caffau, E.; Ludwig, H. -G.; Steffen, M.; Monaco, L.; Cayrel, R. Bibcode: 2019A&A...628A.111G Altcode: 2019arXiv190705109G
Aims: We present high-resolution and high-quality UVES spectroscopic data of the metal-poor double-lined spectroscopic binary CS 22876-032 ([Fe/H] approximately -3.7 dex). Our goal is to derive the 6Li/7Li isotopic ratio by analysing the Li I λ 670.8 nm doublet.
Methods: We co-added all 28 useful spectra normalised and corrected for radial velocity to the rest frame of the primary star. We fitted the Li profile with a grid of the 3D non-local thermodynamic equilibrium (NLTE) synthetic spectra to take into account the line profile asymmetries induced by stellar convection, and performed Monte Carlo simulations to evaluate the uncertainty of the fit of the Li line profile.
Results: We checked that the veiling factor does not affect the derived isotopic ratio, 6 Li/7Li, and only modifies the Li abundance, A(Li), by about 0.15 dex. The best fit of the Li profile of the primary star provides A(Li) = 2.17 ± 0.01 dex and 6 Li/7Li = 8-5+2% at 68% confidence level. In addition, we improved the Li abundance of the secondary star at A(Li) = 1.55 ± 0.04 dex, which is about 0.6 dex lower than that of the primary star.
Conclusions: The analysis of the Li profile of the primary star is consistent with no detection of 6 Li and provides an upper limit to the isotopic ratio of 6 Li/7Li < 10% at this very low metallicity, about 0.5 dex lower in metallicity than previous attempts for detection of 6 Li in extremely metal poor stars. These results do not solve or worsen the cosmological 7 Li problem, nor do they support the need for non-standard 6Li production in the early Universe.

The two averaged spectra are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/628/A111Based on observations made with the Very Large Telescope (VLT) at ESO Paranal Observatory, Chile, Programme 080.D-0333. Title: VizieR Online Data Catalog: Li in BPS CS22876-032 spectrum (Gonzalez Hernandez+, 2019) Authors: Gonzalez Hernandez, J. I.; Bonifacio, P.; Caffau, E.; Ludwig, H. -G.; Steffen, M.; Monaco, L.; Cayrel, R. Bibcode: 2019yCat..36280111G Altcode: Average co-added, rebinned spectra in the region around the LiI 670.8nm resonance line of the two stellar components of the metal-poor binary CS 22876-032 A and CS 22876-032 B. For each star, wavelength, normalised flux and flux error are given.

(2 data files). Title: Masses of the Hyades white dwarfs. A gravitational redshift measurement Authors: Pasquini, L.; Pala, A. F.; Ludwig, H. -G.; Lẽao, I. C.; de Medeiros, J. R.; Weiss, A. Bibcode: 2019A&A...627L...8P Altcode: 2019arXiv190701265P Context. It is possible to accurately measure the masses of the white dwarfs (WDs) in the Hyades cluster using gravitational redshift, because the radial velocity of the stars can be obtained independently of spectroscopy from astrometry and the cluster has a low velocity dispersion.
Aims: We aim to obtain an accurate measurement of the Hyades WD masses by determining the mass-to-radius ratio (M/R) from the observed gravitational redshift, and to compare them with masses derived from other methods.
Methods: We analyse archive high-resolution UVES-VLT spectra of six WDs belonging to the Hyades to measure their Doppler shift, from which M/R is determined after subtracting the astrometric radial velocity. We estimate the radii using Gaia photometry as well as literature data.
Results: The M/R error associated to the gravitational redshift measurement is about 5%. The radii estimates, evaluated with different methods, are in very good agreement, though they can differ by up to 4% depending on the quality of the data. The masses based on gravitational redshift are systematically smaller than those derived from other methods, by a minimum of ∼0.02 up to 0.05 solar masses. While this difference is within our measurement uncertainty, the fact that it is systematic indicates a likely real discrepancy between the different methods.
Conclusions: We show that the M/R derived from gravitational redshift measurements is a powerful tool to determine the masses of the Hyades WDs and could reveal interesting properties of their atmospheres. The technique can be improved by using dedicated spectrographs, and can be extended to other clusters, making it unique in its ability to accurately and empirically determine the masses of WDs in open clusters. At the same time we prove that gravitational redshift in WDs agrees with the predictions of stellar evolution models to within a few percent.

Based on UVES data from the ESO VLT archive. Title: Analysis of surface effect on solar-like oscillation frequencies using 3D hydrodynamical models Authors: Sonoi, T.; Samadi, R.; Belkacem, K.; Ludwig, H. -G.; Caffau, E.; Mosser, B. Bibcode: 2019EAS....82..253S Altcode: We evaluate the frequency difference between standard stellar models and models patched with 3D hydrodynamical models across the Teff-g plane. It allows us to constrain frequency corrections for surface effect. The coefficients in the correction functionals are thus provided as functions of effective temperature and surface gravity. Title: Accurate effective temperature from Hα profiles Authors: Giribaldi, R. E.; Ubaldo-Melo, M. L.; Porto de Mello, G. F.; Pasquini, L.; Ludwig, H. -G.; Ulmer-Moll, S.; Lorenzo-Oliveira, D. Bibcode: 2019A&A...624A..10G Altcode: 2018arXiv181112274G Context. The determination of stellar effective temperature (Teff) in F, G, and K stars using Hα profile fitting is a quite remarkable and powerful tool because it does not depend on reddening and is only slightly sensitive to other atmospheric parameters. Nevertheless, this technique is not frequently used because of the complex procedure needed to recover the profile of broad lines in echelle spectra. As a consequence, tests performed on different models have sometimes provided ambiguous results.
Aims: The main aim of this work is to test the ability of the Hα profile fitting technique to derive Teff. We also aim to improve the applicability of this technique to echelle spectra and to test how well 1D + LTE models perform on a variety of F-K stars. We also apply the technique to HARPS spectra and test the reliability and the stability of the HARPS response over several years using the Sun.
Methods: We have developed a normalization method for recovering undistorted Hα profiles and we have first applied it to spectra acquired with the single-order Coudé instrument (resolution R = 45 000) at do Pico dos Dias Observatory to avoid the problem of blaze correction. The continuum location around Hα is optimised using an iterative procedure, where the identification of minute telluric features is performed. A set of spectra was acquired with the MUSICOS echelle spectrograph (R = 40 000) to independently validate the normalization method. The accuracy of the method and of the 1D + LTE model is determined using Coudé/HARPS/MUSICOS spectra of the Sun and Coudé-only spectra of a sample of ten Gaia Benchmark Stars with Teff determined from interferometric measurements. HARPS, Coudé, and MUSICOS spectra are used to determine Teff of 43 sample stars.
Results: We find that a proper choice of spectral windows of fits plus the identification of telluric features allow for a very careful normalization of the spectra and produce reliable Hα profiles. We also find that the most used solar atlases cannot be used as templates for Hα temperature diagnostics without renormalization. The comparison with the Sun shows that Hα profiles from 1D + LTE models underestimate the solar Teff by 28 K. We find the same agreement between Hα and interferometry and between Hα and Infrared Flux Method: a shallow dependency on metallicity according to the relation Teff = TeffHα - 159[Fe/H] + 28 K within the metallicity range - 0.70 to + 0.40 dex. The comparison with the Infrared Flux Method shows a scatter of 59 K dominated by photometric errors (52 K). In order to investigate the origin of this dependency, we analyzed spectra from 3D models and found that they produce hotter temperatures, and that their use largely improves the agreement with the interferometric and Infrared Flux Method measurements. Finally, we find HARPS spectra to be fully suitable for Hα profile temperature diagnostics; they are perfectly compatible with the Coudé spectra, and lead to the same Teff for the Sun as that found when analysing HARPS spectra over a timespan of more than 7 yr.

A copy of the spectra is available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/624/A10Based on observations collected at Observatório do Pico dos Dias (OPD), operated by the Laboratório Nacional de Astrofísica, CNPq, Brazil and on data from the ESO Science Archive Facility. Title: Two-dimensional non-LTE O I 777 nm line formation in radiation hydrodynamics simulations of Cepheid atmospheres Authors: Vasilyev, V.; Amarsi, A. M.; Ludwig, H. -G.; Lemasle, B. Bibcode: 2019A&A...624A..85V Altcode: 2019arXiv190302109V Oxygen abundance measurements are important for understanding stellar structure and evolution. Measured in Cepheids, they further provide clues on the metallicity gradient and chemo-dynamical evolution in the Galaxy. However, most of the abundance analyses of Cepheids to date have been based on one-dimensional (1D) hydrostatic model atmospheres. Here, we test the validity of this approach for the key oxygen abundance diagnostic, the O I 777 nm triplet lines. We carry out two-dimensional (2D) non-LTE radiative transfer calculations across two different 2D radiation hydrodynamics simulations of Cepheid atmospheres, having stellar parameters of Teff = 5600 K, solar chemical compositions, and log g = 1.5 and 2.0, corresponding to pulsation periods of 9 and 3 days, respectively. We find that the 2D non-LTE versus 1D LTE abundance differences range from -1.0 to -0.25 dex depending on pulsational phase. The 2D non-LTE versus 1D non-LTE abundance differences range from -0.2 to 0.8 dex. The abundance differences are smallest when the Cepheid atmospheres are closest to hydrostatic equilibrium, corresponding to phases of around 0.3-0.8, and we recommend these phases for observers deriving the oxygen abundance from O I 777 nm triplet with 1D hydrostatic models. Title: 4MOST: Project overview and information for the First Call for Proposals Authors: de Jong, R. S.; Agertz, O.; Berbel, A. A.; Aird, J.; Alexander, D. A.; Amarsi, A.; Anders, F.; Andrae, R.; Ansarinejad, B.; Ansorge, W.; Antilogus, P.; Anwand-Heerwart, H.; Arentsen, A.; Arnadottir, A.; Asplund, M.; Auger, M.; Azais, N.; Baade, D.; Baker, G.; Baker, S.; Balbinot, E.; Baldry, I. K.; Banerji, M.; Barden, S.; Barklem, P.; Barthélémy-Mazot, E.; Battistini, C.; Bauer, S.; Bell, C. P. M.; Bellido-Tirado, O.; Bellstedt, S.; Belokurov, V.; Bensby, T.; Bergemann, M.; Bestenlehner, J. M.; Bielby, R.; Bilicki, M.; Blake, C.; Bland-Hawthorn, J.; Boeche, C.; Boland, W.; Boller, T.; Bongard, S.; Bongiorno, A.; Bonifacio, P.; Boudon, D.; Brooks, D.; Brown, M. J. I.; Brown, R.; Brüggen, M.; Brynnel, J.; Brzeski, J.; Buchert, T.; Buschkamp, P.; Caffau, E.; Caillier, P.; Carrick, J.; Casagrande, L.; Case, S.; Casey, A.; Cesarini, I.; Cescutti, G.; Chapuis, D.; Chiappini, C.; Childress, M.; Christlieb, N.; Church, R.; Cioni, M. -R. L.; Cluver, M.; Colless, M.; Collett, T.; Comparat, J.; Cooper, A.; Couch, W.; Courbin, F.; Croom, S.; Croton, D.; Daguisé, E.; Dalton, G.; Davies, L. J. M.; Davis, T.; de Laverny, P.; Deason, A.; Dionies, F.; Disseau, K.; Doel, P.; Döscher, D.; Driver, S. P.; Dwelly, T.; Eckert, D.; Edge, A.; Edvardsson, B.; Youssoufi, D. E.; Elhaddad, A.; Enke, H.; Erfanianfar, G.; Farrell, T.; Fechner, T.; Feiz, C.; Feltzing, S.; Ferreras, I.; Feuerstein, D.; Feuillet, D.; Finoguenov, A.; Ford, D.; Fotopoulou, S.; Fouesneau, M.; Frenk, C.; Frey, S.; Gaessler, W.; Geier, S.; Gentile Fusillo, N.; Gerhard, O.; Giannantonio, T.; Giannone, D.; Gibson, B.; Gillingham, P.; González-Fernández, C.; Gonzalez-Solares, E.; Gottloeber, S.; Gould, A.; Grebel, E. K.; Gueguen, A.; Guiglion, G.; Haehnelt, M.; Hahn, T.; Hansen, C. J.; Hartman, H.; Hauptner, K.; Hawkins, K.; Haynes, D.; Haynes, R.; Heiter, U.; Helmi, A.; Aguayo, C. H.; Hewett, P.; Hinton, S.; Hobbs, D.; Hoenig, S.; Hofman, D.; Hook, I.; Hopgood, J.; Hopkins, A.; Hourihane, A.; Howes, L.; Howlett, C.; Huet, T.; Irwin, M.; Iwert, O.; Jablonka, P.; Jahn, T.; Jahnke, K.; Jarno, A.; Jin, S.; Jofre, P.; Johl, D.; Jones, D.; Jönsson, H.; Jordan, C.; Karovicova, I.; Khalatyan, A.; Kelz, A.; Kennicutt, R.; King, D.; Kitaura, F.; Klar, J.; Klauser, U.; Kneib, J. -P.; Koch, A.; Koposov, S.; Kordopatis, G.; Korn, A.; Kosmalski, J.; Kotak, R.; Kovalev, M.; Kreckel, K.; Kripak, Y.; Krumpe, M.; Kuijken, K.; Kunder, A.; Kushniruk, I.; Lam, M. I.; Lamer, G.; Laurent, F.; Lawrence, J.; Lehmitz, M.; Lemasle, B.; Lewis, J.; Li, B.; Lidman, C.; Lind, K.; Liske, J.; Lizon, J. -L.; Loveday, J.; Ludwig, H. -G.; McDermid, R. M.; Maguire, K.; Mainieri, V.; Mali, S.; Mandel, H.; Mandel, K.; Mannering, L.; Martell, S.; Martinez Delgado, D.; Matijevic, G.; McGregor, H.; McMahon, R.; McMillan, P.; Mena, O.; Merloni, A.; Meyer, M. J.; Michel, C.; Micheva, G.; Migniau, J. -E.; Minchev, I.; Monari, G.; Muller, R.; Murphy, D.; Muthukrishna, D.; Nandra, K.; Navarro, R.; Ness, M.; Nichani, V.; Nichol, R.; Nicklas, H.; Niederhofer, F.; Norberg, P.; Obreschkow, D.; Oliver, S.; Owers, M.; Pai, N.; Pankratow, S.; Parkinson, D.; Paschke, J.; Paterson, R.; Pecontal, A.; Parry, I.; Phillips, D.; Pillepich, A.; Pinard, L.; Pirard, J.; Piskunov, N.; Plank, V.; Plüschke, D.; Pons, E.; Popesso, P.; Power, C.; Pragt, J.; Pramskiy, A.; Pryer, D.; Quattri, M.; Queiroz, A. B. d. A.; Quirrenbach, A.; Rahurkar, S.; Raichoor, A.; Ramstedt, S.; Rau, A.; Recio-Blanco, A.; Reiss, R.; Renaud, F.; Revaz, Y.; Rhode, P.; Richard, J.; Richter, A. D.; Rix, H. -W.; Robotham, A. S. G.; Roelfsema, R.; Romaniello, M.; Rosario, D.; Rothmaier, F.; Roukema, B.; Ruchti, G.; Rupprecht, G.; Rybizki, J.; Ryde, N.; Saar, A.; Sadler, E.; Sahlén, M.; Salvato, M.; Sassolas, B.; Saunders, W.; Saviauk, A.; Sbordone, L.; Schmidt, T.; Schnurr, O.; Scholz, R. -D.; Schwope, A.; Seifert, W.; Shanks, T.; Sheinis, A.; Sivov, T.; Skúladóttir, Á.; Smartt, S.; Smedley, S.; Smith, G.; Smith, R.; Sorce, J.; Spitler, L.; Starkenburg, E.; Steinmetz, M.; Stilz, I.; Storm, J.; Sullivan, M.; Sutherland, W.; Swann, E.; Tamone, A.; Taylor, E. N.; Teillon, J.; Tempel, E.; ter Horst, R.; Thi, W. -F.; Tolstoy, E.; Trager, S.; Traven, G.; Tremblay, P. -E.; Tresse, L.; Valentini, M.; van de Weygaert, R.; van den Ancker, M.; Veljanoski, J.; Venkatesan, S.; Wagner, L.; Wagner, K.; Walcher, C. J.; Waller, L.; Walton, N.; Wang, L.; Winkler, R.; Wisotzki, L.; Worley, C. C.; Worseck, G.; Xiang, M.; Xu, W.; Yong, D.; Zhao, C.; Zheng, J.; Zscheyge, F.; Zucker, D. Bibcode: 2019Msngr.175....3D Altcode: 2019arXiv190302464D We introduce the 4-metre Multi-Object Spectroscopic Telescope (4MOST), a new high-multiplex, wide-field spectroscopic survey facility under development for the four-metre-class Visible and Infrared Survey Telescope for Astronomy (VISTA) at Paranal. Its key specifications are: a large field of view (FoV) of 4.2 square degrees and a high multiplex capability, with 1624 fibres feeding two low-resolution spectrographs (R = λ/Δλ 6500), and 812 fibres transferring light to the high-resolution spectrograph (R 20 000). After a description of the instrument and its expected performance, a short overview is given of its operational scheme and planned 4MOST Consortium science; these aspects are covered in more detail in other articles in this edition of The Messenger. Finally, the processes, schedules, and policies concerning the selection of ESO Community Surveys are presented, commencing with a singular opportunity to submit Letters of Intent for Public Surveys during the first five years of 4MOST operations. Title: Spectroscopic and astrometric radial velocities: Hyades as a benchmark Authors: Leão, I. C.; Pasquini, L.; Ludwig, H. -G.; de Medeiros, J. R. Bibcode: 2019MNRAS.483.5026L Altcode: 2018arXiv181108771L; 2018MNRAS.tmp.3060L We study the accuracy of spectroscopic radial velocities (RVs) by comparing spectroscopic and astrometric RVs for stars of the Hyades open cluster. We obtained High-Accuracy Radial velocity Planet Searcher (HARPS) spectra for a large sample of Hyades stars and combined them with accurate astrometric RVs based on Hipparcos and Gaia data. After cleaning the sample from binaries, RV variables, and outliers, 71 stars remained. The distribution of the observed RV difference (between spectroscopic and astrometric) is skewed and depends on the star right ascension. This is consistent with the Hyades cluster rotating at 42.3 m s-1pc-1. The two Hyades giants in the sample show, as predicted by gravitational redshift (GR), a spectroscopic RV that is blue-shifted with respect to the dwarfs, and the empirical GR slope is of 626 ± 131 m s-1, in agreement with the theoretical prediction. The difference between spectroscopic and astrometric RVs is close to zero when considering the Gaia-based RVs corrected for cluster rotation. The mean difference is of -33 m s-1 and the median is of -16 m s-1, with a standard deviation of 347 m s-1 being close to the expected cluster velocity dispersion. We also determine a new value of the cluster centroid spectroscopic RV: 39.36 ± 0.26 km s-1. We finally discuss other phenomena that can influence the RV difference, such as cluster expansion, stellar rotation, stellar activity, general relativity, and Galactic potential. Clusters within the reach of current telescopes are expected to show differences of several hundreds m s-1, depending on their position in the Galaxy. Title: VizieR Online Data Catalog: Normalized Halpha line profiles of FGK stars (Giribaldi+, 2019) Authors: Giribaldi, R. E.; Ubaldo-Melo, M. L.; Porto de Mello, G. F.; Pasquini, L.; Ludwig, H. -G.; Ulmer-Moll, S.; Lorenzo-Oliveira, D. Bibcode: 2019yCat..36240010G Altcode: Normalized observational Hα line profiles of 43 F- ,G- , and K-type stars, including the Sun. The profiles were recorded in spectra acquired by the single-order coude instrument at do Pico dos Dias Observatory. The spectra are provided in .fits format, and their observation conditions are registered in the headers of the files. For some spectra, a version corrected from telluric contamination is also provided but in ascii format only. These files contain three columns: Wavelength (Angstrom), telluric-corrected flux, and non telluric-corrected flux. Some telluric-corrected spectra may present obvious correction errors. Some spectra may be contaminated by cosmic rays. The names of the files indicate the Henry Draper catalog number of the star, except for the Sun, for which the names of the solar surrogates are indicated. The name of the stars are followed by a number 1, 2, 3, or 4, which indicates a different spectrum of the same star.

(3 data files). Title: 4MOST Consortium Survey 2: The Milky Way Halo High-Resolution Survey Authors: Christlieb, N.; Battistini, C.; Bonifacio, P.; Caffau, E.; Ludwig, H. -G.; Asplund, M.; Barklem, P.; Bergemann, M.; Church, R.; Feltzing, S.; Ford, D.; Grebel, E. K.; Hansen, C. J.; Helmi, A.; Kordopatis, G.; Kovalev, M.; Korn, A.; Lind, K.; Quirrenbach, A.; Rybizki, J.; Skúladóttir, Á.; Starkenburg, E. Bibcode: 2019Msngr.175...26C Altcode: 2019arXiv190302468C We will study the formation history of the Milky Way, and the earliest phases of its chemical enrichment, with a sample of more than 1.5 million stars at high galactic latitude. Elemental abundances of up to 20 elements with a precision of better than 0.2 dex will be derived for these stars. The sample will include members of kinematically coherent substructures, which we will associate with their possible birthplaces by means of their abundance signatures and kinematics, allowing us to test models of galaxy formation. Our target catalogue is also expected to contain 30 000 stars at a metallicity of less than one hundredth that of the Sun. This sample will therefore be almost a factor of 100 larger than currently existing samples of metal-poor stars for which precise elemental abundances are available (determined from high-resolution spectroscopy), enabling us to study the early chemical evolution of the Milky Way in unprecedented detail. Title: Systematic investigation of chemical abundances derived using IR spectra obtained with GIANO Authors: Caffau, E.; Bonifacio, P.; Oliva, E.; Korotin, S.; Capitanio, L.; Andrievsky, S.; Collet, R.; Sbordone, L.; Duffau, S.; Sanna, N.; Tozzi, A.; Origlia, L.; Ryde, N.; Ludwig, H. -G. Bibcode: 2019A&A...622A..68C Altcode: 2018arXiv181205100C Context. Detailed chemical abundances of Galactic stars are needed in order to improve our knowledge of the formation and evolution of our galaxy, the Milky Way.
Aims: We took advantage of the GIANO archive spectra to select a sample of Galactic disc stars in order to derive their chemical inventory and to compare the abundances we derived from these infrared spectra to the chemical pattern derived from optical spectra.
Methods: We analysed high-quality spectra of 40 stars observed with GIANO. We derived the stellar parameters from the photometry and the Gaia data-release 2 (DR2) parallax; the chemical abundances were derived with the code MyGIsFOS. For a subsample of stars we compared the chemical pattern derived from the GIANO spectra with the abundances derived from optical spectra. We derived P abundances for all 40 stars, increasing the number of Galactic stars for which phosphorus abundance is known.
Results: We could derive abundances of 14 elements, 8 of which are also derived from optical spectra. The comparison of the abundances derived from infrared and optical spectra is very good. The chemical pattern of these stars is the one expected for Galactic disc stars and is in agreement with the results from the literature.
Conclusions: GIANO is providing the astronomical community with an extremely useful instrument, able to produce spectra with high resolution and a wide wavelength range in the infrared.

GIANO programme A31TAC. Title: Calibration of mixing-length parameter α for MLT and FST models by matching with CO5BOLD models Authors: Sonoi, T.; Ludwig, H. -G.; Dupret, M. -A.; Montalbán, J.; Samadi, R.; Belkacem, K.; Caffau, E.; Goupil, M. -J. Bibcode: 2019A&A...621A..84S Altcode: 2018arXiv181105229S Context. Space observations by the CoRoT and Kepler missions have provided a wealth of high-quality seismic data for a large number of stars from the main sequence to the red giant phases. One main goal of these missions is to take advantage of the rich spectra of solar-like oscillations to perform precise determinations of stellar characteristic parameters. To make the best of such data, we need theoretical stellar models with a precise near-surface structure since a near-surface structure of a solar-like star has significant influence on solar-like oscillation frequencies. The mixing-length parameter is a key factor to determine the near-surface structure of stellar models. In current versions of the convection formulations used in stellar evolution codes, the mixing-length parameter is a free parameter that needs to be properly specified.
Aims: We aim at determining appropriate values of the mixing-length parameter, α, to be used consistently with the adopted convection formulation when computing stellar evolution models across the Hertzsprung-Russell diagram. This determination is based on 3D hydrodynamical simulation models.
Methods: We calibrated α values by matching entropy profiles of 1D envelope models with those of hydrodynamical 3D models of solar-like stars produced by the CO5BOLD code. For such calibration, previous works concentrated on the classical mixing-length theory (MLT). We also analyzed full spectrum turbulence (FST) models. To construct the atmosphere in the 1D models, we used the Eddington gray T(τ) relation and that with the solar-calibrated Hopf-like function.
Results: For both MLT and FST models with a mixing length l = αHp, calibrated α values increase with increasing surface gravity or decreasing effective temperature. For the FST models, we carried out an additional calibration using an α* value defined as l = rtop - r + α*Hp, top, where α* is found to increase with surface gravity and effective temperature. We provide tables of the calibrated α values across the Teff-log g plane for solar metallicity. By computing stellar evolution with varying α based on our 3D α calibration, we find that the change from solar α to varying α shifts evolutionary tracks particularly for the FST model. As for the correspondence to the 3D models, the solar Hopf-like function generally gives a photospheric-minimum entropy closer to a 3D model than the Eddington T(τ). The structure below the photosphere depends on the adopted convection model. However, we cannot obtain a definitive conclusion about which convection model gives the best correspondence to the 3D models. This is because each 1D physical quantity is related via an equation of state (EoS), but it is not the case for the averaged 3D quantities. Although the FST models with l = rtop - r + α*Hp, top are found to give the oscillation frequencies closest to the solar observed frequencies, their acoustic cavities are formed with compensatory effects between deviating density and temperature profiles near the top of the convective envelope. In future work, an appropriate treatment of the top part of the 1D convective envelope is necessary, for example, by considering turbulent pressure and overshooting. Title: TOPoS. V. Abundance ratios in a sample of very metal-poor turn-off stars Authors: François, P.; Caffau, E.; Bonifacio, P.; Spite, M.; Spite, F.; Cayrel, R.; Christlieb, N.; Gallagher, A. J.; Klessen, R.; Koch, A.; Ludwig, H. -G.; Monaco, L.; Plez, B.; Steffen, M.; Zaggia, S. Bibcode: 2018A&A...620A.187F Altcode: 2018arXiv181100035F Context. Extremely metal-poor stars are keys to understand the early evolution of our Galaxy. The ESO large programme TOPoS has been tailored to analyse a new set of metal-poor turn-off stars, whereas most of the previously known extremely metal-poor stars are giant stars.
Aims: Sixty five turn-off stars (preselected from SDSS spectra) have been observed with the X-shooter spectrograph at the ESO VLT Unit Telescope 2, to derive accurate and detailed abundances of magnesium, silicon, calcium, iron, strontium and barium.
Methods: We analysed medium-resolution spectra (R ≃ 10 000) obtained with the ESO X-shooter spectrograph and computed the abundances of several α and neutron-capture elements using standard one-dimensional local thermodynamic equilibrium (1D LTE) model atmospheres.
Results: Our results confirms the super-solar [Mg/Fe] and [Ca/Fe] ratios in metal-poor turn-off stars as observed in metal-poor giant stars. We found a significant spread of the [α/Fe] ratios with several stars showing subsolar [Ca/Fe] ratios. We could measure the abundance of strontium in 12 stars of the sample, leading to abundance ratios [Sr/Fe] around the Solar value. We detected barium in two stars of the sample. One of the stars (SDSS J114424-004658) shows both very high [Ba/Fe] and [Sr/Fe] abundance ratios (>1 dex).

Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programme ID 189.D-0165.

Equivalent widths of the Fe lines are only, and Tables A.1 and A.2 are also available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/620/A187 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: Influence of metallicity on the near-surface effect on oscillation frequencies Authors: Manchon, L.; Belkacem, K.; Samadi, R.; Sonoi, T.; Marques, J. P. C.; Ludwig, H. -G.; Caffau, E. Bibcode: 2018A&A...620A.107M Altcode: 2018arXiv180908904M Context. The CoRoT and Kepler missions have provided high-quality measurements of the frequency spectra of solar-like pulsators, enabling us to probe stellar interiors with a very high degree of accuracy by comparing the observed and modelled frequencies. However, the frequencies computed with 1D models suffer from systematic errors related to the poor modelling of the uppermost layers of stars. These biases are what is commonly named the near-surface effect. The dominant effect is thought to be related to the turbulent pressure that modifies the hydrostatic equilibrium and thus the frequencies. This has already been investigated using grids of 3D hydrodynamical simulations, which also were used to constrain the parameters of the empirical correction models. However, the effect of metallicity has not been considered so far.
Aims: We aim to study the impact of metallicity on the surface effect, investigating its influence across the Hertzsprung-Russell diagram, and providing a method for accounting for it when using the empirical correction models.
Methods: We computed a grid of patched 1D stellar models with the stellar evolution code CESTAM in which poorly modelled surface layers have been replaced by averaged stratification computed with the 3D hydrodynamical code CO5BOLD. It allowed us to investigate the dependence of both the surface effect and the empirical correction functions on the metallicity.
Results: We found that metallicity has a strong impact on the surface effect: keeping Teff and log g constant, the frequency residuals can vary by up to a factor of two (for instance from [Fe/H] = + 0.0 to [Fe/H] = + 0.5). Therefore, the influence of metallicity cannot be neglected. We found that the correct way of accounting for it is to consider the surface Rosseland mean opacity. It allowed us to give a physically grounded justification as well as a scaling relation for the frequency differences at νmax as a function of Teff, log g and κ. Finally, we provide prescriptions for the fitting parameters of the most commonly used correction functions.
Conclusions: We show that the impact of metallicity through the Rosseland mean opacity must be taken into account when studying and correcting the surface effect. Title: VizieR Online Data Catalog: Very metal-poor turn-off stars abundances (Francois+, 2018) Authors: Francois, P.; Caffau, E.; Bonifacio, P.; Spite, M.; Spite, F.; Cayrel, R.; Christlieb, N.; Gallagher, A.; Klessen, R.; Koch, A.; Ludwig, H. -G.; Monaco, L.; Plez, B.; Steffen, M.; Zaggia, S. Bibcode: 2018yCat..36200187F Altcode: Sixty five turn-off stars (preselected from SDSS spectra) have been observed with the X-Shooter spectrograph at the ESO VLT Unit Telescope 2, to derive accurate and detailed abundances of magnesium, silicon, calcium, iron, strontium and barium. We analysed medium-resolution spectra (R~10000) obtained with the ESO X-Shooter spectrograph and computed the abundances of several alpha and neutron-capture elements using standard one-dimensional local thermodynamic equilibrium (1D LTE) model atmospheres.

(3 data files). Title: Calibration of the mixing length of the MLT and FST models using 3D hydrodynamical models Authors: Sonoi, T.; Ludwig, H. -G.; Dupret, M. -A.; Montalban, J.; Belkacem, K.; Caffau, E. Bibcode: 2018phos.confE..27S Altcode: Rich spectra of solar-like oscillations obtained with space observations are expected to enable us to perform precise determinations of stellar properties. To make the best of the spectra, we need theoretical stellar models with precise near-surface structure, since the near-surface structure has significant influence on solar-like oscillation frequencies. The mixing-length parameter, α, is a key factor to determine the near-surface structure. We aimed at determining appropriate α values based on 3D radiation-coupled hydrodynamical models produced by the CO^5BOLD code. For such calibration, previous works concentrated on the classical mixing-length theory (MLT). Here we also analyzed the full spectrum turbulence (FST) models. The trends of the calibrated α values in the Teff-g plane is found to be similar to those of previous calibrations with the other grids of RHD models. A T(τ) relation based on the so-called VAL-C solar-atmosphere model is found to give better correspondence to the photospheric-minimum entropy in the 3D model than the Eddington T(τ) relation. Although the structure below the photosphere depends on convection models, not a single convection model gives the best correspondence to the 3D model since physical quantities in the 3D models are not necessarily related via an equation of states unlike those in the 1D models. Although the FST model with a form of a mixing length (l=rtop-r+α*Hp,{top}) is found to give solar-oscillation frequencies apparently closest to the observed ones, the acoustic cavity of this model is formed with compensatory effects between deviating density and temperature profiles just below the top of the convective envelope. In future work, a more sophisticated treatment of the top part of the 1D convective envelope is necessary. Title: A physically-grounded relation between the metallicity and the surface term affecting stellar oscillation frequencies Authors: Manchon, Louis; Belkacem, Kevin; Samadi, Reza; Sonoi, Takafumi; Marques, J. P. C.; Ludwig, Hans-Gunter; Caffau, E. Bibcode: 2018phos.confE..36M Altcode: The CoRoT and Kepler missions have provided high-quality measurements of the frequency spectra of solar-like pulsators, enabling us to probe stellar interiors with a very high degree of accuracy by comparing the observed and modeled frequencies. However, the frequencies computed with 1D models suffer from systematic errors related to the poor modeling of the uppermost layers of stars. These biases are what is commonly named the near surface effect. The dominant effect is thought to be related to the turbulent pressure that modifies the hydrostatic equilibrium and thus the frequencies. This has already been investigated using grids of 3D hydrodynamical simulations, however, the effect of metallicity has not been considered so far. We aim at studying the impact of metallicity on the surface effect, investigating its influence across the Hertzsprung–Russell diagram, and providing a relation between the frequency differences and global parameters. We computed a grid of 29 patched 1D stellar models with the stellar evolution code CESTAM in which poorly modeled surface layers have been replaced by averaged stratification computed with the 3D hydrodynamical code CO 5 BOLD. It allowed us to study the dependence of the surface effect on the metallicity. We found that a correct way of accounting for it is to consider the surface Rosseland mean opacity. It allowed us to give a physically-grounded justification as well as a scaling relation for the frequency differences at ν max as a function of T eff , log g and κ. Title: Abundance of zinc in the red giants of Galactic globular cluster 47 Tucanae Authors: Černiauskas, A.; Kučinskas, A.; Klevas, J.; Bonifacio, P.; Ludwig, H. -G.; Caffau, E.; Steffen, M. Bibcode: 2018A&A...616A.142C Altcode: 2018arXiv180603132C
Aims: We investigate possible relations between the abundances of zinc and the light elements sodium, magnesium, and potassium in the atmospheres of red giant branch (RGB) stars of the Galactic globular cluster 47 Tuc and study connections between the chemical composition and dynamical properties of the cluster RGB stars.
Methods: The abundance of zinc was determined in 27 RGB stars of 47 Tuc using 1D local thermal equilibrium (LTE) synthetic line profile fitting to the high-resolution 2dF/HERMES spectra obtained with the Anglo-Australian Telescope (AAT). Synthetic spectra used in the fitting procedure were computed with the SYNTHE code and 1D ATLAS9 stellar model atmospheres.
Results: The average 1D LTE zinc-to-iron abundance ratio and its RMS variations due to star-to-star abundance spread determined in the sample of 27 RGB stars is <[Zn/Fe]>1D LTE = 0.11 ± 0.09. We did not detect any statistically significant relations between the abundances of zinc and those of light elements. Neither did we find any significant correlation or anticorrelation between the zinc abundance in individual stars and their projected distance from the cluster center. Finally, no statistically significant relation between the absolute radial velocities of individual stars and the abundance of zinc in their atmospheres was detected. The obtained average [Zn/Fe]1DLTE ratio agrees well with those determined in this cluster in earlier studies and nearly coincides with that of Galactic field stars at this metallicity. All these results suggest that nucleosynthesis of zinc and light elements proceeded in separate, unrelated pathways in 47 Tuc. Title: Spatially resolved spectroscopy across stellar surfaces. III. Photospheric Fe I lines across HD 189733A (K1 V) Authors: Dravins, Dainis; Gustavsson, Martin; Ludwig, Hans-Günter Bibcode: 2018A&A...616A.144D Altcode: 2018arXiv180600012D Context. Spectroscopy across spatially resolved stellar surfaces reveals spectral line profiles free from rotational broadening, whose gradual changes from disk center toward the stellar limb reflect an atmospheric fine structure that is possible to model by 3D hydrodynamics.
Aims: Previous studies of photospheric spectral lines across stellar disks exist for the Sun and HD 209458 (G0 V) and are now extended to the planet-hosting HD 189733A to sample a cooler K-type star and explore the future potential of the method.
Methods: During exoplanet transit, stellar surface portions successively become hidden and differential spectroscopy between various transit phases uncovers spectra of small surface segments temporarily hidden behind the planet. The method was elaborated in Paper I, in which observable signatures were predicted quantitatively from hydrodynamic simulations.
Results: From observations of HD 189733A with the ESO HARPS spectrometer at λ/Δλ 115 000, profiles for stronger and weaker Fe I lines are retrieved at several center-to-limb positions, reaching adequate S/N after averaging over numerous similar lines.
Conclusions: Retrieved line profile widths and depths are compared to synthetic ones from models with parameters bracketing those of the target star and are found to be consistent with 3D simulations. Center-to-limb changes strongly depend on the surface granulation structure and much greater line-width variation is predicted in hotter F-type stars with vigorous granulation than in cooler K-types. Such parameters, obtained from fits to full line profiles, are realistic to retrieve for brighter planet-hosting stars, while their hydrodynamic modeling offers previously unexplored diagnostics for stellar atmospheric fine structure and 3D line formation. Precise modeling may be required in searches for Earth-analog exoplanets around K-type stars, whose more tranquil surface granulation and lower ensuing microvariability may enable such detections. Title: Carbon-enhanced metal-poor 3D model atmospheres Authors: Steffen, M.; Gallagher, A. J.; Caffau, E.; Bonifacio, P.; Ludwig, H. -G. Bibcode: 2018IAUS..334..364S Altcode: 2017arXiv170805686S We present our latest 3D model atmospheres for carbon-enhanced metal-poor (CEMP) stars computed with the CO5BOLD code. The stellar parameters are representative of hot turn-off objects (Teff ~ 6250 K, log g = 4.0, [Fe/H]=-3). The main purpose of these models is to investigate the role of 3D effects on synthetic spectra of the CH G-band (4140-4400 Å), the CN BX-band (3870-3890 Å), and several UV OH transitions (3122-3128 Å). By comparison with the synthetic spectra from standard 1D model atmospheres (assuming local thermodynamic equilibrium, LTE), we derive 3D abundance corrections for carbon and oxygen of up to -0.5 and -0.7 dex, respectively. Title: Abundances of Mg and K in the atmospheres of turn-off starsin Galactic globular cluster 47 Tucanae Authors: Černiauskas, A.; Kučinskas, A.; Klevas, J.; Dobrovolskas, V.; Korotin, S.; Bonifacio, P.; Ludwig, H. -G.; Caffau, E.; Steffen, M. Bibcode: 2018A&A...615A.173C Altcode: 2018arXiv180410033C
Aims: We determined abundances of Mg and K in the atmospheres of 53 (Mg) and 75 (K) turn-off (TO) stars of the Galactic globular cluster 47 Tuc. The obtained abundances, together with those of Li, O, and Na that we had earlier determined for the same sample of stars, were used to search for possible relations between the abundances of K and other light elements, Li, O, Na, and Mg, as well as the connections between the chemical composition of TO stars and their kinematical properties.
Methods: Abundances of Mg and K were determined using archival high resolution VLT FLAMES/GIRAFFE spectra, in combination with the one-dimensional (1D) non-local thermodynamic equilibrium (NLTE) spectral synthesis methodology. Spectral line profiles were computed with the MULTI code, using 1D hydrostatic ATLAS9 stellar model atmospheres. We also utilized three-dimensional (3D) hydrodynamical CO5BOLD and 1D hydrostatic LHD model atmospheres for computing 3D-1D LTE abundance corrections for the spectral lines of Mg and K, in order to assess the influence of convection on their formation in the atmospheres of TO stars.
Results: The determined average abundance-to-iron ratios and their root mean square variations due to star-to-star abundance spreads were <[Mg/Fe]>1D NLTE = 0.47 ± 0.12, and <[K/Fe]>1D NLTE = 0.39 ± 0.09. Although the data suggest the possible existence of a weak correlation in the [K/Fe]-[Na/Fe] plane, its statistical significance is low. No statistically significant relations between the abundance of K and other light elements were detected. Also, we did not find any significant correlations or anti-correlations between the [Mg/Fe] and [K/Fe] ratios and projected distance from the cluster center. Similarly, no relations between the absolute radial velocities of individual stars and abundances of Mg and K in their atmospheres were detected. The 3D-1D abundance corrections were found to be small (≤0.1 dex) for the lines of Mg and K used in this study, thus indicating that the influence of convection on their formation is small. Title: Observations of the solar chromosphere with ALMA and comparison with theoretical models Authors: Brajsa, Roman; Sudar, Davor; Skokic, Ivica; Benz, Arnold O.; Kuhar, Matej; Kobelski, Adam; Wedemeyer, Sven; White, Stephen M.; Ludwig, Hans-G.; Temmer, Manuela; Saar, Steven H.; Selhorst, Caius L. Bibcode: 2018csss.confE..37B Altcode: 2018arXiv181207293B In this work we use solar observations with the ALMA radio telescope at the wavelength of 1.21 mm. The aim of the analysisis to improve understanding of the solar chromosphere, a dynamic layer in the solar atmosphere between the photosphere andcorona. The study has an observational and a modeling part. In the observational part full-disc solar images are analyzed.Based on a modied FAL atmospheric model, radiation models for various observed solar structures are developed. Finally, theobservational and modeling results are compared and discussed. 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: Using the CIFIST grid of CO5BOLD 3D model atmospheres to study the effects of stellar granulation on photometric colours. II. The role of convection across the H-R diagram Authors: Kučinskas, A.; Klevas, J.; Ludwig, H. -G.; Bonifacio, P.; Steffen, M.; Caffau, E. Bibcode: 2018A&A...613A..24K Altcode: 2018arXiv180200073K
Aims: We studied the influence of convection on the spectral energy distributions (SEDs), photometric magnitudes, and colour indices of different types of stars across the H-R diagram.
Methods: The 3D hydrodynamical CO5BOLD, averaged ⟨3D⟩, and 1D hydrostatic LHD model atmospheres were used to compute SEDs of stars on the main sequence (MS), main sequence turn-off (TO), subgiant branch (SGB), and red giant branch (RGB), in each case at two different effective temperatures and two metallicities, [M/H] = 0.0 and - 2.0. Using the obtained SEDs, we calculated photometric magnitudes and colour indices in the broad-band Johnson-Cousins UBVRI and 2MASS JHKs, and the medium-band Strömgren uvby photometric systems.
Results: The 3D-1D differences in photometric magnitudes and colour indices are small in both photometric systems and typically do not exceed ± 0.03 mag. Only in the case of the coolest giants located on the upper RGB are the differences in the U and u bands able reach ≈-0.2 mag at [M/H] = 0.0 and ≈-0.1 mag at [M/H] = -2.0. Generally, the 3D-1D differences are largest in the blue-UV part of the spectrum and decrease towards longer wavelengths. They are also sensitive to the effective temperature and are significantly smaller in hotter stars. Metallicity also plays a role and leads to slightly larger 3D-1D differences at [M/H] = 0.0. All these patterns are caused by a complex interplay between the radiation field, opacities, and horizontal temperature fluctuations that occur due to convective motions in stellar atmospheres. Although small, the 3D-1D differences in the magnitudes and colour indices are nevertheless comparable to or larger than typical photometric uncertainties and may therefore cause non-negligible systematic differences in the estimated effective temperatures. Title: TOPoS. IV. Chemical abundances from high-resolution observations of seven extremely metal-poor stars Authors: Bonifacio, P.; Caffau, E.; Spite, M.; Spite, F.; Sbordone, L.; Monaco, L.; François, P.; Plez, B.; Molaro, P.; Gallagher, A. J.; Cayrel, R.; Christlieb, N.; Klessen, R. S.; Koch, A.; Ludwig, H. -G.; Steffen, M.; Zaggia, S.; Abate, C. Bibcode: 2018A&A...612A..65B Altcode: 2018arXiv180103935B Context. Extremely metal-poor (EMP) stars provide us with indirect information on the first generations of massive stars. The TOPoS survey has been designed to increase the census of these stars and to provide a chemical inventory that is as detailed as possible.
Aims: Seven of the most iron-poor stars have been observed with the UVES spectrograph at the ESO VLT Kueyen 8.2 m telescope to refine their chemical composition.
Methods: We analysed the spectra based on 1D LTE model atmospheres, but also used 3D hydrodynamical simulations of stellar atmospheres.
Results: We measured carbon in six of the seven stars: all are carbon-enhanced and belong to the low-carbon band, defined in the TOPoS II paper. We measured lithium (A(Li) = 1.9) in the most iron-poor star (SDSS J1035+0641, [Fe/H] <-5.2). We were also able to measure Li in three stars at [Fe/H] -4.0, two of which lie on the Spite plateau. We confirm that SDSS J1349+1407 is extremely rich in Mg, but not in Ca. It is also very rich in Na. Several of our stars are characterised by low α-to-iron ratios.
Conclusions: The lack of high-carbon band stars at low metallicity can be understood in terms of evolutionary timescales of binary systems. The detection of Li in SDSS J1035+0641 places a strong constraint on theories that aim at solving the cosmological lithium problem. The Li abundance of the two warmer stars at [Fe/H] -4.0 places them on the Spite plateau, while the third, cooler star, lies below. We argue that this suggests that the temperature at which Li depletion begins increases with decreasing [Fe/H]. SDSS J1349+1407 may belong to a class of Mg-rich EMP stars. We cannot assess if there is a scatter in α-to-iron ratios among the EMP stars or if there are several discrete populations. However, the existence of stars with low α-to-iron ratios is supported by our observations.

Based on observations obtained at ESO Paranal Observatory, Programmes 189.D-0165,090.D-0306, 093.D-0136, and 096.D-0468. 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: Impact of magnetic fields on the structure of convective atmospheres of red giant stars Authors: Klevas, J.; Kučinskas, A.; Wedemeyer, S.; Ludwig, H. -G. Bibcode: 2018CoSka..48..280K Altcode: We use 3D magnetohydrodynamic CO5BOLD model atmospheres to study the interplay between magnetic fields and convection in the atmospheres of red giant stars. We find that vortex-like structures occur prominently in stars with stronger magnetic fields and lead to alterations of their thermal structures. Title: A comparison of solar ALMA observations and model based predictions of the brightness temperature Authors: Brajša, R.; Kuhar, M.; Benz, A. O.; Skokić, I.; Sudar, D.; Wedemeyer, S.; Báarta, M.; De Pontieu, B.; Kim, S.; Kobelski, A.; Shimojo, M.; White, S.; Yagoubov, P.; Yan, Y.; Ludwig, H. G.; Temmer, M.; Saar, S. H.; Selhorst, C. L.; Beuc, R. Bibcode: 2018CEAB...42....1B Altcode: The new facility Atacama Large Millimeter/submillimeter Array (ALMA) is capable of observing the Sun in the wavelength range from 0.3 mm to 10 mm with an unprecedented spatial, temporal and spectral resolution. The first aim of the present work is to identify different structures in the solar atmosphere (quiet Sun, active regions, filaments on the disc, and coronal holes) in a full disc solar ALMA image at 1.21 mm obtained on December 18, 2015 during a CSV-EOC campaign. It is compared with full disc solar images from the same day in the Hα line (Cerro Tololo Observatory, NISP), and at three EUV wavelengths (30.4 nm, 21.1 nm, 17.1 nm; a composite SDO image). Positions of the quiet Sun areas, active regions, filaments on the disc, and coronal holes are identified in the ALMA image. To interpret solar observations with ALMA it is important to compare the measured and calculated intensities of various solar structures. So, the second aim of this work is to calculate the intensity (brightness temperature) for those structures (quiet Sun, active regions, filaments on the disc, and coronal holes) for a broad wavelength range (from 0.3 mm to 10 mm), closely related to that of the ALMA, and to compare the results with available ALMA observations. Thermal bremsstrahlung is the dominant radiation mechanism for explanation of the observed phenomena. A procedure for calculating the brightness temperature for a given wavelength and model atmosphere, which integrates the radiative transfer equation for thermal bremsstrahlung, is used. At the wavelength of 1.21 mm active regions appear as bright areas, while filaments on the disc and coronal holes are not discernible from the quiet Sun background. The models generally agree with the observed results: Active regions are bright primarily due to higher densities, filaments can appear bright, dark or not at all and coronal holes cannot be easily identified. 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: Three-dimensional hydrodynamical CO5BOLD model atmospheres of red giant stars. VI. First chromosphere model of a late-type giant Authors: Wedemeyer, Sven; Kučinskas, Arūnas; Klevas, Jonas; Ludwig, Hans-Günter Bibcode: 2017A&A...606A..26W Altcode: 2017arXiv170509641W
Aims: Although observational data unequivocally point to the presence of chromospheres in red giant stars, no attempts have been made so far to model them using 3D hydrodynamical model atmospheres. We therefore compute an exploratory 3D hydrodynamical model atmosphere for a cool red giant in order to study the dynamical and thermodynamic properties of its chromosphere, as well as the influence of the chromosphere on its observable properties.
Methods: Three-dimensional radiation hydrodynamics simulations are carried out with the CO5BOLD model atmosphere code for a star with the atmospheric parameters (Teff ≈ 4010 K, log g = 1.5, [ M / H ] = 0.0), which are similar to those of the K-type giant star Aldebaran (α Tau). The computational domain extends from the upper convection zone into the chromosphere (7.4 ≥ log τRoss ≥ - 12.8) and covers several granules in each horizontal direction. Using this model atmosphere, we compute the emergent continuum intensity maps at different wavelengths, spectral line profiles of Ca II K, the Ca II infrared triplet line at 854.2 nm, and Hα, as well as the spectral energy distribution (SED) of the emergent radiative flux.
Results: The initial model quickly develops a dynamical chromosphere that is characterised by propagating and interacting shock waves. The peak temperatures in the chromospheric shock fronts reach values of up to 5000 K, although the shock fronts remain quite narrow. Similar to the Sun, the gas temperature distribution in the upper layers of red giant stars is composed of a cool component due to adiabatic cooling in the expanding post-shock regions and a hot component due to shock waves. For this red giant model, the hot component is a rather flat high-temperature tail, which nevertheless affects the resulting average temperatures significantly.
Conclusions: The simulations show that the atmospheres of red giant stars are dynamic and intermittent. Consequently, many observable properties cannot be reproduced with static 1D models, but require advanced 3D hydrodynamical modelling. Furthermore, including a chromosphere in the models might produce significant contributions to the emergent UV flux. 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: Stellar atmospheres behind transiting exoplanets Authors: Dravins, D.; Ludwig, H. -G.; Dahlén, E.; Gustavsson, M.; Pazira, H. Bibcode: 2017EPSC...11...21D Altcode: Stellar surfaces are covered with brighter and darker structures, just like on the Sun. While solar surface details can be easily studied with telescopes, stellar surfaces cannot thus be resolved. However, one can use planets that happen to pass in front of distant stars as "shades" that successively block out small portions of the stellar surface behind. By measuring how the light from the star changes during such a transit, one can deduce stellar surface properties. Knowing those is required not only to study the star as such, but also to deduce the chemical composition of the planet that is passing in front of it, where some of the detected starlight has been filtered through the planet's atmosphere. Title: Spatially resolved spectroscopy across stellar surfaces. II. High-resolution spectra across HD 209458 (G0 V) Authors: Dravins, Dainis; Ludwig, Hans-Günter; Dahlén, Erik; Pazira, Hiva Bibcode: 2017A&A...605A..91D Altcode: 2017arXiv170801618D Context. High-resolution spectroscopy across spatially resolved stellar surfaces aims at obtaining spectral-line profiles that are free from rotational broadening; the gradual changes of these profiles from disk center toward the stellar limb reveal properties of atmospheric fine structure, which are possible to model with 3D hydrodynamics.
Aims: Previous such studies have only been carried out for the Sun but are now extended to other stars. In this work, profiles of photospheric spectral lines are retrieved across the disk of the planet-hosting star HD 209458 (G0 V).
Methods: During exoplanet transit, stellar surface portions successively become hidden and differential spectroscopy provides spectra of small surface segments temporarily hidden behind the planet. The method was elaborated in Paper I, with observable signatures quantitatively predicted from hydrodynamic simulations.
Results: From observations of HD 209458 with spectral resolution λ/ Δλ 80 000, photospheric Fe I line profiles are obtained at several center-to-limb positions, reaching adequately high S/N after averaging over numerous similar lines.
Conclusions: Retrieved line profiles are compared to synthetic line profiles. Hydrodynamic 3D models predict, and current observations confirm, that photospheric absorption lines become broader and shallower toward the stellar limb, reflecting that horizontal velocities in stellar granulation are greater than vertical velocities. Additional types of 3D signatures will become observable with the highest resolution spectrometers at large telescopes. Title: Spatially resolved spectroscopy across stellar surfaces. I. Using exoplanet transits to analyze 3D stellar atmospheres Authors: Dravins, Dainis; Ludwig, Hans-Günter; Dahlén, Erik; Pazira, Hiva Bibcode: 2017A&A...605A..90D Altcode: 2017arXiv170801616D Context. High-precision stellar analyses require hydrodynamic modeling to interpret chemical abundances or oscillation modes. Exoplanet atmosphere studies require stellar background spectra to be known along the transit path while detection of Earth analogs require stellar microvariability to be understood. Hydrodynamic 3D models can be computed for widely different stars but have been tested in detail only for the Sun with its resolved surface features. Model predictions include spectral line shapes, asymmetries, and wavelength shifts, and their center-to-limb changes across stellar disks.
Aims: We observe high-resolution spectral line profiles across spatially highly resolved stellar surfaces, which are free from the effects of spatial smearing and rotational broadening present in full-disk spectra, enabling comparisons to synthetic profiles from 3D models.
Methods: During exoplanet transits, successive stellar surface portions become hidden and differential spectroscopy between various transit phases provides spectra of small surface segments temporarily hidden behind the planet. Planets cover no more than 1% of any main-sequence star, enabling high spatial resolution but demanding very precise observations. Realistically measurable quantities are identified through simulated observations of synthetic spectral lines.
Results: In normal stars, line profile ratios between various transit phases may vary by 0.5%, requiring S/N ≳ 5000 for meaningful spectral reconstruction. While not yet realistic for individual spectral lines, this is achievable for cool stars by averaging over numerous lines with similar parameters.
Conclusions: For bright host stars of large transiting planets, spatially resolved spectroscopy is currently practical. More observable targets are likely to be found in the near future by ongoing photometric searches. Title: The Gaia-ESO Survey: Galactic evolution of sulphur and zinc Authors: Duffau, S.; Caffau, E.; Sbordone, L.; Bonifacio, P.; Andrievsky, S.; Korotin, S.; Babusiaux, C.; Salvadori, S.; Monaco, L.; François, P.; Skúladóttir, Á.; Bragaglia, A.; Donati, P.; Spina, L.; Gallagher, A. J.; Ludwig, H. -G.; Christlieb, N.; Hansen, C. J.; Mott, A.; Steffen, M.; Zaggia, S.; Blanco-Cuaresma, S.; Calura, F.; Friel, E.; Jiménez-Esteban, F. M.; Koch, A.; Magrini, L.; Pancino, E.; Tang, B.; Tautvaišienė, G.; Vallenari, A.; Hawkins, K.; Gilmore, G.; Randich, S.; Feltzing, S.; Bensby, T.; Flaccomio, E.; Smiljanic, R.; Bayo, A.; Carraro, G.; Casey, A. R.; Costado, M. T.; Damiani, F.; Franciosini, E.; Hourihane, A.; Jofré, P.; Lardo, C.; Lewis, J.; Morbidelli, L.; Sousa, S. G.; Worley, C. C. Bibcode: 2017A&A...604A.128D Altcode: 2017arXiv170402981D Context. Due to their volatile nature, when sulphur and zinc are observed in external galaxies, their determined abundances represent the gas-phase abundances in the interstellar medium. This implies that they can be used as tracers of the chemical enrichment of matter in the Universe at high redshift. Comparable observations in stars are more difficult and, until recently, plagued by small number statistics.
Aims: We wish to exploit the Gaia-ESO Survey (GES) data to study the behaviour of sulphur and zinc abundances of a large number of Galactic stars, in a homogeneous way.
Methods: By using the UVES spectra of the GES sample, we are able to assemble a sample of 1301 Galactic stars, including stars in open and globular clusters in which both sulphur and zinc were measured.
Results: We confirm the results from the literature that sulphur behaves as an α-element. We find a large scatter in [Zn/Fe] ratios among giant stars around solar metallicity. The lower ratios are observed in giant stars at Galactocentric distances less than 7.5 kpc. No such effect is observed among dwarf stars, since they do not extend to that radius.
Conclusions: Given the sample selection, giants and dwarfs are observed at different Galactic locations, and it is plausible, and compatible with simple calculations, that Zn-poor giants trace a younger population more polluted by SN Ia yields. It is necessary to extend observations in order to observe both giants and dwarfs at the same Galactic location. Further theoretical work on the evolution of zinc is also necessary.

Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programmes 188.B-3002, 193.B-0936.The full table of S abundances is 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/604/A128 Title: An Investigation of the Formation and Line Properties of MgH in 3D Hydrodynamical Model Stellar Atmospheres Authors: Thygesen, Anders O.; Kirby, Evan N.; Gallagher, Andrew J.; Ludwig, Hans-G.; Caffau, Elisabetta; Bonifacio, Piercarlo; Sbordone, Luca Bibcode: 2017ApJ...843..144T Altcode: 2017arXiv170604218T Studies of the isotopic composition of magnesium in cool stars have so far relied upon the use of 1D model atmospheres. Since the isotopic ratios derived are based on asymmetries of optical MgH lines, it is important to test the impact from other effects affecting line asymmetries, like stellar convection. Here, we present a theoretical investigation of the effects of including self-consistent modeling of convection. Using spectral syntheses based on 3D hydrodynamical CO5BOLD models of dwarfs (4000 K ≲ T eff ≲ 5160 K, 4.0 ≤ {log}g ≤ 4.5, -3.0≤slant [{Fe}/{{H}}]≤slant -1.0) and giants (T eff ∼ 4000 K, {log}g = 1.5, -3.0≤slant [{Fe}/{{H}}]≤slant -1.0), we perform a detailed analysis comparing 3D and 1D syntheses. We describe the impact on the formation and behavior of MgH lines from using 3D models, and perform a qualitative assessment of the systematics introduced by the use of 1D syntheses. Using 3D model atmospheres significantly affect the strength of the MgH lines, especially in dwarfs, with 1D syntheses requiring an abundance correction of up to +0.69 dex, with the largest for our 5000 K models. The corrections are correlated with T eff and are also affected by the metallicity. The shape of the strong 24MgH component in the 3D syntheses is poorly reproduced in 1D. This results in 1D syntheses underestimating 25Mg by up to ∼5 percentage points and overestimating 24Mg by a similar amount for dwarfs. This discrepancy increases with decreasing metallicity. 26Mg is recovered relatively well, with the largest difference being ∼2 percentage points. The use of 3D for giants has less impact, due to smaller differences in the atmospheric structure and a better reproduction of the line shape in 1D. Title: Abundances of Na, Mg, and K in the atmospheres of red giant branch stars of Galactic globular cluster 47 Tucanae Authors: Černiauskas, A.; Kučinskas, A.; Klevas, J.; Prakapavičius, D.; Korotin, S.; Bonifacio, P.; Ludwig, H. -G.; Caffau, E.; Steffen, M. Bibcode: 2017A&A...604A..35C Altcode: 2017arXiv170402751C
Aims: We study the abundances of Na, Mg, and K in the atmospheres of 32 red giant branch (RGB) stars in the Galactic globular cluster (GGC) 47 Tuc, with the goal to investigate the possible existence of Na-K and Mg-K correlations/anti-correlations, similar to those that were recently discovered in two other GGCs, NGC 2419 and 2808.
Methods: The abundances of K, Na, and Mg were determined using high-resolution 2dF/HERMES spectra obtained with the Anglo-Australian Telescope (AAT). The one-dimensional (1D) NLTE abundance estimates were obtained using 1D hydrostatic ATLAS9 model atmospheres and spectral line profiles synthesized with the MULTI package. We also used three-dimensional (3D) hydrodynamical CO5BOLD and 1D hydrostatic LHD model atmospheres to compute 3D-1D LTE abundance corrections, Δ3D - 1D LTE, for the spectral lines of Na, Mg, and K used in our study. These abundance corrections were used to understand the role of convection in the formation of spectral lines, as well as to estimate the differences in the abundances obtained with the 3D hydrodynamical and 1D hydrostatic model atmospheres.
Results: The average element-to-iron abundance ratios and their RMS variations due to star-to-star abundance spreads determined in our sample of RGB stars were ⟨ [ Na / Fe ] ⟩ 1D NLTE = 0.42 ± 0.13, ⟨ [ Mg / Fe ] ⟩ 1D NLTE = 0.41 ± 0.11, and ⟨ [ K / Fe ] ⟩ 1D NLTE = 0.05 ± 0.14. We found no statistically significant relations between the abundances of the three elements studied here. Also, there were no abundance trends with the distance from the cluster center, nor any statistically significant relations between the abundance/abundance ratios and absolute radial velocities of individual stars. All these facts suggest the similarity of K abundance in stars that belong to different generations in 47 Tuc which, in turn, may hint that evolution of K in this particular cluster was unrelated to the nucleosynthesis of Na and/or Mg. Title: VizieR Online Data Catalog: S abundances for 1301 stars from GES (Duffau+, 2017) Authors: Duffau, S.; Caffau, E.; Sbordone, L.; Bonifacio, P.; Andrievsky, S.; Korotin, S.; Babusiaux, C.; Salvadori, S.; Monaco, L.; Francois, P.; Skuladottir, A.; Bragaglia, A.; Donati, P.; Spina, L.; Gallagher, A. J.; Ludwig, H. -G.; Christlieb, N.; Hansen, C. J.; Mott, A.; Steffen, M.; Zaggia, S.; Blanco-Cuaresma, S.; Calura, F.; Friel, E.; Jimenez-Esteban, F. M.; Koch, A.; Magrini, L.; Pancino, E.; Tang, B.; Tautvaisiene, G.; Vallenari, A.; Hawkins, K.; Gilmore, G.; Randich, S.; Feltzing, S.; Bensby, T.; Flaccomio, E.; Smiljanic, R.; Bayo, A.; Carraro, G.; Casey, A. R.; Costado, M. T.; Damiani, F.; Franciosini, E.; Hourihane, A.; Jofre, P.; Lardo, C.; Lewis, J.; Morbidelli, L.; Sousa, S. G.; Worley, C. C. Bibcode: 2017yCat..36040128D Altcode: GES internal star identifier (CNAME), Sulphur abundances and NLTE corrections to the Sulphur abundances for 1301 stars. Sulphur abundances are expressed in the customary logarithmic form: A(S)=log_10(N(S)/N(H))+12. The abundances delivered are the LTE ones. NLTEabundances can be determined by directly summing the NLTE correction delivered: A(S)NLTE = A(S) + NLTEC. So that a negative NLTE correction indicates that the NLTE abundance is lower than the LTE one.

(1 data file). Title: Computation of eigenfrequencies for equilibrium models including turbulent pressure Authors: Sonoi, T.; Belkacem, K.; Dupret, M. -A.; Samadi, R.; Ludwig, H. -G.; Caffau, E.; Mosser, B. Bibcode: 2017A&A...600A..31S Altcode: 2017arXiv170107244S Context. The space-borne missions CoRoT and Kepler have provided a wealth of highly accurate data. However, our inability to properly model the upper-most region of solar-like stars prevents us from making the best of these observations. This problem is called "surface effect" and a key ingredient to solve it is turbulent pressure for the computation of both the equilibrium models and the oscillations. While 3D hydrodynamic simulations help to include properly the turbulent pressure in the equilibrium models, the way this surface effect is included in the computation of stellar oscillations is still subject to uncertainties.
Aims: We aim at determining how to properly include the effect of turbulent pressure and its Lagrangian perturbation in the adiabatic computation of the oscillations. We also discuss the validity of the gas-gamma model and reduced gamma model approximations, which have been used to compute adiabatic oscillations of equilibrium models including turbulent pressure.
Methods: We use a patched model of the Sun with an inner part constructed by a 1D stellar evolution code (CESTAM) and an outer part by the 3D hydrodynamical code (CO5BOLD). Then, the adiabatic oscillations are computed using the ADIPLS code for the gas-gamma and reduced gamma model approximations and with the MAD code imposing the adiabatic condition on an existing time-dependent convection formalism. Finally, all those results are compared to the observed solar frequencies.
Results: We show that the computation of the oscillations using the time-dependent convection formalism in the adiabatic limit improves significantly the agreement with the observed frequencies compared to the gas-gamma and reduced gamma model approximations. Of the components of the perturbation of the turbulent pressure, the perturbation of the density and advection term is found to contribute most to the frequency shift.
Conclusions: The turbulent pressure is certainly the dominant factor responsible for the surface effects. Its inclusion into the equilibrium models is thus necessary but not sufficient. Indeed, the perturbation of the turbulent pressure must be properly taken into account for computing adiabatic oscillation frequencies. We propose a formalism to evaluate the frequency shift due to the inclusion of the term with the turbulent pressure perturbation in the variational principle in order to extrapolate our result to other stars at various evolutionary stages. Although this work is limited to adiabatic oscillations and the inclusion of the turbulent pressure, future works will have to account for the nonadiabatic effect and convective backwarming. Title: Three-dimensional hydrodynamical CO5BOLD model atmospheres of red giant stars. V. Oxygen abundance in the metal-poor giant HD 122563 from OH UV lines Authors: Prakapavičius, D.; Kučinskas, A.; Dobrovolskas, V.; Klevas, J.; Steffen, M.; Bonifacio, P.; Ludwig, H. -G.; Spite, M. Bibcode: 2017A&A...599A.128P Altcode: 2016arXiv161103283P Context. Although oxygen is an important tracer of the early Galactic evolution, its abundance trends with metallicity are still relatively poorly known at [Fe/H] ≲ -2.5. This is in part due to a lack of reliable oxygen abundance indicators in the metal-poor stars, and in part due to shortcomings in 1D LTE abundance analyses where different abundance indicators, such as OH lines located in the UV and IR or the forbidden [O I] line at 630 nm, frequently provide inconsistent results.
Aims: In this study, we determined the oxygen abundance in the metal-poor halo giant HD 122563 using a 3D hydrodynamical CO5BOLD model atmosphere. Our main goal was to understand whether a 3D LTE analysis can help to improve the reliability of oxygen abundances that are determined from OH UV lines in comparison to those obtained using standard 1D LTE methodology.
Methods: The oxygen abundance in HD 122563 was determined using 71 OH UV lines located in the wavelength range between 308-330 nm. The analysis was performed using a high-resolution VLT UVES spectrum with a 1D LTE spectral line synthesis performed using the SYNTHE package and classical ATLAS9 model atmosphere. Subsequently, a 3D hydrodynamical CO5BOLD and 1D hydrostatic LHD model atmospheres were used to compute 3D-1D abundance corrections. For this, the microturbulence velocity used with the 1D LHD model atmosphere was derived from the hydrodynamical CO5BOLD model atmosphere of HD 122563. The obtained abundance corrections were then applied to determine 3D LTE oxygen abundances from each individual OH UV line.
Results: As in previous studies, we found trends in the 1D LTE oxygen abundances determined from OH UV lines with line parameters, such as the line excitation potential, χ, and the line equivalent width, W. These trends become significantly less pronounced in 3D LTE. Using OH UV lines, we determined a 3D LTE oxygen abundance in HD 122563 of A(O)3D LTE = 6.23 ± 0.13 ([O/Fe] = 0.07 ± 0.13). This is in fair agreement with the oxygen abundance obtained from OH IR lines, A(O)3D LTE = 6.39 ± 0.11 ([O/Fe] = 0.23 ± 0.11), but it is noticeably lower than that determined when using the forbidden [O I] line, A(O)3D LTE = 6.53 ± 0.15 ([O/Fe] = 0.37 ± 0.15). While the exact cause of this discrepancy remains unclear, it is very likely that non-LTE effects may play a decisive role here. Oxygen-to-iron ratios determined in HD 122563 using OH UV/IR lines and the forbidden [O I] line fall on the lower boundary of the [O/Fe] distribution as observed in the Galactic field stars at this metallicity and suggest a very mild oxygen overabundance with respect to iron, [O/Fe] ≲ 0.4. Title: VizieR Online Data Catalog: NGC104 RGB Na, Mg, and K abundances (Cerniauskas+, 2017) Authors: Cerniauskas, A.; Kucinskas, A.; Klevas, J.; Prakapavicius, D.; Korotin, S.; Bonifacio, P.; Ludwig, H. -G.; Caffau, E.; Steffen, M. Bibcode: 2017yCat..36040035C Altcode: We used 2dF/HERMES spectra obtained in two wavelength regions, 564.9-587.3nm (GREEN) and 758.5-788.7nm (IR), using the spectral resolution of R~28000 and exposure time of 1200s. The observations were carried out during the period of Oct 22 - Dec 20, 2013

(1 data file). Title: An in-depth spectroscopic examination of molecular bands from 3D hydrodynamical model atmospheres. II. Carbon-enhanced metal-poor 3D model atmospheres Authors: Gallagher, A. J.; Caffau, E.; Bonifacio, P.; Ludwig, H. -G.; Steffen, M.; Homeier, D.; Plez, B. Bibcode: 2017A&A...598L..10G Altcode: 2017arXiv170109102G Context. Tighter constraints on metal-poor stars we observe are needed to better understand the chemical processes of the early Universe. Computing a stellar spectrum in 3D allows one to model complex stellar behaviours, which cannot be replicated in 1D.
Aims: We examine the effect that the intrinsic CNO abundances have on a 3D model structure and the resulting 3D spectrum synthesis.
Methods: Model atmospheres were computed in 3D for three distinct CNO chemical compositions using the CO5BOLD model atmosphere code, and their internal structures were examined. Synthetic spectra were computed from these models using Linfor3D and they were compared. New 3D abundance corrections for the G-band and a selection of UV OH lines were also computed.
Results: The varying CNO abundances change the metal content of the 3D models. This had an effect on the model structure and the resulting synthesis. However, it was found that the C/O ratio had a larger effect than the overall metal content of a model.
Conclusions: Our results suggest that varying the C/O ratio has a substantial impact on the internal structure of the 3D model, even in the hot turn-off star models explored here. This suggests that bespoke 3D models, for specific CNO abundances should be sought. Such effects are not seen in 1D at these temperature regimes. 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: 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: The influence of convection on OH UV line formation in the atmosphere of the metal-poor red giant HD 122563. Authors: Prakapavičius, D.; Kučinskas, A.; Dobrovolskas, V.; Klevas, J.; Steffen, M.; Bonifacio, P.; Ludwig, H. -G.; Spite, M. Bibcode: 2017MmSAI..88...77P Altcode: We utilized high-resolution spectra of the metal-poor red giant star HD 122563 and classical 1D hydrostatic ATLAS9 model atmosphere to derive the 1D LTE oxygen abundance from OH UV lines. The obtained average 1D LTE oxygen abundance is xtmean {ensuremath {A(O)_{1D LTE}} = 6.41 ± 0.16. We also used 3D hydrodynamical COBOLD and 1D hydrostatic LHD model atmospheres to correct the 1D LTE abundances for convection-related effects and to determine average 3D LTE abundance, xtmean {ensuremath {A(O)_{3D LTE}} = 6.23 ± 0.13. We found that while the oxygen abundances determined using 1D hydrostatic model atmospheres showed trends both with the line strength and excitation potential, these trends essentially disappeared in the 3D LTE case. The average 3D LTE oxygen abundance obtained from the OH UV lines agrees reasonably well with the oxygen abundances obtained in the earlier analyses from the OH IR lines and the 630.0 nm [O I] line, while the remaining small discrepancies may possibly be attributed to NLTE effects. Title: Investigation of the solar centre-to-limb variation of oxygen and lithium spectral features Authors: Caffau, E.; Malherbe, J. -M.; Steffen, M.; Ludwig, H. -G.; Mott, A. Bibcode: 2017MmSAI..88...45C Altcode: We compare intensity spectra of the Sun observed at different limb angles in the wavelength range covering the forbidden oxygen lines and the lithium resonance feature with line formation computations performed on a CO5BOLD 3D hydrodynamical simulation of the solar atmosphere. Among the prime oxygen abundance indicators, the forbidden line at 630 nm is contaminated with a significant Ni I blend. The availability of observations at different positions on the solar disc allows us to disentangle the contributions of oxygen and nickel and to derive their individual abundances. We derived in the past, from the [OI] line, A(O)=8.73± 0.05 with a nickel abundance of A(Ni)=6.1± 0.04. From the observations here presented, we obtain A(O)=8.71 and A(Ni)=6.09, in excellent agreement with the previous result. For lithium, we investigated the Li doublet at 670.7 nm and compared synthetic spectra of the Li spectra range based on different line-lists available in the literature to the observed data. With these observations, we are still unable to conclude on which is the best line-list to be used for the blending lines. Title: 3D hydrodynamical COBOLD simulations of a chromosphere of a red giant Authors: Klevas, J.; Wedemeyer, S.; Kučinskas, A.; Ludwig, H. -G. Bibcode: 2017MmSAI..88..100K Altcode: We present the results of a 3D hydrodynamical simulation of a cool red giant star with a chromosphere. The simulation was performed using a 3D hydrodynamic COBOLD model atmosphere (Teff=4000 K, {log g}=1.5 cgs, [M/H]=0.0 [dex]), which was extended outwards to include chromospheric layers. We synthesized the spectral energy distribution of a model atmosphere including chromosphere and compare it to the spectral energy distribution of a model atmosphere without the chromosphere. We find that adding a model chromosphere leads to a significant increase of the radiative flux at wavelengths smaller than 300 nm. The increase in the UV flux is attributed to the presence of shock waves, which provide additional heating in the chromosphere. Title: TOPoS. III. An ultra iron-poor multiple CEMP system Authors: Caffau, E.; Bonifacio, P.; Spite, M.; Spite, F.; Monaco, L.; Sbordone, L.; François, P.; Gallagher, A. J.; Plez, B.; Zaggia, S.; Ludwig, H. -G.; Cayrel, R.; Koch, A.; Steffen, M.; Salvadori, S.; Klessen, R.; Glover, S.; Christlieb, N. Bibcode: 2016A&A...595L...6C Altcode: 2016arXiv161004106C
Aims: One of the primary objectives of the TOPoS survey is to search for the most metal-poor stars. Our search has led to the discovery of one of the most iron-poor objects known, SDSS J092912.32+023817.0. This object is a multiple system, in which two components are clearly detected in the spectrum.
Methods: We have analysed 16 high-resolution spectra obtained using the UVES spectrograph at the ESO 8.2 m VLT telescope to measure radial velocities and determine the chemical composition of the system.
Results: Cross correlation of the spectra with a synthetic template yields a double-peaked cross-correlation function (CCF) for eight spectra, and in one case there is evidence for the presence of a third peak. Chemical analysis of the spectrum obtained by averaging all the spectra for which the CCF showed a single peak found that the iron abundance is [Fe/H] = -4.97. The system is also carbon enhanced with [C/Fe] = +3.91 (A(C) = 7.44). From the permitted oxygen triplet we determined an upper limit for oxygen of [O/Fe] < +3.52 such that C/O > 1.3. We are also able to provide more stringent upper limits on the Sr and Ba abundances ([Sr/Fe] < +0.70, and [Ba/Fe] < +1.46, respectively).

Based on observations made with ESO Telescopes at the La Silla Paranal Observatory under programme ID 094.D-0488 and 096.D-0616. Title: An in-depth spectroscopic examination of molecular bands from 3D hydrodynamical model atmospheres. I. Formation of the G-band in metal-poor dwarf stars Authors: Gallagher, A. J.; Caffau, E.; Bonifacio, P.; Ludwig, H. -G.; Steffen, M.; Spite, M. Bibcode: 2016A&A...593A..48G Altcode: 2016arXiv160507215G Context. Recent developments in the three-dimensional (3D) spectral synthesis code Linfor3D have meant that for the first time, large spectral wavelength regions, such as molecular bands, can be synthesised with it in a short amount of time.
Aims: A detailed spectral analysis of the synthetic G-band for several dwarf turn-off-type 3D atmospheres (5850 ≲ Teff [ K ] ≲ 6550, 4.0 ≤ log g ≤ 4.5, - 3.0 ≤ [Fe/H] ≤-1.0) was conducted, under the assumption of local thermodynamic equilibrium. We also examine carbon and oxygen molecule formation at various metallicity regimes and discuss the impact it has on the G-band.
Methods: Using a qualitative approach, we describe the different behaviours between the 3D atmospheres and the traditional one-dimensional (1D) atmospheres and how the different physics involved inevitably leads to abundance corrections, which differ over varying metallicities. Spectra computed in 1D were fit to every 3D spectrum to determine the 3D abundance correction.
Results: Early analysis revealed that the CH molecules that make up the G-band exhibited an oxygen abundance dependency; a higher oxygen abundance leads to weaker CH features. Nitrogen abundances showed zero impact to CH formation. The 3D corrections are also stronger at lower metallicity. Analysis of the 3D corrections to the G-band allows us to assign estimations of the 3D abundance correction to most dwarf stars presented in the literature.
Conclusions: The 3D corrections suggest that A(C) in carbon-enhanced metal-poor (CEMP) stars with high A(C) would remain unchanged, but would decrease in CEMP stars with lower A(C). It was found that the C/O ratio is an important parameter to the G-band in 3D. Additional testing confirmed that the C/O ratio is an equally important parameter for OH transitions under 3D. This presents a clear interrelation between the carbon and oxygen abundances in 3D atmospheres through their molecular species, which is not seen in 1D. Title: Hydrodynamical model atmospheres: Their impact on stellar spectroscopy and asteroseismology of late-type stars Authors: Ludwig, H. -G.; Steffen, M. Bibcode: 2016AN....337..844L Altcode: 2016arXiv160900219L Hydrodynamical, i.e. multi-dimensional and time-dependent, model atmospheres of late-type stars have reached a high level of realism. They are commonly applied in high-fidelity work on stellar abundances but also allow the study of processes that are not modelled in standard, one-dimensional hydrostatic model atmospheres. Here, we discuss two observational aspects that emerge from such processes, the photometric granulation background and the spectroscopic microturbulence. We use CO5BOLD hydrodynamical model atmospheres to characterize the total granular brightness fluctuations and characteristic time scale for FGK stars. Emphasis is put on the diagnostic potential of the granulation background for constraining the fundamental atmospheric parameters. We find a clear metallicity dependence of the granulation background. The comparison between the model predictions and available observational constraints at solar metallicity shows significant differences, that need further clarification. Concerning microturbulence, we report on the derivation of a theoretical calibration based on CO5BOLD models, which shows good correspondence with the measurements for stars in the Hyades. We emphasize the importance of a consistent procedure when determining the microturbulence, and point to limitations of the commonly applied description of microturbulence in hydrostatic model atmospheres. Title: Exoplanet Transits Enable High-Resolution Spectroscopy Across Spatially Resolved Stellar Surfaces Authors: Dravins, Dainis; Ludwig, Hans-Günter; Dahlén, Erik; Pazira, Hiva Bibcode: 2016csss.confE..66D Altcode: 2016arXiv160703489D Observations of stellar surfaces ndash; except for the Sun ndash; are hampered by their tiny angular extent, while observed spectral lines are smeared by averaging over the stellar surface, and by stellar rotation. Exoplanet transits can be used to analyze stellar atmospheric structure, yielding high-resolution spectra across spatially highly resolved stellar surfaces, free from effects of spatial smearing and the rotational wavelength broadening present in full-disk spectra. During a transit, stellar surface portions successively become hidden, and differential spectroscopy between various transit phases provides spectra of those surface segments then hidden behind the planet. The small area subtended by even a large planet (about 1% of a main-sequence star) offers high spatial resolution but demands very precise observations. We demonstrate the reconstruction of photospheric Fe I line profilesnbsp;at a spectral resolution R=80,000 across the surface of the solar-type star HD 209458. Any detailed understanding of stellar atmospheres requires modeling with 3-dimensional hydrodynamics. The properties predicted by such models are mapped onto the precise spectral-line shapes, asymmetries and wavelength shifts, and their variation from the center to the limb across any stellar disk. This method provides a tool for testing and verifying such models. The method will soon become applicable to more diverse types of stars, thanks to new spectrometers on very large telescopes, and since ongoing photometric searches are expected to discover additional bright host stars of transiting exoplanets.> Title: Spatially Resolved Spectroscopy Across HD189733 (K1V) Using Exoplanet Transits Authors: Gustavsson, Martin; Dravins, Dainis; Ludwig, Hans-Günter Bibcode: 2016csss.confE..53G Altcode: For testing 3-dimensional models of stellar atmospheres, spectroscopy across spatially resolved stellar surfaces would be desired with a spectral resolution of(R = 100,000) or more. Hydrodynamic models predict variations in line profile shapes, strengths, wavelength positions and asymmetries. These variations vary systematically between disk center and limb and as a function of line strength, excitation potential and wavelength region. However, except for a few supergiants and the Sun, current telescopes are not yet capable of resolving any stellar surfaces. One alternative method to resolve distant stellar surfaces, feasible already now, is differential spectroscopy of transiting exoplanet systems. By subtracting in-transit spectra from the spectrum outside of transit, the spectra from stellar surface portions temporarily hidden behind the planet can be disentangled. Since transiting planets cover only a small portion of the stellar surface, the method requires a very high signal-to-noise ratio, obtainable by averaging numerous similar spectral lines. We apply such differential spectroscopy on the 7.7 mag K1V star HD 189733 ('Alopex'*); its transiting planet covers ∼ 3% of its host star's surface, which is the deepest known transit among the brighter systems. Archival data from the ESO HARPS spectrometerare used to construct averaged profiles of photospheric Fe I lines, with the aim of comparing spatially resolved profiles to analogous synthetic line profiles computed from the 3-dimensional hydrodynamic CO5BOLD model.
* We refer to HD 189733 as 'Alopex' (from the Greek 'αλɛπού'), denoting a fox related to the one that gave name to its constellation of Vulpecula. Title: Chemical abundances in a high-velocity RR Lyrae star near the bulge Authors: Hansen, C. J.; Rich, R. M.; Koch, A.; Xu, S.; Kunder, A.; Ludwig, H. -G. Bibcode: 2016A&A...590A..39H Altcode: 2016arXiv160305654H Low-mass variable high-velocity stars are interesting study cases for many aspects of Galactic structure and evolution. Until recently, the only known high- or hyper-velocity stars were young stars thought to originate from the Galactic center. Wide-area surveys such as APOGEE and BRAVA have found several low-mass stars in the bulge with Galactic rest-frame velocities higher than 350 km s-1. In this study we present the first abundance analysis of a low-mass RR Lyrae star that is located close to the Galactic bulge, with a space motion of ~-400 km s-1. Using medium-resolution spectra, we derived abundances (including upper limits) of 11 elements. These allowed us to chemically tag the star and discuss its origin, although our derived abundances and metallicity, at [Fe/H] =-0.9 dex, do not point toward one unambiguous answer. Based on the chemical tagging, we cannot exclude that it originated in the bulge. However, its retrograde orbit and the derived abundances combined suggest that the star was accelerated from the outskirts of the inner (or even outer) halo during many-body interactions. Other possible origins include the bulge itself, or the star might have been stripped from a stellar cluster or the Sagittarius dwarf galaxy when it merged with the Milky Way.

The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. Title: The chemical composition of red giants in 47 Tucanae. II. Magnesium isotopes and pollution scenarios Authors: Thygesen, A. O.; Sbordone, L.; Ludwig, H. -G.; Ventura, P.; Yong, D.; Collet, R.; Christlieb, N.; Melendez, J.; Zaggia, S. Bibcode: 2016A&A...588A..66T Altcode: 2016arXiv160200058T Context. The phenomenon of multiple populations in globular clusters is still far from understood, with several proposed mechanisms to explain the observed behaviour. The study of elemental and isotopic abundance patterns are crucial for investigating the differences among candidate pollution mechanisms.
Aims: We derive magnesium isotopic ratios for 13 stars in the globular cluster 47 Tucanae (NGC 104) to provide new, detailed information about the nucleosynthesis that has occurred within the cluster. For the first time, the impact of 3D model stellar atmospheres on the derived Mg isotopic ratios is investigated.
Methods: Using both tailored 1D atmospheric models and 3D hydrodynamical models, we derive magnesium isotopic ratios from four features of MgH near 5135 Å in 13 giants near the tip of the red giant branch, using high signal-to-noise, high-resolution spectra.
Results: We derive the magnesium isotopic ratios for all stars and find no significant offset of the isotopic distribution between the pristine and the polluted populations. Furthermore, we do not detect any statistically significant differences in the spread in the Mg isotopes in either population. No trends were found between the Mg isotopes and [Al/Fe]. The inclusion of 3D atmospheres has a significant impact on the derived 25Mg/24Mg ratio, increasing it by a factor of up to 2.5, compared to 1D. The 26Mg/24Mg ratio, on the other hand, essentially remains unchanged.
Conclusions: We confirm the results seen from other globular clusters, where no strong variation in the isotopic ratios is observed between stellar populations, for observed ranges in [Al/Fe]. We see no evidence for any significant activation of the Mg-Al burning chain. The use of 3D atmospheres causes an increase of a factor of up to 2.5 in the fraction of 25Mg, resolving part of the discrepancy between the observed isotopic fraction and the predictions from pollution models.

Based on observations made with the ESO Very Large Telescope at Paranal Observatory, Chile (Programmes 084.B-0810 and 086.B-0237). Title: Solar Science with the Atacama Large Millimeter/Submillimeter Array—A New View of Our Sun Authors: Wedemeyer, S.; Bastian, T.; Brajša, R.; Hudson, H.; Fleishman, G.; Loukitcheva, M.; Fleck, B.; Kontar, E. P.; De Pontieu, B.; Yagoubov, P.; Tiwari, S. K.; Soler, R.; Black, J. H.; Antolin, P.; Scullion, E.; Gunár, S.; Labrosse, N.; Ludwig, H. -G.; Benz, A. O.; White, S. M.; Hauschildt, P.; Doyle, J. G.; Nakariakov, V. M.; Ayres, T.; Heinzel, P.; Karlicky, M.; Van Doorsselaere, T.; Gary, D.; Alissandrakis, C. E.; Nindos, A.; Solanki, S. K.; Rouppe van der Voort, L.; Shimojo, M.; Kato, Y.; Zaqarashvili, T.; Perez, E.; Selhorst, C. L.; Barta, M. Bibcode: 2016SSRv..200....1W Altcode: 2015SSRv..tmp..118W; 2015arXiv150406887W The Atacama Large Millimeter/submillimeter Array (ALMA) is a new powerful tool for observing the Sun at high spatial, temporal, and spectral resolution. These capabilities can address a broad range of fundamental scientific questions in solar physics. The radiation observed by ALMA originates mostly from the chromosphere—a complex and dynamic region between the photosphere and corona, which plays a crucial role in the transport of energy and matter and, ultimately, the heating of the outer layers of the solar atmosphere. Based on first solar test observations, strategies for regular solar campaigns are currently being developed. State-of-the-art numerical simulations of the solar atmosphere and modeling of instrumental effects can help constrain and optimize future observing modes for ALMA. Here we present a short technical description of ALMA and an overview of past efforts and future possibilities for solar observations at submillimeter and millimeter wavelengths. In addition, selected numerical simulations and observations at other wavelengths demonstrate ALMA's scientific potential for studying the Sun for a large range of science cases. Title: Lithium spectral line formation in stellar atmospheres. The impact of convection and NLTE effects Authors: Klevas, J.; Kučinskas, A.; Steffen, M.; Caffau, E.; Ludwig, H. -G. Bibcode: 2016A&A...586A.156K Altcode: 2015arXiv151208999K
Aims: Because of the complexities involved in treating spectral line formation in full 3D and non-local thermodynamic equilibrium (NLTE), different simplified approaches are sometimes used to account for the NLTE effects with 3D hydrodynamical model atmospheres. In certain cases, chemical abundances are derived in 1D NLTE and then corrected for the 3D effects by adding 3D-1D LTE (Local Thermodynamic Equilibrium, LTE) abundance corrections (3D+NLTE approach). Alternatively, average ⟨3D⟩ model atmospheres are sometimes used to substitute for the full 3D hydrodynamical models.
Methods: In this work we tested whether the results obtained using these simplified schemes (3D+NLTE, ⟨3D⟩ NLTE) may reproduce those derived using the full 3D NLTE computations. The tests were made using 3D hydrodynamical CO5BOLD model atmospheres of the main sequence (MS), main sequence turn-off (TO), subgiant (SGB), and red giant branch (RGB) stars, all at two metallicities, [ M / H ] = 0.0 and -2.0. Our goal was to investigate the role of 3D and NLTE effects on the formation of the 670.8 nm lithium resonance line. This was done by assessing differences in the strengths of synthetic 670.8 nm line profiles, which were computed using 3D/1D NLTE/LTE approaches.
Results: Our results show that Li 670.8 nm line strengths obtained using different methodologies differ only slightly in most of the models at solar metallicity studied here. However, the line strengths predicted with the 3D NLTE and 3D+NLTE approaches become significantly different at subsolar metallicities. At [ M / H ] = -2.0, this may lead to (3D NLTE) - (3D+NLTE) differences in the predicted lithium abundance of ~0.46 and ~0.31 dex in the TO and RGB stars respectively. On the other hand, NLTE line strengths computed with the average ⟨3D⟩ and 1D model atmospheres are similar to those obtained with the full 3D NLTE approach for MS, TO, SGB, and RGB stars, at all metallicities; 3D - ⟨3D⟩ and 3D - 1D differences in the predicted abundances are always less than ~0.04 dex and ~0.08 dex, respectively. However, neither of the simplified approaches can reliably substitute 3D NLTE spectral synthesis when precision is required. Title: New insights on pulsating white dwarfs from 3D radiation-hydrodynamical simulations Authors: Tremblay, Pier-Emmanuel; Fontaine, Gilles; Ludwig, Hans-Günter; Gianninas, Alexandros; Kilic, Mukremin Bibcode: 2016IAUFM..29B.667T Altcode: We have recently computed a grid of 3D radiation-hydrodynamical simulations for the atmosphere of pure-hydrogen DA white dwarfs in the range 5.0 < log g < 9.0. Our grid covers the full ZZ Ceti instability strip where pulsating DA white dwarfs are located. We have significantly improved the theoretical framework to study these objects by removing the free parameters of 1D convection, which were previously a major modeling hurdle. We present improved atmospheric parameter determinations based on spectroscopic fits with 3D model spectra, allowing for an updated definition of the empirical edges of the ZZ Ceti instability strip. Our 3D simulations also precisely predict the depth of the convection zones, narrowing down the internal layers where pulsation are being driven. We hope that these 3D effects will be included in asteroseismic models in the future to predict the region of the HR diagram where white dwarfs are expected to pulsate. Title: GIANO Y-band spectroscopy of dwarf stars: Phosphorus, sulphur, and strontium abundances Authors: Caffau, E.; Andrievsky, S.; Korotin, S.; Origlia, L.; Oliva, E.; Sanna, N.; Ludwig, H. -G.; Bonifacio, P. Bibcode: 2016A&A...585A..16C Altcode: 2015arXiv151006396C Context. In recent years a number of poorly studied chemical elements, such as phosphorus, sulphur, and strontium, have received special attention as important tracers of the Galactic chemical evolution.
Aims: By exploiting the capabilities of the infrared echelle spectrograph GIANO mounted at the Telescopio Nazionale Galileo, we acquired high resolution spectra of four Galactic dwarf stars spanning the metallicity range between about one-third and twice the solar value. We performed a detailed feasibility study about the effectiveness of the P, S, and Sr line diagnostics in the Y band between 1.03 and 1.10 μm.
Methods: Accurate chemical abundances have been derived using one-dimensional model atmospheres computed in local thermodynamic equilibrium (LTE). We computed the line formation assuming LTE for P, while we performed non-LTE analysis to derive S and Sr abundances.
Results: We were able to derive phosphorus abundance for three stars and an upper limit for one star, while we obtained the abundance of sulphur and strontium for all of the stars. We find [P/Fe] and [S/Fe] abundance ratios consistent with solar-scaled or slightly depleted values, while the [Sr/Fe] abundance ratios are more scattered (by ±0.2 dex) around the solar-scaled value. This is fully consistent with previous studies using both optical and infrared spectroscopy.
Conclusions: We verified that high-resolution, Y-band spectroscopy as provided by GIANO is a powerful tool to study the chemical evolution of P, S, and Sr in dwarf stars.

Based on observations obtained with GIANO. Title: Synthetic activity indicators for M-type dwarf stars Authors: Wedemeyer, Sven; Ludwig, Hans-Günter Bibcode: 2016IAUS..320..303W Altcode: 2015arXiv151106153W Here, we present a set of time-dependent 3D RMHD simulations of a M-dwarf star representative of AD Leo, which extend from the upper convection zone into the chromosphere. The 3D model atmospheres are characterized by a very dynamic and intermittent structure on small spatial and temporal scales and a wealth of physical processes, which by nature cannot be described by means of 1D static model atmospheres. Artificial observations of these models imply that a combination of complementary diagnostics such as Ca II lines and the continuum intensity from UV to millimeter wavelengths, probe various properties of the dynamics, thermal and magnetic structure of the photosphere and the chromosphere and thus provide measures of stellar activity, which can be compared to observations. The complicated magnetic field structure and its imprint in synthetic diagnostics may have important implications for the understanding and characterization of stellar activity and with it possibly for the evaluation of planetary habitability around active M-dwarf stars. Title: Book Review: Astronomical Spectroscopy — An Introduction to the Atomic and Molecular Physics of Astronomical Spectra Authors: Ludwig, Hans-Günter Bibcode: 2015JAI.....480001L Altcode: No abstract at ADS Title: The photospheric solar oxygen project. IV. 3D-NLTE investigation of the 777 nm triplet lines Authors: Steffen, M.; Prakapavičius, D.; Caffau, E.; Ludwig, H. -G.; Bonifacio, P.; Cayrel, R.; Kučinskas, A.; Livingston, W. C. Bibcode: 2015A&A...583A..57S Altcode: 2015arXiv150803487S Context. The solar photospheric oxygen abundance is still widely debated. Adopting the solar chemical composition based on the "low" oxygen abundance, as determined with the use of three-dimensional (3D) hydrodynamical model atmospheres, results in a well-known mismatch between theoretical solar models and helioseismic measurements that is so far unresolved.
Aims: We carry out an independent redetermination of the solar oxygen abundance by investigating the center-to-limb variation of the O i IR triplet lines at 777 nm in different sets of spectra.
Methods: The high-resolution and high signal-to-noise solar center-to-limb spectra are analyzed with the help of detailed synthetic line profiles based on 3D hydrodynamical CO5BOLD model atmospheres and 3D non-LTE line formation calculations with NLTE3D. The idea is to exploit the information contained in the observations at different limb angles to simultaneously derive the oxygen abundance, A(O), and the scaling factor SH that describes the cross-sections for inelastic collisions with neutral hydrogen relative to the classical Drawin formula. Using the same codes and methods, we compare our 3D results with those obtained from the semi-empirical Holweger-Müller model atmosphere as well as from different one-dimensional (1D) reference models.
Results: With the CO5BOLD 3D solar model, the best fit of the center-to-limb variation of the triplet lines is obtained when the collisions by neutral hydrogen atoms are assumed to be efficient, i.e., when the scaling factor SH is between 1.2 and 1.8, depending on the choice of the observed spectrum and the triplet component used in the analysis. The line profile fits achieved with standard 1D model atmospheres (with fixed microturbulence, independent of disk position μ) are clearly of inferior quality compared to the 3D case, and give the best match to the observations when ignoring collisions with neutral hydrogen (SH = 0). The results derived with the Holweger-Müller model are intermediate between 3D and standard 1D.
Conclusions: The analysis of various observations of the triplet lines with different methods yields oxygen abundance values (on a logarithmic scale where A(H) = 12) that fall in the range 8.74 <A(O) < 8.78, and our best estimate of the 3D non-LTE solar oxygen abundance is A(O) = 8.76 ± 0.02. All 1D non-LTE models give much lower oxygen abundances, by up to -0.15 dex. This is mainly a consequence of the assumption of a μ-independent microturbulence. An independent determination of the relevant collisional cross-sections is essential to substantially improve the accuracy of the oxygen abundance derived from the O i IR triplet.

Appendices E and F are available in electronic form at http://www.aanda.org Title: Surface-effect corrections for solar-like oscillations using 3D hydrodynamical simulations. I. Adiabatic oscillations Authors: Sonoi, T.; Samadi, R.; Belkacem, K.; Ludwig, H. -G.; Caffau, E.; Mosser, B. Bibcode: 2015A&A...583A.112S Altcode: 2015arXiv151000300S Context. The CoRoT and Kepler space-borne missions have provided us with a wealth of high-quality observational data that allows for seismic inferences of stellar interiors. This requires the computation of precise and accurate theoretical frequencies, but imperfect modeling of the uppermost stellar layers introduces systematic errors. To overcome this problem, an empirical correction has been introduced by Kjeldsen et al. (2008, ApJ, 683, L175) and is now commonly used for seismic inferences. Nevertheless, we still lack a physical justification allowing for the quantification of the surface-effect corrections.
Aims: Our aim is to constrain the surface-effect corrections across the Hertzsprung-Russell (HR) diagram using a set of 3D hydrodynamical simulations.
Methods: We used a grid of these simulations computed with the CO5BOLD code to model the outer layers of solar-like stars. Upper layers of the corresponding 1D standard models were then replaced by the layers obtained from the horizontally averaged 3D models. The frequency differences between these patched models and the 1D standard models were then calculated using the adiabatic approximation and allowed us to constrain the Kjeldsen et al. power law, as well as a Lorentzian formulation.
Results: We find that the surface effects on modal frequencies depend significantly on both the effective temperature and the surface gravity. We further provide the variation in the parameters related to the surface-effect corrections using their power law as well as a Lorentzian formulation. Scaling relations between these parameters and the elevation (related to the Mach number) is also provided. The Lorentzian formulation is shown to be more robust for the whole frequency spectrum, while the power law is not suitable for the frequency shifts in the frequency range above νmax. Finally, we show that, owing to turbulent pressure, the elevation of the uppermost layers modifies the location of the hydrogen ionization zone and consequently introduces glitches in the surface effects for models with high (low) effective temperature (surface gravity).
Conclusions: Surface-effect corrections vary significantly across the HR diagram. Therefore, empirical relations like those by Kjeldsen et al. must not be calibrated on the Sun but should instead be constrained using realistic physical modeling as provided by 3D hydrodynamical simulations. 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: Stellar science from a blue wavelength range. A possible design for the blue arm of 4MOST Authors: Hansen, C. J.; Ludwig, H. -G.; Seifert, W.; Koch, A.; Xu, W.; Caffau, E.; Christlieb, N.; Korn, A. J.; Lind, K.; Sbordone, L.; Ruchti, G.; Feltzing, S.; de Jong, R. S.; Barden, S. Bibcode: 2015AN....336..665H Altcode: 2015arXiv150802714H From stellar spectra, a variety of physical properties of stars can be derived. In particular, the chemical composition of stellar atmospheres can be inferred from absorption line analyses. These provide key information on large scales, such as the formation of our Galaxy, down to the small-scale nucleosynthesis processes that take place in stars and supernovae. By extending the observed wavelength range toward bluer wavelengths, we optimize such studies to also include critical absorption lines in metal-poor stars, and allow for studies of heavy elements (Z\ensuremath{g}e 38) whose formation processes remain poorly constrained. In this context, spectrographs optimized for observing blue wavelength ranges are essential, since many absorption lines at redder wavelengths are too weak to be detected in metal-poor stars. This means that some elements cannot be studied in the visual-redder regions, and important scientific tracers and science cases are lost. The present era of large public surveys will target millions of stars. It is therefore important that the next generation of spectrographs are designed such that they cover a wide wavelength range and can observe a large number of stars simultaneously. Only then, we can gain the full information from stellar spectra, from both metal-poor to metal-rich ones, that will allow us to understand the aforementioned formation scenarios in greater detail. Here we describe the requirements driving the design of the forthcoming survey instrument 4MOST, a multi-object spectrograph commissioned for the ESO VISTA 4 m-telescope. While 4MOST is also intended for studies of active galactic nuclei, baryonic acoustic oscillations, weak lensing, cosmological constants, supernovae and other transients, we focus here on high-density, wide-area survey of stars and the science that can be achieved with high-resolution stellar spectroscopy. Scientific and technical requirements that governed the design are described along with a thorough line blending analysis. For the high-resolution spectrograph, we find that a sampling of {\ensuremath{g}e 2.5} (pixels per resolving element), spectral resolution of 18 000 or higher, and a wavelength range covering 393-436 nm, is the most well-balanced solution for the instrument. A spectrograph with these characteristics will enable accurate abundance analysis (± 0.1 dex) in the blue and allow us to confront the outlined scientific questions. Title: Collective electronic pulsation around giant nuclei in the Thomas-Fermi model Authors: Ludwig, H.; Ruffini, R.; Xue, S. -S. Bibcode: 2015NuPhA.941....1L Altcode: 2014arXiv1402.3468L Based on the Thomas-Fermi solution for compressed electron gas around a giant nucleus, we study electric pulsations of electron number-density, pressure and electric fields, which could be caused by an external perturbations acting on the nucleus or the electrons themselves. We numerically obtain the eigen-frequencies and eigen-functions for stationary pulsation modes that fulfill the boundary-value problem established by electron-number and energy-momentum conservation, equation of state, and Maxwell's equations, as well as physical boundary conditions, and assume the nucleons in β-equilibrium at nuclear density. We particularly study the configuration of ultra-relativistic electrons with a large fraction contained within the nucleus. Such configurations can be realized for a giant nucleus or high external compression on the electrons. The lowest modes turn out to be heavily influenced by the relativistic plasma frequency induced by the positive charge background in the nucleus. Our results can be applied to heavy nuclei in the neutron star crust, as well as to the whole core of a neutron star. We discuss the possibility to apply our results to dynamic nuclei using the spectral method. Title: Synthetic activity indicators for M-type dwarf stars Authors: Wedemeyer, Sven; Ludwig, Hans-Günter; Hauschildt, Peter; De Gennaro Aquino, Ivan Bibcode: 2015IAUGA..2255174W Altcode: Our understanding of the Sun has been substantially progressed owing to the advances in high-resolution observations during the last decades. These observations guided the development of numerical simulation codes for stellar atmospheres towards unprecedented levels of realism and complexity. Such 3D radiation magnetohydrodynamic (RMHD) codes can be applied and adapted to cooler stars. Here, we present a set of time-dependent 3D RMHD simulations for dwarf stars of spectral type M (representative of AD Leo). "M-dwarfs" are the most abundant stars in our galaxy and known to exhibit mega-flares. Comparisons of M-dwarf models with the Sun as fundamental reference case reveal differences and similarities, which lead to important insights into the structure and dynamics of quiescent "background" atmospheres. The models, which extend from the upper convection zone into the chromosphere, have different initial magnetic field strengths (up to 500G) and topologies, representing regions with different activity levels. The 3D model atmospheres are characterized by a very dynamic and intermittent structure on small spatial and temporal scales, final field strengths reaching a few kG and a wealth of physical processes, which by nature cannot be described by means of 1D static model atmospheres.Synthetic observables, i.e. spectra and intensity images, are calculated by using these models as input for detailed radiative transfer calculations and can be combined into synthetic full stellar disks, thus simulating spatially unresolved observations of M-dwarfs. The considered diagnostics, like, e.g., Halpha, Ca II lines, or the continuum intensity from UV to millimeter wavelengths, sample various properties of the dynamics, thermal and magnetic structure of the photosphere and the chromosphere and thus provide measures of stellar activity, which can be compared to observations. The complicated magnetic field structure and its imprint in synthetic diagnostics may have important implications for the understanding and characterization of stellar activity and with it possibly for the evaluation of planetary habitability around active M-dwarf stars. 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: Stellar Spectroscopy during Exoplanet Transits: Revealing structures across stellar surfaces Authors: Dravins, Dainis; Ludwig, Hans-Günter; Dahlén, Erik Bibcode: 2015IAUGA..2233688D Altcode: Exoplanet transits permit to study stellar surface portions that successively become hidden behind the planet. Differential spectroscopy between various transit phases reveals spectra of those stellar surface segments that were hidden. The deduced center-to-limb behavior of stellar spectral line shapes, asymmetries and wavelength shifts enables detailed tests of 3-dimensional hydrodynamic models of stellar atmospheres, such that are required for any precise determination of abundances or seismic properties. Such models can now be computed for widely different classes of stars (including metal-poor ones and white dwarfs), but have been feasible to test and verify only for the Sun with its resolved surface structure. Exoplanet transits may also occur across features such as starspots, whose magnetic signatures will be retrieved from spectra of sufficient fidelity.Knowing the precise background stellar spectra, also properties of exoplanet atmospheres are better constrained: e.g., the Rossiter-McLaughlin effect becomes resolved as not only a simple change of stellar wavelength, but as a variation of the full line profiles and their asymmetries.Such studies are challenging since exoplanets cover only a tiny fraction of the stellar disk. Current work, analyzing sequences of high-fidelity ESO UVES spectra, demonstrate that such spatially resolved stellar spectra can already be (marginally) retrieved in a few cases with the brightest host stars. Already in a near future, ongoing exoplanet surveys are likely to find further bright hosts that will enable such studies for various stellar types. http://arxiv.org/abs/1408.1402 Title: New Insights on Pulsating White Dwarfs from 3D Radiation-Hydrodynamical Simulations Authors: Tremblay, Pier-Emmanuel; Fontaine, Gilles; Ludwig, Hans-Günter Bibcode: 2015IAUGA..2253532T Altcode: We have recently computed a grid of 3D radiation-hydrodynamical simulations for the atmosphere of 70 pure-hydrogen DA white dwarfs in the range 7.0 < log g < 9.0. This includes the full ZZ Ceti instability strip where DA white dwarfs are pulsating, by far the most common type of degenerate pulsators. We have significantly improved the theoretical framework to study these objects by removing the free parameters of 1D convection, which were previously a major modeling hurdle. We will compare our new models with the observed sample of ZZ Ceti stars and highlight the improved derived properties of these objects. In particular, the new spectroscopically determined 3D atmospheric parameters allow for an improved definition of instability strip edges. We have also made new predictions for the size of convection zones, which significantly impact the position where the pulsations are driven, and the region of the HR diagram where white dwarfs are expected to pulsate. Finally, we will present new results from non-adiabatic pulsation calculations. Title: A three-dimensional hydrodynamical line profile analysis of iron lines and barium isotopes in HD 140283 Authors: Gallagher, A. J.; Ludwig, H. -G.; Ryan, S. G.; Aoki, W. Bibcode: 2015A&A...579A..94G Altcode: 2015arXiv150402353G Context. Heavy-elements, i.e. those beyond the iron peak, mostly form via two neutron capture processes: the slow (s-) and the rapid (r-) process. Metal-poor stars should contain fewer isotopes that form via the s-process, according to currently accepted theory. It has been shown in several investigations that theory and observation do not agree well, raising questions on the validity of either the methodology or the theory.
Aims: We analyse the metal-poor star HD 140283, for which we have a high quality spectrum. We test whether a three-dimensional (3D) local thermodynamic equilibrium (LTE) stellar atmosphere and spectrum synthesis code permits a more reliable analysis of the iron abundance and barium isotope ratio than a one-dimensional (1D) LTE analysis.
Methods: Using 3D hydrodynamical model atmospheres, we examine 91 iron lines of varying strength and formation depth. This provides us with the star's rotational speed. With this, we model the barium isotope ratio by exploiting the hyperfine structure of the singly ionised 4554 Å resonance line, and study the impact of the uncertainties in the stellar parameters.
Results: The star's rotational speed was found to be 1.65 ± 0.05 km s-1. Barium isotopes under the 3D paradigm show a dominant r-process signature as 77 ± 6 ± 17% (fodd = 0.38 ± 0.02 ± 0.06) of barium isotopes form via the r-process, where errors represent the assigned random and systematic errors, respectively. We find that 3D LTE fits reproduce iron line profiles better than those in 1D, but do not provide a unique abundance (within the uncertainties). However, we demonstrate that the isotopic ratio is robust against this shortcoming.
Conclusions: Our barium isotope result agrees well with currently accepted theory regarding the formation of the heavy-elements during the early Galaxy. The improved fit to the asymmetric iron line profiles suggests that the current state of 3D LTE modelling provides excellent simulations of fluid flows. However, the abundances they provide are not yet self-consistent. This may improve with non-LTE considerations and higher resolution models.

Based on observations taken using the Subaru Telescope High Dispersion Spectrograph (HDS), operated by the National Astronomical Observatory of Japan. Title: The photospheric solar oxygen project. III. Investigation of the centre-to-limb variation of the 630 nm [O I]-Ni I blend Authors: Caffau, E.; Ludwig, H. -G.; Steffen, M.; Livingston, W.; Bonifacio, P.; Malherbe, J. -M.; Doerr, H. -P.; Schmidt, W. Bibcode: 2015A&A...579A..88C Altcode: 2015arXiv150600931C Context. The solar photospheric abundance of oxygen is still a matter of debate. For about ten years some determinations have favoured a low oxygen abundance which is at variance with the value inferred by helioseismology. Among the oxygen abundance indicators, the forbidden line at 630 nm has often been considered the most reliable even though it is blended with a Ni i line. In Papers I and II of this series we reported a discrepancy in the oxygen abundance derived from the 630 nm and the subordinate [O I] line at 636 nm in dwarf stars, including the Sun.
Aims: Here we analyse several, in part new, solar observations of the centre-to-limb variation of the spectral region including the blend at 630 nm in order to separate the individual contributions of oxygen and nickel.
Methods: We analyse intensity spectra observed at different limb angles in comparison with line formation computations performed on a CO5BOLD 3D hydrodynamical simulation of the solar atmosphere.
Results: The oxygen abundances obtained from the forbidden line at different limb angles are inconsistent if the commonly adopted nickel abundance of 6.25 is assumed in our local thermodynamic equilibrium computations. With a slightly lower nickel abundance, A(Ni) ≈ 6.1, we obtain consistent fits indicating an oxygen abundance of A(O) = 8.73 ± 0.05. At this value the discrepancy with the subordinate oxygen line remains.
Conclusions: The derived value of the oxygen abundance supports the notion of a rather low oxygen abundance in the solar photosphere. However, it is disconcerting that the forbidden oxygen lines at 630 and 636 nm give noticeably different results, and that the nickel abundance derived here from the 630 nm blend is lower than expected from other nickel lines. Title: TOPoS . II. On the bimodality of carbon abundance in CEMP stars Implications on the early chemical evolution of galaxies Authors: Bonifacio, P.; Caffau, E.; Spite, M.; Limongi, M.; Chieffi, A.; Klessen, R. S.; François, P.; Molaro, P.; Ludwig, H. -G.; Zaggia, S.; Spite, F.; Plez, B.; Cayrel, R.; Christlieb, N.; Clark, P. C.; Glover, S. C. O.; Hammer, F.; Koch, A.; Monaco, L.; Sbordone, L.; Steffen, M. Bibcode: 2015A&A...579A..28B Altcode: 2015arXiv150405963B Context. In the course of the Turn Off Primordial Stars (TOPoS) survey, aimed at discovering the lowest metallicity stars, we have found several carbon-enhanced metal-poor (CEMP) stars. These stars are very common among the stars of extremely low metallicity and provide important clues to the star formation processes. We here present our analysis of six CEMP stars.
Aims: We want to provide the most complete chemical inventory for these six stars in order to constrain the nucleosynthesis processes responsible for the abundance patterns.
Methods: We analyse both X-Shooter and UVES spectra acquired at the VLT. We used a traditional abundance analysis based on OSMARCS 1D local thermodynamic equilibrium (LTE) model atmospheres and the turbospectrum line formation code.
Results: Calcium and carbon are the only elements that can be measured in all six stars. The range is -5.0 ≤ [Ca/H] <-2.1 and 7.12 ≤ A(C) ≤ 8.65. For star SDSS J1742+2531 we were able to detect three Fe i lines from which we deduced [Fe/H] = -4.80, from four Ca ii lines we derived [Ca/H] = -4.56, and from synthesis of the G-band we derived A(C) = 7.26. For SDSS J1035+0641 we were not able to detect any iron lines, yet we could place a robust (3σ) upper limit of [Fe/H] < -5.0 and measure the Ca abundance, with [Ca/H] = -5.0, and carbon, A(C) = 6.90, suggesting that this star could be even more metal-poor than SDSS J1742+2531. This makes these two stars the seventh and eighth stars known so far with [Fe/H] < -4.5, usually termed ultra-iron-poor (UIP) stars. No lithium is detected in the spectrum of SDSS J1742+2531 or SDSS J1035+0641, which implies a robust upper limit of A(Li) < 1.8 for both stars.
Conclusions: Our measured carbon abundances confirm the bimodal distribution of carbon in CEMP stars, identifying a high-carbon band and a low-carbon band. We propose an interpretation of this bimodality according to which the stars on the high-carbon band are the result of mass transfer from an AGB companion, while the stars on the low-carbon band are genuine fossil records of a gas cloud that has also been enriched by a faint supernova (SN) providing carbon and the lighter elements. The abundance pattern of the UIP stars shows a large star-to-star scatter in the [X/Ca] ratios for all elements up to aluminium (up to 1 dex), but this scatter drops for heavier elements and is at most of the order of a factor of two. We propose that this can be explained if these stars are formed from gas that has been chemically enriched by several SNe, that produce the roughly constant [X/Ca] ratios for the heavier elements, and in some cases the gas has also been polluted by the ejecta of a faint SN that contributes the lighter elements in variable amounts. The absence of lithium in four of the five known unevolved UIP stars can be explained by a dominant role of fragmentation in the formation of these stars. This would result either in a destruction of lithium in the pre-main-sequence phase, through rotational mixing or to a lack of late accretion from a reservoir of fresh gas. The phenomenon should have varying degrees of efficiency.

Based on observations obtained at ESO Paranal Observatory, programme 091.D-0288, 091.D-0305, 189.D-0165.Appendix A is available in electronic form at http://www.aanda.orgTables 4 is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/579/A28 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: VizieR Online Data Catalog: Abundances of 3 CEMP stars (Bonifacio+, 2015) Authors: Bonifacio, P.; Caffau, E.; Spite, M.; Limongi, M.; Chieffi, A.; Klessen, R. S.; Francois, P.; Molaro, P.; Ludwig, H. -G.; Zaggia, S.; Spite, F.; Plez, B.; Cayrel, R.; Christlieb, N.; Clark, P. C.; Glover, S. C. O.; Hammer, F.; Koch, A.; Monaco, L.; Sbordone, L.; Steffen, M. Bibcode: 2015yCat..35790028B Altcode: We analyse both X-Shooter and UVES spectra acquired at the VLT. We used a traditional abundance analysis based on OSMARCS 1D Local Thermodynamic Equilibrium (LTE) model atmospheres and the TURBOSPECTRUM line formation code.

(2 data files). Title: Three-dimensional hydrodynamical CO5BOLD model atmospheres of red giant stars. IV. Oxygen diagnostics in extremely metal-poor red giants with infrared OH lines Authors: Dobrovolskas, V.; Kučinskas, A.; Bonifacio, P.; Caffau, E.; Ludwig, H. -G.; Steffen, M.; Spite, M. Bibcode: 2015A&A...576A.128D Altcode: 2015arXiv150206587D Context. Although oxygen is an important tracer of Galactic chemical evolution, measurements of its abundance in the atmospheres of the oldest Galactic stars are still scarce and rather imprecise. This is mainly because only a few spectral lines are available for the abundance diagnostics. At the lowest end of the metallicity scale, oxygen can only be measured in giant stars and in most of cases such measurements rely on a single forbidden [O i] 630 nm line that is very weak and frequently blended with telluric lines. Although molecular OH lines located in the ultraviolet and infrared could also be used for the diagnostics, oxygen abundances obtained from the OH lines and the [O i] 630 nm line are usually discrepant to a level of ~ 0.3-0.4 dex.
Aims: We study the influence of convection on the formation of the infrared (IR) OH lines and the forbidden [O i] 630 nm line in the atmospheres of extremely metal-poor (EMP) red giant stars. Our ultimate goal is to clarify whether a realistic treatment of convection with state-of-the-art 3D hydrodynamical model atmospheres may help to bring the oxygen abundances obtained using the two indicators into closer agreement.
Methods: We used high-resolution (R = 50 000) and high signal-to-noise ratio (S/N ≈ 200-600) spectra of four EMP red giant stars obtained with the VLT CRIRES spectrograph. For each EMP star, 4-14 IR OH vibrational-rotational lines located in the spectral range of 1514-1548 and 1595-1632 nm were used to determine oxygen abundances by employing standard 1D local thermodynamic equilibrium (LTE) abundance analysis methodology. We then corrected the 1D LTE abundances obtained from each individual OH line for the 3D hydrodynamical effects, which was done by applying 3D-1D LTE abundance corrections that were determined using 3D hydrodynamical CO5BOLD and 1D hydrostatic LHD model atmospheres.
Results: We find that the influence of convection on the formation of [O i] 630 nm line in the atmospheres of EMP giants studied here is minor, which leads to very small 3D-1D abundance corrections (Δ3D-1D ≤ -0.01 dex). On the contrary, IR OH lines are strongly affected by convection and thus the abundance corrections for these lines are significant, Δ3D-1D ≈ -0.2···-0.3 dex. These abundance corrections do indeed bring the 1D LTE oxygen abundances of EMP red giants obtained using IR OH lines into better agreement with those determined from the [O i] 630 nm line. Since in the EMP red giants IR OH lines are typically at least a factor of two stronger than the [O i] line, OH lines may be useful indicators of oxygen abundances in the EMP stars, provided that the analysis is based on 3D hydrodynamical model atmospheres.

Based on observations obtained at the European Southern Observatory (ESO) Very Large Telescope (VLT) at Paranal Observatory, Chile (observing programme 089.D-0079).Appendices 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: Oxygen in the Early Galaxy: OH Lines as Tracers of Oxygen Abundance in Extremely Metal-Poor Giant Stars Authors: Kucinskas, A.; Dobrovolskas, V.; Bonifacio, P.; Caffau, E.; Ludwig, H. -G.; Steffen, M.; Spite, M. Bibcode: 2015csss...18..327K Altcode: 2014arXiv1409.3153K Oxygen is a powerful tracer element of Galactic chemical evolution. Unfortunately, only a few oxygen lines are available in the ultraviolet-infrared stellar spectra for the reliable determination of its abundance. Moreover, oxygen abundances obtained using different spectral lines often disagree significantly. In this contribution we therefore investigate whether the inadequate treatment of convection in 1D hydrostatic model atmospheres used in the abundance determinations may be responsible for this disagreement. For this purpose, we used VLT CRIRES spectra of three EMP giants, as well as 3D hydrodynamical COBOLD and 1D hydrostatic LHD model atmospheres, to investigate the role of convection in the formation of infrared (IR) OH lines. Our results show that the presence of convection leads to significantly stronger IR OH lines. As a result, the difference in the oxygen abundance determined from IR OH lines with 3D hydrodynamical and classical 1D hydrostatic model atmospheres may reach -0.2 dots -0.3 dex. In case of the three EMP giants studied here, we obtain a good agrement between the 3D LTE oxygen abundances determined by us using vibrational-rotational IR OH lines in the spectral range of 1514-1626 nm, and oxygen abundances determined from forbidden [O I] 630 nm line in previous studies. Title: Stellar Spectroscopy During Exoplanet Transits: Dissecting Fine Structure Across Stellar Surfaces Authors: Dravins, Dainis; Ludwig, Hans-Gunter; Dahlen, Erik; Pazira, Hiva Bibcode: 2015csss...18..853D Altcode: 2014arXiv1408.1402D Differential spectroscopy during exoplanet transits permits to reconstruct spectra of small stellar surface portions that successively become hidden behind the planet. The center-to-limb behavior of stellar line shapes, asymmetries and wavelength shifts will enable detailed tests of 3-dimensional hydrodynamic models of stellar atmospheres, such that are required for any precise determination of abundances or seismic properties. Such models can now be computed for widely different stars but have been feasible to test in detail only for the Sun with its resolved surface structure. Although very high quality spectra are required, already current data permit reconstructions of line profiles in the brightest transit host stars such as HD 209458 (G0 V). Title: Nucleus Driven Electronic Pulsation Authors: Ludwig, H.; Ruffini, R.; Xue, S. -S. Bibcode: 2014arXiv1412.5455L Altcode: We derive and solve by the spectral method the equations for a neutral system of ultra-relativistic electrons that are compressed to the radius of the nucleus and subject to a driving force. This driving force can be thought of as originating from a nuclear breathing mode, a possibility we discuss in detail. Title: The chemical composition of red giants in 47 Tucanae. I. Fundamental parameters and chemical abundance patterns Authors: Thygesen, A. O.; Sbordone, L.; Andrievsky, S.; Korotin, S.; Yong, D.; Zaggia, S.; Ludwig, H. -G.; Collet, R.; Asplund, M.; Ventura, P.; D'Antona, F.; Meléndez, J.; D'Ercole, A. Bibcode: 2014A&A...572A.108T Altcode: 2014arXiv1409.4694T Context. The study of chemical abundance patterns in globular clusters is key importance to constraining the different candidates for intracluster pollution of light elements.
Aims: We aim at deriving accurate abundances for a wide range of elements in the globular cluster 47 Tucanae (NGC 104) to add new constraints to the pollution scenarios for this particular cluster, expanding the range of previously derived element abundances.
Methods: Using tailored 1D local thermodynamic equilibrium (LTE) atmospheric models, together with a combination of equivalent width measurements, LTE, and NLTE synthesis, we derive stellar parameters and element abundances from high-resolution, high signal-to-noise spectra of 13 red giant stars near the tip of the RGB.
Results: We derive abundances of a total 27 elements (O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Mo, Ru, Ba, La, Ce, Pr, Nd, Eu, Dy). Departures from LTE were taken into account for Na, Al, and Ba. We find a mean [Fe/H] = -0.78 ± 0.07 and [ α/ Fe ] = 0.34 ± 0.03 in good agreement with previous studies. The remaining elements show good agreement with the literature, but including NLTE for Al has a significant impact on the behavior of this key element.
Conclusions: We confirm the presence of an Na-O anti-correlation in 47 Tucanae found by several other works. Our NLTE analysis of Al shifts the [Al/Fe] to lower values, indicating that this may be overestimated in earlier works. No evidence of an intrinsic variation is found in any of the remaining elements.

Based on observations made with the ESO Very Large Telescope at Paranal Observatory, Chile (Programmes 084.B-0810 and 086.B-0237).Full Tables 2, 5, and 9 are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/572/A108Appendix A is available in electronic form at http://www.aanda.org Title: The Gaia-ESO Survey: The analysis of high-resolution UVES spectra of FGK-type stars Authors: Smiljanic, R.; Korn, A. J.; Bergemann, M.; Frasca, A.; Magrini, L.; Masseron, T.; Pancino, E.; Ruchti, G.; San Roman, I.; Sbordone, L.; Sousa, S. G.; Tabernero, H.; Tautvaišienė, G.; Valentini, M.; Weber, M.; Worley, C. C.; Adibekyan, V. Zh.; Allende Prieto, C.; Barisevičius, G.; Biazzo, K.; Blanco-Cuaresma, S.; Bonifacio, P.; Bragaglia, A.; Caffau, E.; Cantat-Gaudin, T.; Chorniy, Y.; de Laverny, P.; Delgado-Mena, E.; Donati, P.; Duffau, S.; Franciosini, E.; Friel, E.; Geisler, D.; González Hernández, J. I.; Gruyters, P.; Guiglion, G.; Hansen, C. J.; Heiter, U.; Hill, V.; Jacobson, H. R.; Jofre, P.; Jönsson, H.; Lanzafame, A. C.; Lardo, C.; Ludwig, H. -G.; Maiorca, E.; Mikolaitis, Š.; Montes, D.; Morel, T.; Mucciarelli, A.; Muñoz, C.; Nordlander, T.; Pasquini, L.; Puzeras, E.; Recio-Blanco, A.; Ryde, N.; Sacco, G.; Santos, N. C.; Serenelli, A. M.; Sordo, R.; Soubiran, C.; Spina, L.; Steffen, M.; Vallenari, A.; Van Eck, S.; Villanova, S.; Gilmore, G.; Randich, S.; Asplund, M.; Binney, J.; Drew, J.; Feltzing, S.; Ferguson, A.; Jeffries, R.; Micela, G.; Negueruela, I.; Prusti, T.; Rix, H. -W.; Alfaro, E.; Babusiaux, C.; Bensby, T.; Blomme, R.; Flaccomio, E.; François, P.; Irwin, M.; Koposov, S.; Walton, N.; Bayo, A.; Carraro, G.; Costado, M. T.; Damiani, F.; Edvardsson, B.; Hourihane, A.; Jackson, R.; Lewis, J.; Lind, K.; Marconi, G.; Martayan, C.; Monaco, L.; Morbidelli, L.; Prisinzano, L.; Zaggia, S. Bibcode: 2014A&A...570A.122S Altcode: 2014arXiv1409.0568S Context. The ongoing Gaia-ESO Public Spectroscopic Survey is using FLAMES at the VLT to obtain high-quality medium-resolution Giraffe spectra for about 105 stars and high-resolution UVES spectra for about 5000 stars. With UVES, the Survey has already observed 1447 FGK-type stars.
Aims: These UVES spectra are analyzed in parallel by several state-of-the-art methodologies. Our aim is to present how these analyses were implemented, to discuss their results, and to describe how a final recommended parameter scale is defined. We also discuss the precision (method-to-method dispersion) and accuracy (biases with respect to the reference values) of the final parameters. These results are part of the Gaia-ESO second internal release and will be part of its first public release of advanced data products.
Methods: The final parameter scale is tied to the scale defined by the Gaia benchmark stars, a set of stars with fundamental atmospheric parameters. In addition, a set of open and globular clusters is used to evaluate the physical soundness of the results. Each of the implemented methodologies is judged against the benchmark stars to define weights in three different regions of the parameter space. The final recommended results are the weighted medians of those from the individual methods.
Results: The recommended results successfully reproduce the atmospheric parameters of the benchmark stars and the expected Teff-log g relation of the calibrating clusters. Atmospheric parameters and abundances have been determined for 1301 FGK-type stars observed with UVES. The median of the method-to-method dispersion of the atmospheric parameters is 55 K for Teff, 0.13 dex for log g and 0.07 dex for [Fe/H]. Systematic biases are estimated to be between 50-100 K for Teff, 0.10-0.25 dex for log g and 0.05-0.10 dex for [Fe/H]. Abundances for 24 elements were derived: C, N, O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Mo, Ba, Nd, and Eu. The typical method-to-method dispersion of the abundances varies between 0.10 and 0.20 dex.
Conclusions: The Gaia-ESO sample of high-resolution spectra of FGK-type stars will be among the largest of its kind analyzed in a homogeneous way. The extensive list of elemental abundances derived in these stars will enable significant advances in the areas of stellar evolution and Milky Way formation and evolution.

Based on observations made with the ESO/VLT, at Paranal Observatory, under program 188.B-3002 (The Gaia-ESO Public Spectroscopic Survey, PIs Gilmore and Randich). Appendices are available in electronic form at http://www.aanda.org Title: VizieR Online Data Catalog: 47 Tuc red giants chemical composition (Thygesen+, 2014) Authors: Thygesen, A. O.; Sbordone, L.; Andrievsky, S.; Korotin, S.; Yong, D.; Zaggia, S.; Ludwig, H. -G.; Collet, R.; Asplund, M.; Ventura, P.; D'Antona, F.; Melendez, J.; D'Ercole, A. Bibcode: 2014yCat..35720108T Altcode: 2014yCat..35729108T Table 2 contains linelist of the non-HFS elements. These lines were used for equivalent width measurements. Sorted by element number. Table 5 contains all the lines used for HFS synthesis. With the exception of Ba, all lines are split into individual HFS components, including isotopes. log(gf) values reflect this. Solar isotope mixture is assumed. Sorted by element number. Table 8 contains the fundamental stellar parameters. Table 9 contains [Fe/H] as well as abundance ratios [X/Fe] for all stars. All measurements have an associated uncertainty sig[X/Fe] and the number of lines used in each individual star. In cases where no measurement exist, the value "-9.99" is given.

(5 data files). Title: Clues on the Galactic evolution of sulphur from star clusters Authors: Caffau, E.; Monaco, L.; Spite, M.; Bonifacio, P.; Carraro, G.; Ludwig, H. -G.; Villanova, S.; Beletsky, Y.; Sbordone, L. Bibcode: 2014A&A...568A..29C Altcode: 2014arXiv1407.0485C Context. The abundances of α-elements are a powerful diagnostic of the star formation history and chemical evolution of a galaxy. Sulphur, being moderately volatile, can be reliably measured in the interstellar medium (ISM) of damped Ly-α galaxies and extragalactic H ii regions. Measurements in stars of different metallicity in our Galaxy can then be readily compared to the abundances in external galaxies. Such a comparison is not possible for Si or Ca that suffer depletion onto dust in the ISM. Furthermore, studying sulphur is interesting because it probes nucleosynthetic conditions that are very different from those of O or Mg. In this context measurements in star clusters are a reliable tracers of the Galactic evolution of sulphur.
Aims: The aim of this paper is to determine sulphur abundances in several Galactic clusters that span a metallicity range -1.5 < [Fe/H] < 0.0.
Methods: We use a standard abundance analysis, based on 1D model atmospheres in local thermodynamical equilibrium (LTE) and literature corrections for non-LTE (NLTE), as well as 3D corrections based on hydrodynamical model atmospheres, to derive sulphur abundances in a sample of stars in the globular cluster M 4, and the open clusters Trumpler 5, NGC 2477, and NGC 5822.
Results: We find ⟨ A(S) ⟩ NLTE = 6.11 ± 0.04 for M 4, ⟨ A(S) ⟩ NLTE = 7.17 ± 0.02 for NGC 2477, and ⟨ A(S) ⟩ NLTE = 7.13 ± 0.06 for NGC 5822. For the only star studied in Trumpler 5 we find A(S)NLTE = 6.43 ± 0.03 and A(S)LTE = 6.94 ± 0.05.
Conclusions: Our measurements show that, by and large, the S abundances in Galactic clusters trace reliably those in field stars. The only possible exception is Trumpler 5, for which the NLTE sulphur abundance implies an [S/Fe] ratio lower by roughly 0.4 dex than observed in field stars of comparable metallicity, even though its LTE sulphur abundance is in line with abundances of field stars. Moreover the LTE sulphur abundance is consistent only with the abundance of another α-element, Mg, in the same star, while the low NLTE value is consistent with Si and Ca. We believe that further investigation of departures from LTE is necessary, as well as observation of other S i lines in this star and in other stars of the same cluster, before one can conclude that the sulphur abundance in Trumpler 5 is indeed 0.4 dex lower than in field stars of comparable metallicity. The S abundances in our sample of stars in clusters imply that the clusters are chemically homogeneous for S within 0.05 dex.

Based on observations made with ESO Telescopes at the La Silla Paranal Observatory under programme ID 085.D-0537(A), 088.D-0045(A), 089.D-0062(B). Title: 4MOST: 4-metre Multi-Object Spectroscopic Telescope Authors: de Jong, Roelof S.; Barden, Sam; Bellido-Tirado, Olga; Brynnel, Joar; Chiappini, Cristina; Depagne, Éric; Haynes, Roger; Johl, Diana; Phillips, Daniel P.; Schnurr, Olivier; Schwope, Axel D.; Walcher, Jakob; Bauer, Svend M.; Cescutti, Gabriele; Cioni, Maria-Rosa L.; Dionies, Frank; Enke, Harry; Haynes, Dionne M.; Kelz, Andreas; Kitaura, Francisco S.; Lamer, Georg; Minchev, Ivan; Müller, Volker; Nuza, Sebastián. E.; Olaya, Jean-Christophe; Piffl, Tilmann; Popow, Emil; Saviauk, Allar; Steinmetz, Matthias; Ural, Uǧur; Valentini, Monica; Winkler, Roland; Wisotzki, Lutz; Ansorge, Wolfgang R.; Banerji, Manda; Gonzalez Solares, Eduardo; Irwin, Mike; Kennicutt, Robert C.; King, David M. P.; McMahon, Richard; Koposov, Sergey; Parry, Ian R.; Sun, Xiaowei; Walton, Nicholas A.; Finger, Gert; Iwert, Olaf; Krumpe, Mirko; Lizon, Jean-Louis; Mainieri, Vincenzo; Amans, Jean-Philippe; Bonifacio, Piercarlo; Cohen, Matthieu; François, Patrick; Jagourel, Pascal; Mignot, Shan B.; Royer, Frédéric; Sartoretti, Paola; Bender, Ralf; Hess, Hans-Joachim; Lang-Bardl, Florian; Muschielok, Bernard; Schlichter, Jörg; Böhringer, Hans; Boller, Thomas; Bongiorno, Angela; Brusa, Marcella; Dwelly, Tom; Merloni, Andrea; Nandra, Kirpal; Salvato, Mara; Pragt, Johannes H.; Navarro, Ramón; Gerlofsma, Gerrit; Roelfsema, Ronald; Dalton, Gavin B.; Middleton, Kevin F.; Tosh, Ian A.; Boeche, Corrado; Caffau, Elisabetta; Christlieb, Norbert; Grebel, Eva K.; Hansen, Camilla J.; Koch, Andreas; Ludwig, Hans-G.; Mandel, Holger; Quirrenbach, Andreas; Sbordone, Luca; Seifert, Walter; Thimm, Guido; Helmi, Amina; trager, Scott C.; Bensby, Thomas; Feltzing, Sofia; Ruchti, Gregory; Edvardsson, Bengt; Korn, Andreas; Lind, Karin; Boland, Wilfried; Colless, Matthew; Frost, Gabriella; Gilbert, James; Gillingham, Peter; Lawrence, Jon; Legg, Neville; Saunders, Will; Sheinis, Andrew; Driver, Simon; Robotham, Aaron; Bacon, Roland; Caillier, Patrick; Kosmalski, Johan; Laurent, Florence; Richard, Johan Bibcode: 2014SPIE.9147E..0MD Altcode: 4MOST is a wide-field, high-multiplex spectroscopic survey facility under development for the VISTA telescope of the European Southern Observatory (ESO). Its main science drivers are in the fields of galactic archeology, high-energy physics, galaxy evolution and cosmology. 4MOST will in particular provide the spectroscopic complements to the large area surveys coming from space missions like Gaia, eROSITA, Euclid, and PLATO and from ground-based facilities like VISTA, VST, DES, LSST and SKA. The 4MOST baseline concept features a 2.5 degree diameter field-of-view with ~2400 fibres in the focal surface that are configured by a fibre positioner based on the tilting spine principle. The fibres feed two types of spectrographs; ~1600 fibres go to two spectrographs with resolution R<5000 (λ~390-930 nm) and ~800 fibres to a spectrograph with R>18,000 (λ~392-437 nm and 515-572 nm and 605-675 nm). Both types of spectrographs are fixed-configuration, three-channel spectrographs. 4MOST will have an unique operations concept in which 5 year public surveys from both the consortium and the ESO community will be combined and observed in parallel during each exposure, resulting in more than 25 million spectra of targets spread over a large fraction of the southern sky. The 4MOST Facility Simulator (4FS) was developed to demonstrate the feasibility of this observing concept. 4MOST has been accepted for implementation by ESO with operations expected to start by the end of 2020. This paper provides a top-level overview of the 4MOST facility, while other papers in these proceedings provide more detailed descriptions of the instrument concept[1], the instrument requirements development[2], the systems engineering implementation[3], the instrument model[4], the fibre positioner concepts[5], the fibre feed[6], and the spectrographs[7]. Title: VizieR Online Data Catalog: Abundances of 47 Tuc turn-off stars (Dobrovolskas+, 2014) Authors: Dobrovolskas, V.; Kucinskas, A.; Bonifacio, P.; Korotin, S. A.; Steffen, M.; Sbordone, L.; Caffau, E.; Ludwig, H. -G.; Royer, F.; Prakapavicius, D. Bibcode: 2014yCat..35650121D Altcode: 2014yCat..35659121D Spectra of the TO stars in 47 Tuc investigated in this work were obtained with the GIRAFFE spectrograph in August-September, 2008, under the programme 081.D-0287(A) (PI: Shen). The same data set was independently analysed by D'Orazi et al. (2010ApJ...713L...1D, Cat. J/ApJ/713/L1).

(1 data file). 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: Abundances of lithium, oxygen, and sodium in the turn-off stars of Galactic globular cluster 47 Tucanae Authors: Dobrovolskas, V.; Kučinskas, A.; Bonifacio, P.; Korotin, S. A.; Steffen, M.; Sbordone, L.; Caffau, E.; Ludwig, H. -G.; Royer, F.; Prakapavičius, D. Bibcode: 2014A&A...565A.121D Altcode: 2013arXiv1311.1072D Context. The cluster 47 Tuc is among the most metal-rich Galactic globular clusters and its metallicity is similar to that of metal-poor disc stars and open clusters. Like other globular clusters, it displays variations in the abundances of elements lighter than Si, which is generally interpreted as evidence of the presence of multiple stellar populations.
Aims: We aim to determine abundances of Li, O, and Na in a sample of of 110 turn-off (TO) stars, in order to study the evolution of light elements in this cluster and to put our results in perspective with observations of other globular and open clusters, as well as with field stars.
Methods: We use medium resolution spectra obtained with the GIRAFFE spectrograph at the ESO 8.2 m Kueyen VLT telescope and use state of the art 1D model atmospheres and NLTE line transfer to determine the abundances. We also employ CO5BOLD hydrodynamical simulations to assess the impact of stellar granulation on the line formation and inferred abundances.
Results: Our results confirm the existence of Na-O abundance anti-correlation and hint towards a possible Li-O anti-correlation in the TO stars of 47 Tuc. At the same time, we find no convincing evidence supporting the existence of Li-Na correlation. The obtained 3D NLTE mean lithium abundance in a sample of 94 TO stars where Li lines were detected reliably, ⟨A(Li)3D NLTE⟩ = 1.78 ± 0.18 dex, appears to be significantly lower than what is observed in other globular clusters. At the same time, star-to-star spread in Li abundance is also larger than seen in other clusters. The highest Li abundance observed in 47 Tuc is about 0.1 dex lower than the lowest Li abundance observed among the un-depleted stars of the metal-poor open cluster NGC 2243.
Conclusions: The correlations/anti-correlations among light element abundances confirm that chemical enrichment history of 47 Tuc was similar to that of other globular clusters, despite the higher metallicity of 47 Tuc. The lithium abundances in 47 Tuc, when put into context with observations in other clusters and field stars, suggest that stars that are more metal-rich than [Fe/H] ~ -1.0 experience significant lithium depletion during their lifetime on the main sequence, while the more metal-poor stars do not. Rather strikingly, our results suggest that initial lithium abundance with which the star was created may only depend on its age (the younger the star, the higher its Li content) and not on its metallicity.

Appendices are available in electronic form at http://www.aanda.org 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: The first generations of stars Authors: Caffau, E.; Gallagher, A.; Bonifacio, P.; Cayrel, R.; Christlieb, N.; Clark, P. C.; Francois, P.; Glover, S.; Klessen, R. S.; Koch, A.; Ludwig, H. G.; Monaco, L.; Plez, B.; Sbordone, L.; Spite, M.; Spite, F.; Steffen, M.; Zaggia, S. Bibcode: 2014nic..confE..53C Altcode: 2014PoS...204E..53C No abstract at ADS Title: TOPoS: chemical study of extremely metal-poor stars. Authors: Caffau, E.; Sbordone, L.; Bonifacio, P.; Cayrel, R.; Christlieb, N.; Clark, P.; François, P.; Glover, S.; Klessen, R.; Koch, A.; Ludwig, H. -G.; Monaco, L.; Plez, B.; Spite, F.; Spite, M.; Steffen, M.; Zaggia, S. Bibcode: 2014MmSAI..85..222C Altcode: The extremely metal-poor (EMP) stars hold in their atmospheres the fossil record of the chemical composition of the early phases of the Galactic evolution. The chemical analysis of such objects provides important constraints on these early phases. EMP stars are very rare objects; to dig them out, large amounts of data have to be processed. With an automatic procedure, we analysed objects with colours of Turn-Off stars from the Sloan Digital Sky Survey to select a sample of good candidate EMP stars. In the latest years, we observed a sample of these candidates with X-Shooter and UVES, and we have an ongoing ESO large programme to use these spectrographs to observe EMP stars. I will report here the results on metallicity and Strontium abundance.

Based on observations obtained at ESO Paranal Observatory, programme 189.D-0165(A) Title: 6Li/7Li isotopic ratio in the most metal-poor binary CS22876-032 Authors: Gonzalez-Hernandez, J.; Caffau, E.; Ludwig, H. G.; Bonifacio, P.; Steffen, M.; Monaco, L.; Cayrel, R. Bibcode: 2014nic..confE..23G Altcode: 2014PoS...204E..23G No abstract at ADS Title: Isotope spectroscopy Authors: Caffau, E.; Steffen, M.; Bonifacio, P.; Ludwig, H. -G.; Monaco, L.; Lo Curto, G.; Kamp, I. Bibcode: 2014AN....335...59C Altcode: 2013arXiv1310.6058C The measurement of isotopic ratios provides a privileged insight both into nucleosynthesis and into the mechanisms operating in stellar envelopes, such as gravitational settling. In this article, we give a few examples of how isotopic ratios can be determined from high-resolution, high-quality stellar spectra. We consider examples of the lightest elements, H and He, for which the isotopic shifts are very large and easily measurable, and examples of heavier elements for which the determination of isotopic ratios is more difficult. The presence of 6Li in the stellar atmospheres causes a subtle extra depression in the red wing of the 7Li 670.7 nm doublet which can only be detected in spectra of the highest quality. But even with the best spectra, the derived 6Li abundance can only be as good as the synthetic spectra used for their interpretation. It is now known that 3D non-LTE modelling of the lithium spectral line profiles is necessary to account properly for the intrinsic line asymmetry, which is produced by convective flows in the atmospheres of cool stars, and can mimic the presence of 6Li. We also discuss briefly the case of the carbon isotopic ratio in metal-poor stars, and provide a new determination of the nickel isotopic ratios in the solar atmosphere. Title: X-shooter GTO: evidence for a population of extremely metal-poor, alpha-poor stars Authors: Caffau, E.; Bonifacio, P.; François, P.; Sbordone, L.; Spite, M.; Monaco, L.; Plez, B.; Spite, F.; Zaggia, S.; Ludwig, H. -G.; Cayrel, R.; Molaro, P.; Randich, S.; Hammer, F.; Hill, V. Bibcode: 2013A&A...560A..15C Altcode: 2013arXiv1309.4913C Context. The extremely metal-poor stars are the direct descendants of the first generation stars. They carry the chemical signature of the pristine Universe at the time they formed, shortly after the Big Bang.
Aims: We aim to derive information about extremely metal-poor stars from their observed spectra.
Methods: Four extremely metal-poor stars were selected from the Sloan Digital Sky Survey (SDSS) and observed during the guaranteed observing time of X-shooter. The X-shooter spectra were analysed using an automatic code, MyGIsFOS, which is based on a traditional analysis method. It makes use of a synthetic grid computed from one-dimensional, plane-parallel, hydrostatic model atmospheres.
Results: The low metallicity derived from the SDSS spectra is confirmed here. Two kinds of stars are found. Two stars are confirmed to be extremely metal-poor, with no evidence of any enhancement in carbon. The two other stars are strongly enhanced in carbon. We could not derive iron abundance for one of them, while [Ca/H] is below -4.5. Two of the stars are members of the rare population of extremely metal-poor stars low in alpha elements.

Based on observations obtained at ESO Paranal Observatory, GTO programme 089.D-0039. Title: On the Seismic Scaling Relations Δν — ρ and νmax — νc Authors: Belkacem, K.; Samadi, R.; Mosser, B.; Goupil, M. -J.; Ludwig, H. -G. Bibcode: 2013ASPC..479...61B Altcode: 2013arXiv1307.3132B Scaling relations between asteroseismic quantities and stellar parameters are essential tools for studying stellar structure and evolution. We will address two of them, namely, the relation between the large frequency separation (Δν) and the mean density (ρ) as well as the relation between the frequency of the maximum in the power spectrum of solar-like oscillations (νmax) and the cut-off frequency (νc). For the first relation, we will consider the possible sources of uncertainties and explore them with the help of a grid of stellar models. For the second one, we will show that the basic physical picture is understood and that departure from the observed relation arises from the complexity of non-adiabatic processes involving the time-dependent treatment of convection. This will be further discussed on the basis of a set of three-dimensional (3D) hydrodynamical simulations of surface convection. Title: TOPoS. I. Survey design and analysis of the first sample Authors: Caffau, E.; Bonifacio, P.; Sbordone, L.; François, P.; Monaco, L.; Spite, M.; Plez, B.; Cayrel, R.; Christlieb, N.; Clark, P.; Glover, S.; Klessen, R.; Koch, A.; Ludwig, H. -G.; Spite, F.; Steffen, M.; Zaggia, S. Bibcode: 2013A&A...560A..71C Altcode: 2013arXiv1310.6963C Context. The metal-weak tail of the metallicity distribution function (MDF) of the Galactic Halo stars contains crucial information on the formation mode of the first generation of stars. To determine this observationally, it is necessary to observe large numbers of extremely metal-poor stars.
Aims: We present here the Turn-Off Primordial Stars survey (TOPoS) that is conducted as an ESO Large Programme at the VLT. This project has four main goals: (i) to understand the formation of low-mass stars in a low-metallicity gas: determine the metal-weak tail of the halo MDF below [M/H] = -3.5; in particular, we aim at determining the critical metallicity, that is the lowest metallicity sufficient for the formation of low-mass stars; (ii) to determine in extremely metal-poor stars the relative abundances of the elements that are the signature of the massive first stars; (iii) to determine the trend of the lithium abundance at the time when the Galaxy formed; and (iv) to derive the fraction of C-enhanced extremely metal-poor stars with respect to normal extremely metal-poor stars. The large number of stars observed in the SDSS provides a good sample of candidate stars at extremely low metallicity.
Methods: Candidates with turn-off colours down to magnitude g = 20 were selected from the low-resolution spectra of SDSS by means of an automated procedure. X-Shooter has the potential of performing the necessary follow-up spectroscopy, providing accurate metallicities and abundance ratios for several key elements for these stars.
Results: We present here the stellar parameters of the first set of stars. The nineteen stars range in iron abundance between -4.1 and -2.9 dex relative to the Sun. Two stars have a high radial velocity and, according to our estimate of their kinematics, appear to be marginally bound to the Galaxy and are possibly accreted from another galaxy.

Based on observations obtained at ESO Paranal Observatory, GTO programme 189.D-0165(A). Title: Stellar granulation as seen in disk-integrated intensity. II. Theoretical scaling relations compared with observations Authors: Samadi, R.; Belkacem, K.; Ludwig, H. -G.; Caffau, E.; Campante, T. L.; Davies, G. R.; Kallinger, T.; Lund, M. N.; Mosser, B.; Baglin, A.; Mathur, S.; Garcia, R. A. Bibcode: 2013A&A...559A..40S Altcode: 2013arXiv1309.1488S Context. A large set of stars observed by CoRoT and Kepler shows clear evidence for the presence of a stellar background, which is interpreted to arise from surface convection, i.e., granulation. These observations show that the characteristic time-scale (τeff) and the root-mean-square (rms) brightness fluctuations (σ) associated with the granulation scale as a function of the peak frequency (νmax) of the solar-like oscillations.
Aims: We aim at providing a theoretical background to the observed scaling relations based on a model developed in Paper I.
Methods: We computed for each 3D model the theoretical power density spectrum (PDS) associated with the granulation as seen in disk-integrated intensity on the basis of the theoretical model published in Paper I. For each PDS we derived the associated characteristic time (τeff) and the rms brightness fluctuations (σ) and compared these theoretical values with the theoretical scaling relations derived from the theoretical model and the measurements made on a large set of Kepler targets.
Results: We derive theoretical scaling relations for τeff and σ, which show the same dependence on νmax as the observed scaling relations. In addition, we show that these quantities also scale as a function of the turbulent Mach number (ℳa) estimated at the photosphere. The theoretical scaling relations for τeff and σ match the observations well on a global scale. Quantitatively, the remaining discrepancies with the observations are found to be much smaller than previous theoretical calculations made for red giants.
Conclusions: Our modelling provides additional theoretical support for the observed variations of σ and τeff with νmax. It also highlights the important role of ℳa in controlling the properties of the stellar granulation. However, the observations made with Kepler on a wide variety of stars cannot confirm the dependence of our scaling relations on ℳa. Measurements of the granulation background and detections of solar-like oscillations in a statistically sufficient number of cool dwarf stars will be required for confirming the dependence of the theoretical scaling relations with ℳa.

Appendices are available in electronic form at http://www.aanda.org Title: Stellar granulation as seen in disk-integrated intensity. I. Simplified theoretical modeling Authors: Samadi, R.; Belkacem, K.; Ludwig, H. -G. Bibcode: 2013A&A...559A..39S Altcode: 2013arXiv1309.1620S Context. Solar granulation has been known for a long time to be a surface manifestation of convection. The space-borne missions CoRoT and Kepler enable us to observe the signature of this phenomena in disk-integrated intensity on a large number of stars.
Aims: The space-based photometric measurements show that the global brightness fluctuations and the lifetime associated with granulation obeys characteristic scaling relations. We thus aimed at providing simple theoretical modeling to reproduce these scaling relations, and subsequently at inferring the physical properties of granulation across the Hertzsprung-Russell diagram.
Methods: We developed a simple 1D theoretical model. The input parameters were extracted from 3D hydrodynamical models of the surface layers of stars, and the free parameters involved in the model were calibrated with solar observations. Two different prescriptions for representing the Fourier transform of the time-correlation of the eddy velocity were compared: a Lorentzian and an exponential form. Finally, we compared our theoretical prediction with 3D radiative hydrodynamical (RHD) numerical modeling of stellar granulation (hereafter ab initio approach).
Results: Provided that the free parameters are appropriately adjusted, our theoretical model reproduces the observed solar granulation spectrum quite satisfactorily; the best agreement is obtained for an exponential form. Furthermore, our model results in granulation spectra that agree well with the ab initio approach using two 3D RHD models that are representative of the surface layers of an F-dwarf and a red-giant star.
Conclusions: We have developed a theoretical model that satisfactory reproduces the solar granulation spectrum and gives results consistent with the ab initio approach. The model is used in a companion paper as theoretical framework for interpretating the observed scaling relations.

Appendices are available in electronic form at http://www.aanda.org 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: Three-dimensional hydrodynamical CO5BOLD model atmospheres of red giant stars. III. Line formation in the atmospheres of giants located close to the base of the red giant branch Authors: Dobrovolskas, V.; Kučinskas, A.; Steffen, M.; Ludwig, H. -G.; Prakapavičius, D.; Klevas, J.; Caffau, E.; Bonifacio, P. Bibcode: 2013A&A...559A.102D Altcode: 2013arXiv1310.7791D
Aims: We utilize state-of-the-art three-dimensional (3D) hydrodynamical and classical 1D stellar model atmospheres to study the influence of convection on the formation properties of various atomic and molecular spectral lines in the atmospheres of four red giant stars, located close to the base of the red giant branch, RGB (Teff ≈ 5000 K, log g = 2.5), and characterized by four different metallicities, [M/H] = 0.0, -1.0, -2.0, -3.0.
Methods: The role of convection in the spectral line formation is assessed with the aid of abundance corrections, i.e., the differences in abundances predicted for a given equivalent width of a particular spectral line with the 3D and 1D model atmospheres. The 3D hydrodynamical and classical 1D model atmospheres used in this study were calculated with the CO5BOLD and 1D LHD codes, respectively. Identical atmospheric parameters, chemical composition, equation of state, and opacities were used with both codes, therefore allowing a strictly differential analysis of the line formation properties in the 3D and 1D models.
Results: We find that for lines of certain neutral atoms, such as Mg i, Ti i, Fe i, and Ni i, the abundance corrections strongly depend both on the metallicity of a given model atmosphere and the line excitation potential, χ. While abundance corrections for all lines of both neutral and ionized elements tend to be small at solar metallicity (≤±0.1 dex), for lines of neutral elements with low ionization potential and low-to-intermediate χ they quickly increase with decreasing metallicity, reaching in their extremes -0.6 to -0.8 dex. In all such cases the large abundance corrections are due to horizontal temperature fluctuations in the 3D hydrodynamical models. Lines of neutral elements with higher ionization potentials (Eion ≳ 10 eV) generally behave very similarly to lines of ionized elements characterized by low ionization potentials (Eion ≲ 6 eV). In the latter case, the abundance corrections are small (generally, ≤±0.1 dex) and are caused by approximately equal contributions from the horizontal temperature fluctuations and differences between the temperature profiles in the 3D and 1D model atmospheres. Abundance corrections of molecular lines are very sensitive to the metallicity of the underlying model atmosphere and may be larger (in absolute value) than ~-0.5 dex at [M/H] = -3.0 (~-1.5 dex in the case of CO). At fixed metallicity and excitation potential, the abundance corrections show little variation within the wavelength range studied here, 400-1600 nm. We also find that an approximate treatment of scattering in the 3D model calculations (i.e., ignoring the scattering opacity in the outer, optically thin, atmosphere) leads to abundance corrections that are altered by less than ~0.1 dex, both for atomic and molecular (CO) lines, with respect to the model where scattering is treated as true absorption throughout the entire atmosphere, with the largest differences for the resonance and low-excitation lines.

Appendices and Figs. 3, 5, 6, 8, 9, 11 are available in electronic form at http://www.aanda.org 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: The photospheric solar oxygen project. II. Non-concordance of the oxygen abundance derived from two forbidden lines Authors: Caffau, E.; Ludwig, H. -G.; Malherbe, J. -M.; Bonifacio, P.; Steffen, M.; Monaco, L. Bibcode: 2013A&A...554A.126C Altcode: 2013arXiv1305.1763C Context. In the Sun, the two forbidden [O i] lines at 630 and 636 nm were previously found to provide discrepant oxygen abundances.
Aims: We investigate whether this discrepancy is peculiar to the Sun or whether it is also observed in other stars.
Methods: We make use of high-resolution, high signal-to-noise ratio spectra of four dwarf to turn-off stars, five giant stars, and one sub-giant star observed with THEMIS, HARPS, and UVES to investigate the coherence of the two lines.
Results: The two lines provide oxygen abundances that are consistent, within observational errors, in all the giant stars examined by us. On the other hand, for the two dwarf stars for which a measurement was possible, for Procyon, and for the sub-giant star Capella, the 636 nm line provides systematically higher oxygen abundances, as already seen for the Sun.
Conclusions: The only two possible reasons for the discrepancy are a serious error in the oscillator strength of the Ni i line blending the 630 nm line or the presence of an unknown blend in the 636 nm line, which makes the feature stronger. The CN lines blending the 636 nm line cannot be responsible for the discrepancy. The Ca i autoionisation line, on the red wing of which the 636 nm line is formed, is not well modelled by our synthetic spectra. However, a better reproduction of this line would result in even higher abundances from the 636 nm, thus increasing the discrepancy.

Based on observations collected at ESO Paranal Observatory, Programme 182.D-5053(A). Title: Carbon-enhanced metal-poor stars: the most pristine objects? Authors: Spite, M.; Caffau, E.; Bonifacio, P.; Spite, F.; Ludwig, H. -G.; Plez, B.; Christlieb, N. Bibcode: 2013A&A...552A.107S Altcode: 2013arXiv1303.1791S Context. Carbon-enhanced metal-poor stars (CEMP) form a significant proportion of the metal-poor stars, their origin is not well understood, and this carbon-enhancement appears in stars that exhibit different abundance patterns.
Aims: Three very metal-poor C-rich turnoff stars were selected from the SDSS survey, observed with the ESO VLT (UVES) to precisely determine the element abundances. In turnoff stars (unlike giants) the carbon abundance has not been affected by mixing with deep layers and is therefore easier to interpret.
Methods: The analysis was performed with one dimensional (1D) local thermodynamical equilibrium (LTE) static model atmospheres. When available, non-LTE corrections were applied to the classical LTE abundances. The 3D effects on the CH and CN molecular bands were computed using hydrodynamical simulations of the stellar atmosphere (CO5BOLD) and are found to be very important.
Results: To facilitate a comparison with previous results, only 1D abundances are used in the discussion. The abundances (or upper limits) of the elements enable us to place these stars in different CEMP classes. The carbon abundances confirm the existence of a plateau at A(C)= 8.25 for [Fe/H] ≥ -3.4. The most metal-poor stars ([Fe/H] < -3.4) have significantly lower carbon abundances, suggesting a lower plateau at A(C) ≈ 6.5. Detailed analyses of a larger sample of very low metallicity carbon-rich stars are required to confirm (or refute) this possible second plateau and specify the behavior of the CEMP stars at very low metallicity.

Based on observations obtained with the ESO Very Large Telescope at Paranal Observatory, Chile (ID 087.D-0123(A).Table 5 is available in electronic form at http://www.aanda.org 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: Fluorine Abundances of Galactic Low-metallicity Giants Authors: Li, H. N.; Ludwig, H. -G.; Caffau, E.; Christlieb, N.; Zhao, G. Bibcode: 2013ApJ...765...51L Altcode: 2013arXiv1302.3928L With abundances and 2σ upper limits of fluorine (F) in seven metal-poor field giants, nucleosynthesis of stellar F at low metallicity is discussed. The measurements are derived from the HF(1-0) R9 line at 23358 Å using near-infrared K-band high-resolution spectra obtained with CRIRES at the Very Large Telescope. The sample reaches lower metallicities than previous studies on F of field giants, ranging from [Fe/H] = -1.56 down to -2.13. Effects of three-dimensional model atmospheres on the derived F and O abundances are quantitatively estimated and shown to be insignificant for the program stars. The observed F yield in the form of [F/O] is compared with two sets of Galactic chemical evolution models, which quantitatively demonstrate the contribution of Type II supernova (SN II) ν-process and asymptotic giant branch/Wolf-Rayet stars. It is found that at this low-metallicity region, models cannot well predict the observed distribution of [F/O], while the observations are better fit by models considering an SN II ν-process with a neutrino energy of E ν = 3 × 1053 erg. Our sample contains HD 110281, a retrograde orbiting low-α halo star, showing a similar F evolution as globular clusters. This supports the theory that such halo stars are possibly accreted from dwarf galaxy progenitors of globular clusters in the halo. Title: Is the Sun Lighter than the Earth? Isotopic CO in the Photosphere, Viewed through the Lens of Three-dimensional Spectrum Synthesis Authors: Ayres, Thomas R.; Lyons, J. R.; Ludwig, H. -G.; Caffau, E.; Wedemeyer-Böhm, S. Bibcode: 2013ApJ...765...46A Altcode: 2013arXiv1301.5281A We consider the formation of solar infrared (2-6 μm) rovibrational bands of carbon monoxide (CO) in CO5BOLD 3D convection models, with the aim of refining abundances of the heavy isotopes of carbon (13C) and oxygen (18O, 17O), to compare with direct capture measurements of solar wind light ions by the Genesis Discovery Mission. We find that previous, mainly 1D, analyses were systematically biased toward lower isotopic ratios (e.g., R 23 ≡ 12C/13C), suggesting an isotopically "heavy" Sun contrary to accepted fractionation processes that were thought to have operated in the primitive solar nebula. The new 3D ratios for 13C and 18O are R 23 = 91.4 ± 1.3 (R ⊕ = 89.2) and R 68 = 511 ± 10 (R ⊕ = 499), where the uncertainties are 1σ and "optimistic." We also obtained R 67 = 2738 ± 118 (R ⊕ = 2632), but we caution that the observed 12C17O features are extremely weak. The new solar ratios for the oxygen isotopes fall between the terrestrial values and those reported by Genesis (R 68 = 530, R 67 = 2798), although including both within 2σ error flags, and go in the direction favoring recent theories for the oxygen isotope composition of Ca-Al inclusions in primitive meteorites. While not a major focus of this work, we derive an oxygen abundance, epsilonO ~ 603 ± 9 ppm (relative to hydrogen; log epsilon ~ 8.78 on the H = 12 scale). The fact that the Sun is likely lighter than the Earth, isotopically speaking, removes the necessity of invoking exotic fractionation processes during the early construction of the inner solar system. Title: The potential of 3D radiation-hydrodynamics models for white dwarf asteroseismology Authors: Tremblay, P. -E.; Ludwig, H. -G. Bibcode: 2013EPJWC..4305008T Altcode: White dwarfs with hydrogen-rich atmospheres (DA) are the most abundant of all degenerate objects. In recent years work has been dedicated to increase the accuracy of their model atmospheres. Most notably, convective motions are now treated with 3D radiation-hydrodynamics instead of the standard mixing-length theory. We present and describe selected 3D model atmospheres close and within the instability strip of the pulsating ZZ Ceti white dwarfs. Our 3D simulations depend only weakly on numerical parameters and compared to 1D models, they provide more realistic determinations of the depth of the convective zone. The 3D structures can then be adopted as input for asteroseismology. Title: Velocity and abundance precisions for future high-resolution spectroscopic surveys: A study for 4MOST Authors: Caffau, E.; Koch, A.; Sbordone, L.; Sartoretti, P.; Hansen, C. J.; Royer, F.; Leclerc, N.; Bonifacio, P.; Christlieb, N.; Ludwig, H. -G.; Grebel, E. K.; de Jong, R. S.; Chiappini, C.; Walcher, J.; Mignot, S.; Feltzing, S.; Cohen, M.; Minchev, I.; Helmi, A.; Piffl, T.; Depagne, E.; Schnurr, O. Bibcode: 2013AN....334..197C Altcode: 2012arXiv1211.1406C In preparation for future, large-scale, multi-object, high-resolution spectroscopic surveys of the Galaxy, we present a series of tests of the precision in radial velocity and chemical abundances that any such project can achieve at a 4 m class telescope. We briefly discuss a number of science cases that aim at studying the chemo-dynamical history of the major Galactic components (bulge, thin and thick disks, and halo) - either as a follow-up to the Gaia mission or on their own merits. Based on a large grid of synthetic spectra that cover the full range in stellar parameters of typical survey targets, we devise an optimal wavelength range and argue for a moderately high-resolution spectrograph. As a result, the kinematic precision is not limited by any of these factors, but will practically only suffer from systematic effects, easily reaching uncertainties <1 km s-1. Under realistic survey conditions (namely, considering stars brighter than r=16 mag with reasonable exposure times) we prefer an ideal resolving power of R∼20 000 on average, for an overall wavelength range (with a common two-arm spectrograph design) of [395;456.5] nm and [587;673] nm. We show for the first time on a general basis that it is possible to measure chemical abundance ratios to better than 0.1 dex for many species (Fe, Mg, Si, Ca, Ti, Na, Al, V, Cr, Mn, Co, Ni, Y, Ba, Nd, Eu) and to an accuracy of about 0.2 dex for other species such as Zr, La, and Sr. While our feasibility study was explicitly carried out for the 4MOST facility, the results can be readily applied to and used for any other conceptual design study for high-resolution spectrographs. Title: Amplitudes of solar-like oscillations in red giants: Departures from the quasi-adiabatic approximation Authors: Samadi, R.; Belkacem, K.; Dupret, M. -A.; Goupil, M. J.; Ludwig, H. -G.; Barban, C.; Baudin, F.; Caffau, E. Bibcode: 2013EPJWC..4303008S Altcode: CoRoT and Kepler measurements reveal us that the amplitudes of solar-like oscillations detected in red giant stars scale from stars to stars in a characteristic way. This observed scaling relation is not yet fully understood but constitutes potentially a powerful diagnostic about mode physics. Quasi-adiabatic theoretical scaling relations in terms of mode amplitudes result in systematic and large differences with the measurements performed for red giant stars. The use of a non-adiabatic intensity-velocity relation derived from a non-adiabatic pulsation code significantly reduces the discrepancy with the CoRoT measurements. The origin of the remaining difference is still unknown. Departure from adiabatic eigenfunction is a very likely explanation that is investigated in the present work using a 3D hydrodynamical model of the surface layers of a representative red giant star. Title: Isotopic CO in the Solar Photosphere, Viewed Through the Lens of 3D Spectrum Synthesis Authors: Ayres, T. R.; Lyons, J. R.; Ludwig, H. -G.; Caffau, E.; Wedemeyer-Bohm, S. Bibcode: 2013LPI....44.3038A Altcode: 2013LPICo1719.3038A New analyses of CO isotopologue abundances in the solar photosphere are now consistent with Genesis solar wind results, although ^17O error bars are still large. Title: Three-dimensional magnetohydrodynamic simulations of M-dwarf chromospheres Authors: Wedemeyer, S.; Ludwig, H. -G.; Steiner, O. Bibcode: 2013AN....334..137W Altcode: 2013csss...17..137W; 2012arXiv1207.2342W We present first results from three-dimensional radiation magnetohydrodynamic simulations of M-type dwarf stars with CO5BOLD. The local models include the top of the convection zone, the photosphere, and the chromosphere. The results are illustrated for models with an effective temperature of 3240 K and a gravitational acceleration of {log g = 4.5}, which represent analogues of AD Leo. The models have different initial magnetic field strengths and field topologies. This first generation of models demonstrates that the atmospheres of M dwarfs are highly dynamic and intermittent. Magnetic fields and propagating shock waves produce a complicated fine-structure, which is clearly visible in synthetic intensity maps in the core of the Ca II K spectral line and also at millimeter wavelengths. The dynamic small-scale pattern cannot be described by means of one-dimensional models, which has important implications for the construction of semi-empirical model atmospheres and thus for the interpretation of observations in general. Detailed three-dimensional numerical simulations are valuable in this respect. Furthermore, such models facilitate the analysis of small-scale processes, which cannot be observed on stars but nevertheless might be essential for understanding M-dwarf atmospheres and their activity. An example are so-called ``magnetic tornadoes'', which have recently been found on the Sun and are presented here in M-dwarf models for the first time. 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: Three-dimensional hydrodynamical CO5BOLD model atmospheres of red giant stars. II. Spectral line formation in the atmosphere of a giant located near the RGB tip Authors: Kučinskas, A.; Steffen, M.; Ludwig, H. -G.; Dobrovolskas, V.; Ivanauskas, A.; Klevas, J.; Prakapavičius, D.; Caffau, E.; Bonifacio, P. Bibcode: 2013A&A...549A..14K Altcode: 2012arXiv1211.7313K
Aims: We investigate the role of convection in the formation of atomic and molecular lines in the atmosphere of a red giant star. For this purpose we study the formation properties of spectral lines that belong to a number of astrophysically important tracer elements, including neutral and singly ionized atoms (Li I, N I, O I, Na I, Mg I, Al I, Si I, Si II, S I, K I, Ca I, Ca II, Ti I, Ti II, Cr I, Cr II, Mn I, Fe I, Fe II, Co I, Ni I, Zn I, Sr II, Ba II, and Eu II), and molecules (CH, CO, C2, NH, CN, and OH).
Methods: We focus our investigation on a prototypical red giant located close to the red giant branch (RGB) tip (Teff = 3660 K, log g = 1.0, [M/H] = 0.0). We used two types of model atmospheres, 3D hydrodynamical and classical 1D, calculated with the CO5BOLD and LHD stellar atmosphere codes, respectively. Both codes share the same atmospheric parameters, chemical composition, equation of state, and opacities, which allowed us to make a strictly differential comparison between the line formation properties predicted in 3D and 1D. The influence of convection on the spectral line formation was assessed with the aid of 3D-1D abundance corrections, which measure the difference between the abundances of chemical species derived with the 3D hydrodynamical and 1D classical model atmospheres.
Results: We find that convection plays a significant role in the spectral line formation in this particular red giant. The derived 3D-1D abundance corrections rarely exceed ± 0.1 dex when lines of neutral atoms and molecules are considered, which is in line with the previous findings for solar-metallicity red giants located on the lower RGB. The situation is different with lines that belong to ionized atoms, or to neutral atoms with high ionization potential. In both cases, the corrections for high-excitation lines (χ > 8 eV) may amount to Δ3D-1D ~ -0.4 dex. The 3D-1D abundance corrections generally show a significant wavelength dependence; in most cases they are smaller in the near-infrared, at 1600-2500 nm.

Appendices are available in electronic form at http://www.aanda.org 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: Micro- and macroturbulence predictions from CO5BOLD 3D stellar atmospheres . Authors: Steffen, M.; Caffau, E.; Ludwig, H. -G. Bibcode: 2013MSAIS..24...37S Altcode: 2013arXiv1306.4307S We present an overview of the current status of our efforts to derive the microturbulence and macroturbulence parameters (xi_mic and xi_mac) from the CIFIST grid of CO5BOLD 3D model atmospheres as a function of the basic stellar parameters T_{eff}, log g, and [M/H]. The latest results for the Sun and Procyon show that the derived microturbulence parameter depends significantly on the numerical resolution of the underlying 3D simulation, confirming that `low-resolution' models tend to underestimate the true value of xi_mic . Extending the investigation to 12 further simulations with different T_{eff}, log g, and [M/H], we obtain a first impression of the predicted trend of xi_mic over the Hertzsprung-Russell diagram: in agreement with empirical evidence, microturbulence increases towards higher effective temperature and lower gravity. The metallicity dependence of xi_mic must be interpreted with care, since it also reflects the deviation between the 1D and 3D photospheric temperature stratifications that increases systematically towards lower [M/H]. Title: The influence of convection on the atmospheric structures and observable properties of red giant stars. Authors: Kučinskas, A.; Ludwig, H. -G.; Steffen, M.; Dobrovolskas, V.; Klevas, J.; Prakapavičius, D.; Caffau, E.; Bonifacio, P. Bibcode: 2013MSAIS..24...68K Altcode: 2013arXiv1305.3441K During the recent years significant progress has been made in the modeling of red giant atmospheres with the aid of 3D hydrodynamical model atmosphere codes. In this contribution we provide an overview of selected results obtained in this context by utilizing 3D hydrodynamical CO5BOLD stellar model atmospheres. Hydrodynamical simulations show that convective motions lead to significant differences in the atmospheric structures of red giants with respect to those predicted by the classical 1D model atmospheres. Results of these simulations also show that in certain cases 1D models fail to reproduce even the average properties of the 3D hydrodynamical models, such as P-T profiles. Large horizontal temperature fluctuations in the 3D model atmospheres, as well as differences between the temperature profiles of the average xtmean {3D} and 1D models, lead to large discrepancies in the strengths of spectral lines predicted by the 3D and 1D model atmospheres. This is especially important in models at lowest metallicities ([M/H]<-2.0) where the 3D-1D abundance differences may reach (or even exceed) -0.6 dex for lines of neutral atoms and molecules. We also discuss several simplifications and numerical aspects involved in the present 3D hydrodynamical modeling of red giant atmospheres, and briefly address several issues where urgent progress may be needed. Title: Molecular bands in extremely metal-poor stars: Granulation effects Authors: Bonifacio, P.; Caffau, E.; Ludwig, H. -G.; Spite, M.; Plez, B.; Steffen, M.; Spite, F. Bibcode: 2013MSAIS..24..138B Altcode: 2013arXiv1305.2065B The bands of diatomic molecules are important abundance indicators, especially in metal-poor stars, where they are still measurable in metallicity regimes where the atomic lines of their constituting metallic elements have become vanishingly small. In order to use them for abundance determinations it is imperative to understand the formation of these bands. In this contribution we report on our results obtained using CO^5{BOLD} hydrodynamical simulations. Some effects that are qualitatively different from what found in 1D computations are highlighted. Due to the large number of lines that form the bands, their spectrum synthesis is computationally challenging. We discuss some of the computational strategies we employed to parallelise the computation and possible future developments. Title: Signs of atmospheric inhomogeneities in cool stars from 1D-NLTE analysis of iron lines Authors: Mashonkina, L.; Ludwig, H. -G.; Korn, A.; Sitnova, T.; Caffau, E. Bibcode: 2013MSAIS..24..120M Altcode: 2013arXiv1303.0357M For the well studied halo star HD 122563 and the four stars in the globular cluster NGC 6397, we determine NLTE abundances of iron using classical plane-parallel model atmospheres. Each star reveals a discrepancy in abundances between the Fe I lines arising from the ground state and the other Fe I lines, in qualitative agreement with the 3D-LTE line formation predictions, however, the magnitude of the observed effect is a factor of 2 smaller compared with the predicted one. When ignoring the Fe I low-excitation lines, the NLTE abundances from the two ionization stages, Fe I and Fe II are consistent in each investigated star. For the subgiants in NGC 6397, this is only true when using the cooler effective temperature scale of \citet{Alonso1999}. We also present full 3D-LTE line formation calculations for some selected iron lines in the solar and metal-poor 4480/2/-3 models and NLTE calculations with the corresponding spatial and temporal average <{3D}> models. The use of the <{3D}> models is justified only for particular Fe I lines in particular physical conditions. Our NLTE calculations reproduce well the centre-to-limb variation of the solar Fe I 7780 Å line, but they are unsuccessful for Fe I 6151 Å. The metal-poor <{3D}> model was found to be adequite for the strong Fe I 5166 Å (E_exc = 0) line, but inadequite in all other investigated cases. Title: Solar carbon monoxide: poster child for 3D effects . Authors: Ayres, T. R.; Lyons, J. R.; Ludwig, H. -G.; Caffau, E.; Wedemeyer-Böhm, S. Bibcode: 2013MSAIS..24...85A Altcode: Photospheric infrared (2-6 mu m) rovibrational bands of carbon monoxide (CO) provide a tough test for 3D convection models such as those calculated using CO5BOLD. The molecular formation is highly temperature-sensitive, and thus responds in an exaggerated way to thermal fluctuations in the dynamic atmosphere. CO, itself, is an important tracer of the oxygen abundance, a still controversial issue in solar physics; as well as the heavy isotopes of carbon (13C) and oxygen (18O, 17O), which, relative to terrestrial values, are fingerprints of fractionation processes that operated in the primitive solar nebula. We show how 3D models impact the CO line formation, and add in a second constraint involving the near-UV Ca RIPTSIZE II line wings, which also are highly temperature sensitive, but in the opposite sense to the molecules. We find that our reference CO5BOLD snapshots appear to be slightly too cool on average in the outer layers of the photosphere where the CO absorptions and Ca RIPTSIZE II wing emissions arise. We show, further, that previous 1D modeling was systematically biased toward higher oxygen abundances and lower isotopic ratios (e.g., R23equiv 12C/13C), suggesting an isotopically ``heavy'' Sun contrary to direct capture measurements of solar wind light ions by the Genesis Discovery Mission. New 3D ratios for the oxygen isotopes are much closer to those reported by Genesis, and the associated oxygen abundance from CO now is consistent with the recent Caffau et al. study of atomic oxygen. Some lingering discrepancies perhaps can be explained by magnetic bright points. Solar CO demonstrates graphically the wide gulf that can occur between a 3D analysis and 1D. Title: White Dwarfs Escaping From the Hyades Authors: Tremblay, P. -E.; Schilbach, E.; Röser, S.; Jordan, S.; Ludwig, H. -G.; Goldman, B. Bibcode: 2013ASPC..469..105T Altcode: Ten white dwarfs are currently known to be associated with the Hyades cluster. This number of degenerate objects is too low when compared to the predictions of numerical simulations. Using the PPMXL catalog of proper motions and positions, 17 new Hyades white dwarf candidates have recently been proposed, most of them outside of the tidal radius. They could potentially explain the discrepancy between the observed and predicted number of associated objects. We review the Hyades membership of these candidates by using spectroscopic and photometric observations, as well as by simulating the contamination from field white dwarfs. We find that six objects remain candidates, and three more objects have an uncertain membership status due to their unknown or imprecise atmospheric parameters. Title: Opacities in CO5BOLD Authors: Ludwig, H. -G.; Steffen, M. Bibcode: 2013MSAIS..24...53L Altcode: We describe the present treatment of the frequency-dependence of the radiative transfer in CO5BOLD. This mostly refers to the way opacities are binned into groups. We discuss the basic ideas behind the Opacity Binning Method, give some details of the practical implementation in CO5BOLD, and point to some issues where improvements are needed. 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: Three-dimensional hydrodynamical CO5BOLD model atmospheres of red giant stars. I. Atmospheric structure of a giant located near the RGB tip Authors: Ludwig, H. -G.; Kučinskas, A. Bibcode: 2012A&A...547A.118L Altcode: 2012arXiv1211.7304L Context. Red giant stars are important tracers of stellar populations in the Galaxy and beyond, thus accurate modeling of their structure and related observable properties is of great importance. Three-dimensional (3D) hydrodynamical stellar atmosphere models offer a new level of realism in the modeling of red giant atmospheres but still need to be established as standard tools.
Aims: We investigate the character and role of convection in the atmosphere of a prototypical red giant located close to the red giant branch (RGB) tip with atmospheric parameters, Teff = 3660 K, log g = 1.0, [M/H] = 0.0.
Methods: Differential analysis of the atmospheric structures is performed using the 3D hydrodynamical and 1D classical atmosphere models calculated with the CO5BOLD and LHD codes, respectively. All models share identical atmospheric parameters, elemental composition, opacities and equation-of-state.
Results: We find that the atmosphere of this particular red giant consists of two rather distinct regions: the lower atmosphere dominated by convective motions and the upper atmosphere dominated by wave activity. Convective motions form a prominent granulation pattern with an intensity contrast (~18%) which is larger than in the solar models (~15%). The upper atmosphere is frequently traversed by fast shock waves, with vertical and horizontal velocities of up to Mach ~2.5 and ~6.0, respectively. The typical diameter of the granules amounts to ~5 Gm which translates into ~400 granules covering the whole stellar surface. The turbulent pressure in the giant model contributes up to ~35% to the total (i.e., gas plus turbulent) pressure which shows that it cannot be neglected in stellar atmosphere and evolutionary modeling. However, there exists no combination of the mixing-length parameter, αMLT, and turbulent pressure, Pturb, that would allow to satisfactorily reproduce the 3D temperature-pressure profile with 1D atmosphere models based on a standard formulation of mixing-length theory. Title: Spectroscopic and photometric studies of white dwarfs in the Hyades Authors: Tremblay, P. -E.; Schilbach, E.; Röser, S.; Jordan, S.; Ludwig, H. -G.; Goldman, B. Bibcode: 2012A&A...547A..99T Altcode: 2012arXiv1209.4309T The Hyades cluster is known to harbour ten so-called classical white dwarf members. Numerous studies through the years have predicted that more than twice this amount of degenerate stars should be associated with the cluster. Using the PPMXL Catalog of proper motions and positions, a recent study proposed 17 new white dwarf candidates. We review the membership of these candidates by using published spectroscopic and photometric observations, as well as by simulating the contamination from field white dwarfs. In addition to the ten classical Hyades white dwarfs, we find six white dwarfs that may be of Hyades origin and three more objects that have an uncertain membership status due to their unknown or imprecise atmospheric parameters. Among those, two to three are expected as field stars contamination. Accurate radial velocity measurements will confirm or reject the candidates. One consequence is that the longstanding problem that no white dwarf older than ~340 Myr appears to be associated with the cluster remains unsolved. Title: 4MOST: 4-metre multi-object spectroscopic telescope Authors: de Jong, Roelof S.; Bellido-Tirado, Olga; Chiappini, Cristina; Depagne, Éric; Haynes, Roger; Johl, Diana; Schnurr, Olivier; Schwope, Axel; Walcher, Jakob; Dionies, Frank; Haynes, Dionne; Kelz, Andreas; Kitaura, Francisco S.; Lamer, Georg; Minchev, Ivan; Müller, Volker; Nuza, Sebastián. E.; Olaya, Jean-Christophe; Piffl, Tilmann; Popow, Emil; Steinmetz, Matthias; Ural, Ugur; Williams, Mary; Winkler, Roland; Wisotzki, Lutz; Ansorge, Wolfgang R.; Banerji, Manda; Gonzalez Solares, Eduardo; Irwin, Mike; Kennicutt, Robert C.; King, Dave; McMahon, Richard G.; Koposov, Sergey; Parry, Ian R.; Sun, David; Walton, Nicholas A.; Finger, Gert; Iwert, Olaf; Krumpe, Mirko; Lizon, Jean-Louis; Vincenzo, Mainieri; Amans, Jean-Philippe; Bonifacio, Piercarlo; Cohen, Mathieu; Francois, Patrick; Jagourel, Pascal; Mignot, Shan B.; Royer, Frédéric; Sartoretti, Paola; Bender, Ralf; Grupp, Frank; Hess, Hans-Joachim; Lang-Bardl, Florian; Muschielok, Bernard; Böhringer, Hans; Boller, Thomas; Bongiorno, Angela; Brusa, Marcella; Dwelly, Tom; Merloni, Andrea; Nandra, Kirpal; Salvato, Mara; Pragt, Johannes H.; Navarro, Ramón; Gerlofsma, Gerrit; Roelfsema, Ronald; Dalton, Gavin B.; Middleton, Kevin F.; Tosh, Ian A.; Boeche, Corrado; Caffau, Elisabetta; Christlieb, Norbert; Grebel, Eva K.; Hansen, Camilla; Koch, Andreas; Ludwig, Hans-G.; Quirrenbach, Andreas; Sbordone, Luca; Seifert, Walter; Thimm, Guido; Trifonov, Trifon; Helmi, Amina; Trager, Scott C.; Feltzing, Sofia; Korn, Andreas; Boland, Wilfried Bibcode: 2012SPIE.8446E..0TD Altcode: 2012arXiv1206.6885D The 4MOST consortium is currently halfway through a Conceptual Design study for ESO with the aim to develop a wide-field ( < 3 square degree, goal < 5 square degree), high-multiplex ( < 1500 fibres, goal 3000 fibres) spectroscopic survey facility for an ESO 4m-class telescope (VISTA). 4MOST will run permanently on the telescope to perform a 5 year public survey yielding more than 20 million spectra at resolution R∼5000 (λ=390-1000 nm) and more than 2 million spectra at R~20,000 (395-456.5 nm and 587-673 nm). The 4MOST design is especially intended to complement three key all-sky, space-based observatories of prime European interest: Gaia, eROSITA and Euclid. Initial design and performance estimates for the wide-field corrector concepts are presented. Two fibre positioner concepts are being considered for 4MOST. The first one is a Phi-Theta system similar to ones used on existing and planned facilities. The second one is a new R-Theta concept with large patrol area. Both positioner concepts effectively address the issues of fibre focus and pupil pointing. The 4MOST spectrographs are fixed configuration two-arm spectrographs, with dedicated spectrographs for the high- and low-resolution fibres. A full facility simulator is being developed to guide trade-off decisions regarding the optimal field-of-view, number of fibres needed, and the relative fraction of high-to-low resolution fibres. The simulator takes mock catalogues with template spectra from Design Reference Surveys as starting point, calculates the output spectra based on a throughput simulator, assigns targets to fibres based on the capabilities of the fibre positioner designs, and calculates the required survey time by tiling the fields on the sky. The 4MOST consortium aims to deliver the full 4MOST facility by the end of 2018 and start delivering high-level data products for both consortium and ESO community targets a year later with yearly increments. Title: Detailed Abundances in Extremely Metal Poor Dwarf Stars Extracted from SDSS Authors: Sbordone, L.; Bonifacio, P.; Caffau, E.; Ludwig, H. -G. Bibcode: 2012ASPC..458...69S Altcode: 2012arXiv1201.1044S We report on the result of an ongoing campaign to determine chemical abundances in extremely metal poor (EMP) turn-off (TO) stars selected from the Sloan Digital Sky Survey (SDSS) low resolution spectra. This contribution focuses principally on the largest part of the sample (18 stars out of 29), observed with UVES@VLT and analyzed by means of the automatic abundance analysis code MyGIsFOS to derive atmosphere parameters and detailed compositions. The most significant findings include i) the detection of a C-rich, strongly Mg-enhanced star ([Mg/Fe]=1.45); ii) a group of Mn-rich stars ([Mn/Fe]>-0.4); iii) a group of Ni-rich stars ([Ni/Fe]>0.2). Li is measured in twelve stars, while for three upper limits are derived. Title: Amplitudes of solar-like oscillations in red giant stars. Evidence for non-adiabatic effects using CoRoT observations Authors: Samadi, R.; Belkacem, K.; Dupret, M. -A.; Ludwig, H. -G.; Baudin, F.; Caffau, E.; Goupil, M. -J.; Barban, C. Bibcode: 2012A&A...543A.120S Altcode: 2012arXiv1205.4846S Context. A growing number of solar-like oscillations has been detected in red giant stars thanks to the CoRoT and Kepler space-crafts. In the same way as for main-sequence stars, mode driving is attributed to turbulent convection in the uppermost convective layers of those stars.
Aims: The seismic data gathered by CoRoT on red giant stars allow us to test the mode driving theory in physical conditions different from main-sequence stars.
Methods: Using a set of 3D hydrodynamical models representative of the upper layers of sub- and red giant stars, we computed the acoustic mode energy supply rate ({p_max}). Assuming adiabatic pulsations and using global stellar models that assume that the surface stratification comes from the 3D hydrodynamical models, we computed the mode amplitude in terms of surface velocity. This was converted into intensity fluctuations using either a simplified adiabatic scaling relation or a non-adiabatic one.
Results: From L and M (the luminosity and mass), the energy supply rate {p_max} is found to scale as (L/M)2.6 for both main-sequence and red giant stars, extending previous results. The theoretical amplitudes in velocity under-estimate the Doppler velocity measurements obtained so far from the ground for red giant stars by about 30%. In terms of intensity, the theoretical scaling law based on the adiabatic intensity-velocity scaling relation results in an under-estimation by a factor of about 2.5 with respect to the CoRoT seismic measurements. On the other hand, using the non-adiabatic intensity-velocity relation significantly reduces the discrepancy with the CoRoT data. The theoretical amplitudes remain 40% below, however, the CoRoT measurements.
Conclusions: Our results show that scaling relations of mode amplitudes cannot be simply extended from main-sequence to red giant stars in terms of intensity on the basis of adiabatic relations because non-adiabatic effects for red giant stars are important and cannot be neglected. We discuss possible reasons for the remaining differences. Title: Chemical abundances of distant extremely metal-poor unevolved stars Authors: Bonifacio, P.; Sbordone, L.; Caffau, E.; Ludwig, H. -G.; Spite, M.; González Hernández, J. I.; Behara, N. T. Bibcode: 2012A&A...542A..87B Altcode: 2012arXiv1204.1641B Context. The old Galactic halo stars hold the fossil record of the interstellar medium chemical composition at the time of their formation. Most of the stars studied so far are relatively near to the Sun, this prompts the study of more distant stars, both to increase the size of the sample and to search for possible variations of abundance patterns at greater distances.
Aims: The purpose of our study is to determine the chemical composition of a sample of 16 candidate extremely metal-poor (EMP) dwarf stars, extracted from the Sloan Digital Sky Survey (SDSS). There are two main purposes: in the first place to verify the reliability of the metallicity estimates derived from the SDSS spectra; in the second place to see if the abundance trends found for the brighter nearer stars studied previously also hold for this sample of fainter, more distant stars.
Methods: We used the UVES at the VLT to obtain high-resolution spectra of the programme stars. The abundances were determined by an automatic analysis with the MyGIsFOS code, with the exception of lithium, for which the abundances were determined from the measured equivalent widths of the Li i resonance doublet.
Results: All candidates are confirmed to be EMP stars, with [Fe/H] ≤ -3.0. The chemical composition of the sample of stars is similar to that of brighter and nearer samples. We measured the lithium abundance for 12 stars and provide stringent upper limits for three other stars, for a fourth star the upper limit is not significant, owing to the low signal-to noise ratio of the spectrum. The "meltdown" of the Spite plateau is confirmed, but some of the lowest metallicity stars of the sample lie on the plateau.
Conclusions: The concordance of the metallicities derived from high-resolution spectra and those estimated from the SDSS spectra suggests that the latter may be used to study the metallicity distribution of the halo. The abundance pattern suggests that the halo was well mixed for all probed metallicities and distances. The fact that at the lowest metallicities we find stars on the Spite plateau suggests that the meltdown depends on at least another parameter, besides metallicity.

Based on spectra obtained with UVES at the 8.2 m Kueyen ESO telescope, programmes 078.D-0217 and 081.D.0373.Table 1 is available in electronic form at http://www.aanda.org Title: A primordial star in the heart of the Lion Authors: Caffau, E.; Bonifacio, P.; François, P.; Spite, M.; Spite, F.; Zaggia, S.; Ludwig, H. -G.; Steffen, M.; Mashonkina, L.; Monaco, L.; Sbordone, L.; Molaro, P.; Cayrel, R.; Plez, B.; Hill, V.; Hammer, F.; Randich, S. Bibcode: 2012A&A...542A..51C Altcode: 2012arXiv1203.2607C Context. The discovery and chemical analysis of extremely metal-poor stars permit a better understanding of the star formation of the first generation of stars and of the Universe emerging from the Big Bang.
Aims: We report the study of a primordial star situated in the centre of the constellation Leo (SDSS J102915+172927).
Methods: The star, selected from the low-resolution spectrum of the Sloan Digital Sky Survey, was observed at intermediate (with X-Shooter at VLT) and at high spectral resolution (with UVES at VLT). The stellar parameters were derived from the photometry. The standard spectroscopic analysis based on 1D ATLAS models was completed by applying 3D and non-LTE corrections.
Results: An iron abundance of [Fe/H ] = -4.89 makes SDSS J102915+172927 one of the lowest [Fe/H] stars known. However, the absence of measurable C and N enhancements indicates that it has the lowest metallicity, Z ≤ 7.40 × 10-7 (metal-mass fraction), ever detected. No oxygen measurement was possible.
Conclusions: The discovery of SDSS J102915+172927 highlights that low-mass star formation occurred at metallicities lower than previously assumed. Even lower metallicity stars may yet be discovered, with a chemical composition closer to the composition of the primordial gas and of the first supernovae.

Based on observations obtained at ESO Paranal Observatory, GTO programme 086.D-0094 and programme 286.D-5045. Title: NLTE determination of the calcium abundance and 3D corrections in extremely metal-poor stars Authors: Spite, M.; Andrievsky, S. M.; Spite, F.; Caffau, E.; Korotin, S. A.; Bonifacio, P.; Ludwig, H. -G.; François, P.; Cayrel, R. Bibcode: 2012A&A...541A.143S Altcode: 2012arXiv1204.1139S Context. Calcium is a key element for constraining the models of chemical enrichment of the Galaxy.
Aims: Extremely metal-poor stars contain the fossil records of the chemical composition of the early Galaxy and it is important to compare Ca abundance with abundances of other light elements, that are supposed to be synthesized in the same stellar evolution phases.
Methods: The NLTE profiles of the calcium lines were computed in a sample of 53 extremely metal-poor stars with a modified version of the program MULTI, which allows a very good description of the radiation field.
Results: With our new model atom we are able to reconcile the abundance of Ca deduced from the Ca I and Ca II lines in Procyon. This abundance is found to be solar. We find that [Ca/Fe] = 0.50±0.09 in the early Galaxy, a value slightly higher than the previous LTE estimations. The scatter of the ratios [X/Ca] is generally smaller than the scatter of the ratio [X/Mg] where X is a "light metal" (O, Na, Mg, Al, S, and K) with the exception of Al. These scatters cannot be explained by error of measurements, except for oxygen. Surprisingly, the scatter of [X/Fe] is always equal to, or even smaller than, the scatter around the mean value of [X/Ca]. We note that at low metallicity, the wavelength of the Ca I resonance line is shifted relative to the (weaker) subordinate lines, a signature of the effect of convection. The Ca abundance deduced from the Ca I resonance line (422.7 nm) is found to be systematically smaller at very low metallicity than the abundance deduced from the subordinate lines. Our computations of the effects of convection (3D effects) are not able to explain this difference. A fully consistent 3D NLTE model atmosphere and line formation scheme would be necessary to fully capture the physics of the stellar atmosphere.

Based on observations obtained with the ESO Very Large Telescope at Paranal Observatory, Chile (Large Programme "First Stars", ID 165.N-0276(A); P.I.: R. Cayrel).The NLTE corrections of the Ca lines are 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/541/A143 Title: VizieR Online Data Catalog: Li and Na in globular cluster M4 (Monaco+, 2012) Authors: Monaco, L.; Villanova, S.; Bonifacio, P.; Caffau, E.; Geisler, D.; Marconi, G.; Momany, Y.; Ludwig, H. -G. Bibcode: 2012yCat..35390157M Altcode: 2012yCat..35399157M We observed stars along the M4 MS and SGB using the FLAMES/GIRAFFE spectrograph at ESO Paranal. Observations were conducted in service mode between April and July 2010 using the HR12 and HR15N settings.

(1 data file). Title: Barium abundance in red giants of NGC 6752. Non-local thermodynamic equilibrium and three-dimensional effects Authors: Dobrovolskas, V.; Kučinskas, A.; Andrievsky, S. M.; Korotin, S. A.; Mishenina, T. V.; Bonifacio, P.; Ludwig, H. -G.; Caffau, E. Bibcode: 2012A&A...540A.128D Altcode: 2012arXiv1203.3124D
Aims: We study the effects related to departures from non-local thermodynamic equilibrium (NLTE) and homogeneity in the atmospheres of red giant stars, to assess their influence on the formation of Ba II lines. We estimate the impact of these effects on the barium abundance determinations for 20 red giants in Galactic globular cluster NGC 6752.
Methods: One-dimensional (1D) local thermodynamic equilibrium (LTE) and 1D NLTE barium abundances were derived using classical 1D ATLAS9 stellar model atmospheres. The three-dimensional (3D) LTE abundances were obtained for 8 red giants on the lower RGB, by adjusting their 1D LTE abundances using 3D-1D abundance corrections, i.e., the differences between the abundances obtained from the same spectral line using the 3D hydrodynamical and classical 1D stellar model atmospheres. The 3D-1D abundance corrections were obtained in a strictly differential way using the 3D hydrodynamical and classical 1D codes CO5BOLD and LHD. Both codes utilized identical stellar atmospheric parameters, opacities, and equation of state.
Results: The mean 1D barium-to-iron abundance ratios derived for 20 giants are ⟨[Ba/Fe]⟩1D LTE = 0.24 ± 0.05(stat.) ± 0.08(sys.) and ⟨[Ba/Fe]⟩1D NLTE = 0.05 ± 0.06(stat.) ± 0.08(sys.). The 3D-1D abundance correction obtained for 8 giants is small (~+0.05 dex), thus leads to only minor adjustment when applied to the mean 1D NLTE barium-to-iron abundance ratio for the 20 giants, ⟨[Ba/Fe]⟩3D + NLTE = 0.10 ± 0.06(stat.) ± 0.10(sys.). The intrinsic abundance spread between the individual cluster stars is small and can be explained in terms of uncertainties in the abundance determinations.
Conclusions: Deviations from LTE play an important role in the formation of barium lines in the atmospheres of red giants studied here. The role of 3D hydrodynamical effects should not be dismissed either, even if the obtained 3D-1D abundance corrections are small. This result is a consequence of subtle fine-tuning of individual contributions from horizontal temperature fluctuations and differences between the average temperature profiles in the 3D and 1D model atmospheres: owing to the comparable size and opposite sign, their contributions nearly cancel each other. This fine-tuning is characteristic of the particular set of atmospheric parameters and the element investigated, hence should not necessarily be a general property of spectral line formation in the atmospheres of red giant stars. Title: LTE Model Atmospheres: MARCS, ATLAS and CO5BOLD Authors: Bonifacio, P.; Caffau, E.; Ludwig, H. -G.; Steffen, M. Bibcode: 2012IAUS..282..213B Altcode: In this talk, we review the basic assumptions and physics covered by classical 1D LTE model atmospheres. We will focus on ATLAS and MARCS models of F-G-K stars and describe what resources are available through the web, both in terms of codes and model-atmosphere grids. We describe the advances made in hydrodynamical simulations of convective stellar atmospheres with the CO5BOLD code and what grids and resources are available, with a prospect of what will be available in the near future. Title: Chapter 1 : How to Derive Oxygen Abundances Authors: Stasińska, G.; Prantzos, N.; Meynet, G.; Simón-Díaz, S.; Chiappini, C.; Dessauges-Zavadsky, M.; Charbonnel, C.; Ludwig, H. -G.; Mendoza, C.; Grevesse, N.; Arnould, M.; Barbuy, B.; Lebreton, Y.; Decourchelle, A.; Hill, V.; Ferrando, P.; Hébrard, G.; Durret, F.; Katsuma, M.; Zeippen, C. J. Bibcode: 2012EAS....54....3S Altcode: No abstract at ADS Title: VizieR Online Data Catalog: NLTE Corrections of the Ca lines (Spite+, 2012) Authors: Spite, M.; Andrievsky, S. M.; Spite, F.; Caffau, E.; Korotin, S. A.; Bonifacio, P.; Ludwig, H. -G.; Francois, P.; Cayrel, R. Bibcode: 2012yCat..35410143S Altcode: 2012yCat..35419143S The NLTE corrections were computed for 51 CaI lines and 16 CaII lines for a grid of models with different metallicities from [Fe/H]=0.0 to [Fe/H]=-3. These corrections must be added to the LTE value of [Ca/H], they were computed only if the equivalent width of the Ca line was stronger than 3mÅ. In the tables the model is given in the form (Teff, logg, [Fe/H], [Ca/Fe]) where Teff is the effective temperature, and logg the logarithm of the surface gravity)

(5 data files). Title: Chapter 2 : A Panorama of Oxygen in the Universe Authors: Stasińska, G.; Prantzos, N.; Meynet, G.; Simón-Díaz, S.; Chiappini, C.; Dessauges-Zavadsky, M.; Charbonnel, C.; Ludwig, H. -G.; Mendoza, C.; Grevesse, N.; Arnould, M.; Barbuy, B.; Lebreton, Y.; Decourchelle, A.; Hill, V.; Ferrando, P.; Hébrard, G.; Durret, F.; Katsuma, M.; Zeippen, C. J. Bibcode: 2012EAS....54...65S Altcode: No abstract at ADS Title: The Gaia-ESO Public Spectroscopic Survey Authors: Gilmore, G.; Randich, S.; Asplund, M.; Binney, J.; Bonifacio, P.; Drew, J.; Feltzing, S.; Ferguson, A.; Jeffries, R.; Micela, G.; Negueruela, I.; Prusti, T.; Rix, H. -W.; Vallenari, A.; Alfaro, E.; Allende-Prieto, C.; Babusiaux, C.; Bensby, T.; Blomme, R.; Bragaglia, A.; Flaccomio, E.; François, P.; Irwin, M.; Koposov, S.; Korn, A.; Lanzafame, A.; Pancino, E.; Paunzen, E.; Recio-Blanco, A.; Sacco, G.; Smiljanic, R.; Van Eck, S.; Walton, N.; Aden, D.; Aerts, C.; Affer, L.; Alcala, J. -M.; Altavilla, G.; Alves, J.; Antoja, T.; Arenou, F.; Argiroffi, C.; Asensio Ramos, A.; Bailer-Jones, C.; Balaguer-Nunez, L.; Bayo, A.; Barbuy, B.; Barisevicius, G.; Barrado y Navascues, D.; Battistini, C.; Bellas Velidis, I.; Bellazzini, M.; Belokurov, V.; Bergemann, M.; Bertelli, G.; Biazzo, K.; Bienayme, O.; Bland-Hawthorn, J.; Boeche, C.; Bonito, S.; Boudreault, S.; Bouvier, J.; Brandao, I.; Brown, A.; de Bruijne, J.; Burleigh, M.; Caballero, J.; Caffau, E.; Calura, F.; Capuzzo-Dolcetta, R.; Caramazza, M.; Carraro, G.; Casagrande, L.; Casewell, S.; Chapman, S.; Chiappini, C.; Chorniy, Y.; Christlieb, N.; Cignoni, M.; Cocozza, G.; Colless, M.; Collet, R.; Collins, M.; Correnti, M.; Covino, E.; Crnojevic, D.; Cropper, M.; Cunha, M.; Damiani, F.; David, M.; Delgado, A.; Duffau, S.; Edvardsson, B.; Eldridge, J.; Enke, H.; Eriksson, K.; Evans, N. W.; Eyer, L.; Famaey, B.; Fellhauer, M.; Ferreras, I.; Figueras, F.; Fiorentino, G.; Flynn, C.; Folha, D.; Franciosini, E.; Frasca, A.; Freeman, K.; Fremat, Y.; Friel, E.; Gaensicke, B.; Gameiro, J.; Garzon, F.; Geier, S.; Geisler, D.; Gerhard, O.; Gibson, B.; Gomboc, A.; Gomez, A.; Gonzalez-Fernandez, C.; Gonzalez Hernandez, J.; Gosset, E.; Grebel, E.; Greimel, R.; Groenewegen, M.; Grundahl, F.; Guarcello, M.; Gustafsson, B.; Hadrava, P.; Hatzidimitriou, D.; Hambly, N.; Hammersley, P.; Hansen, C.; Haywood, M.; Heber, U.; Heiter, U.; Held, E.; Helmi, A.; Hensler, G.; Herrero, A.; Hill, V.; Hodgkin, S.; Huelamo, N.; Huxor, A.; Ibata, R.; Jackson, R.; de Jong, R.; Jonker, P.; Jordan, S.; Jordi, C.; Jorissen, A.; Katz, D.; Kawata, D.; Keller, S.; Kharchenko, N.; Klement, R.; Klutsch, A.; Knude, J.; Koch, A.; Kochukhov, O.; Kontizas, M.; Koubsky, P.; Lallement, R.; de Laverny, P.; van Leeuwen, F.; Lemasle, B.; Lewis, G.; Lind, K.; Lindstrom, H. P. E.; Lobel, A.; Lopez Santiago, J.; Lucas, P.; Ludwig, H.; Lueftinger, T.; Magrini, L.; Maiz Apellaniz, J.; Maldonado, J.; Marconi, G.; Marino, A.; Martayan, C.; Martinez-Valpuesta, I.; Matijevic, G.; McMahon, R.; Messina, S.; Meyer, M.; Miglio, A.; Mikolaitis, S.; Minchev, I.; Minniti, D.; Moitinho, A.; Momany, Y.; Monaco, L.; Montalto, M.; Monteiro, M. J.; Monier, R.; Montes, D.; Mora, A.; Moraux, E.; Morel, T.; Mowlavi, N.; Mucciarelli, A.; Munari, U.; Napiwotzki, R.; Nardetto, N.; Naylor, T.; Naze, Y.; Nelemans, G.; Okamoto, S.; Ortolani, S.; Pace, G.; Palla, F.; Palous, J.; Parker, R.; Penarrubia, J.; Pillitteri, I.; Piotto, G.; Posbic, H.; Prisinzano, L.; Puzeras, E.; Quirrenbach, A.; Ragaini, S.; Read, J.; Read, M.; Reyle, C.; De Ridder, J.; Robichon, N.; Robin, A.; Roeser, S.; Romano, D.; Royer, F.; Ruchti, G.; Ruzicka, A.; Ryan, S.; Ryde, N.; Santos, N.; Sanz Forcada, J.; Sarro Baro, L. M.; Sbordone, L.; Schilbach, E.; Schmeja, S.; Schnurr, O.; Schoenrich, R.; Scholz, R. -D.; Seabroke, G.; Sharma, S.; De Silva, G.; Smith, M.; Solano, E.; Sordo, R.; Soubiran, C.; Sousa, S.; Spagna, A.; Steffen, M.; Steinmetz, M.; Stelzer, B.; Stempels, E.; Tabernero, H.; Tautvaisiene, G.; Thevenin, F.; Torra, J.; Tosi, M.; Tolstoy, E.; Turon, C.; Walker, M.; Wambsganss, J.; Worley, C.; Venn, K.; Vink, J.; Wyse, R.; Zaggia, S.; Zeilinger, W.; Zoccali, M.; Zorec, J.; Zucker, D.; Zwitter, T.; Gaia-ESO Survey Team Bibcode: 2012Msngr.147...25G Altcode: The Gaia-ESO Public Spectroscopic Survey has begun and will obtain high quality spectroscopy of some 100000 Milky Way stars, in the field and in open clusters, down to magnitude 19, systematically covering all the major components of the Milky Way. This survey will provide the first homogeneous overview of the distributions of kinematics and chemical element abundances in the Galaxy. The motivation, organisation and implementation of the Gaia-ESO Survey are described, emphasising the complementarity with the ESA Gaia mission. Spectra from the very first observing run of the survey are presented. Title: Lithium and sodium in the globular cluster M 4. Detection of a Li-rich dwarf star: preservation or pollution? Authors: Monaco, L.; Villanova, S.; Bonifacio, P.; Caffau, E.; Geisler, D.; Marconi, G.; Momany, Y.; Ludwig, H. -G. Bibcode: 2012A&A...539A.157M Altcode: 2011arXiv1108.0138M Context. The abundance inhomogeneities of light elements observed in globular clusters (GCs), and notably the ubiquitous Na-O anti-correlation, are generally interpreted as evidence that GCs comprise several generations of stars. There is an on-going debate as to the nature of the stars, which produce the inhomogeneous elements, and investigating the behavior of several elements is a way to shed new light on this problem.
Aims: We aim at investigating the Li and Na content of the GC M 4, that is known to have a well defined Na-O anti-correlation.
Methods: We obtained moderate resolution (R = 17 000-18 700) spectra for 91 main sequence (MS)/sub-giant branch stars of M 4 with the Giraffe spectrograph at the FLAMES/VLT ESO facility. Using model atmospheres analysis we measured lithium and sodium abundances.
Results: We detect a weak Li-Na anti-correlation among un-evolved MS stars. One star in the sample, # 37934, shows the remarkably high lithium abundance A(Li) = 2.87, compatible with current estimates of the primordial lithium abundance.
Conclusions: The shallow slope found for the Li-Na anti-correlation suggests that lithium is produced in parallel to sodium. This evidence, coupled with its sodium-rich nature, suggests that the high lithium abundance of star # 37934 may originate by pollution from a previous generations of stars. The recent detection of a Li-rich dwarf of pollution origin in the globular cluster NGC 6397 may also point in this direction. Still, no clear cut evidence is available against a possible preservation of the primordial lithium abundance for star # 37934.

Based on observations taken at ESO VLT Kueyen telescope (Cerro Paranal, Chile, program: 085.D-0537A).Table A.1 is available in electronic form at http://www.aanda.org Title: Oxygen in the Universe Authors: Stasińska, G.; Prantzos, N.; Meynet, G.; Simón-Díaz, S.; Chiappini, C.; Dessauges-Zavadsky, M.; Charbonnel, C.; Ludwig, H. -G.; Mendoza, C.; Grevesse, N.; Arnould, M.; Barbuy, B.; Lebreton, Y.; Decourchelle, A.; Hill, V.; Ferrando, P.; Hébrard, G.; Durret, F.; Katsuma, M.; Zeippen, C. J. Bibcode: 2012EAS....54.....S Altcode: No abstract at ADS Title: Foreword Authors: Stasińska, G.; Prantzos, N.; Meynet, G.; Simón-Díaz, S.; Chiappini, C.; Dessauges-Zavadsky, M.; Charbonnel, C.; Ludwig, H. -G.; Mendoza, C.; Grevesse, N.; Arnould, M.; Barbuy, B.; Lebreton, Y.; Decourchelle, A.; Hill, V.; Ferrando, P.; Hébrard, G.; Durret, F.; Katsuma, M.; Zeippen, C. J. Bibcode: 2012EAS....54....1S Altcode: No abstract at ADS Title: Appendix A : The atomic physics of oxygen Authors: Stasińska, G.; Prantzos, N.; Meynet, G.; Simón-Díaz, S.; Chiappini, C.; Dessauges-Zavadsky, M.; Charbonnel, C.; Ludwig, H. -G.; Mendoza, C.; Grevesse, N.; Arnould, M.; Barbuy, B.; Lebreton, Y.; Decourchelle, A.; Hill, V.; Ferrando, P.; Hébrard, G.; Durret, F.; Katsuma, M.; Zeippen, C. J. Bibcode: 2012EAS....54..319S Altcode: No abstract at ADS 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: Chapter 4 : The Evolution of Oxygen in Galaxies Authors: Stasińska, G.; Prantzos, N.; Meynet, G.; Simón-Díaz, S.; Chiappini, C.; Dessauges-Zavadsky, M.; Charbonnel, C.; Ludwig, H. -G.; Mendoza, C.; Grevesse, N.; Arnould, M.; Barbuy, B.; Lebreton, Y.; Decourchelle, A.; Hill, V.; Ferrando, P.; Hébrard, G.; Durret, F.; Katsuma, M.; Zeippen, C. J. Bibcode: 2012EAS....54..255S Altcode: No abstract at ADS Title: Chapter 3 : Oxygen Production and Destruction Authors: Stasińska, G.; Prantzos, N.; Meynet, G.; Simón-Díaz, S.; Chiappini, C.; Dessauges-Zavadsky, M.; Charbonnel, C.; Ludwig, H. -G.; Mendoza, C.; Grevesse, N.; Arnould, M.; Barbuy, B.; Lebreton, Y.; Decourchelle, A.; Hill, V.; Ferrando, P.; Hébrard, G.; Durret, F.; Katsuma, M.; Zeippen, C. J. Bibcode: 2012EAS....54..187S Altcode: No abstract at ADS 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: 6Li detection in metal-poor stars: can 3D model atmospheres solve the second lithium problem? Authors: Steffen, M.; Cayrel, R.; Caffau, E.; Bonifacio, P.; Ludwig, H. -G.; Spite, M. Bibcode: 2012MSAIS..22..152S Altcode: 2012arXiv1206.2239S The presence of 6Li in the atmospheres of metal-poor halo stars is usually inferred from the detection of a subtle extra depression in the red wing of the 7Li doublet line at 670.8 nm. However, as pointed out recently by \cite{Cayrel2007}, the intrinsic line asymmetry caused by convective flows in the photospheres of cool stars is almost indistinguishable from the asymmetry produced by a weak 6Li blend on a (presumed) symmetric 7Li profile. Previous determinations of the 6Li/ 7Li isotopic ratio based on 1D model atmospheres, ignoring the convection-induced line asymmetry, must therefore be considered as upper limits. By comparing synthetic 1D LTE and 3D non-LTE line profiles of the iLi 670.8 nm feature, we quantify the differential effect of the convective line asymmetry on the derived 6Li abundance as a function of effective temperature, gravity, and metallicity. As expected, we find that the asymmetry effect systematically reduces the resulting 6Li/7Li ratios. Depending on the stellar parameters, the 3D-1D offset in 6Li/7Li ranges between -0.005 and -0.020. When this purely theoretical correction is taken into account for the \cite{A2006} sample of stars, the number of significant 6Li detections decreases from 9 to 5 (2sigma criterion), or from 5 to 2 (3sigma criterion).

We also present preliminary results of a re-analysis of high-resolution, high S/N spectra of individual metal-poor turn-off stars, to see whether the second Lithium problem actually disappears when accounting properly for convection and non-LTE line formation in 3D stellar atmospheres. Out of 8 stars, HD 84937 seems to be the only significant (2sigma ) detection of 6Li. In view of our results, the existence of a 6Li plateau appears questionable. Title: 3D Model Atmospheres of Red Giant Stars Authors: Ludwig, Hans-Günter; Steffen, Matthias Bibcode: 2012ASSP...26..125L Altcode: 2011arXiv1108.4554L; 2012rgps.book..125L We provide a brief overview of the modelling of the atmospheres of red giant stars with the 3D radiation-hydrodynamics code CO5BOLD. We emphasize aspects where 3D modelling provides additional insight beyond standard hydrostatic 1D models, and comment on present modelling challenges. 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: X-shooter Finds an Extremely Primitive Star Authors: Caffau, E.; Bonifacio, P.; François, P.; Sbordone, L.; Monaco, L.; Spite, M.; Spite, F.; Ludwig, H. -G.; Cayrel, R.; Zaggia, S.; Hammer, F.; Randich, S.; Molaro, P.; Hill, V. Bibcode: 2011Msngr.146...28C Altcode: Low-mass extremely metal-poor (EMP) stars hold the fossil record of the chemical composition of the early phases of the Universe in their atmospheres. Chemical analysis of such objects provides important constraints on these early phases. EMP stars are rather rare objects: to dig them out, large amounts of data have to be considered. We have analysed stars from the Sloan Digital Sky Survey using an automatic procedure and selected a sample of good candidate EMP stars, which we observed with the spectrographs X-shooter and UVES. We could confirm the low metallicity of our sample of stars, and we succeeded in finding a record metal-poor star. Title: X-Shooter GTO: chemical analysis of a sample of EMP candidates Authors: Caffau, E.; Bonifacio, P.; François, P.; Spite, M.; Spite, F.; Zaggia, S.; Ludwig, H. -G.; Monaco, L.; Sbordone, L.; Cayrel, R.; Hammer, F.; Randich, S.; Hill, V.; Molaro, P. Bibcode: 2011A&A...534A...4C Altcode: 2011arXiv1109.0992C Context. Extremely metal-poor stars (EMP) are very rare objects that hold in their atmospheres the fossil record of the chemical composition of the early phases of Galactic evolution. Finding these objects and determining their chemical composition provides important constraints on these early phases.
Aims: Using a carefully designed selection method, we chose a sample of candidate EMP stars from the low resolution spectra of the Sloan Digital Sky Survey and observed them with X-Shooter at the VLT to confirm their metallicities and determine abundances for as many elements as possible.
Methods: The X-Shooter spectra are analysed by means of one-dimensional, plane-parallel, hydrostatic model atmospheres. Corrections for the granulation effects are computed using CO5BOLD hydrodynamical simulations.
Results: All the candidates are confirmed to be EMP stars, proving the efficiency of our selection method within about 0.5 dex. The chemical composition of this sample is compatible with those of brighter samples, suggesting that the stars in the Galactic halo are well mixed.
Conclusions: These observations show that it is feasible to observe, in a limited amount of time, a large sample of about one hundred stars among EMP candidates selected from the SDSS. Such a size of sample will allow us, in particular, to confirm or refute the existence of a vertical drop in the Galactic halo metallicity distribution function around [Fe/H] ~ -3.5.

Based on observations obtained at ESO Paranal Observatory, GTO programme 086.D-0094. Title: An extremely primitive star in the Galactic halo Authors: Caffau, Elisabetta; Bonifacio, Piercarlo; François, Patrick; Sbordone, Luca; Monaco, Lorenzo; Spite, Monique; Spite, François; Ludwig, Hans-G.; Cayrel, Roger; Zaggia, Simone; Hammer, François; Randich, Sofia; Molaro, Paolo; Hill, Vanessa Bibcode: 2011Natur.477...67C Altcode: 2012arXiv1203.2612C The early Universe had a chemical composition consisting of hydrogen, helium and traces of lithium; almost all other elements were subsequently created in stars and supernovae. The mass fraction of elements more massive than helium, Z, is known as `metallicity'. A number of very metal-poor stars has been found, some of which have a low iron abundance but are rich in carbon, nitrogen and oxygen. For theoretical reasons and because of an observed absence of stars with Z<1.5×10-5, it has been suggested that low-mass stars cannot form from the primitive interstellar medium until it has been enriched above a critical value of Z, estimated to lie in the range 1.5×10-8 to 1.5×10-6 (ref. 8), although competing theories claiming the contrary do exist. (We use `low-mass' here to mean a stellar mass of less than 0.8 solar masses, the stars that survive to the present day.) Here we report the chemical composition of a star in the Galactic halo with a very low Z (<=6.9×10-7, which is 4.5×10-5 times that of the Sun) and a chemical pattern typical of classical extremely metal-poor stars--that is, without enrichment of carbon, nitrogen and oxygen. This shows that low-mass stars can be formed at very low metallicity, that is, below the critical value of Z. Lithium is not detected, suggesting a low-metallicity extension of the previously observed trend in lithium depletion. Such lithium depletion implies that the stellar material must have experienced temperatures above two million kelvin in its history, given that this is necessary to destroy lithium. Title: LTE model atmopsheres MARCS, ATLAS and CO5BOLD Authors: Bonifacio, Piercarlo; Caffau, Elisabetta; Ludwig, Hans-Guenter; Steffen, Matthias Bibcode: 2011arXiv1109.0717B Altcode: In this talk we review the basic assumptions and physics covered by classical 1D LTE model atmospheres. We will focus on ATLAS and MARCS models of F-G-K stars and describe what resources are available through the web, both in terms of codes and model-atmosphere grids. We describe the advances made in hydrodynamical simulations of convective stellar atmospheres with the CO5BOLD code and what grids and resources are available, with a prospect of what will be available in the near future. 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: First stars. XIV. Sulfur abundances in extremely metal-poor stars Authors: Spite, M.; Caffau, E.; Andrievsky, S. M.; Korotin, S. A.; Depagne, E.; Spite, F.; Bonifacio, P.; Ludwig, H. -G.; Cayrel, R.; François, P.; Hill, V.; Plez, B.; Andersen, J.; Barbuy, B.; Beers, T. C.; Molaro, P.; Nordström, B.; Primas, F. Bibcode: 2011A&A...528A...9S Altcode: 2010arXiv1012.4358S Context. Precise S abundances are important in the study of the early chemical evolution of the Galaxy. In particular the site of the formation remains uncertain because, at low metallicity, the trend of this α-element versus [Fe/H] remains unclear. Moreover, although sulfur is not bound significantly in dust grains in the ISM, it seems to behave differently in DLAs and old metal-poor stars.
Aims: We attempt a precise measurement of the S abundance in a sample of extremely metal-poor stars observed with the ESO VLT equipped with UVES, taking into account NLTE and 3D effects.
Methods: The NLTE profiles of the lines of multiplet 1 of S I were computed with a version of the program MULTI, including opacity sources from ATLAS9 and based on a new model atom for S. These profiles were fitted to the observed spectra.
Results: We find that sulfur in EMP stars behaves like the other α-elements, with [S/Fe] remaining approximately constant below [Fe/H] = -3. However, [S/Mg] seems to decrease slightly with increasing [Mg/H]. The overall abundance patterns of O, Na, Mg, Al, S, and K are most closely matched by the SN model yields by Heger & Woosley. The [S/Zn] ratio in EMP stars is solar, as also found in DLAs. We derive an upper limit to the sulfur abundance [S/Fe] < +0.5 for the ultra metal-poor star CS 22949-037. This, along with a previously reported measurement of zinc, argues against the conjecture that the light-element abundance pattern of this star (and by analogy, the hyper iron-poor stars HE 0107-5240 and HE 1327-2326) would be due to dust depletion.

Based on observations obtained with the ESO Very Large Telescope at Paranal (Large Programme "First Stars", ID 165, N-0276, P.I.: Cayrel. Title: Extremely metal-poor stars in SDSS fields Authors: Bonifacio, P.; Caffau, E.; François, P.; Sbordone, L.; Ludwig, H. -G.; Spite, M.; Molaro, P.; Spite, F.; Cayrel, R.; Hammer, F.; Hill, V.; Nonino, M.; Randich, S.; Stelzer, B.; Zaggia, S. Bibcode: 2011AN....332..251B Altcode: 2011arXiv1101.3139B Some insight on the first generation of stars can be obtained from the chemical composition of their direct descendants, extremely metal-poor stars (EMP), with metallicity less than or equal to 1/1000 of the solar metallicity. Such stars are exceedingly rare, the most successful surveys, for this purpose, have so far provided only about 100 stars with 1/1 000 the solar metallicity and 4 stars with about 1/10 000 of the solar metallicity. The Sloan Digital Sky Survey has the potential to provide a large number of candidates of extremely low metallicity. X-shooter has the unique capability of performing the necessary follow-up spectroscopy providing accurate metallicities and abundance ratios for several elements (Mg, Al, Ca, Ti, Cr, Sr, ...) for EMP candidates. We here report on the results for the first two stars observed in the course of our Franco-Italian X-shooter GTO. The two stars were targeted to be of metallicity around -3.0, the analysis of the X-shooter spectra showed them to be of metallicity around -2.0, but with a low α to iron ratio, which explains the underestimate of the metallicity from the SDSS spectra. The efficiency of X-shooter allows an in situ study of the outer halo, for the two stars studied here we estimate distances of 3.9 and 9.1 kpc, these are likely the most distant dwarf stars studied in detail to date.

Based on spectra obtained with X-shooter at the 8.2-m Kueyen ESO telescope, GTO programmes 085.D-0194 and 086.D.0094. Title: The solar photospheric abundance of zirconium Authors: Caffau, E.; Faraggiana, R.; Ludwig, H. -G.; Bonifacio, P.; Steffen, M. Bibcode: 2011AN....332..128C Altcode: 2010arXiv1012.1038C Zirconium (Zr), together with strontium and yttrium, is an important element in the understanding of the Galactic nucleosynthesis. In fact, the triad Sr-Y-Zr constitutes the first peak of s-process elements. Despite its general relevance not many studies of the solar abundance of Zr were conducted. We derive the zirconium abundance in the solar photosphere with the same CO5BOLD hydrodynamical model of the solar atmosphere that we previously used to investigate the abundances of C-N-O. We review the zirconium lines available in the observed solar spectra and select a sample of lines to determine the zirconium abundance, considering lines of neutral and singly ionised zirconium. We apply different line profile fitting strategies for a reliable analysis of Zr lines that are blended by lines of other elements. The abundance obtained from lines of neutral zirconium is very uncertain because these lines are commonly blended and weak in the solar spectrum. However, we believe that some lines of ionised zirconium are reliable abundance indicators. Restricting the set to Zr II lines, from the CO5BOLD 3D model atmosphere we derive A(Zr) {=2.62± 0.06}, where the quoted error is the RMS line-to-line scatter. Title: Gravitational redshifts in main-sequence and giant stars Authors: Pasquini, L.; Melo, C.; Chavero, C.; Dravins, D.; Ludwig, H. -G.; Bonifacio, P.; de La Reza, R. Bibcode: 2011A&A...526A.127P Altcode: 2010arXiv1011.4635P Context. Precise analyses of stellar radial velocities is able to reveal intrinsic causes of the wavelength shifts of spectral lines (other than Doppler shifts due to radial motion), such as gravitational redshifts and convective blueshifts.
Aims: Gravitational redshifts in solar-type main-sequence stars are expected to be some 500 m s-1 greater than those in giants. We search for this difference in redshifts among groups of open-cluster stars that share the same average space motion and thus have the same average Doppler shift.
Methods: We observed 144 main-sequence stars and cool giants in the M 67 open cluster using the ESO FEROS spectrograph and obtained radial velocities by means of cross-correlation with a spectral template. Binaries and doubtful members were not analyzed, and average spectra were created for different classes of stars.
Results: The M 67 dwarf and giant radial-velocity distributions are each well represented by Gaussian functions, which share the same apparent average radial velocity to within ≃100 m s-1. In addition, dwarfs in M 67 appear to be dynamically hotter (σ = 0.90 km s-1) than giants (σ = 0.68 km s-1).
Conclusions: We fail to detect any difference in the gravitational redshifts of giants and MS stars. This is probably because of the differential wavelength shifts produced by the different hydrodynamics of dwarf and giant atmospheres. Radial-velocity differences measured between unblended lines in averaged spectra vary with line-strength: stronger lines are more blueshifted in dwarfs than in giants, apparently removing any effect of the gravitational redshift. Synthetic high-resolution spectra are computed from three dimensional (3D) hydrodynamic model atmospheres for both giants and dwarfs, and synthetic wavelength shifts obtained. In agreement with observations, 3D models predict substantially smaller wavelength-shift differences than expected from gravitational redshifts only. The procedures developed could be used to test 3D models for different classes of stars, but will ultimately require high-fidelity spectra for measurements of wavelength shifts in individual spectral lines.

Based on observations collected at ESO, La Silla, Chile, during the agreement between the Observatorio Nacional at Rio de Janeiro and ESO.Table 1 is available in electronic form at http://www.aanda.org and also 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/526/A127 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: VizieR Online Data Catalog: Velocities of M67 main-sequence and giant stars (Pasquini+, 2011) Authors: Pasquini, L.; Melo, C.; Chavero, C.; Dravins, D.; Ludwig, H. -G.; Bonifacio, P.; de La, Reza R. Bibcode: 2011yCat..35260127P Altcode: 2011yCat..35269127P We observed 144 main-sequence stars and cool giants in the M67 open cluster using the ESO FEROS spectrograph and obtained radial velocities by means of cross-correlation with a spectral template. Binaries and doubtful members were not analyzed, and average spectra were created for different classes of stars.

(1 data file). Title: Asteroseismology of solar-type stars with Kepler I: Data analysis Authors: Karoff, C.; Chaplin, W. J.; Appourchaux, T.; Elsworth, Y.; Garcia, R. A.; Houdek, G.; Metcalfe, T. S.; Molenda-Żakowicz, J.; Monteiro, M. J. P. F. G.; Thompson, M. J.; Christensen-Dalsgaard, J.; Gilliland, R. L.; Kjeldsen, H.; Basu, S.; Bedding, T. R.; Campante, T. L.; Eggenberger, P.; Fletcher, S. T.; Gaulme, P.; Handberg, R.; Hekker, S.; Martic, M.; Mathur, S.; Mosser, B.; Regulo, C.; Roxburgh, I. W.; Salabert, D.; Stello, D.; Verner, G. A.; Belkacem, K.; Biazzo, K.; Cunha, M. S.; Gruberbauer, M.; Guzik, J. A.; Kupka, F.; Leroy, B.; Ludwig, H. -G.; Mathis, S.; Noels, A.; Noyes, R. W.; Roca Cortes, T.; Roth, M.; Sato, K. H.; Schmitt, J.; Suran, M. D.; Trampedach, R.; Uytterhoeven, K.; Ventura, R. Bibcode: 2010AN....331..972K Altcode: 2010arXiv1005.0507K We report on the first asteroseismic analysis of solar-type stars observed by Kepler. Observations of three G-type stars, made at one-minute cadence during the first 33.5 days of science operations, reveal high signal-to-noise solar-like oscillation spectra in all three stars: About 20 modes of oscillation can clearly be distinguished in each star. We discuss the appearance of the oscillation spectra, including the presence of a possible signature of faculae, and the presence of mixed modes in one of the three stars. Title: Cu I resonance lines in turn-off stars of NGC 6752 and NGC 6397. Effects of granulation from CO5BOLD models Authors: Bonifacio, P.; Caffau, E.; Ludwig, H. -G. Bibcode: 2010A&A...524A..96B Altcode: 2010arXiv1009.1848B Context. Copper is an element whose interesting evolution with metallicity is not fully understood. Observations of copper abundances rely on a very limited number of lines, the strongest are the Cu I lines of Mult. 1 at 324.7 nm and 327.3 nm which can be measured even at extremely low metallicities.
Aims: We investigate the quality of these lines as abundance indicators.
Methods: We measure these lines in two turn-off (TO) stars in the Globular Cluster NGC 6752 and two TO stars in the Globular Cluster NGC 6397 and derive abundances with 3D hydrodynamical model atmospheres computed with the CO5BOLD code. These abundances are compared to the Cu abundances measured in giant stars of the same clusters, using the lines of Mult. 2 at 510.5 nm and 578.2 nm.
Results: The abundances derived from the lines of Mult. 1 in TO stars differ from the abundances of giants of the same clusters. This is true both using CO5BOLD models and using traditional 1D model atmospheres. The LTE 3D corrections for TO stars are large, while they are small for giant stars.
Conclusions: The Cu I resonance lines of Mult. 1 are not reliable abundance indicators. It is likely that departures from LTE should be taken into account to properly describe these lines, although it is not clear if these alone can account for the observations. An investigation of these departures is indeed encouraged for both dwarfs and giants. Our recommendation to those interested in the study of the evolution of copper abundances is to rely on the measurements in giants, based on the lines of Mult. 2. We caution, however, that NLTE studies may imply a revision in all the Cu abundances, both in dwarfs and giants.

Based on observations made with the ESO Very Large Telescope at Paranal Observatory, Chile (Programmes 71.D-0155, 75.D-0807, 76.B-0133). 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: Joint Discussion 10: 3D views on cool stellar atmospheres - theory meets observation Authors: Nagendra, K. N.; Bonifacio, P.; Ludwig, H. -G. Bibcode: 2010HiA....15..331N Altcode: Much of what we know about the chemical composition of the Universe actually stems from the chemical composition of stars, which is often deciphered from the spectra emerging from their atmospheres. Cool, low-mass and long-living stars allow to study the evolution of the Universe's chemistry from a time shortly after the big bang until today. The observation and interpretation of stellar spectra is a classical field in astronomy but is still undergoing vivid developments. The enormous increase in available computational resources opened-up possibilities which led to a revolution in the degree of realism to which modelers can mimic Nature. High-resolution, high-stability, high-efficiency spectrographs are now routinely providing stellar spectra whose full information content can only be exploited if a very much refined description of a stellar atmosphere is at hand. 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: 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: Sulphur abundances in halo stars from multiplet 3 at 1045 nm Authors: Caffau, E.; Sbordone, L.; Ludwig, H. -G.; Bonifacio, P.; Spite, M. Bibcode: 2010AN....331..725C Altcode: 2010arXiv1003.4914C Sulphur is a volatile α-element which is not locked into dust grains in the interstellar medium (ISM). Hence, its abundance does not need to be corrected for dust depletion when comparing the ISM to the stellar atmospheres. The abundance of sulphur in the photosphere of metal-poor stars is a matter of debate: according to some authors, [S/Fe] versus [Fe/H] forms a plateau at low metallicity, while, according to other studies, there is a large scatter or perhaps a bimodal distribution. In metal-poor stars sulphur is detectable by its lines of multiplet 1 at 920 nm, but this range is heavily contaminated by telluric absorptions, and one line of the multiplet is blended by the hydrogen Paschen ζ line. We study the possibility of using multiplet 3 (at 1045 nm) for deriving the sulphur abundance because this range, now observable at the VLT with the infra-red spectrograph CRIRES, is little contaminated by telluric absorption and not affected by blends at least in metal-poor stars. We compare the abundances derived from multiplets 1 and 3, taking into account NLTE corrections and 3D effects. Here we present the results for a sample of four stars, although the scatter is less pronounced than in previous analysis, we cannot find a plateau in [S/Fe], and confirm the scatter of the sulphur abundance at low metallicity.

Using data from CRIRES at the ESO-VLT, Programme 079.D-0434. 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: Commission 36: Theory of Stellar Atmospheres Authors: Asplund, Martin; Puls, Joachim; Landstreet, John; Allende Prieto, Carlos; Ayres, Thomas; Berdyugina, Svetlana; Gustafsson, Bengt; Hubeny, Ivan; Ludwig, Hans Günter; Mashonkina, Lyudmila; Randich, Sofia Bibcode: 2010IAUTB..27..197A Altcode: The members of the Commission 36 Organizing Committee attending the IAU General Assembly in Rio de Janeiro met for a business session on August 7. Both members from the previous (2006-2009) and the new (2009-2012) Organizing Committee partook in the discussions. Past president John Landstreet described the work he had done over the past three years in terms of supporting proposed conferences on the topic. He has also spent significant amount of time establishing an updated mailing list of all >350 members of the commission, which is unfortunately not provided automatically by the IAU. Such a list is critical for a rapid dissemination of information to the commission members and for a correct and smooth running of elections of IAU officials. Everyone present thanked John effusively for all of his hard work over the past three years to stimulate a high level of activity within the discipline. Title: A 3D-NLTE study of the 670 nm solar lithium feature Authors: Caffau, Elisabetta; Ludwig, Hans-Günter; Steffen, Matthias; Bonifacio, Piercarlo Bibcode: 2010IAUS..268..329C Altcode: We derive the 3D-NLTE lithium abundance in the solar photosphere from the Lii line at 670 nm as measured in several solar atlases. The Li abundance is obtained from line profile fitting with 1D/3D-LTE/3D-NLTE synthetic spectra, considering several possibilities for the atomic parameters of the lines blending the Li feature. The 670 nm spectral region shows considerable differences in the two available disc-centre solar atlases, while the two integrated disc spectra are very similar. We obtain A(Li)3D-NLTE = 1.03. The 1D-LTE abundance is 0.07 dex smaller. The line-lists giving the best fit for the Sun may fail for other stars, while some line-lists fail to reproduce the solar profile satisfactorily. We need a better knowledge of the atomic parameters of the lines blending the Li feature in order to be able to reproduce both the solar spectrum and the spectra of other stars. An improved line-list is also required to derive reliable estimates of the isotopic Li ratio in solar-metallicity stars. 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: Three carbon-enhanced metal-poor dwarf stars from the SDSS. Chemical abundances from CO5BOLD 3D hydrodynamical model atmospheres Authors: Behara, N. T.; Bonifacio, P.; Ludwig, H. -G.; Sbordone, L.; González Hernández, J. I.; Caffau, E. Bibcode: 2010A&A...513A..72B Altcode: 2010arXiv1002.1670B Context. The origin of carbon-enhanced metal-poor stars enriched with both s and r elements is highly debated. Detailed abundances of these types of stars are crucial to understand the nature of their progenitors.
Aims: The aim of this investigation is to study in detail the abundances of SDSS J1349-0229, SDSS J0912+0216 and SDSS J1036+1212, three dwarf CEMP stars, selected from the Sloan Digital Sky Survey.
Methods: Using high resolution VLT/UVES spectra (R ~ 30 000) we determine abundances for Li, C, N, O, Na, Mg, Al, Ca, Sc, Ti, Cr, Mn, Fe, Co, Ni and 21 neutron-capture elements. We made use of CO5BOLD 3D hydrodynamical model atmospheres in the analysis of the carbon, nitrogen and oxygen abundances. NLTE corrections for Ci and Oi lines were computed using the Kiel code.
Results: We classify SDSS J1349-0229 and SDSS J0912+0216 as CEMP-r+s stars. SDSS J1036+1212 belongs to the class CEMP-no/s, with enhanced Ba, but deficient Sr, of which it is the third member discovered to date. Radial-velocity variations have been observed in SDSS J1349-0229, providing evidence that it is a member of a binary system.
Conclusions: The chemical composition of the three stars is generally compatible with mass transfer from an AGB companion. However, many details remain difficult to explain. Most notably of those are the abundance of Li at the level of the Spite plateau in SDSS J1036+1212 and the large over-abundance of the pure r-process element Eu in all three stars.

Based on observations obtained with the ESO Very Large Telescope at Paranal Observatory, Chile (programmes 078.D-0217 and 383.D-0927). Title: Convection and 6Li in the atmospheres of metal-poor halo stars Authors: Steffen, Matthias; Cayrel, R.; Bonifacio, P.; Ludwig, H. -G.; Caffau, E. Bibcode: 2010IAUS..268..215S Altcode: 2010arXiv1001.3274S Based on 3D hydrodynamical model atmospheres computed with the CO5BOLD code and 3D non-LTE (NLTE) line formation calculations, we study the effect of the convection-induced line asymmetry on the derived 6Li abundance for a range in effective temperature, gravity, and metallicity covering the stars of the Asplund et al. (2006) sample. When the asymmetry effect is taken into account for this sample of stars, the resulting 6Li/7Li ratios are reduced by about 1.5% on average with respect to the isotopic ratios determined by Asplund et al. (2006). This purely theoretical correction diminishes the number of significant 6Li detections from 9 to 4 (2σ criterion), or from 5 to 2 (3σ criterion). In view of this result the existence of a 6Li plateau appears questionable. A careful reanalysis of individual objects by fitting the observed lithium 6707 Å doublet both with 3D NLTE and 1D LTE synthetic line profiles confirms that the inferred 6Li abundance is systematically lower when using 3D NLTE instead of 1D LTE line fitting. Nevertheless, halo stars with unquestionable 6Li detection do exist even if analyzed in 3D-NLTE, the most prominent example being HD 84937. Title: The Asteroseismic Potential of Kepler: First Results for Solar-Type Stars Authors: Chaplin, W. J.; Appourchaux, T.; Elsworth, Y.; García, R. A.; Houdek, G.; Karoff, C.; Metcalfe, T. S.; Molenda-Żakowicz, J.; Monteiro, M. J. P. F. G.; Thompson, M. J.; Brown, T. M.; Christensen-Dalsgaard, J.; Gilliland, R. L.; Kjeldsen, H.; Borucki, W. J.; Koch, D.; Jenkins, J. M.; Ballot, J.; Basu, S.; Bazot, M.; Bedding, T. R.; Benomar, O.; Bonanno, A.; Brandão, I. M.; Bruntt, H.; Campante, T. L.; Creevey, O. L.; Di Mauro, M. P.; Doǧan, G.; Dreizler, S.; Eggenberger, P.; Esch, L.; Fletcher, S. T.; Frandsen, S.; Gai, N.; Gaulme, P.; Handberg, R.; Hekker, S.; Howe, R.; Huber, D.; Korzennik, S. G.; Lebrun, J. C.; Leccia, S.; Martic, M.; Mathur, S.; Mosser, B.; New, R.; Quirion, P. -O.; Régulo, C.; Roxburgh, I. W.; Salabert, D.; Schou, J.; Sousa, S. G.; Stello, D.; Verner, G. A.; Arentoft, T.; Barban, C.; Belkacem, K.; Benatti, S.; Biazzo, K.; Boumier, P.; Bradley, P. A.; Broomhall, A. -M.; Buzasi, D. L.; Claudi, R. U.; Cunha, M. S.; D'Antona, F.; Deheuvels, S.; Derekas, A.; García Hernández, A.; Giampapa, M. S.; Goupil, M. J.; Gruberbauer, M.; Guzik, J. A.; Hale, S. J.; Ireland, M. J.; Kiss, L. L.; Kitiashvili, I. N.; Kolenberg, K.; Korhonen, H.; Kosovichev, A. G.; Kupka, F.; Lebreton, Y.; Leroy, B.; Ludwig, H. -G.; Mathis, S.; Michel, E.; Miglio, A.; Montalbán, J.; Moya, A.; Noels, A.; Noyes, R. W.; Pallé, P. L.; Piau, L.; Preston, H. L.; Roca Cortés, T.; Roth, M.; Sato, K. H.; Schmitt, J.; Serenelli, A. M.; Silva Aguirre, V.; Stevens, I. R.; Suárez, J. C.; Suran, M. D.; Trampedach, R.; Turck-Chièze, S.; Uytterhoeven, K.; Ventura, R.; Wilson, P. A. Bibcode: 2010ApJ...713L.169C Altcode: 2010arXiv1001.0506C We present preliminary asteroseismic results from Kepler on three G-type stars. The observations, made at one-minute cadence during the first 33.5 days of science operations, reveal high signal-to-noise solar-like oscillation spectra in all three stars: about 20 modes of oscillation may be clearly distinguished in each star. We discuss the appearance of the oscillation spectra, use the frequencies and frequency separations to provide first results on the radii, masses, and ages of the stars, and comment in the light of these results on prospects for inference on other solar-type stars that Kepler will observe. 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: Main-sequence and sub-giant stars in the globular cluster NGC 6397: The complex evolution of the lithium abundance Authors: González Hernández, J. I.; Bonifacio, P.; Caffau, E.; Steffen, M.; Ludwig, H. -G.; Behara, N.; Sbordone, L.; Cayrel, R.; Zaggia, S. Bibcode: 2010IAUS..268..257G Altcode: 2009arXiv0912.4105G Thanks to the high multiplex and efficiency of Giraffe at the VLT we have been able for the first time to observe the Li I doublet in the Main Sequence stars of a globular cluster. At the same time we observed Li in a sample of Sub-Giant stars of the same B-V colour.

Our final sample is composed of 84 SG stars and 79 MS stars. In spite of the fact that SG and MS span the same temperature range we find that the equivalent widths of the Li I doublet in SG stars are systematically larger than those in MS stars, suggesting a higher Li content among SG stars. This is confirmed by our quantitative analysis carried out making use of 1D hydrostatic plane-parallel models and 3D hydrodynamical simulations of the stellar atmospheres.

We derived the effective temperatures of stars in our the sample from Hα fitting. Theoretical profiles were computed using 3D hydrodynamical simulations and 1D ATLAS models. Therefore, we are able to determined 1D and 3D-based effective temperatures. We then infer Li abundances taking into account non-local thermodynamical equilibrium effects when using both 1D and 3D models.

We find that SG stars have a mean Li abundance higher by 0.1 dex than MS stars. This result is obtained using both 1D and 3D models. We also detect a positive slope of Li abundance with effective temperature, the higher the temperature the higher the Li abundance, both for SG and MS stars, although the slope is slightly steeper for MS stars. These results provide an unambiguous evidence that the Li abundance changes with evolutionary status.

The physical mechanisms responsible for this behaviour are not yet clear, and none of the existing models seems to describe accurately these observations. Based on these conclusions, we believe that the cosmological lithium problem still remains an open question. Title: 6Li in metal-poor halo stars: real or spurious? Authors: Steffen, M.; Cayrel, R.; Bonifacio, P.; Ludwig, H. -G.; Caffau, E. Bibcode: 2010IAUS..265...23S Altcode: 2009arXiv0910.5917S The presence of convective motions in the atmospheres of metal-poor halo stars leads to systematic asymmetries of the emergent spectral line profiles. Since such line asymmetries are very small, they can be safely ignored for standard spectroscopic abundance analysis. However, when it comes to the determination of the 6Li/7Li isotopic ratio, q(Li)=n(6Li)/n(7Li), the intrinsic asymmetry of the 7Li line must be taken into account, because its signature is essentially indistinguishable from the presence of a weak 6Li blend in the red wing of the 7Li line. In this contribution we quantity the error of the inferred 6Li/7Li isotopic ratio that arises if the convective line asymmetry is ignored in the fitting of the λ6707 Å lithium blend. Our conclusion is that 6Li/7Li ratios derived by Asplund et al. (2006), using symmetric line profiles, must be reduced by typically Δq(Li) ≈ 0.015. This diminishes the number of certain 6Li detections from 9 to 4 stars or less, casting some doubt on the existence of a 6Li plateau. Title: Can we trust elemental abundances derived in late-type giants with the classical 1D stellar atmosphere models? Authors: Kučinskas, A.; Dobrovolskas, V.; Ivanauskas, A.; Ludwig, H. -G.; Caffau, E.; Blaževičius, K.; Klevas, J.; Prakapavičius, D. Bibcode: 2010IAUS..265..209K Altcode: 2009arXiv0910.3397K We compare the abundances of various chemical species as derived with 3D hydrodynamical and classical 1D stellar atmosphere codes in a late-type giant characterized by Teff =3640 K, log g = 1.0, [M/H]= 0.0. For this particular set of atmospheric parameters the 3D-1D abundance differences are generally small for neutral atoms and molecules but they may reach up to 0.3-0.4 dex in case of ions. The 3D-1D differences generally become increasingly more negative at higher excitation potentials and are typically largest in the optical wavelength range. Their sign can be both positive and negative, and depends on the excitation potential and wavelength of a given spectral line. While our results obtained with this particular late-type giant model suggest that 1D stellar atmosphere models may be safe to use with neutral atoms and molecules, care should be taken if they are exploited with ions. Title: Detailed analyses of three neutron-capture-rich carbon-enhanced metal-poor stars Authors: Behara, N. T.; Bonifacio, P.; Ludwig, H. -G.; Sbordone, L.; González Hernández, J. I.; Caffau, E. Bibcode: 2010IAUS..265..122B Altcode: 2009arXiv0909.0180B Approximately 20% of very metal-poor stars ([Fe/H] < -2.0) are strongly enhanced in carbon ([C/Fe] > +1.0). Such stars are referred to as carbon-enhanced metal-poor (CEMP) stars. We present a chemical abundance analysis based on high resolution spectra acquired with UVES at the VLT of three dwarf CEMP stars: SDSS J1349-0229, SDSS J0912+0216 and SDSS J1036+1212. These very metal-poor stars, with [Fe/H] < -2.5, were selected from our ongoing survey of extremely metal-poor dwarf candidates from the SDSS.

Among these CEMPs, SDSS J1349-0229 has been identified as a carbon star ([C/O] > +1.0). First and second peak s-process elements, as well as second peak r-process elements have been detected in all stars. In addition, elements from the third r-process peak were detected in one of the stars, SDSS J1036+1212. We present the abundance results of these stars in the context of neutron-capture nucleosynthesis theories. 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: Accuracy of spectroscopy-based radioactive dating of stars Authors: Ludwig, H. -G.; Caffau, E.; Steffen, M.; Bonifacio, P.; Sbordone, L. Bibcode: 2010A&A...509A..84L Altcode: 2009arXiv0911.4251L Context. Combined spectroscopic abundance analyses of stable and radioactive elements can be applied for deriving stellar ages. The achievable precision depends on factors related to spectroscopy, nucleosynthesis, and chemical evolution.
Aims: We quantify the uncertainties arising from the spectroscopic analysis, and compare these to the other error sources.
Methods: We derive formulae for the age uncertainties arising from the spectroscopic abundance analysis, and apply them to spectroscopic and nucleosynthetic data compiled from the literature for the Sun and metal-poor stars.
Results: We obtained ready-to-use analytic formulae of the age uncertainty for the cases of stable+unstable and unstable+unstable chronometer pairs, and discuss the optimal relation between to-be-measured age and mean lifetime of a radioactive species. Application to the literature data indicates that, for a single star, the achievable spectroscopic accuracy is limited to about ±20% for the foreseeable future. At present, theoretical uncertainties in nucleosynthesis and chemical evolution models form the precision bottleneck. For stellar clusters, isochrone fitting provides a higher accuracy than radioactive dating, but radioactive dating becomes competitive when applied to many cluster members simultaneously, reducing the statistical errors by a factor √{N}.
Conclusions: Spectroscopy-based radioactive stellar dating would benefit from improvements in the theoretical understanding of nucleosynthesis and chemical evolution. Its application to clusters can provide strong constraints for nucleosynthetic models. Title: The CoRoT target HD 49933 . II. Comparison of theoretical mode amplitudes with observations Authors: Samadi, R.; Ludwig, H. -G.; Belkacem, K.; Goupil, M. J.; Benomar, O.; Mosser, B.; Dupret, M. -A.; Baudin, F.; Appourchaux, T.; Michel, E. Bibcode: 2010A&A...509A..16S Altcode: 2009arXiv0910.4037S Context. The seismic data obtained by CoRoT for the star HD 49933 enable us for the first time to measure directly the amplitudes and linewidths of solar-like oscillations for a star other than the Sun. From those measurements it is possible, as was done for the Sun, to constrain models of the excitation of acoustic modes by turbulent convection.
Aims: We compare a stochastic excitation model described in Paper I with the asteroseismology data for HD 49933, a star that is rather metal poor and significantly hotter than the Sun.
Methods: Using the seismic determinations of the mode linewidths detected by CoRoT for HD 49933 and the theoretical mode excitation rates computed in Paper I for the specific case of HD 49933, we derive the expected surface velocity amplitudes of the acoustic modes detected in HD 49933. Using a calibrated quasi-adiabatic approximation relating the mode amplitudes in intensity to those in velocity, we derive the expected values of the mode amplitude in intensity.
Results: Except at rather high frequency, our amplitude calculations are within 1-σ error bars of the mode surface velocity spectrum derived with the HARPS spectrograph. The same is found with respect to the mode amplitudes in intensity derived for HD 49933 from the CoRoT data. On the other hand, at high frequency (ν ⪆ 1.9 mHz), our calculations depart significantly from the CoRoT and HARPS measurements. We show that assuming a solar metal abundance rather than the actual metal abundance of the star would result in a larger discrepancy with the seismic data. Furthermore, we present calculations which assume the “new” solar chemical mixture to be in better agreement with the seismic data than those that assumed the “old” solar chemical mixture.
Conclusions: These results validate in the case of a star significantly hotter than the Sun and α Cen A the main assumptions in the model of stochastic excitation. However, the discrepancies seen at high frequency highlight some deficiencies of the modelling, whose origin remains to be understood. We also show that it is important to take the surface metal abundance of the solar-like pulsators into account.

The CoRoT space mission, launched on December 27 2006, has been developped and is operated by CNES, with the contribution of Austria, Belgium, Brasil, ESA, Germany and Spain. Title: The CoRoT target HD 49933 . I. Effect of the metal abundance on the mode excitation rates Authors: Samadi, R.; Ludwig, H. -G.; Belkacem, K.; Goupil, M. J.; Dupret, M. -A. Bibcode: 2010A&A...509A..15S Altcode: 2009arXiv0910.4027S Context. Solar-like oscillations are stochastically excited by turbulent convection at the surface layers of the stars.
Aims: We study the role of the surface metal abundance on the efficiency of the stochastic driving in the case of the CoRoT target HD 49933.
Methods: We compute two 3D hydrodynamical simulations representative - in effective temperature and gravity - of the surface layers of the CoRoT target HD 49933, a star that is rather metal poor and significantly hotter than the Sun. One 3D simulation has a solar metal abundance, and the other has a surface iron-to-hydrogen, [Fe/H], abundance ten times smaller. For each 3D simulation we match an associated global 1D model, and we compute the associated acoustic modes using a theoretical model of stochastic excitation validated in the case of the Sun and α Cen A.
Results: The rate at which energy is supplied per unit time into the acoustic modes associated with the 3D simulation with [Fe/H] = -1 is found to be about three times smaller than those associated with the 3D simulation with [Fe/H] = 0. As shown here, these differences are related to the fact that low metallicity implies surface layers with a higher mean density. In turn, a higher mean density favors smaller convective velocities and hence less efficient driving of the acoustic modes.
Conclusions: Our result shows the importance of taking the surface metal abundance into account in the modeling of the mode driving by turbulent convection. A comparison with observational data is presented in a companion paper using seismic data obtained for the CoRoT target HD 49933.

The CoRoT space mission, launched on December 27, 2006, has been developped and is operated by CNES, with the contribution of Austria, Belgium, Brasil, ESA, Germany and Spain. Title: Chemical abundances in metal-poor giants: limitations imposed by the use of classical 1D stellar atmosphere models Authors: Dobrovolskas, V.; Kucinskas, A.; Ludwig, H. G.; Caffau, E.; Klevas, J.; Prakapavicius, D. Bibcode: 2010nuco.confE.288D Altcode: 2010arXiv1010.2507D; 2010PoS...100E.288D In this work we have used 3D hydrodynamical (CO5BOLD) and 1D hydrostatic (LHD) stellar atmosphere models to study the importance of convection and horizontal temperature inhomogeneities in stellar abundance work related to late-type giants. We have found that for a number of key elements, such as Na, Mg, Si, Ca, Ti, Fe, Ni, Zn, Ba, Eu, differences in abundances predicted by 3D and 1D models are typically minor (< 0.1 dex) at solar metallicity. However, at [M/H] = -3 they become larger and reach to -0.5...-0.8 dex. In case of neutral atoms and fixed metallicity, the largest abundance differences were obtained for the spectral lines with lowest excitation potential, while for ionized species the largest 3D-1D abundance differences were found for lines of highest excitation potential. The large abundance differences at low metallicity are caused by large horizontal temperature fluctuations and lower mean temperature in the outer layers of the 3D hydrodynamical model compared with its 1D counterpart. Title: Lithium abundances of main-sequence and subgiant stars in the globular cluster NGC 6397 Authors: González Hernández, J. I.; Bonifacio, P.; Caffau, E.; Steffen, M.; Ludwig, H. -G.; Behara, N.; Sbordone, L.; Cayrel, R.; Zaggia, S. Bibcode: 2010IAUS..266..407G Altcode: 2009arXiv0910.2305G We present FLAMES/GIRAFFE spectroscopy obtained with the Very Large Telescope (VLT). Using these observations, we have been able (for the first time) to observe the Lii doublet in the main-sequence (MS) stars of a globular cluster. We also observed Li in a sample of subgiant (SG) stars of the same B - V colour. Our final sample is composed of 84 SG and 79 MS stars. In spite of the fact that SG and MS stars span the same temperature range, we find that the equivalent widths of the Lii doublet in SG stars are systematically greater than in MS stars, suggesting a higher Li content among SG stars. This is confirmed by our quantitative analysis, which makes use of both 1D and 3D model atmospheres. We find that SG stars show, on average, a higher Li abundance, by 0.1 dex, than MS stars. We also detect a positive slope of Li abundance with effective temperature: the higher the temperature the higher the Li abundance, both for SG and MS stars, although the slope is slightly steeper for MS stars. These results provide unambiguous evidence that the Li abundance changes with evolutionary state. The physical mechanisms that contribute to this are not yet clear, since none of the proposed models seem to describe accurately the observations. Whether such a mechanism can explain the cosmological lithium problem is still an open question. Title: Perspectives for Determining Stellar Surface Parameters Authors: Ludwig, H. -G. Bibcode: 2010EAS....45..251L Altcode: 2011EAS....45..251L I present ideas of Gaia's impact on the determination of the properties of stars primarily connected to the study of their atmospheres. This mainly relates to effective temperatures, gravities and high-fidelity chemical abundances obtained by combining envisioned Gaia measurements with ground-based spectroscopy ranging from single objects to well-selected stellar populations. I further discuss the impact of Gaia on the study of the kinematics of atmospheric flows. Title: Sulfur in the globular clusters 47 Tuc and NGC 6752 Authors: Sbordone, L.; Chieffi, A.; Limongi, M.; Caffau, E.; Ludwig, H. -G.; Bonifacio, P. Bibcode: 2010IAUS..266..537S Altcode: The light elements Li, O, Na, Al, and Mg are known to show star-to-star variations in the globular clusters 47 Tuc and NGC 6752. We have investigated the behavior of the α element sulfur, for which no previous measurements exist in any Galactic globular cluster. We used high-resolution UVES spectra of Si multiplet 1 around 923 nm, and determined S abundances by means of ATLAS static plano-parallel models. NLTE corrections were applied and 3D corrections were also computed from co5bold 3D hydrodynamical models. Sulfur has been measured in four subgiant stars in NGC 6752, leading to an average value of [S/Fe] = +0.49 ± 0.15 dex, consistent with what is observed in field stars of similar metallicity. In 47 Tuc, we measured S in four turnoff (TO) and five subgiant (SG) stars, for an average value of [S/Fe] = 0.18 ± 0.14 dex. While the measurement errors are consistent with a constant value among all cluster stars analyzed, we detected a highly significant correlation with sodium abundance, as well as a tentative one with silicon. The sulfur-sodium correlation is difficult to explain in terms of nucleosynthesis. Given its high statistical significance, it is also difficult to dismiss it as fortuitous. Until better data for more stars are available, the question as to its origin remains open. Title: Local stars formed at z>10: a sample extracted from the SDSS Authors: Sbordone, L.; Bonifacio, P.; Caffau, E.; Ludwig, H. G. Bibcode: 2010nuco.confE.294S Altcode: 2010arXiv1009.5210S; 2010PoS...100E.294S As the Universe emerged from its initial hot and dense phase, its chemical composition was extremely simple, being limited to stable H and He isotopes, and traces of Li. The first stars that formed had such initial composition. However, they quickly began to produce a whole array of heavier nuclei, polluting the interstellar medium. While none among these first stars has been detected to date, an increasing sample exists of their direct descendant, stars with heavy elements content of the order of 1/1000 of the solar value, or less. In most cases, such stars should have formed at redshift of about 10 or beyond, and their chemical composition can provide crucial constraints to the nature of the very first stars. Extremely metal poor (EMP) stars are exceedingly rare. We used the low resolution spectra obtained by the Sloan Digital Sky Survey (SDSS) to search for EMP candidates: results of VLT-UVES high resolution follow-up for 16 of them is presented here. A newly developed automatic abundance analysis and parameter determination code, MyGIsFOS, has been employed to analyze the detailed chemical abundances of such stars. Title: Gaia spectroscopy: processing, performances and scientific returns Authors: Katz, D.; Cropper, M.; Meynadier, F.; Jean-Antoine, A.; Allende Prieto, C.; Baker, S.; Benson, K.; Berthier, J.; Bigot, L.; Blomme, R.; Boudreault, S.; Chemin, L.; Crifo, F.; Damerdji, Y.; David, M.; David, P.; Delle Luche, C.; Dolding, C.; Frémat, Y.; Gerbier, N.; Gerssen, J.; Gómez, A.; Gosset, E.; Guerrier, A.; Guy, L.; Hall, D.; Hestroffer, D.; Huckle, H.; Jasniewicz, G.; Ludwig, H. -G.; Martayan, C.; Morel, T.; Nguyen, A. -T.; Ocvirk, P.; Parr, C.; Royer, F.; Sartoretti, P.; Seabroke, G.; Simon, E.; Smith, M.; Soubiran, C.; Steinmetz, M.; Thévenin, F.; Turon, C.; Udry, S.; Veltz, L.; Viala, Y. Bibcode: 2010EAS....45..189K Altcode: 2011EAS....45..189K During the five years of the mission, the Gaia spectrograph, the Radial Velocity Spectrometer (RVS) will repeatedly survey the celestial sphere down to magnitude V ~ 17-18. This talk presents: (i) the system which is currently developed within the Gaia Data Processing and Analysis Consortium (DPAC) to reduce and calibrate the spectra and to derive the radial and rotational velocities, (ii) the RVS expected performances and (iii) scientific returns. Title: Radial Velocity Standard Stars for the Gaia RVS Authors: Jasniewicz, G.; Crifo, F.; Soubiran, C.; Hestroffer, D.; Siebert, A.; Veltz, L.; Bigot, L.; Chemin, L.; David, P.; Guerrier, A.; Katz, D.; Ludwig, H. -G.; Richard, P.; Royer, F.; Sartoretti, P.; Udry, S. Bibcode: 2010EAS....45..195J Altcode: 2011EAS....45..195J The calibration of the Radial Velocity Zero-Point (RVZP) of the Radial Velocity Spectrometer (RVS) will be performed with the help of Radial Velocity (RV) standard stars and asteroids. A full-sky list of RV standard stars candidates has been built for this purpose within the Development Unit (DU) 640 of the Gaia DPAC Consortium. A ground-based campaign of RV observations has been initiated to eliminate unsuitable candidates. Simulations have also been performed in order to estimate in advance the number of RV standard stars and asteroids crossing the RVS field of view. These standard stars and asteroids will also be very helpful for controlling any harmful effect (especially the CCD radiation damage) on calibration and RVZP during the lifetime of the Gaia project. Kinematic RV are expected to be published at the end of the project for the brightest stars, taking into account gravitational redshift and convective shifts for the lines present in the RVS spectral wavelength range. Title: 3D hydrodynamical CO5BOLD model atmospheres of late-type giants: stellar abundances from molecular lines Authors: Ivanauskas, A.; Kucinskas, A.; Ludwig, H. G.; Caffau, E. Bibcode: 2010nuco.confE.290I Altcode: 2010PoS...100E.290I; 2010arXiv1010.1722I We investigate the influence of convection on the formation of molecular spectral lines in the atmospheres of late-type giants. For this purpose we use the 3D hydrodynamical CO5BOLD and classical 1D LHD stellar atmosphere codes and synthesize a number of fictitious lines belonging to a number of astrophysically relevant molecules, C2, CH, CN, CO, NH, OH. We find that differences between the abundances obtained from molecular lines using the 3D and 1D model atmospheres are generally small at [M/H]=0.0, but they quickly increase at sub-solar metallicities where for certain molecules they may reach -2.0 dex. The 3D-1D abundance differences show a significant dependence on the spectral line parameters, such as wavelength and excitation potential. Our comparison, therefore, reveals a complex interplay between the spectral line formation and convection that can not be properly accounted for with the classical 1D model atmospheres. Title: 3D simulations of M star atmosphere velocities and their influence on molecular FeH lines Authors: Wende, S.; Reiners, A.; Ludwig, H. -G. Bibcode: 2009A&A...508.1429W Altcode: 2009arXiv0910.3493W Context: The measurement of line broadening in cool stars is in general a difficult task. In order to detect slow rotation or weak magnetic fields, an accuracy of 1 km s-1 is needed. In this regime the broadening from convective motion becomes important. We present an investigation of the velocity fields in early to late M-type star hydrodynamic models, and we simulate their influence on FeH molecular line shapes. The M star model parameters range between log{g} of 3.0-5.0 and effective temperatures from 2500 K to 4000 K.
Aims: Our aim is to characterize the Teff- and log{g}-dependence of the velocity fields and express them in terms of micro- and macro-turbulent velocities in the one dimensional sense. We present a direct comparison between 3D hydrodynamical velocity fields and 1D turbulent velocities. The velocity fields strongly affect the line shapes of FeH, and it is our goal to give a rough estimate of the log{g} and Teff parameter range in which 3D spectral synthesis is necessary and where 1D synthesis suffices. We want to distinguish between the velocity-broadening from convective motion and the rotational- or Zeeman-broadening in M-type stars we are planning to measure. For the latter, FeH lines are an important indicator.
Methods: In order to calculate M-star structure models, we employ the 3D radiative-hydrodynamics (RHD) code CO^5BOLD. The spectral synthesis in these models is performed with the line synthesis code LINFOR3D. We describe the 3D velocity fields in terms of a Gaussian standard deviations and project them onto the line of sight to include geometrical and limb-darkening effects. The micro- and macro-turbulent velocities are determined with the “curve of growth” method and convolution with a Gaussian velocity profile, respectively. To characterize the log{g} and Teff dependence of FeH lines, the equivalent width, line width, and line depth are examined.
Results: The velocity fields in M-stars strongly depend on log{g} and Teff. They become stronger with decreasing log{g} and increasing Teff. The projected velocities from the 3D models agree within 100 m s-1 with the 1D micro- and macro-turbulent velocities. The FeH line quantities systematically depend on log{g} and Teff.
Conclusions: The influence of hydrodynamic velocity fields on line shapes of M-type stars can well be reproduced with 1D broadening methods. FeH lines turn out to provide a means to measure log{g} and Teff in M-type stars. Since different FeH lines all behave in a similar manner, they provide an ideal measure for rotational and magnetic broadening. Title: GAIA RVS data reduction : the 6^{th} dimension Authors: Meynadier, F.; Crifo, F.; Katz, D.; Thévenin, F.; Berthier, J.; Bigot, L.; Delle Luche, C.; Doressoundiram, A.; Gomez, A.; Guerrier, A.; Hestroffer, D.; Hubert, A. -M. .; Jasniewicz, G.; Jean-Antoine, A.; Ludwig, H.; Martayan, C.; Nguyen, A. -T.; Ocvirk, P.; Pichon, B.; Royer, F.; Sartoretti, P.; Siebert, A.; Soubiran, C.; Turon, C.; Veltz, L.; Viala, Y. Bibcode: 2009sf2a.conf...63M Altcode: This poster describes the current organisation of RVS data processing among the Gaia-DPAC (Data Processing & Analysis Consortium), with a particular focus on the French community's contribution. Title: Hydrodynamical simulations of convection-related stellar micro-variability. II. The enigmatic granulation background of the CoRoT target HD 49933 Authors: Ludwig, H. -G.; Samadi, R.; Steffen, M.; Appourchaux, T.; Baudin, F.; Belkacem, K.; Boumier, P.; Goupil, M. -J.; Michel, E. Bibcode: 2009A&A...506..167L Altcode: 2009arXiv0905.2695L Context: Local-box hydrodynamical model atmospheres provide statistical information about a star's emergent radiation field which allows one to predict the level of its granulation-related micro-variability. Space-based photometry is now sufficiently accurate to test model predictions.
Aims: We aim to model the photometric granulation background of HD 49933 as well as the Sun, and compare the predictions to the measurements obtained by the <sf>CoRoT</sf> and <sf>SOHO</sf> satellite missions.
Methods: We construct hydrodynamical model atmospheres representing HD 49933 and the Sun, and use a previously developed scaling technique to obtain the observable disk-integrated brightness fluctuations. We further performed exploratory magneto-hydrodynamical simulations to gauge the impact of small scale magnetic fields on the synthetic light-curves.
Results: We find that the granulation-related brightness fluctuations depend on metallicity. We obtain a satisfactory correspondence between prediction and observation for the Sun, validating our approach. For HD 49933, we arrive at a significant over-estimation by a factor of two to three in total power. Locally generated magnetic fields are unlikely to be responsible, otherwise existing fields would need to be rather strong to sufficiently suppress the granulation signal. Presently suggested updates on the fundamental stellar parameters do not improve the correspondence; however, an ad-hoc increase of the HD 49933 surface gravity by about 0.2 dex would eliminate most of the discrepancy.
Conclusions: We diagnose a puzzling discrepancy between the predicted and observed granulation background in HD 49933, with only rather ad-hoc ideas for remedies at hand.

The <sf>CoRoT</sf> space mission, launched on December 27th 2006, has been developed and is operated by CNES, with the contribution of Austria, Belgium, Brazil, ESA, Germany and Spain. CIFIST Marie Curie Excellence Team. Title: Theoretical amplitudes and lifetimes of non-radial solar-like oscillations in red giants Authors: Dupret, M. -A.; Belkacem, K.; Samadi, R.; Montalban, J.; Moreira, O.; Miglio, A.; Godart, M.; Ventura, P.; Ludwig, H. -G.; Grigahcène, A.; Goupil, M. -J.; Noels, A.; Caffau, E. Bibcode: 2009A&A...506...57D Altcode: 2009arXiv0906.3951D Context: Solar-like oscillations have been observed in numerous red giants from ground and from space. An important question arises: could we expect to detect non-radial modes probing the internal structure of these stars?
Aims: We investigate under what physical circumstances non-radial modes could be observable in red giants; what would be their amplitudes, lifetimes and heights in the power spectrum (PS)?
Methods: Using a non-radial non-adiabatic pulsation code including a non-local time-dependent treatment of convection, we compute the theoretical lifetimes of radial and non-radial modes in several red giant models. Next, using a stochastic excitation model, we compute the amplitudes of these modes and their heights in the PS.
Results: Distinct cases appear. Case A corresponds to subgiants and stars at the bottom of the ascending giant branch. Our results show that the lifetimes of the modes are mainly proportional to the inertia I, which is modulated by the mode trapping. The predicted amplitudes are lower for non-radial modes. But the height of the peaks in the PS are of the same order for radial and non-radial modes as long as they can be resolved. The resulting frequency spectrum is complex. Case B corresponds to intermediate models in the red giant branch. In these models, the radiative damping becomes high enough to destroy the non-radial modes trapped in the core. Hence, only modes trapped in the envelope have significant heights in the PS and could be observed. The resulting frequency spectrum of detectable modes is regular for ℓ=0 and 2, but a little more complex for ℓ=1 modes because of less efficient trapping. Case C corresponds to models of even higher luminosity. In these models the radiative damping of non-radial modes is even larger than in the previous case and only radial and non-radial modes completely trapped in the envelope could be observed. The frequency pattern is very regular for these stars. The comparison between the predictions for radial and non-radial modes is very different if we consider the heights in the PS instead of the amplitudes. This is important as the heights (not the amplitudes) are used as detection criterion.

CIFIST Marie Curie Excellence Team. Title: Lithium in the globular cluster NGC 6397. Evidence for dependence on evolutionary status Authors: González Hernández, J. I.; Bonifacio, P.; Caffau, E.; Steffen, M.; Ludwig, H. -G.; Behara, N. T.; Sbordone, L.; Cayrel, R.; Zaggia, S. Bibcode: 2009A&A...505L..13G Altcode: 2009arXiv0909.0983G Context: Most globular clusters are believed to host a single stellar population. They can thus be considered a good place to study the Spite plateau and to search for possible evolutionary modifications of the Li content.
Aims: We want to determine the Li content of subgiant (SG) and main sequence (MS) stars of the old, metal-poor globular cluster NGC 6397. This work was aimed not only at studying possible Li abundance variations but also to investigate the cosmological Li discrepancy.
Methods: Here, we present FLAMES/GIRAFFE observations of a sample of 84 SG and 79 MS stars in NGC 6397 selected in a narrow range of B-V colour and, therefore, effective temperatures. We determine both effective temperatures and Li abundances using three-dimensional hydrodynamical model atmospheres for all the MS and SG stars of the sample.
Results: We find a significant difference in the Li abundance between SG stars and MS stars, the SG stars having an abundance higher by almost 0.1 dex on average. We also find a decrease in the lithium abundance with decreasing effective temperature, both in MS and SG stars, albeit with a significantly different slope for the two classes of stars. This suggests that the lithium abundance in these stars is, indeed, altered by some process, which is temperature-dependent.
Conclusions: The lithium abundance pattern observed in NGC 6397 is different from what is found among field stars, casting some doubt on the use of globular cluster stars as representative of Population II with respect to the lithium abundance. None of the available theories of Li depletion appears to satisfactorily describe our observations.

Based on observations obtained with FLAMES/GIRAFFE at VLT Kueyen 8.2 m telescope in programme 079.D-0399(A). Table and Figs. 3-10 are only available in electronic form at http://www.aanda.org Table 2 is available in electronic form at http://www.aanda.org and at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/505/L13 Title: The Solar Photospheric Nitrogen Abundance: Determination with 3D and 1D Model Atmospheres Authors: Maiorca, E.; Caffau, E.; Bonifacio, P.; Busso, M.; Faraggiana, R.; Steffen, M.; Ludwig, H. -G.; Kamp, I. Bibcode: 2009PASA...26..345M Altcode: 2009arXiv0912.0375M We present a new determination of the solar nitrogen abundance making use of 3D hydrodynamical modelling of the solar photosphere, which is more physically motivated than traditional static 1D models. We selected suitable atomic spectral lines, relying on equivalent width measurements already existing in the literature. For atmospheric modelling we used the co 5 bold 3D radiation hydrodynamics code. We investigated the influence of both deviations from local thermodynamic equilibrium (non-LTE effects) and photospheric inhomogeneities (granulation effects) on the resulting abundance. We also compared several atlases of solar flux and centre-disc intensity presently available. As a result of our analysis, the photospheric solar nitrogen abundance is A(N) = 7.86 +/- 0.12. Title: Teff and log g dependence of velocity fields in M-stars Authors: Wende, S.; Reiners, A.; Ludwig, H. -G. Bibcode: 2009AIPC.1171..323W Altcode: 2009arXiv0908.0820W We present an investigation of velocity fields in early to late M-type hydrodynamic stellar atmosphere models. These velocities will be expressed in classical terms of micro- and macro-turbulent velocities for usage in 1D spectral synthesis. The M-star model parameters range between log g of 3.0-5.0 and Teff of 2500 K-4000 K. We characterize the Teff- and log g-dependence of the hydrodynamical velocity fields in these models with a binning method, and for the determination of micro-turbulent velocities, the Curve of Growth method is used. The macro-turbulent velocities are obtained by convolutions with Gaussian profiles. Velocity fields in M-stars strongly depend on log g and Teff. Their velocity amplitudes increase with decreasing log g and increasing Teff. The 3D hydrodynamical and 1D macro-turbulent velocities range from ~100 m/s for cool high gravity models to ~800 m/s-1000 m/s for hot models or models with low log g. The micro-turbulent velocities range in the order of ~100 m/s for cool models, to ~600 m/s for hot or low log g models. Our M-star structure models are calculated with the 3D radiative-hydrodynamics (RHD) code CO5BOLD. The spectral synthesis on these models is performed with the line synthesis code LINFOR3D. Title: VizieR Online Data Catalog: Lithium in NGC 6397 (Gonzalez Hernandez+, 2009) Authors: Gonzalez Hernandez, J. I.; Bonifacio, P.; Caffau, E.; Steffen, M.; Ludwig, H. -G.; Behara, N. T.; Sbordone, L.; Cayrel, R.; Zaggia, S. Bibcode: 2009yCat..35059013G Altcode: Photometric data of the dwarf and subgiant stars of the globular cluster NGC 6397. We also provide the signal-to-noise of the spectra, the 3D and 1D Halpha-based effective temperatures, 3D Li abundances, and the equivalent widths and errors: dEWa: Error of the equivalent width measurements estimated from a fitting routine that uses as free parameters the velocity shift, the continuum location, and the equivalent width of the Li line. dEWb: Error of the equivalent width associated to the signal-to-noise ratio and the wavelength dispersion of the spectra, derived using Cayrel's formula (Cayrel, 1988, IAU Symp. 132: The Impact of Very High S/N Spectroscopy on Stellar Physics, 132, 345).

(1 data file). Title: Impact of granulation effects on the use of Balmer lines as temperature indicators Authors: Ludwig, H. -G.; Behara, N. T.; Steffen, M.; Bonifacio, P. Bibcode: 2009A&A...502L...1L Altcode: 2009arXiv0906.4697L Context: Balmer lines serve as important indicators of stellar effective temperatures in late-type stellar spectra. One of their modelling uncertainties is the influence of convective flows on their shape.
Aims: We aim to characterize the influence of convection on the wings of Balmer lines.
Methods: We perform a differential comparison of synthetic Balmer line profiles obtained from 3D hydrodynamical model atmospheres and 1D hydrostatic standard ones. The model parameters are appropriate for F, G, K dwarf and subgiant stars of metallicity ranging from solar to 10-3 solar.
Results: The shape of the Balmer lines predicted by 3D models can never be exactly reproduced by a 1D model, irrespective of its effective temperature. We introduce the concept of a 3D temperature correction, as the effective temperature difference between a 3D model and a 1D model which provides the closest match to the 3D profile. The temperature correction is different for the different members of the Balmer series and depends on the adopted mixing-length parameter αMLT in the 1D model. Among the investigated models, the 3D correction ranges from -300 K to +300 K. Horizontal temperature fluctuations tend to reduce the 3D correction.
Conclusions: Accurate effective temperatures cannot be derived from the wings of Balmer lines, unless the effects of convection are properly accounted for. The 3D models offer a physically well justified way of doing so. The use of 1D models treating convection with the mixing-length theory do not appear to be suitable for this purpose. In particular, there are indications that it is not possible to determine a single value of αMLT which will optimally reproduce the Balmer lines for any choice of atmospheric parameters. The investigation of a more extended grid and direct comparison with observed Balmer profiles will be carried out in the near future.

Appendices is only available in electronic form at http://www.aanda.org Title: Sulfur in the globular clusters 47 Tucanae and NGC 6752 Authors: Sbordone, L.; Limongi, M.; Chieffi, A.; Caffau, E.; Ludwig, H. -G.; Bonifacio, P. Bibcode: 2009A&A...503..121S Altcode: 2009arXiv0904.1417S Context: The light elements Li, O, Na, Al, and Mg are known to show star-to-star variations in the globular clusters 47 Tuc and NGC 6752. Such variations are interpreted as coming from processing in a previous generation of stars.
Aims: In this paper we investigate the abundances of the α-element sulfur, for which no previous measurements exist. In fact this element has not been investigated in any Galactic globular cluster so far. The only globular cluster for which such measurements are available is Terzan 7, which belongs to the Sgr dSph.
Methods: We use high-resolution spectra of the S i Mult. 1, acquired with the UVES spectrograph at the 8.2 m VLT-Kueyen telescope, for turn-off and giant stars in the two globular clusters. The spectra were analysed making use of ATLAS static plane parallel model atmospheres and SYNTHE spectrum synthesis. We also compute 3D corrections from CO^5BOLD hydrodynamic models and apply corrections due to NLTE effects taken from the literature.
Results: In the cluster NGC 6752 sulfur has been measured only in four subgiant stars. We find no significant star-to-star scatter and a mean <[S/Fe]> = +0.49 ± 0.15, consistent with what is observed in field stars of the same metallicity. In the cluster 47 Tuc we measured S in 4 turn-off and 5 subgiant stars with a mean <[S/Fe]> = +0.18 ± 0.14. While this result is compatible with no star-to-star scatter we notice a statistically significant correlation of the sulfur abundance with the sodium abundance and a tentative correlation with the silicon abundance.
Conclusions: The sulfur-sodium correlation is not easily explained in terms of nucleosynthesis. An origin due to atomic diffusion can be easily dismissed. The correlation cannot be easily dismissed either, in view of its statistical significance, until better data for more stars is available.

Based on observations made with the ESO VLT-Kueyen telescope at the Paranal Observatory, Chile, in the course of the ESO-Large Programme 165.L-0263. Title: VizieR Online Data Catalog: Extremely metal-poor turnoff stars abundances (Bonifacio+, 2009) Authors: Bonifacio, P.; Spite, M.; Cayrel, R.; Hill, V.; Spite, F.; Francois, P.; Plez, B.; Ludwig, H. -G.; Caffau, E.; Molaro, P.; Depagne, E.; Andersen, J.; Barbuy, B.; Beers, T. C.; Nordstroem, B.; Primas, F. Bibcode: 2009yCat..35010519B Altcode: The detailed chemical abundances of extremely metal-poor (EMP) stars are key guides to understanding the early chemical evolution of the Galaxy. Most existing data, however, treat giant stars that may have experienced internal mixing later. We aim to compare the results for giants with new, accurate abundances for all observable elements in 18 EMP turnoff stars. VLT/UVES spectra at ~45000 and S/N ~130 per pixel (330-1000nm) are analysed with OSMARCS model atmospheres and the TURBOSPECTRUM code to derive abundances for C, Mg, Si, Ca, Sc, Ti, Cr, Mn, Co, Ni, Zn, Sr, and Ba. For Ca, Ni, Sr, and Ba, we find excellent consistency with our earlier sample of EMP giants, at all metallicities. However, our abundances of C, Sc, Ti, Cr, Mn and Co are ~0.2dex larger than in giants of similar metallicity. Mg and Si abundances are ~0.2dex lower (the giant [Mg/Fe] values are slightly revised), while Zn is again ~0.4dex higher than in giants of similar [Fe/H] (6 stars only). For C, the dwarf/giant discrepancy could possibly have an astrophysical cause, but for the other elements it must arise from shortcomings in the analysis. Approximate computations of granulation (3D) effects yield smaller corrections for giants than for dwarfs, but suggest that this is an unlikely explanation, except perhaps for C, Cr, and Mn. NLTE computations for Na and Al provide consistent abundances between dwarfs and giants, unlike the LTE results, and would be highly desirable for the other discrepant elements as well. Meanwhile, we recommend using the giant abundances as reference data for Galactic chemical evolution models.

(3 data files). Title: First stars XII. Abundances in extremely metal-poor turnoff stars, and comparison with the giants Authors: Bonifacio, P.; Spite, M.; Cayrel, R.; Hill, V.; Spite, F.; François, P.; Plez, B.; Ludwig, H. -G.; Caffau, E.; Molaro, P.; Depagne, E.; Andersen, J.; Barbuy, B.; Beers, T. C.; Nordström, B.; Primas, F. Bibcode: 2009A&A...501..519B Altcode: 2009arXiv0903.4174B Context: The detailed chemical abundances of extremely metal-poor (EMP) stars are key guides to understanding the early chemical evolution of the Galaxy. Most existing data, however, treat giant stars that may have experienced internal mixing later.
Aims: We aim to compare the results for giants with new, accurate abundances for all observable elements in 18 EMP turnoff stars.
Methods: VLT/UVES spectra at R ~ 45 000 and S/N ~ 130 per pixel (λλ 330-1000 nm) are analysed with OSMARCS model atmospheres and the TURBOSPECTRUM code to derive abundances for C, Mg, Si, Ca, Sc, Ti, Cr, Mn, Co, Ni, Zn, Sr, and Ba.
Results: For Ca, Ni, Sr, and Ba, we find excellent consistency with our earlier sample of EMP giants, at all metallicities. However, our abundances of C, Sc, Ti, Cr, Mn and Co are ~0.2 dex larger than in giants of similar metallicity. Mg and Si abundances are ~0.2 dex lower (the giant [Mg/Fe] values are slightly revised), while Zn is again ~0.4 dex higher than in giants of similar [Fe/H] (6 stars only).
Conclusions: For C, the dwarf/giant discrepancy could possibly have an astrophysical cause, but for the other elements it must arise from shortcomings in the analysis. Approximate computations of granulation (3D) effects yield smaller corrections for giants than for dwarfs, but suggest that this is an unlikely explanation, except perhaps for C, Cr, and Mn. NLTE computations for Na and Al provide consistent abundances between dwarfs and giants, unlike the LTE results, and would be highly desirable for the other discrepant elements as well. Meanwhile, we recommend using the giant abundances as reference data for Galactic chemical evolution models.

Based on observations obtained with the ESO Very Large Telescope at Paranal Observatory, Chile (Large Programme “First Stars”, ID 165.N-0276; P.I.: R. Cayrel, and Programme 078.B-0238; P.I.: M. Spite). Appendices A-C are only available in electronic form at http://www.aanda.org Table 7 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/501/519 Title: The solar photospheric nitrogen abundance. Analysis of atomic transitions with 3D and 1D model atmospheres Authors: Caffau, E.; Maiorca, E.; Bonifacio, P.; Faraggiana, R.; Steffen, M.; Ludwig, H. -G.; Kamp, I.; Busso, M. Bibcode: 2009A&A...498..877C Altcode: 2009arXiv0903.3406C Context: In recent years, the solar chemical abundances have been studied in considerable detail because of discrepant values of solar metallicity inferred from different indicators, i.e., on the one hand, the “sub-solar” photospheric abundances resulting from spectroscopic chemical composition analyses with the aid of 3D hydrodynamical models of the solar atmosphere, and, on the other hand, the high metallicity inferred by helioseismology.
Aims: After investigating the solar oxygen abundance using a CO^5BOLD 3D hydrodynamical solar model in previous work, we undertake a similar approach studying the solar abundance of nitrogen, since this element accounts for a significant fraction of the overall solar metallicity, Z.
Methods: We used a selection of atomic spectral lines to determine the solar nitrogen abundance, relying mainly on equivalent width measurements in the literature. We investigate the influence on the abundance analysis, of both deviations from local thermodynamic equilibrium (“NLTE effects”) and photospheric inhomogeneities (“granulation effects”).
Results: We recommend use of a solar nitrogen abundance of A(N) = 7.86 ± 0.12, whose error bar reflects the line-to-line scatter.
Conclusions: The solar metallicity implied by the CO^5BOLD-based nitrogen and oxygen abundances is in the range 0.0145≤ Z ≤ 0.0167. This result is a step towards reconciling photospheric abundances with helioseismic constraints on Z. Our most suitable estimates are Z=0.0156 and Z/X=0.0213. Title: Near-surface stellar magneto-convection: simulations for the Sun and a metal-poor solar analog Authors: Steffen, Matthias; Ludwig, H. -G.; Steiner, O. Bibcode: 2009IAUS..259..233S Altcode: 2009arXiv0902.2753S We present 2D local box simulations of near-surface radiative magneto-convection with prescribed magnetic flux, carried out with the MHD version of the CO5BOLD code for the Sun and a solar-like star with a metal-poor chemical composition (metal abundances reduced by a factor 100, [M/H] = -2). The resulting magneto-hydrodynamical models can be used to study the influence of the metallicity on the properties of magnetized stellar atmospheres. A preliminary analysis indicates that the horizontal magnetic field component tends to be significantly stronger in the optically thin layers of metal-poor stellar atmospheres. Title: Observable properties of late-type giants predicted by 3D hydrodynamical and 1D stellar atmosphere models Authors: Kucinskas, A.; Ludwig, H. -G.; Ivanauskas, A.; Caffau, E. Bibcode: 2009IAUS..254P..37K Altcode: No abstract at ADS Title: Halo chemistry and first stars. The chemical composition of the matter in the early Galaxy, from C to Mg† Authors: Spite, M.; Bonifacio, P.; Cayrel, R.; Spite, F.; Francois, P.; Ludwig, H. G.; Caffau, E.; Andrievsky, S.; Barbuy, B.; Plez, B.; Molaro, P.; Andersen, J.; Beers, T.; Depagne, E.; Nordström, B.; Primas, F. Bibcode: 2009IAUS..254..349S Altcode: From NLTE computations of the magnesium abundance in a sample of extremely metal-poor giants we derive [Mg/Fe]=+0.7, leading to [Al/Mg]=-0.80 and [Na/Mg]=-0.85 in the early Galaxy. The ratio [O/Mg] should be near to the solar value. Measurements of nitrogen abundances derived from the analysis of the NH band in eight more stars confirm the large scatter of the ratios [N/Fe] and [N/O] in the early Galaxy. 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: Effective temperatures of cool metal-poor stars derived from the analysis of 3D Balmer lines Authors: Behara, N. T.; Ludwig, H. -G.; Steffen, M.; Bonifacio, P. Bibcode: 2009AIPC.1094..784B Altcode: 2009csss...15..784B Balmer lines are recognized as accurate indicators of the effective temperature of late-type stars. The influence of convection on the shape of Balmer line profiles has been investigated using LTE 3D hydrodynamical model atmospheres and `classical' LTE 1D stellar atmospheres, where convection is modeled within the simplistic picture of mixing-length theory. Models and line profiles computed with the CO5BOLD and Linfor3D codes have been used to determine the effective temperatures of the Sun and three well known metal-poor stars HD84937, HD74000, and HD140283. Our 3D fit provides the best fit thus far for the solar Hα temperature using the Barklem theory. The resulting (3D-1D) Teff biases related to the different treatment of convection in the 1D and 3D models are presented. Title: Teff and log g dependence of FeH in M-dwarfs Authors: Wende, S.; Reiners, A.; Ludwig, H. -G. Bibcode: 2009AIPC.1094..816W Altcode: 2008arXiv0812.2146W; 2009csss...15..816W We present synthetic FeH band spectra in the z-filter range for several M-dwarf models with log g = 3.0-5.0 [cgs] and Teff = 2800 K-3450 K. Our aim is to characterize convective velocities in M-dwarfs and to give a rough estimate of the range in which 3D-atmosphere treatment is necessary and where 1D-atmosphere models suffice for the interpretation of molecular spectral features. This is also important in order to distinguish between the velocity-broadening and the rotational- or Zeeman-broadening. The synthetic spectra were calculated using 3D CO5BOLD radiative-hydrodynamic (RHD) models and the line synthesis code LINFOR3D. We used complete 3D-models and high resolution 3D spectral synthesis for the detailed study of some well isolated FeH lines. The FeH line strength shows a dependence on surface gravity and effective temperature and could be employed to measure both quantities in M-type objects. The line width is related to the velocity-field in the model stars, which depends strongly on surface gravity. Furthermore, we investigate the velocity-field in the 3D M-dwarf models together with the related micro- and macro-turbulent velocities in the 1D case. We also search for effects on the lineshapes. Title: Commission 36: Theory of Stellar Atmospheres Authors: Landstreet, John D.; Asplund, Martin; Spite, Monique; Balachandran, Suchitra B.; Berdyugina, Svetlana V.; Hauschildt, Peter H.; Ludwig, Hans G.; Mashonkina, Lyudmila I.; Nagendra, K. N.; Puls, Joachim; Randich, M. Sofia; Tautvaisiene, Grazina Bibcode: 2009IAUTA..27..222L Altcode: Commission 36 covers the whole field of the physics of stellar atmospheres. The scientific activity in this large subject has been very intense during the last triennium and led to the publication of a large number of papers, which makes a complete report quite impractical. We have therefore decided to keep the format of the preceding report: first a list of areas of current research, then Web links for obtaining further information. Title: Micro- and macroturbulence derived from 3D hydrodynamical stellar atmospheres . Authors: Steffen, M.; Ludwig, H. -G.; Caffau, E. Bibcode: 2009MmSAI..80..731S Altcode: 2009arXiv0909.2831S The theoretical prediction of micro- and macroturbulence (xi_mic and xi_mac ) as a function of stellar parameters can be useful for spectroscopic work based on 1D model atmospheres in cases where an empirical determination of xi_mic is impossible due to a lack of suitable lines and/or macroturbulence and rotational line broadening are difficult to separate. In an effort to exploit the CIFIST 3D model atmosphere grid for deriving the theoretical dependence of xi_mic and xi_mac on effective temperature, gravity, and metallicity, we discuss different methods to derive xi_mic from the numerical simulations, and report first results for the Sun and Procyon. In both cases the preliminary analysis indicates that the microturbulence found in the simulations is significantly lower than in the real stellar atmospheres. Title: The ESO Large Programme ``First Stars'' Authors: Bonifacio, P.; Andersen, J.; Andrievsky, S. M.; Barbuy, B.; Beers, T. C.; Caffau, E.; Cayrel, R.; Depagne, E.; François, P.; González Hernández, J. I.; Hansen, C. J.; Herwig, F.; Hill, V.; Korotin, S. A.; Ludwig, H. -G.; Molaro, P.; Nordström, B.; Plez, B.; Primas, F.; Sivarani, T.; Spite, F.; Spite, M. Bibcode: 2009ASSP....9...31B Altcode: 2008arXiv0801.1293B; 2009svlt.conf...31B In ESO period 65 (April-September 2000) the large programme 165.N-0276, led by Roger Cayrel, began making use of UVES at the Kueyen VLT telescope. Known within the Team and outside as "First Stars", it was aimed at obtaining high resolution, high signal-to-noise ratio spectra in the range 320 nm-1000 nm for a large sample of extremely metal-poor (EMP) stars identified from the HK objective prism survey [T.C. Beers, G.W. Preston, S.A. Shectman in Astron. J. 90, 2089 (1985); T.C. Beers, G.W. Preston, S.A. Shectman in Astron. J. 103, 1987 (1992)]. The goal was to use these spectra to determine accurate atmospheric parameters and chemical composition of these stars which are among the oldest objects amenable to our detailed study. Although these stars are not the first generation of stars they must be very close descendants of the first generation. One may hope to gain insight on the nature of the progenitors from detailed information on the descendants. 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: Accounting for convective blue-shifts in the determination of absolute stellar radial velocities. Authors: Allende Prieto, C.; Koesterke, L.; Ramírez, I.; Ludwig, H. -G.; Asplund, M. Bibcode: 2009MmSAI..80..622A Altcode: 2009arXiv0909.0470A For late-type non-active stars, gravitational redshifts and convective blueshifts are the main source of biases in the determination of radial velocities. If ignored, these effects can introduce systematic errors of the order of ∼ 0.5 km s-1. We demonstrate that three-dimensional hydrodynamical simulations of solar surface convection can be used to predict the convective blue-shifts of weak spectral lines in solar-like stars to ∼ 0.070 km s-1. Using accurate trigonometric parallaxes and stellar evolution models, the gravitational redshifts can be constrained with a similar uncertainty, leading to absolute radial velocities accurate to ∼ 0.1 km s-1. Title: Effects of granulation on neutral copper resonance lines in metal-poor stars Authors: Bonifacio, P.; Caffau, E.; Ludwig, H. -G. Bibcode: 2009MmSAI..80..739B Altcode: 2009arXiv0910.4730B We make use of three dimensional hydrodynamical simulations to investigate the effects of granulation on the Cu I lines of Mult. 1 in the near UV, at 324.7 nm and 327.3 nm. These lines remain strong even at very low metallicity and provide the opportunity to study the chemical evolution of Cu in the metal-poor populations. We find very strong granulation effects on these lines. In terms of abundances the neglect of such effects can lead to an overestimate of the A(Cu) by as much as 0.8 dex in dwarf stars. Comparison of our computations with stars in the metal-poor Globular Clusters NGC 6752 and NGC 6397, show that there is a systematic discrepancy between the copper abundances derived from Mult. 2 in TO stars and those derived in giant stars of the same cluster from the lines of Mult. 2 at at 510.5 nm and 587.2 nm. We conclude that the Cu I resonance lines are not reliable indicators of Cu abundance and we believe that an investigations of departures from LTE is mandatory to make use of these lines. Title: Solar abundances and granulation effects Authors: Caffau, E.; Ludwig, H. -G.; Steffen, M. Bibcode: 2009MmSAI..80..643C Altcode: 2009arXiv0910.4733C The solar abundances have undergone a major downward revision in the last decade, reputedly as a result of employing 3D hydrodynamical simulations to model the inhomogeneous structure of the solar photosphere. The very low oxygen abundance advocated by \citet{asplund04}, A(O)=8.66, together with the downward revision of the carbon and nitrogen abundances, has created serious problems for solar models to explain the helioseismic measurements.

In an effort to contribute to the dispute we have re-derived photospheric abundances of several elements independently of previous analysis. We applied a state-of-the art 3D (CO5BOLD) hydrodynamical simulation of the solar granulation as well as different 1D model atmospheres for the line by line spectroscopic abundance determinations. The analysis is based on both standard disc-centre and disc-integrated spectral atlases; for oxygen we acquired in addition spectra at different heliocentric angles. The derived abundances are the result of equivalent width and/or line profile fitting of the available atomic lines. We discuss the different granulation effects on solar abundances and compare our results with previous investigations. According to our investigations hydrodynamical models are important in the solar abundance determination, but are not responsible for the recent downward revision in the literature of the solar metallicity. Title: NLTE Abundances of Sodium, Magnesium and Barium in the Globular Clusters M10 and M71 Authors: Mishenina, T. V.; Kučinskas, A.; Andrievsky, S. M.; Korotin, S. A.; Dobrovolskas, V.; Ivanauskas, A.; Caffau, E.; Ludwig, H. -G.; Steffen, M.; Sperauskas, J.; Klochkova, V. G.; Panchuk, V. E. Bibcode: 2009BaltA..18..193M Altcode: 2009OAst...18..193M We derive NLTE abundances of Na, Mg and Ba in four late-type giants belonging to globular clusters M10 and M71. The obtained relative [Na/Fe] ratios, which were measured only in M10, are positive, with the average value [Na/Fe] = +0.3. The ratios [Mg/Fe] in both clusters are supersolar, +0.15 to +0.28, while [Ba/Fe] scatter between --0.14 and +0.09. Differences between the NLTE abundances derived in this work and those obtained in LTE by Mishenina et al. (2003) are small, typically within ±0.1 dex. We also perform numerical simulations with the CO5BOLD 3D hydrodynamical stellar atmosphere code to investigate the influence of convection on the formation of spectral lines used in our NLTE study. For this purpose we use a model of late-type giant with T eff = 4020 K, log g = 1.0, [M/H] = --1.0 and find that for Na, Mg and Ba the 3D--1D abundance corrections are below ∼ 0.02 dex. However, their size strongly depends on the value of microturbulent velocity used with the 1D model. 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: 3D hydrodynamical simulations of stellar photospheres with the CO5BOLD code. Photometric colors of a late-type giant Authors: Kučinskas, A.; Ludwig, H. -G.; Caffau, E.; Steffen, M. Bibcode: 2009MmSAI..80..723K Altcode: 2009arXiv0910.3412K We present synthetic broad-band photometric colors of a late-type giant located close to the RGB tip (T_eff≈3640 K, log g=1.0 and [M/H]=0.0). Johnson-Cousins-Glass BVRIJHK colors were obtained from the spectral energy distributions calculated using 3D hydrodynamical and 1D classical stellar atmosphere models. The differences between photometric magnitudes and colors predicted by the two types of models are significant, especially at optical wavelengths where they may reach, e.g., Delta V≈0.16, Delta R≈0.13 and Delta (V-I)≈0.14, Delta (V-K)≈0.20. Differences in the near-infrared are smaller but still non-negligible (e.g., Delta K≈ 0.04). Such discrepancies may lead to noticeably different photometric parameters when these are inferred from photometry (e.g., effective temperature will change by Delta T_eff≈60 K due to difference of Delta (V-K)≈0.20). Title: 3D views on cool stellar atmospheres: theory meets observation Authors: Nagendra, K. N.; Bonifacio, P.; Ludwig, H. -G. Bibcode: 2009MmSAI..80..601N Altcode: No abstract at ADS Title: 3D molecular line formation in dwarf carbon-enhanced metal-poor stars. Authors: Behara, N. T.; Ludwig, H. -G.; Bonifacio, P.; Sbordone, L.; González Hernández, J. I.; Caffau, E. Bibcode: 2009MmSAI..80..735B Altcode: 2009arXiv0909.1010B We present a detailed analysis of the carbon and nitrogen abundances of two dwarf carbon-enhanced metal-poor (CEMP) stars: SDSS J1349-0229 and SDSS J0912+0216. We also report the oxygen abundance of SDSS J1349-0229. These stars are metal-poor, with [Fe/H] < -2.5, and were selected from our ongoing survey of extremely metal-poor dwarf candidates from the Sloan Digital Sky Survey (SDSS). The carbon, nitrogen and oxygen abundances rely on molecular lines which form in the outer layers of the stellar atmosphere. It is known that convection in metal-poor stars induces very low temperatures which are not predicted by `classical' 1D stellar atmospheres. To obtain the correct temperature structure, one needs full 3D hydrodynamical models. Using CO5BOLD 3D hydrodynamical model atmospheres and the Linfor3D line formation code, molecular lines of CH, NH, OH and C_2 were computed, and 3D carbon, nitrogen and oxygen abundances were determined. The resulting carbon abundances were compared to abundances derived using atomic C I lines in 1D LTE and NLTE. For one star, SDSS J1349-0229, we were able to compare the 3D oxygen abundance from OH lines to O I lines in 1D LTE and NLTE. There is not a good agreement between the carbon abundances determined from C_2 bands and from the CH band, and molecular lines do not agree with the atomic C I lines. Although this may be partly due to uncertainties in the transition probabilities of the molecular bands it certainly has to do with the temperature structure of the outer layers of the adopted model atmosphere. In fact the discrepancy between C_2 and CH is in opposite directions when using 3D and 1D models. Confronted with this inconsistency, we explore the influence of the 3D model properties on the molecular abundance determination. In particular, the choice of the number of opacity bins used in the model calculations and its subsequent effects on the temperature structure and molecular line formation is discussed. 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: Extremely metal-poor stars from the SDSS Authors: Ludwig, H. -G.; Bonifacio, P.; Caffau, E.; Behara, N. T.; González Hernández, J. I.; Sbordone, L. Bibcode: 2008PhST..133a4037L Altcode: 2008arXiv0809.2948L We give a progress report on the activities within the CIFIST Team related to the search for extremely metal-poor (EMP) stars in the Sloan Digital Sky Survey's (SDSS) spectroscopic catalogue. So far, the search has provided 25 candidates with metallicities around or smaller than -3. For 15 candidates, high-resolution spectroscopy with UVES at the VLT has confirmed their EMP status. Work is under way to extend the search to the SDSS's photometric catalogue by augmenting the SDSS photometry and by gauging the capabilities of X-shooter when going to significantly fainter targets. Title: Modeling stochastic excitation of acoustic modes in stars: present status and perspectives Authors: Samadi, R.; Belkacem, K.; Goupil, M. -J.; Ludwig, H. -G.; Dupret, M. -A. Bibcode: 2008CoAst.157..130S Altcode: Solar-like oscillations have now been detected for more than ten years and their frequencies measured for a still growing number of stars with various characteristics (e.g. mass, chemical composition, evolutionary stage ...). Excitation of such oscillations is attributed to turbu- lent convection and takes place in the uppermost part of the convective envelope. Since the pioneering work of Goldreich & Keely (1977), more sophisticated theoretical models of stochastic excitation were developed, which differ from each other both by the way turbulent convection is modeled and by the assumed sources of excitation. We briefly review here the different underlying approximations and assumptions of those models. A second part shows that computed mode excitation rates crucially depend on the way time-correlations between eddies are described but also on the surface metal abundance of the star. 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: Radiation-hydrodynamics simulations of surface convection in low-mass stars: connections to stellar structure and asteroseismology Authors: Ludwig, Hans-G.; Caffau, Elisabetta; Kučinskas, A. Bibcode: 2008IAUS..252...75L Altcode: 2008arXiv0809.2939L Radiation-hydrodynamical simulations of surface convection in low-mass stars can be exploited to derive estimates of i) the efficiency of the convective energy transport in the stellar surface layers; ii) the convection-related photometric micro-variability. We comment on the universality of the mixing-length parameter, and point out potential pitfalls in the process of its calibration which may be in part responsible for the contradictory findings about its variability across the Hertzsprung-Russell digramme. We further comment on the modelling of the photometric micro-variability in HD 49933 one of the first main COROT targets. Title: 3D model atmospheres and the solar photospheric oxygen abundance Authors: Caffau, E.; Ludwig, H. -G. Bibcode: 2008IAUS..252...35C Altcode: In recent years the photospheric solar oxygen abundance experienced a significant downward revision. However, a low photospheric abundance is incompatible with the value in the solar interior inferred from helioseismology. For contributing to the dispute whether the solar oxygen abundance is “high” or “low”, we re-derived its photospheric abundance independently of previous analyses. We applied 3D (CO5BOLD) as well as 1D model atmospheres. We considered standard disc-centre and disc-integrated spectral atlases, as well as newly acquired solar intensity spectra at different heliocentric angles. We determined the oxygen abundances from equivalent width and/or line profile fitting of a number of atomic lines. As preliminary result, we find an oxygen abundance in the range 8.73 8.79, encompassing the value obtained by Holweger (2001), and somewhat higher than the value obtained by Asplund et al. (2005). Title: The Solar Photospheric Oxygen Abundance and the Role of 3D Model Atmospheres Authors: Caffau, E.; Steffen, M.; Ludwig, H. -G. Bibcode: 2008ESPM...12..3.7C Altcode: The solar oxygen abundance has undergone a major downward revision in the last decade, reputedly as a result of employing 3D hydrodynamical simulations to model the inhomogeneous structure of the solar photosphere.

The very low oxygen abundance advocated by Asplund et al. 2004, A(O)=8.66, together with the downward revision of the abundances of other key elements, has created serious problems for solar models to explain the helioseismic measurements.

In an effort to contribute to the dispute of whether the Sun has "solar" or "sub-solar" abundances, we have re-derived its photospheric abundance of oxygen, nitrogen, and other elements, independently of previous analyses.

We applied a state-of-the art 3D (CO5BOLD) hydrodynamical simulation of the solar granulation as well as different 1D model atmospheres for the line by line spectroscopic abundance determinations. The analysis is based on both standard disk-center and full-disk spectral atlases; for oxygen we acquired in addition spectra at different heliocentric angles. The derived abundances are the result of equivalent width and/or line profile fitting of the available atomic lines. Our recommended oxygen abundance is A(O)=8.76+- 0.07, 0.1 dex higher than the value of Asplund et al. (2004). Our current estimate of the overall solar metallicity is 0.014< Z<0.016.

Questions we discuss include: (i) Is the general downward revision of the solar abundances a 3D effect? (ii) How large are the abundance corrections due to horizontal inhomogeneities? (iii) What is the main reason for the differences between the abundances obtained in our study and those derived by Apslund and coworkers? (iv) How large are the uncertainties in the observed solar spectra? (v) What is the reason why the two forbidden oxygen lines, [OI] lambda 630 nm and [OI] lambda 636.3 nm, give significantly different answers for the solar oxygen abundance? 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: The solar photospheric abundance of hafnium and thorium. Results from CO5BOLD 3D hydrodynamic model atmospheres Authors: Caffau, E.; Sbordone, L.; Ludwig, H. -G.; Bonifacio, P.; Steffen, M.; Behara, N. T. Bibcode: 2008A&A...483..591C Altcode: 2008arXiv0803.3585C Context: The stable element hafnium (Hf) and the radioactive element thorium (Th) were recently suggested as a suitable pair for radioactive dating of stars. The applicability of this elemental pair needs to be established for stellar spectroscopy.
Aims: We aim at a spectroscopic determination of the abundance of Hf and Th in the solar photosphere based on a CO5BOLD 3D hydrodynamical model atmosphere. We put this into a wider context by investigating 3D abundance corrections for a set of G- and F-type dwarfs.
Methods: High-resolution, high signal-to-noise solar spectra were compared to line synthesis calculations performed on a solar CO5BOLD model. For the other atmospheres, we compared synthetic spectra of CO5BOLD 3D and associated 1D models.
Results: For Hf we find a photospheric abundance A(Hf) = 0.87 ± 0.04, in good agreement with a previous analysis, based on 1D model atmospheres. The weak Th II 401.9 nm line constitutes the only Th abundance indicator available in the solar spectrum. It lies in the red wing of a Ni-Fe blend exhibiting a non-negligible convective asymmetry. Accounting for the asymmetry-related additional absorption, we obtain A(Th) = 0.08 ± 0.03, consistent with the meteoritic abundance, and about 0.1 dex lower than obtained in previous photospheric abundance determinations.
Conclusions: Only for the second time, to our knowledge, has a non-negligible effect of convective line asymmetries on an abundance derivation been highlighted. Three-dimensional hydrodynamical simulations should be employed to measure Th abundances in dwarfs if similar blending is present, as in the solar case. In contrast, 3D effects on Hf abundances are small in G- to mid F-type dwarfs and sub-giants, and 1D model atmospheres can be conveniently used. Title: Radiation-hydrodynamical model atmospheres across the Hertzsprung-Russell diagram Authors: Ludwig, H. -G. Bibcode: 2008iac..talk..177L Altcode: 2008iac..talk....1L No abstract at ADS Title: Hydrodynamical Model Atmospheres of Metal-Poor Stars Authors: Ludwig, Hans-Günter; González Hernández, Jonay I.; Behara, Natalie; Caffau, Elisabetta; Steffen, Matthias Bibcode: 2008AIPC..990..268L Altcode: Standard one-dimensional (1D) model atmospheres rely on the assumption of radiative equilibrium in the non-convective part of the stellar photosphere. However, gas-dynamical effects can lead to dramatic deviations from radiative equilibrium conditions, especially in metal-poor stellar atmospheres. These can be taken into account in 3D stellar atmosphere models representing the detailed interplay of hydrodynamics and radiation. During the last two years efforts have been invested to compute such 3D models for metal-poor atmospheres with the CO5 BOLD code within the CIFIST (Cosmological Impact of the FIrst STars) Team, an European Union funded research group dedicated to the study of metal-poor stars. Based on the available models we will give an account of the radiation-hydrodynamical processes at work, and discuss consequences for the temperature scale and abundance analysis of metal-poor stars. Title: First stars XI. Chemical composition of the extremely metal-poor dwarfs in the binary CS 22876-032 Authors: González Hernández, J. I.; Bonifacio, P.; Ludwig, H. -G.; Caffau, E.; Spite, M.; Spite, F.; Cayrel, R.; Molaro, P.; Hill, V.; François, P.; Plez, B.; Beers, T. C.; Sivarani, T.; Andersen, J.; Barbuy, B.; Depagne, E.; Nordström, B.; Primas, F. Bibcode: 2008A&A...480..233G Altcode: 2007arXiv0712.2949G Context: Unevolved metal-poor stars constitute a fossil record of the early Galaxy, and can provide invaluable information on the properties of the first generations of stars. Binary systems also provide direct information on the stellar masses of their member stars.
Aims: The purpose of this investigation is a detailed abundance study of the double-lined spectroscopic binary CS 22876-032, which comprises the two most metal-poor dwarfs known.
Methods: We used high-resolution, high-S/N ratio spectra from the UVES spectrograph at the ESO VLT telescope. Long-term radial-velocity measurements and broad-band photometry allowed us to determine improved orbital elements and stellar parameters for both components. We used OSMARCS 1D models and the turbospectrum spectral synthesis code to determine the abundances of Li, O, Na, Mg, Al, Si, Ca, Sc, Ti, Cr, Mn, Fe, Co and Ni. We also used the CO^5BOLD model atmosphere code to compute the 3D abundance corrections, notably for Li and O.
Results: We find a metallicity of [Fe/H] ~ -3.6 for both stars, using 1D models with 3D corrections of ~-0.1 dex from averaged 3D models. We determine the oxygen abundance from the near-UV OH bands; the 3D corrections are large, -1 and -1.5 dex for the secondary and primary respectively, and yield [O/Fe] ~ 0.8, close to the high-quality results obtained from the [OI] 630 nm line in metal-poor giants. Other [ α/Fe] ratios are consistent with those measured in other dwarfs and giants with similar [Fe/H], although Ca and Si are somewhat low ([X/Fe] ⪉ 0). Other element ratios follow those of other halo stars. The Li abundance of the primary star is consistent with the Spite plateau, but the secondary shows a lower abundance; 3D corrections are small.
Conclusions: The Li abundance in the primary star supports the extension of the Spite Plateau value at the lowest metallicities, without any decrease. The low abundance in the secondary star could be explained by endogenic Li depletion, due to its cooler temperature. If this is not the case, another, yet unknown mechanism may be causing increased scatter in A(Li) at the lowest metallicities. Title: CS 22876-032: The Most Metal-Poor Dwarfs. Abundances and 3D Effects Authors: González Hernández, J. I.; Bonifacio, P.; Ludwig, H. -G.; Caffau, E.; Spite, M.; Spite, F.; Cayrel, R.; Molaro, P.; Hill, V.; François, P.; Plez, B.; Beers, T. C.; Sivarani, T.; Andersen, J.; Barbuy, B.; Depagne, E.; Nordström, B.; Primas, F. Bibcode: 2008AIPC..990..175G Altcode: 2008AIPC..990..175H Unevolved extremely metal-poor stars offer us a unique tool to infer knowledge of the first generation of stars. We have analysed UVES high-resolution spectra of the double-lined spectroscopic binary CS 22876-032 which comprises the two most metal-poor dwarfs currently known. In particular, we determine the oxygen (from OH lines in the near-UV) and lithium abundances taking into account 3D effects.

The long-time baseline radial velocity measurements and photometric data available allowed us to determine the orbital elements as well as stellar parameters of both components. We use OSMARCS 1D models and the TURBOSPECTRUM spectral synthesis code to determine the abundances of Li, O, Na, Mg, Al, Si, Ca, Sc, Ti, Cr, Mn, Fe, Co and Ni. We also use the CO5 BOLD 3D model atmosphere code to predict the 3D abundance corrections, mainly for Li, O and Fe.

We find a metallicity of [Fe/H]~-3.6 for both stars using 1D models with 3D corrections of ~-0.1 dex from horizontal and temporal averaged 3D models. The [α/Fe] ratios are consistent with those found for metal-poor giants with similar [Fe/H], although Ca and Si are rather low, [X/Fe]~=0. The 1D O abundance, [O/Fe]~2 for both stars, is very large, but 3D models predict abundance corrections of roughly -1.0 dex and -1.5 dex for the secondary and primary stars, respectively. These 3D corrections bring the O abundances derived from near-UV OH bands in these two dwarfs closer to other high-quality measurements from the forbidden [OI] 630 nm line in metal-poor giants. The Li abundance is consistent with the Spite plateau, although the secondary star shows a lower abundance. Title: The Metal-Poor End of the Lithium Plateau Authors: Sbordone, L.; Bonifacio, P.; González Hernández, J. I.; Cayrel, R.; Behara, N.; Molaro, P.; Plez, B.; Francois, P.; Christlieb, N.; Ludwig, H. -G.; Sivarani, T.; Beers, T. C.; van't Veer, C. Bibcode: 2008AIPC..990..339S Altcode: We present our current sample of Lithium abundances in 28 low metallicity dwarf and Turn Off (TO) stars ([Fe/H] between -2.5 and -3.5), based on high resolution, high signal to noise echelle spectra. Nine new stars have been added to the Bonifacio et al. [1] sample, and the full sample has been reanalyzed in order to take into account the effect of two different possible temperature scales. In fact, the Li abundance measurement based on the 670.8 nm line is highly sensitive to temperature, and Teff scales are still poorly calibrated at low metallicities. First, the effective temperature has been derived from Hα profile fitting, and second, directly from the star's infrared flux. The two methods offer similar precision but are affected by different uncertainties and systematics. The infrared flux method (IRFM) leads to a larger Teff dispersion than the Hα profile fitting, while also producing an offset of about 150 K towards hotter temperatures. This leads to a contraction of the metallicity scale of the sample, which encompasses [Fe/H] = -3.7 to -2.5 when using Hα calibrated temperatures, and [Fe/H] = -3.4 to -2.5 when using IRFM. The higher average IRFM temperature increases somewhat the mean Li abundance, changing from A(Li)Hα = 2.10 to A(Li)IRFM = 2.18 Title: Hydrodynamical Model Atmospheres and 3D Spectral Synthesis Authors: Ludwig, Hans-Günter; Steffen, Matthias Bibcode: 2008psa..conf..133L Altcode: 2007arXiv0704.1176L In this paper we discuss three issues in the context of three-dimensional (3D) hydrodynamical model atmospheres for late-type stars, related to spectral line shifts, radiative transfer in metal-poor 3D models, and the solar oxygen abundance. To establish the context we start by giving a brief overview about the model construction, taking the radiation-hydrodynamics code CO5BOLD (Conservative COde for the Computation of COmpressible COnvection in a BOx of L Dimensions with L=2,3; [3]) and the related spectral synthesis package Linfor3D as examples. Title: Spectral analyses of three carbon-enhanced metal-poor stars Authors: Behara, N.; Bonifacio, P.; Ludwig, H. G.; Sbordone, L.; Gonzales Hernandez, J. I.; Caffau, E. Bibcode: 2008nuco.confE..68B Altcode: 2008arXiv0809.4204B; 2008PoS....53E..68B We are conducting a high-resolution follow-up of candidate EMP stars extracted from the Sloan Digital Sky Survey (SDSS; York et al. 2000) using UVES at the VLT. Three of the programme stars, SDSS J0912+0216, SDSS J1036+1212 and SDSS J1349-0229, where deliberately targetted as CEMP stars since a strong $G$ band was evident from the SDSS spectra and the weakness of the Ca {\sc ii} K line testified their very low metallicity. The UVES high resolution follow-up confirmed the original findings ([Fe/H] $<-2.50$) and allowed a more detailed investigation of their chemical composition. We determined the carbon abundance from molecular lines which form in the outer layers of the stellar atmosphere. It is known that convection in metal-poor stars induces very low temperatures which are not predicted by classical 1D stellar atmospheres. To obtain the correct temperature structure, one needs full 3D hydrodynamical models. 3D carbon abundances were determined for all three stars, using CO$^5$BOLD 3D hydrodynamical model atmospheres. 3D effects on the carbon abundance are found to be quite significant for these stars, with 3D corrections of up to --0.7 dex. Two of the stars, SDSS J0912+0216 and SDSS J1349-0229 exhibit an overabundance of neutron capture elements which classifies them as CEMP-s. Star SDSS J1036+1212, instead belongs to the elusive class of CEMP-no/s stars, with enhanced Ba, but deficient Sr, of which it is the third member discovered to date. Title: Overview of the Li problem in metal-poor stars and new results on 6Li Authors: Cayrel, R.; Steffen, M.; Bonifacio, P.; Ludwig, H. -G.; Caffau, E. Bibcode: 2008nuco.confE...2C Altcode: 2008arXiv0810.4290C; 2008PoS....53E...2C Two problems are discussed here. The first one is the 0.4 dex discrepancy between the 7Li abundance derived from the spectra of metal-poor halo stars on the one hand, and from Big Bang nucleosynthesis, based on the cosmological parameters constrained by the WMAP measurements, on the other hand. Lithium, indeed, can be depleted in the convection zone of unevolved stars. The understanding of the hydrodynamics of the crucial zone near the bottom of the convective envelope in dwarfs or turn-off stars of solar metallicity has recently made enormous progress with the inclusion of internal gravity waves. However, similar work for metal-poor stars is still lacking. Therefore it is not yet clear whether the depletion occurring in the metal-poor stars themselves is adequate to produce a 7Li plateau. The second problem pertains to the large amount of 6Li recently found in metal-poor halo stars. The convection-related asymmetry of the 7Li line could mimic the signal attributed so far to the weak blend of 6Li in the red wing of the 7Li line. Theoretical computations show that the signal generated by the asymmetry of 7Li is 2.0, 2.1, and 3.7 per cent for [Fe/H]= -3.0, -2.0, -1.0, respectively (Teff =6250 K and log g=4.0 [cgs]). In addition we re-investigate the statistical properties of the 6Li plateau and show that previous analyses were biased. Our conclusion is that the 6Li plateau can be reinterpreted in terms of intrinsic line asymmetry, without the need to invoke a contribution of 6Li. (abridged) Title: Limb Darkening: Getting Warmer Authors: Aufdenberg, J. P.; Ludwig, H. -G.; Kervella, P.; Mérand, A.; Ridgway, S. T.; Coudé du Foresto, V.; sten Brummelaar, T. A.; Berger, D. H.; Sturmann, J.; Turner, N. H. Bibcode: 2008poii.conf...71A Altcode: 2008poio.conf...71A We present interferometric observations and model atmosphere analyses of three stars: the F-type subgiant Procyon, the A-type supergiant Deneb, and the B-type supergiant Rigel. We use VLTI/VINCI and Mark III observations of Procyon to test recent multiwavelength limb-darkening predictions from 3-D hydrodynamic atmosphere simulations with no free parameters for convection. We also investigate the effects of different 1-D atmospheric convection treatments on limb-darkening predictions. We show that the 3-D model predictions are confirmed and we find that 1-D models fail to reproduce Procyon's UV spectral energy distribution, a result consistent with models of granulation for Procyon's surface. We use observations employing the longest baselines of the CHARA Array together with the FLUOR beam combiner to determine precise angular diameters for the two early-type supergiants and test limb-darkening predictions from expanding atmosphere models of these tars' stellar winds. For Deneb, we derive angular diameters consistent with previous measurements, but which vary with position angle at the ≃3% level. Observations of the 2nd lobe of Deneb's visibility curve are more consistent with expanding atmosphere predictions than hydrostatic atmosphere predictions. For Rigel, we derive from the CHARA/FLUOR observations a limb-darkened angular diameter consistent with a recent VLTI/IONIC measurement and 8% larger than reported from the Intensity Interferometer. Title: Towards the Interferometric Imaging of Red Supergiants Authors: Ludwig, Hans-Günter; Beckers, Jacques Bibcode: 2008poii.conf..485L Altcode: 2008poio.conf..485L No abstract at ADS Title: The solar photospheric abundance of phosphorus: results from CO^5BOLD 3D model atmospheres Authors: Caffau, E.; Steffen, M.; Sbordone, L.; Ludwig, H. -G.; Bonifacio, P. Bibcode: 2007A&A...473L...9C Altcode: 2007arXiv0708.1607C Aims:We determine the solar abundance of phosphorus using CO^5BOLD 3D hydrodynamic model atmospheres.
Methods: High-resolution, high signal-to-noise solar spectra of the P i lines of Multiplet 1 at 1051-1068 nm are compared to line-formation computations performed on a CO^5BOLD solar model atmosphere.
Results: We find A(P) = 5.46 ± 0.04, in good agreement with previous analyses based on 1D model atmospheres, due to the P i lines of Mult. 1 not being affected much by 3D effects. We cannot confirm an earlier claim by other authors of a downward revision of the solar P abundance by 0.1 dex when employing a 3D model atmosphere. Concerning other stars, we find modest (<0.1 dex) 3D abundance corrections for P among four F-dwarf model atmospheres of different metallicities, and these corrections are largest at lowest metallicity.
Conclusions: We conclude that 3D abundance corrections are generally rather small for the P i lines studied in this work. They are marginally relevant for metal-poor stars, but may be neglected in the Sun.

Tables 2-4 are only available in electronic form at http://www.aanda.org Title: Line shift, line asymmetry, and the ^6Li/^7Li isotopic ratio determination Authors: Cayrel, R.; Steffen, M.; Chand, H.; Bonifacio, P.; Spite, M.; Spite, F.; Petitjean, P.; Ludwig, H. -G.; Caffau, E. Bibcode: 2007A&A...473L..37C Altcode: 2007arXiv0708.3819C Context: Line asymmetries are generated by convective Doppler shifts in stellar atmospheres, especially in metal-poor stars, where convective motions penetrate to higher atmospheric levels. Such asymmetries are usually neglected in abundance analyses. The determination of the ^6Li/^7Li isotopic ratio is prone to suffering from such asymmetries, as the contribution of ^6Li is a slight blending reinforcement of the red wing of each component of the corresponding ^7Li line, with respect to its blue wing.
Aims: The present paper studies the halo star HD 74000 and estimates the impact of convection-related asymmetries on the Li isotopic ratio determination.
Methods: Two methods are used to meet this aim. The first, which is purely empirical, consists in deriving a template profile from another element that can be assumed to originate in the same stellar atmospheric layers as Li I, producing absorption lines of approximately the same equivalent width as individual components of the ^7Li I resonance line. The second method consists in conducting the abundance analysis based on NLTE line formation in a 3D hydrodynamical model atmosphere, taking into account the effects of photospheric convection.
Results: The results of the first method show that the convective asymmetry generates an excess absorption in the red wing of the ^7Li absorption feature that mimics the presence of ^6Li at a level comparable to the hitherto published values. This opens the possibility that only an upper limit on ^6Li/^7Li has thus far been derived. The second method confirms these findings.
Conclusions: From this work, it appears that a systematic reappraisal of former determinations of ^6Li abundances in halo stars is warranted.

Based on observations carried out at the European Southern Observatory (ESO), under prog. ID 75.D-0600. Tables 1-3, and additional references are only available in electronic form at http://www.aanda.org Title: 3D spectral synthesis and rotational line broadening Authors: Ludwig, H. -G. Bibcode: 2007A&A...471..925L Altcode: 2007arXiv0707.3347L Context: Spectral synthesis calculations based on three-dimensional stellar atmosphere models are limited by the affordable angular resolution of the radiation field. This hampers an accurate treatment of rotational line broadening.
Aims: We aim to find a treatment of rotational broadening of a spherical star when the radiation field is only available at a modest number of limb-angles.
Methods: We apply a combination of analytical considerations of the line-broadening process and numerical tests.
Results: We obtain a method which is closely related to classical flux convolution and which performs noticeably better than a previously suggested procedure. It can be applied to rigid as well as differential rotation. 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: Sulphur abundances from the S i near-infrared triplet at 1045 nm Authors: Caffau, E.; Faraggiana, R.; Bonifacio, P.; Ludwig, H. -G.; Steffen, M. Bibcode: 2007A&A...470..699C Altcode: 2007arXiv0704.2335C Context: Unlike silicon and calcium, sulphur is an α-element that does not form dust. Some of the available observations of the evolution of sulphur with metallicity indicate an increased scatter of sulphur-to-iron ratios at low metallicities or even a bimodal distribution, with some stars showing constant S/Fe at all metallicities and others showing an increasing S/Fe ratio with decreasing metallicity. In metal-poor stars S i lines of Multiplet 1 at 920 nm are not yet too weak to permit the measurement of the sulphur abundance A(S); however, in ground-based observations they are severely affected by telluric lines.
Aims: We investigate the possibility of measuring sulphur abundances from S iMult. 3 at 1045 nm lines. These lie in the near infrared and are slightly weaker than those of Mult. 1, but lie in a range not affected by telluric lines.
Methods: We investigated the lines of Mult. 3 in the Sun (G2V), Procyon (F5V), HD 33256 (F5V), HD 25069 (G9V), and ɛ Eri (HD 22049, K2V). For the Sun and Procyon the analysis was performed with CO^5BOLD 3D hydrodynamical model atmospheres, while the three other stars, for which hydrodynamical simulations are not available, were analysed using 1D model atmospheres.
Results: For our sample of stars we find a global agreement between A(S) from lines of different multiplets.
Conclusions: Our results suggest that the infrared lines of Mult. 3 are a viable indicator of the sulphur abundance that, because of the intrinsic strength of this multiplet, should be suitable for studying the trend of [S/Fe] at low metallicities.

Based on data from the UVES Paranal Observatory Project (ESO DDT Program ID 266.D-5655). Appendix is only available in electronic form at http://www.aanda.org 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: The forbidden 1082 nm line of sulphur:. the photospheric abundance of sulphur in the Sun and 3D effects Authors: Caffau, E.; Ludwig, H. -G. Bibcode: 2007A&A...467L..11C Altcode: 2007astro.ph..3423C Context: Sulphur is an element which is formed in the α-process and is easily measured in the gaseous phase in external galaxies. Since it does not form dust, it is the preferred indicator for α-elements, rather than Si or Mg, for which dust corrections are necessary. The measurement of the sulphur abundance in stars is not an easy task, relying mainly on high excitation lines with non-negligible deviations from LTE. The 1082 nm sulphur forbidden transition is less sensitive to departures from LTE and is less dependent on temperature uncertainties than other sulphur lines usually employed as abundance indicators. Therefore it should provide a more robust abundance diagnostics.
Aims: To derive the solar photospheric abundance of sulphur from the 1082 nm [SI] line and to investigate 3D effects present in G- and F-type atmospheres at solar and lower metallicity.
Methods: High-resolution, high signal-to-noise solar intensity and flux spectra were used to measure the sulphur abundance from the [SI] 1082 nm line. CO^5BOLD hydrodynamical model atmospheres were applied to predict 3D abundance corrections for the [SI] line.
Results: The solar sulphur abundance is derived to be 7.15± (0.01)_stat ± (0.05)_sys, where the statistical uncertainty represents the scatter in the determination using four different solar spectra and the systematic uncertainty is due to the modelling of the blending lines. Sulphur abundances obtained from this line are insensitive to the micro-turbulence. 3D abundance corrections, found from strictly differential comparisons between 1D and 3D models, are negligible in the Sun, but become sizable for more metal-poor dwarfs. Title: Prospects of using simulations to study the photospheres of brown dwarfs Authors: Ludwig, Hans-Günter Bibcode: 2007IAUS..239..205L Altcode: 2007arXiv0704.1296L We discuss prospects of using multi-dimensional time-dependent simulations to study the atmospheres of brown dwarfs and extrasolar giant planets, including the processes of convection, radiation, dust formation, and rotation. We argue that reasonably realistic simulations are feasible, however, separated into two classes of local and global models. Numerical challenges are related to potentially large dynamic ranges, and the treatment of scattering of radiation in multi-D geometries. Title: Energy transport, overshoot, and mixing in the atmospheres of M-type main- and pre-main-sequence objects Authors: Ludwig, H. -G.; Allard, F.; Hauschildt, P. H. Bibcode: 2006A&A...459..599L Altcode: 2006astro.ph..8264L We constructed hydrodynamical model atmospheres for mid M-type main-, as well as pre-main-sequence (PMS) objects. Despite the complex chemistry encountered in these cool atmospheres a reasonably accurate representation of the radiative transfer is possible, even in the context of time-dependent and three-dimensional models. The models provide detailed information about the morphology of M-type granulation and statistical properties of the convective surface flows. In particular, we determined the efficiency of the convective energy transport, and the efficiency of mixing by convective overshoot. The convective transport efficiency was expressed in terms of an equivalent mixing-length parameter α_MLT in the formulation of mixing-length theory (MLT) given by Mihalas (1978). α_MLT amounts to values around ≈2 for matching the entropy of the deep, adiabatically stratified regions of the convective envelope, and lies between 2.5 and 3.0 for matching the thermal structure of the deep photosphere. For current spectral analysis of PMS objects this implies that MLT models based on α_MLT=2.0 overestimate the effective temperature by 100 K and surface gravities by 0.25 dex. The average thermal structure of the formally convectively stable layers is little affected by convective overshoot and wave heating, i.e., stays close to radiative equilibrium conditions. Our models suggest that the rate of mixing by convective overshoot declines exponentially with geometrical distance to the Schwarzschild stability boundary. It increases at given effective temperature with decreasing gravitational acceleration. Title: Comments on the granulation background in the Sun, Procyon, and η Bootis Authors: Ludwig, H. -G. Bibcode: 2006IAUJD..17E..24L Altcode: We performed radiation-hydrodynamical simulations to derive estimates for the background signal due to granulation in the Sun, Procyon, and η Boo. We present an intercomparison of the theoretical results, and discuss them in the light of recent observational findings, in particular of the MOST photometry mission. Title: Dust in the atmospheres of brown dwarfs and young planets: the effects of gravitational settling and convective overshoot Authors: Homeier, D.; Ludwig, H. -G.; Allard, F.; Hauschildt, P.; Dehn, M. Bibcode: 2006IAUS..232..328H Altcode: Dwarfs of the spectral types late-M, L and T span mass regimes from very-low-mass stars through brown dwarfs down to young planetary objects. They all show massive molecular line-blanketing and the condensation of refractory species with decreasing T_{eff}, leading to changes in chemical equilibrium composition and absorption due to dust grains. The spectral evolution from late-M through L to mid- and late-T classes is now understood as chiefly due to increasing amounts of condensates in the visible photosphere up to mid-L types, and the settling of dust clouds into deeper regions at the transition from L to T, resulting in a depletion of condensable elements in the upper atmosphere. The ensuing photospheric cooling also drives a change in carbon chemistry leading to the hallmark methane absorption features of T dwarfs.Recent observations of brown dwarfs in the L-T spectral sequence and model atmosphere calculations have shown that these changes in spectral features reveal differences in the efficiency of cloud removal that seem to be triggered by an additional parameter besides effective temperature. We present models describing the settling of particle clouds as an equilibrium process between condensation, gravitational sedimentation and convective and turbulent mixing, based on 3D-hydrodynamical simulations for the description of the velocity field. These calculations predict a strong dependence of the settling on gravity, and can therefore explain observed differences between the spectral energy distributions of brown dwarfs of equal luminosity as an effect of different mass. Based on these calculations we would expect even stronger deviations in the colours of young brown dwarfs of very low mass from those commonly observed in the field. Such predictions seem to be in aggreement with the first observations of candidate planetary-mass objects, and would imply that extrasolar planets at very young ages have spectral energy distributions significantly different from previous models. Our models al so predict characteristic time scales for the cloud formation processes that may be compared to observed variability in brown dwarfs. Title: Photometric colors of late-type giants: theory versus observations Authors: Kucinskas, A.; Hauschildt, P. H.; Ludwig, H. -G.; Brott, I.; Vansevicius, V.; Lindegren, L.; Tanabé, T.; Allard, F. Bibcode: 2006IAUS..232..276K Altcode: 2005astro.ph.12354K Late-type giants (i.e., stars on the red and asymptotic giant branches, RGB/AGB, respectively) are dominant contributors to the overall spectral appearance of intermediate age and old stellar populations, especially in the red/near-infrared part of the spectrum. Being intrinsically bright, they are well suited for probing distant/obscured populations, especially those that can not be studied with their fainter members, like main sequence turn-off stars or subgiants. Late-type giants and supergiants will be the only stellar types accessible in intermediate age and old populations beyond the distances of several Mpc with the future 30-50 m class extremely large telescopes (Olsen et al. 2003). Indeed, proper understanding of their observable properties by means of theoretical models is of key importance for studying the evolution of stellar populations and their host galaxies. Title: Hydrodynamical simulations of convection-related stellar micro-variability. I. Statistical relations for photometric and photocentric variability Authors: Ludwig, H. -G. Bibcode: 2006A&A...445..661L Altcode: 2005astro.ph..9441L Local-box hydrodynamical model atmospheres provide statistical information about the spatial dependence, as well as temporal evolution, of a star's emergent radiation field. Here, we consider late-type stellar atmospheres for which temporal changes of the radiative output are primarily related to convective (granular) surface flows. We derived relations for evaluating the granulation-induced, disk-integrated thus observable fluctuations of the stellar brightness and location of the photocenter from radiation intensities available from a local model. Apart from their application in the context of hydrodynamical stellar atmospheres, these formulae provide some broader insight into the nature of the fluctuations under consideration. Brightness fluctuations scale inversely proportional to the square root of the number of convective cells (the statistically independently radiating surface elements) present on the stellar surface and increase with more pronounced limb-darkening. Fluctuations of the stellar photocentric position do not depend on the number of cells and are largely insensitive to the degree of limb-darkening. They amount to a small fraction of the typical cell size, and can become a limiting factor for high-precision astrometry in the case of extreme giants. The temporal brightness and positional fluctuations are statistically uncorrelated but closely related in magnitude. Title: Convection and observable properties of late-type giants Authors: Kucinskas, A.; Ludwig, H. -G.; Hauschildt, P. H. Bibcode: 2006IAUS..232..498K Altcode: 2005astro.ph.12353K We show that contrary to what is expected from 1D stationary model atmospheres, 3D hydrodynamical modeling predicts a considerable influence of convection on the spectral properties of late-type giants. This is due to the fact that convection overshoots into the formally stable outer atmospheric layers producing a notable granulation pattern in the 3D hydrodynamical models, which has a direct influence on the observable spectra and colors. Within the framework of standard 1D model atmospheres the average thermal stratification of the 3D hydro model can not be reproduced with any reasonable choice of the mixing length parameter and formulation of the turbulent pressure. The differences in individual photometric colors - in terms of 3D versus 1D - reach up to ∼0.2 mag, or Δ T_{eff}∼70 K. We discuss the impact of full 3D hydrodynamical models on the interpretation of observable properties of late-type giants, briefly mentioning problems and challenges which need to be solved for bringing these models to a routine use within the astronomical community in 5-10 years from now. Title: Status of the physics of substellar objects Authors: Jones, H. R. A.; Viti, S.; Tennyson, J.; Barber, B.; Harris, G.; Pickering, J. C.; Blackwell-Whitehead, R.; Champion, J. -P.; Allard, F.; Hauschildt, P. H.; Jorgensen, U. G.; Ehrenfreund, P.; Stachowska, E.; Ludwig, H. -G.; Martin, E.; Pavlenko, Ya.; Lyubchik, Yu.; Kurucz, R. L. Bibcode: 2005AN....326..920J Altcode: A full understanding of the properties of substellar objects is one of the major challenges facing astrophysics. Since their discovery in 1995, we have discovered hundreds of brown dwarfs and extrasolar planets. While these discoveries have enabled important comparisons with theory, observational progress has been much more rapid than the theoretical understanding of cool atmospheres. The reliable determination of mass, abundances, gravities and temperatures is not yet possible. The key problem is that substellar objects emit their observable radiation in the infrared region of the spectrum where our knowledge of atomic, molecular and line broadening data is poor. Here we report on the status of PoSSO (Physics of SubStellar Objects). In order to understand brown dwarfs and extrasolar planets increasing more like those in our solar system, we are studying a wide range of processes. Here we give an update on the project and sketch an outline of atoms, molecules and processes requiring study. Title: On the Limb Darkening, Spectral Energy Distribution, and Temperature Structure of Procyon Authors: Aufdenberg, J. P.; Ludwig, H. -G.; Kervella, P. Bibcode: 2005ApJ...633..424A Altcode: 2005astro.ph..7336A We have fit synthetic visibilities from three-dimensional (CO5BOLD+PHOENIX) and one-dimensional (PHOENIX, ATLAS 12) model stellar atmospheres of Procyon (F5 IV) to high-precision interferometric data from the VLT Interferometer (K band) and from the Mark III interferometer (500 and 800 nm). These data sets provide a test of theoretical wavelength-dependent limb-darkening predictions. The work of Allende Prieto et al. has shown that the temperature structure from a spatially and temporally averaged three-dimensional hydrodynamic model produces significantly less limb darkening at 500 nm relative to the temperature structure of a one-dimensional MARCS model atmosphere with a standard mixing-length approximation for convection. Our direct fits to the interferometric data confirm this prediction. A one-dimensional ATLAS 12 model with ``approximate overshooting'' provides the required temperature gradient. We show, however, that one-dimensional models cannot reproduce the ultraviolet spectrophotometry below 160 nm with effective temperatures in the range constrained by the measured bolometric flux and angular diameter. We find that a good match to the full spectral energy distribution can be obtained with a composite model consisting of a weighted average of 12 one-dimensional model atmospheres based on the surface intensity distribution of a three-dimensional granulation simulation. We emphasize that one-dimensional models with overshooting may realistically represent the mean temperature structure of F-type stars such as Procyon, but the same models will predict redder colors than observed because they lack the multicomponent temperature distribution expected for the surfaces of these stars. Title: Broad-band photometric colors and effective temperature calibrations for late-type giants. I. Z = 0.02 Authors: Kučinskas, A.; Hauschildt, P. H.; Ludwig, H. -G.; Brott, I.; Vansevičius, V.; Lindegren, L.; Tanabé, T.; Allard, F. Bibcode: 2005A&A...442..281K Altcode: 2005astro.ph.10434K We present new synthetic broad-band photometric colors for late-type giants based on synthetic spectra calculated with the PHOENIX model atmosphere code. The grid covers effective temperatures T_eff=3000dots 5000 K, gravities log g=-0.5dots{+3.5}, and metallicities [M/H]=+0.5dots{-4.0}. We show that individual broad-band photometric colors are strongly affected by model parameters such as molecular opacities, gravity, microturbulent velocity, and stellar mass. Our exploratory 3D modeling of a prototypical late-type giant shows that convection has a noticeable effect on the photometric colors too, as it alters significantly both the vertical and horizontal thermal structures in the outer atmosphere. The differences between colors calculated with full 3D hydrodynamical and 1D model atmospheres are significant (e.g., Δ(V-K)∼0.2 mag), translating into offsets in effective temperature of up to 70 K. For a sample of 74 late-type giants in the Solar neighborhood, with interferometric effective temperatures and broad-band photometry available in the literature, we compare observed colors with a new PHOENIX grid of synthetic photometric colors, as well as with photometric colors calculated with the MARCS and ATLAS model atmosphere codes. We find good agreement of the new synthetic colors with observations and published T_eff-color and color-color relations, especially in the T_eff-(V-K), T_eff-(J-K) and (J-K)-(V-K) planes. Deviations from the observed trends in the T_eff-color planes are generally within ±100 K for T_eff=3500 to 4800 K. Synthetic colors calculated with different stellar atmosphere models agree to ±100 K, within a large range of effective temperatures and gravities. The comparison of the observed and synthetic spectra of late-type giants shows that discrepancies result from the differences both in the strengths of various spectral lines/bands (especially those of molecular bands, such as TiO, H2O, CO) and the continuum level. Finally, we derive several new T_eff-log g-color relations for late-type giants at solar-metallicity (valid for T_eff=3500 to 4800 K), based both on the observed effective temperatures and colors of the nearby giants, and synthetic colors produced with PHOENIX, MARCS and ATLAS model atmospheres. Title: Model atmospheres of substellar atmospheres at a young age: influence of gravity and dust. Authors: Homeier, D.; Allard, F.; Ludwig, H. -G.; Hauschildt, P.; Dehn, M. Bibcode: 2005AN....326Q.628H Altcode: No abstract at ADS Title: VizieR Online Data Catalog: Late-type giants BVRIJHKL and Teff calibration (Kucinskas+, 2005) Authors: Kucinskas, A.; Hauschildt, P. H.; Ludwig, H. -G.; Brott, I.; Vansevicius, V.; Lindegren, L.; Tanabe, T.; Allard, F. Bibcode: 2005yCat..34420281K Altcode: Table 2 contains synthetic broad-band photometric colors of late-type giants in the Johnson-Cousins-Glass photometric system. Colors are based on the synthetic spectra calculated with the PHOENIX stellar model atmosphere code. Photometric filter definitions used are those from Bessell (1990PASP..102.1181B) for the Johnson-Cousins BVRI bands, and from Bessell & Brett (1988PASP..100.1134B) for the Johnson-Glass JHKL bands.

(1 data file). Title: Dynamo action in M-dwarfs Authors: Dorch, S. B. F.; Gudiksen, B. V.; Ludwig, H. -G. Bibcode: 2005ESASP.560..515D Altcode: 2005csss...13..515D; 2004astro.ph..9219D Magnetic activity in M-dwarfs present enigmatic questions: On the one hand they have higher field strengths and larger filling factors than the magnetic field on the Sun, on the other hand, they are fully convective and their atmospheres are more neutral, hence they do not have an undershoot layer for magnetic flux storage and as we show here, cannot have small-scale dynamo action in their photospheres either. We present a discussion of these facts and propose a new numerical model to investigate M-dwarf magnetism. Title: Hydrodynamical simulations of convection-related stellar micro-variability Authors: Svensson, F.; Ludwig, H. -G. Bibcode: 2005ESASP.560..979S Altcode: 2004astro.ph..9714S; 2005csss...13..979S We used a series of COBOLD hydrodynamical model atmospheres covering stellar objects from white dwarfs to red giants to derive theoretical estimates of the photometric and photocentric stellar variability in wavelength-integrated light across the Hertzsprung-Russell diagram. We validated our models against solar measurements from the SOHO/VIRGO instrument. Within our set of models we find a systematic increase of the photometric as well as photocentric variability - which turn out to be closely connected - with decreasing surface gravity. The estimated absolute levels of the photocentric variability do not affect astrometric observations on a precision level expected to be achieved by the GAIA mission - with the exception of close-by giants. The case of supergiants remains to be investigated. In view of the ongoing debate about the photometric non-detection of p-modes in Procyon by the Canadian MOST satellite we remark that we obtain a factor of ca. 3 in amplitude between the granular background noise in the Sun and Procyon. This statement refers to a particular representation of temporal power spectra as discussed in Sect. 5. 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: Wavelength shifts in solar-type spectra Authors: Dravins, D.; Lindegren, L.; Ludwig, H. -G.; Madsen, S. Bibcode: 2005ESASP.560..113D Altcode: 2004astro.ph..9212D; 2005csss...13..113D Spectral-line displacements away from the wavelengths naively expected from the Doppler shift caused by stellar radial motion may originate as convective shifts (correlated velocity and brightness patterns in the photosphere), as gravitational redshifts, or perhaps be induced by wave motions. Absolute lineshifts, in the past studied only for the Sun, are now accessible also for other stars thanks to astrometric determination of stellar radial motion, and spectrometers with accurate wavelength calibration. Comparisons between spectroscopic apparent radial velocities and astrometrically determined radial motions reveal greater spectral blueshifts in F-type stars than in the Sun (as theoretically expected from their more vigorous convection), further increasing in A-type stars (possibly due to atmospheric shockwaves). An important near-future development to enable a further analysis of stellar surface structure will be the study of wavelength variations across spatially resolved stellar disks, e.g., the center-to-limb wavelength changes along a stellar diameter, and their spatially resolved time variability. Title: Status and future of hydrodynamical model atmospheres Authors: Ludwig, H. -G.; Kučinskas, A. Bibcode: 2005ESASP.560..319L Altcode: 2005csss...13..319L; 2004astro.ph..9712L Since about 25 years ago work has been dedicated to the development of hydrodynamical model atmospheres for cool stars (of A to T spectral type). Despite their obviously sounder physical foundation in comparison with standard hydrostatic models, their general application has been rather limited. In order to understand why this is, and how to progress, we review the present status of hydrodynamical modelling of cool star atmospheres. The development efforts were and are motivated by the theoretical interest of understanding the dynamical processes operating in stellar atmospheres. To show the observational impact, we discuss examples in the fields of spectroscopy and stellar structure where hydrodynamical modelling provided results on a level qualitatively beyond standard models. We stress present modelling challenges, and highlight presently possible and future observations that would be particularly valuable in the interplay between model validation and interpretation of observables, to eventually widen the usage of hydrodynamical model atmospheres within the astronomical community. Title: Excitation of P-Modes in the Sun and Stars Authors: Stein, Robert; Georgobiani, Dali; Trampedach, Regner; Ludwig, Hans-Günter; Nordlund, Åke Bibcode: 2005HiA....13..411S Altcode: We describe the stochastic excitation of p-mode oscillations by solar convection. We discuss the role of Reynolds stresses and entropy fluctuations what controls the excitation spectrum the depth of the driving and the location of the driving. We then present results for a range of other stars and discuss the similarities and differences with the Sun. Title: Theoretical Modelling of Late-Type Giant Atmospheres: Preparing for Gaia Authors: Kucinskas, A.; Brott, I.; Hauschildt, P. H.; Ludwig, H. -G.; Lindegren, L.; Tanabé, T.; Vansevicius, V. Bibcode: 2005ESASP.576..591K Altcode: 2005tdug.conf..591K; 2005astro.ph..3208K Late-type giants (RGB/AGB stars) will be important tracers of the Galactic morphology and evolution in the framework of Gaia, as they are intrinsically bright and thus can probe distant stellar populations or those obscured by interstellar extinction. A realistic representation of their atmospheres and spectra with stellar atmosphere models is thus of crucial importance, both for the design and optimization of Gaia instruments, as well as the interpretation of provided astrophysical data. Our analysis of synthetic photometric colours of latetype giants based on PHOENIX, MARCS and ATLAS model atmospheres indicates a general agreement between the current theoretical predictions and observations in the framework of stationary 1-D model atmospheres. Presently available models allow temperature determinations of RGB/AGB stars to an accuracy of ∼ ±100 K. In an exploratory study we try to quantify possible residual systematic effects due to the approximations made in 1-D models using full 3-D hydrodynamical models. We find that differences in broad-band photometric colours calculated with 1-D and 3-D models are significant, translating to the offsets in effective temperature of up to μTeff ∼70 K. Clearly, full 3-D hydrodynamical models will help to alleviate such ambiguities in current theoretical modelling. Additionally, they will allow to study new phenomena, to open qualitatively new windows for stellar astrophysics in the Gaia-era. Title: PoSSO Physics of SubStellar Objects Authors: Jones, Hugh; Viti, Serena; Tennyson, Jonathan; Barber, Bob; Pickering, Juliet; Blackwell-Whitehead, Richard; Champion, Jean-Paul; Allard, France; Hauschildt, Peter; Jørgensen, Uffe; Ehrenfreund, Pascale; Stachowska, Ewa; Ludwig, Hans-Günter; Pavlenko, Yakiv; Lyubchik, Yuri; Kurucz, Robert Bibcode: 2005hris.conf..477J Altcode: A full understanding of the properties of substellar objects is one of the major challenges facing astrophysics. Since their discovery in 1995, we have discovered hundreds of brown dwarfs and extrasolar planets. While these discoveries have enabled important comparisons with theory, observational progress has been much more rapid than the theoretical understanding of cool atmospheres. The determination of mass, abundances, gravities and temperatures is not yet possible. The key problem is that substellar objects emit their observable radiation in the infrared region of the spectrum where our knowledge of atomic, molecular and line broadening data is poor. In order to understand these objects, and extra-solar planets increasing more like those our Solar System, we urge the wider physical chemistry community to engage in this exciting new field. Here we sketch an outline of the atoms, molecules and processes requiring study. Title: Hypervelocity impacts on HST solar arrays and the debris and meteoroids population Authors: Moussi, A.; Drolshagen, G.; McDonnell, J. A. M.; Mandeville, J. -C.; Kearsley, A. T.; Ludwig, H. Bibcode: 2005AdSpR..35.1243M Altcode: Accurate debris and meteoroid flux models are crucial for the design of manned and unmanned space missions. For the most abundant particle sizes smaller than a few millimetres, knowledge of the populations can only be gained from in situ detectors or the analysis of retrieved space hardware. The measurement of impact flux from exposed surfaces improves with increased surface area and exposure time. A post-flight impact investigation was initiated by the European Space Agency to record and analyse the impact fluxes and any potential resulting damage on the two flexible solar arrays of the Hubble Space Telescope. The arrays were deployed during the first Hubble Space Telescope servicing mission in December 1993 and retrieved in March 2002. They have a total exposed surface area of roughly 120 m 2, including 42 m 2 covered with solar cells. This new Hubble post-flight impact study follows a similar activity undertaken after the retrieval of one of the first solar arrays, in 1993. The earlier study provided the first opportunity for a numerical survey of damage to exposed surfaces from more than 600 km altitude, and of impacts from particles larger than 1 mm. The results have proven very valuable in validation of important flux model regimes. The second set of Hubble solar arrays has again provided an unrivalled opportunity to measure the meteoroid and debris environment, now sampled during a long interval in low Earth orbit, and to identify changes in the space debris environment since the previous survey. The retrieved solar array wings exhibit thousands of craters, many of which are visible to the naked eye. A few hundred impacts have completely penetrated the 0.7 mm thick array. The largest impact features are about 7-8 mm in diameter. The cover glass of the solar cells is particularly well suited to the recognition of small impact features by optical and electron microscopy. In this paper, we present the first results of the impact survey. Data upon the abundance of craters of specific measured size ranges are plotted as cumulative flux curves, and compared to the results of model predictions. The most significant change to the particle flux since 1993 is a decrease in the small debris population. Title: Procyon: Constraining Its Temperature Structure with High-Precision Interferometry and 3-D Model Atmospheres Authors: Aufdenberg, J. P.; Ludwig, H. -G.; Kervella, P. Bibcode: 2004AAS...205.1203A Altcode: 2004BAAS...36.1357A We have fit synthetic visibilities from 3-D (CO5BOLD + PHOENIX) and 1-D (PHOENIX, ATLAS12) model stellar atmospheres for Procyon (F5 IV) to high-precision interferometric data from the VINCI instrument at the VLT Interferometer (K-band) and from the Mark III interferometer (500 nm, 800 nm). These data provide a test of theoretical wavelength-dependent limb-darkening predictions, and therefore Procyon's atmospheric temperature structure. Earlier work (Allende Prieto et al. 2002 ApJ 567, 544) has shown that the temperature structure from a spatially and temporally averaged 3-D hydrodynamical model produces significantly less limb darkening at 500 nm relative to the temperature structure from a 1-D MARCS model atmosphere which uses a mixing-length approximation for convective flux transport. Our direct fits to the interferometric data confirm this prediction, however we find that not all 1-D models fail to reproduce the observations. The key to matching the interferometric data is a shallower temperature gradient than provided by the standard 1-D mixing-length approximation. We find that in addition to our best fitting 3-D hydrodynamical model, a 1-D ATLAS12 model, with an additional free parameter for ``approximate overshooting'', provides the required temperature gradient. We estimate that an interferometric precision better than 0.1% will be required to distinguish between the 3-D model and the ATLAS12 model. This overshooting approximation has been shown to match Solar limb-darkening observations reasonably well (Castelli et al 1997 A&A 324, 432), however published work since using Strömgren photometry of solar-type stars has cast doubt on the importance of overshooting. We have also compared synthetic spectral energy distributions for Procyon to ultraviolet, optical and near-infrared spectrophotometry and find differences from comparisons to Strömgren photometry alone.

This work was performed in part contract with the Jet Propulsion Laboratory (JPL) funded by NASA through the Michelson Fellowship Program. JPL is managed for NASA by the California Institute of Technology. Title: Intrinsic Wavelength Shifts in Stellar Spectra Authors: Dravins, D.; Lindegren, L.; Ludwig, H. -G.; Madsen, S. Bibcode: 2004AAS...20517004D Altcode: 2004BAAS...36.1624D Wavelengths of stellar spectral lines do not have the precise values `naively' expected from laboratory wavelengths merely Doppler-shifted by stellar radial motion. Slight displacements may originate as convective shifts (correlated velocity and brightness patterns in the photosphere), as gravitational redshifts, or perhaps be induced by wave motions. Intrinsic lineshifts thus reveal stellar surface structure, while possible periodic changes (during a stellar activity cycle, say) need to be segregated from variability induced by orbiting exoplanets.

Absolute lineshifts can now be studied also in some stars other than the Sun, thanks to astrometric determinations of stellar radial motion. Comparisons between spectroscopic apparent radial velocities and astrometrically determined radial motions reveal greater spectral blueshifts in F-type stars than in the Sun (as theoretically expected from their more vigorous convection), further increasing in A-type stars (possibly due to atmospheric shockwaves).

Solar spectral atlases, and high-resolution spectra (from UVES on ESO VLT) of a dozen solar-type stars are being surveyed for `unblended' photospheric lines of most atomic species with accurate laboratory wavelengths available. One aim is to understand the ultimate information content of stellar spectra, and in what detail it will be feasible to verify models of stellar atmospheric hydrodynamics. These may predict line asymmetries (bisectors) and shifts for widely different classes of lines, but there will not result any comparison with observations if such lines do not exist in real spectra.

An expected near-future development in stellar physics is spatially resolved spectroscopy across stellar disks, enabled by optical interferometry and adaptive optics on very large telescopes. Stellar surface structure manifests itself in the center-to-limb wavelength changes along a stellar diameter, and their spatially resolved time variability, diagnostics which already now can be theoretically modeled. Title: Excitation of Radial P-Modes in the Sun and Stars Authors: Stein, Robert; Georgobiani, Dali; Trampedach, Regner; Ludwig, Hans-Günter; Nordlund, Åke Bibcode: 2004SoPh..220..229S Altcode: P-mode oscillations in the Sun and stars are excited stochastically by Reynolds stress and entropy fluctuations produced by convection in their outer envelopes. The excitation rate of radial oscillations of stars near the main sequence from K to F and a subgiant K IV star have been calculated from numerical simulations of their surface convection zones. P-mode excitation increases with increasing effective temperature (until envelope convection ceases in the F stars) and also increases with decreasing gravity. The frequency of the maximum excitation decreases with decreasing surface gravity. 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: Hypervelocity impacts on HST solar arrays and the debris population Authors: Drolshagen, G.; McDonnell, J. A. M.; Mandeville, J. -C.; Moussi, A.; Ludwig, H. Bibcode: 2004cosp...35..969D Altcode: 2004cosp.meet..969D Accurate debris and meteoroid flux models are crucial for the design of manned and unmanned space missions. For the most abundant particle sizes smaller than a few millimetres, knowledge on the populations can only be gained by in-situ detectors or the analysis of retrieved space hardware. The impact flux information, which can be obtained from exposed surfaces, increases with surface area and exposure time. A Post-Flight Impact Investigation was initiated by ESA to record and analyze the impact fluxes and any potential resulting damage on the two flexible solar arrays of the Hubble Space Telescope. They were deployed during the first HST servicing mission in December 1993 and retrieved in March 2002. They have a total exposed surface area of roughly 120 m2, including 41 m2 covered with solar cells. The HST post-flight impact study follows a similar activity undertaken after the retrieval of one of the first HST solar arrays. That analysis has been very successful and already resulted in a validation of certain flux model regimes. For the first time exposed surfaces from more then 600 km altitude could be analysed and impacts from particles larger than 1 mm could be observed. The second set of HST solar arrays provide 4 times the area x time product of the first array and extend the measurements to the largest particle sizes ever recorded. The retrieved HST solar array wings exhibit thousands of craters, which are visible to the naked eye. A few hundred impacts have completely penetrated the 0.7 mm thick array. The largest impact features are about 7-8 mm in diameter. Measured fluxes of craters larger than 10 microns and 1 mm are in the order of 3 x 10-5 m-2 s-1 and 1.3 x 10-7 m-2 s-1, respectively. First results of the impact survey are presented here and compared to model predictions. Flux predictions are based on the latest meteoroid and debris (e.g. MASTER 2001) models and on crater size equations derived specifically for the HST solar arrays. A second paper presented during this conference is dealing more specifically on the identification of projectile remnants and discrimination between natural particles and man-made orbital debris. Title: Revising the abundance of Pr in the solar photosphere Authors: Ivarsson, S.; Wahlgren, G. M.; Ludwig, H. -G. Bibcode: 2003AAS...20313404I Altcode: 2003BAAS...35.1421I New atomic data (Ivarsson et al. 2001, Physica Scripta 64, 455) have opened the possibility of re-evaluating the solar photospheric abundance of the rare-earth element praseodymium. The experimental spectra were recorded in the region 2000 to 7000 Å with the Lund UV Fourier transform spectrometer and a hollow-cathode light source. These data allowed us to measure accurate wavelengths, re-analyse energy levels and determine the hyperfine structure for transitions of Pr II and Pr III. In addition, energy level lifetimes were measured at the Lund Laser Centre, which combined with the new experimental branching fractions lead to revisions of the gf values for many lines. The abundance of Pr in the solar photosphere was determined from synthetic spectrum fitting of three Pr II lines found in high-resolution solar observations from Kitt-Peak (NOAO) and Junfraujoch observatories. The 1D modelling was accomplished using the ATLAS/SYNTHE LTE codes (Kurucz 1998) and resulted in a Pr abundance (log ɛ =0.40±0.1) that represents a significant lowering from the current canonical value of the solar photospheric abundance (log ɛ =0.71±0.08, Biemont et al. 1979, Sol. Phys. 61, 17). We also tested the influence of 3D effects on the determination of the Pr abundance using the CO5BOLD radiation-hydrodynamics code (Freytag et al. 2002, AN 323, 213). Our preliminary results show insignificant differences in the equivalent widths of the Pr II lines between the 3D and 1D cases. Our new solar abundance of Pr differs from that determined from a sample of two carbonaceous condrite meteorites (log ɛ =0.78±0.03, Lodder 2003, ApJ 591, 1220) and raises such questions as the appropriateness of assuming the convergence of solar and meteoritic abundances and the accuracy of modelling the solar spectrum. Title: Intrinsic spectral blueshifts in rapidly rotating stars? Authors: Madsen, Søren; Dravins, Dainis; Ludwig, Hans-Günter; Lindegren, Lennart Bibcode: 2003A&A...411..581M Altcode: 2003astro.ph..9346M Spectroscopic radial velocities for several nearby open clusters suggest that spectra of (especially earlier-type) rapidly rotating stars are systematically blueshifted by 3 km s-1 or more, relative to the spectra of slowly rotating ones. Comparisons with astrometrically determined radial motions in the Hyades suggests this to be an absolute blueshift, relative to wavelengths naively expected from stellar radial motion and gravitational redshift. Analogous trends are seen also in most other clusters studied (Pleiades, Coma Berenices, Praesepe, alpha Persei, IC 2391, NGC 6475, IC 4665, NGC 1976 and NGC 2516). Possible mechanisms are discussed, including photospheric convection, stellar pulsation, meridional circulation, and shock-wave propagation, as well as effects caused by template mismatch in determining wavelength displacements. For early-type stars, a plausible mechanism is shock-wave propagation upward through the photospheric line-forming regions. Such wavelength shifts thus permit studies of certain types of stellar atmospheric dynamics and - irrespective of their cause - may influence deduced open-cluster membership (when selected from common velocity) and deduced cluster dynamics (some types of stars might show fortuitous velocity patterns). 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: Convection and Small-scale Magnetic Fields in M-type Atmospheres Authors: Ludwig, Hans-Günter; Dorch, Søren Bertil Fabricius Bibcode: 2003ANS...324...65L Altcode: 2003ANS...324..I06L 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: Model Atmospheres and Spectra: The Role of Dust Authors: Allard, France; Guillot, Tristan; Ludwig, Hans-Günter; Hauschildt, Peter H.; Schweitzer, Andreas; Alexander, David R.; Ferguson, Jason W. Bibcode: 2003IAUS..211..325A Altcode: Brown dwarf atmospheres form molecules, then high temperature condensates (corundum, titanates, silicates, and iron compounds), and then low temperature condensates (ices) as they cool down over time. These produce large opacities which govern entirely their spectral energy distribution. Just as it is important to know molecular opacities (TiO, H_2O, CH_4, etc.) with accuracy, it is imperative to understand the interplay of processes (e.g. condensation, sedimentation, coagulation, convection) that determines the radial and size distribution of grains. Limiting case models have shown that young, hot brown (L) dwarfs form dust mostly in equilibrium, while at much cooler stages (late T dwarfs) all high temperature condensates have sedimented out of their photospheres. But this process is gradual and all intermediate classes of brown dwarfs can partly be understood in terms of partial sedimentation of dust. With new models accounting for these processes, we describe the effects they may have upon brown dwarf spectral properties. Title: Challenges in the Solution of the Transfer Equation in Multi-D Hydrodynamical Model Atmospheres for Cool Stars Authors: Ludwig, H. -G. Bibcode: 2003ASPC..288..537L Altcode: 2003sam..conf..537L My talk is intended to stimulate discussions about methods in radiative transfer for hydrodynamical model atmospheres of late-type stars. I will present a number a number of inherent problems, show how they are presently tackled (if so), and ask for ideas for more efficient approaches. The transparencies of my talk can be found in

Transparencies (4.3Mb) 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: 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: Energy Transport, Overshoot, and Mixing in the Atmospheres of Very Cool Stars Authors: Ludwig, H. -G. Bibcode: 2003IAUS..210..113L Altcode: 2002astro.ph..8583L We constructed hydrodynamical model atmospheres for mid M-type main-, as well as pre-main-sequence objects. Despite the complex chemistry encountered in such cool atmospheres a reasonably accurate representation of the radiative transfer is possible. The detailed treatment of the interplay between radiation and convection in the hydrodynamical models allows to study processes usually not accessible within the framework conventional model atmospheres. In particular, we determined the efficiency of the convective energy transport, and the efficiency of mixing by convective overshoot. The convective transport efficiency expressed in terms of an equivalent mixing-length parameter amounts to values around ~2 in the optically thick, and ~2.8 in the optically thin regime. The thermal structure of the formally convectively stable layers is little affected by convective overshoot and wave heating, i.e. stays close to radiative equilibrium. Mixing by convective overshoot shows an exponential decline with geometrical distance from the Schwarzschild stability boundary. The scale height of the decline varies with gravitational acceleration roughly as g^(-1/2), with 0.5 pressure scale heights at log(g)=5.0. Title: Numerical simulations of surface convection in a late M-dwarf Authors: Ludwig, H. -G.; Allard, F.; Hauschildt, P. H. Bibcode: 2002A&A...395...99L Altcode: 2002astro.ph..8584L Based on detailed 2D and 3D numerical radiation-hydrodynamics (RHD) simulations of time-dependent compressible convection, we have studied the dynamics and thermal structure of the convective surface layers of a prototypical late-type M-dwarf (Teffapprox 2800 K, log g=5.0, solar chemical composition). The RHD models predict stellar granulation qualitatively similar to the familiar solar pattern. Quantitatively, the granular cells show a convective turn-over time scale of ~100 s, and a horizontal scale of 80 km; the relative intensity contrast of the granular pattern amounts to 1.1%, and root-mean-square vertical velocities reach 240 m s-1 at maximum. Deviations from radiative equilibrium in the higher, formally convectively stable atmospheric layers are found to be insignificant allowing a reliable modeling of the atmosphere with 1D standard model atmospheres. A mixing-length parameter of αMLT = 2.1 provides the best representation of the average thermal structure of the RHD model atmosphere while alternative values are found when fitting the asymptotic entropy encountered in deeper layers of the stellar envelope (αMLT = 1.5), or when matching the vertical velocity (αMLT = 3.5). The close correspondence between RHD and standard model atmospheres implies that presently existing discrepancies between observed and predicted stellar colors in the M-dwarf regime cannot be traced back to an inadequate treatment of convection in the 1D standard models. The RHD models predict a modest extension of the convectively mixed region beyond the formal Schwarzschild stability boundary which provides hints for the distribution of dust grains in cooler (brown dwarf) atmospheres. Title: A simulation of solar convection at supergranulation scale Authors: Rieutord, M.; Ludwig, H. -G.; Roudier, T.; Nordlund, .; Stein, R. Bibcode: 2002NCimC..25..523R Altcode: 2001astro.ph.10208R We present here numerical simulations of surface solar convection which cover a box of 30$\times30\times$3.2 Mm$^3$ with a resolution of 315$\times315\times$82, which is used to investigate the dynamics of scales larger than granulation. No structure resembling supergranulation is present; possibly higher Reynolds numbers (i.e. higher numerical resolution), or magnetic fields, or greater depth are necessary. The results also show interesting aspects of granular dynamics which are briefly presented, like extensive p-mode ridges in the k-$\omega$ diagram and a ringlike distribution of horizontal vorticity around granules. At large scales, the horizontal velocity is much larger than the vertical velocity and the vertical motion is dominated by p-mode oscillations. Title: Small-scale magnetic fields on late-type M-dwarfs Authors: Dorch, S. B. F.; Ludwig, H. -G. Bibcode: 2002AN....323..402D Altcode: 2002astro.ph..5458D We performed kinematic studies of the evolution of small-scale magnetic fields in the surface layers of M-dwarfs. We solved the induction equation for a prescribed velocity field, magnetic Reynolds number \ReM, and boundary conditions in a Cartesian box, representing a volume comprising the optically thin stellar atmosphere and the uppermost part of the optically thick convective envelope. The velocity field is spatially and temporally variable, and stems from detailed radiation-hydrodynamics simulations of convective flows in a proto-typical late-type M-dwarf (Teff =2800pun {K}, logg =5.0, solar chemical composition, spectral type ~M6). We find dynamo action for ReM >= 400. Growth time scales of the magnetic field are comparable to the convective turn-over time scale (~ 150pun {sec}). The convective velocity field concentrates the magnetic field in sheets and tubular structures in the inter-granular downflows. Scaling from solar conditions suggests that field strengths as high as 20\pun{kG} might be reached locally. Perhaps surprisingly, \ReM\ is of order unity in the surface layers of cooler M-dwarfs, rendering the dynamo inoperative. In all studied cases we find a rather low spatial filling factor of the magnetic field. Title: Are granules good tracers of solar surface velocity fields? Authors: Rieutord, M.; Roudier, T.; Ludwig, H. -G.; Nordlund, Å.; Stein, R. Bibcode: 2001A&A...377L..14R Altcode: 2001astro.ph..8284R Using a numerical simulation of compressible convection with radiative transfer mimicking the solar photosphere, we compare the velocity field derived from granule motions to the actual velocity field of the plasma. We thus test the idea that granules may be used to trace large-scale velocity fields at the sun's surface. Our results show that this is indeed the case provided the scale separation is sufficient. We thus estimate that neither velocity fields at scales less than 2500 km nor time evolution at scales shorter than 0.5 hr can be faithfully described by granules. At larger scales the granular motions correlate linearly with the underlying fluid motions with a slope of ≲2 reaching correlation coefficients up to ~ 0.9. Title: Radiation-hydrodynamics Simulations of Surface Convection in a Late M-dwarf Authors: Ludwig, Hans-Günter; Hauschildt, Peter Bibcode: 2001AGM....18..P15L Altcode: Based on detailed 2D and 3D numerical radiation hydrodynamics calculations of time-dependent compressible convection, we have studied the dynamics and thermal structure of the convective surface layers of a prototypical late-type M-dwarf at Teff=2800 K, and log g=5.0 with solar chemical composition. The thermal structure of the atmosphere is dominated by molecular absorption, the formation of dust grains is not important for the chosen model parameters. In the multi dimensional models the radiative transfer is treated by a multi-group (4 groups) approach which provides a simplified, nevertheless rather realistic treatment of the complex radiative energy transport. The equation of state includes the important contribution of H2 molecule formation. Our models predict a convective pattern at the surface of an late M-dwarf qualitatively similar to solar granulation. Quantitatively, the convective turn-over timescale amounts to ≈ 100 s, a typical horizontal scale of convective cells to 80 km, and a relative intensity contrast of the granular pattern to 1.1 %. The efficiency of convective energy transport corresponds to an effective mixing-length parameter between 1.5 to 2.1 depending on the thermal property which is represented. The models provide mixing timescales due to atmospheric overshoot which can be extrapolated to lower effective temperatures where dust grains are present and mixed into optically thin layers. 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: The effects of numerical resolution on hydrodynamical surface convection simulations and spectral line formation Authors: Asplund, M.; Ludwig, H. -G.; Nordlund, Å.; Stein, R. F. Bibcode: 2000A&A...359..669A Altcode: 2000astro.ph..5319A The computationally demanding nature of radiative-hydrodynamical simulations of stellar surface convection warrants an investigation of the sensitivity of the convective structure and spectral synthesis to the numerical resolution and dimension of the simulations, which is presented here. With too coarse a resolution the predicted spectral lines tend to be too narrow, reflecting insufficient Doppler broadening from the convective motions, while at the currently highest affordable resolution the line shapes have converged essentially perfectly to the observed profiles. Similar conclusions are drawn from the line asymmetries and shifts. Due to the robustness of the pressure and temperature structures with respect to the numerical resolution, strong Fe lines with pronounced damping wings and H i lines are essentially immune to resolution effects, and can therefore be used for improved T_eff and log g determinations even at very modest resolutions. In terms of abundances, weak Fe i and Fe ii lines show a very small dependence ( =~ 0.02 dex) while for intermediate strong lines with significant non-thermal broadening the sensitivity increases (<~ 0.10 dex). Problems arise when using 2D convection simulations to describe an inherent 3D phenomenon, which translates to inaccurate atmospheric velocity fields and temperature and pressure structures. In 2D the theoretical line profiles tend to be too shallow and broad compared with the 3D calculations and observations, in particular for intermediate strong lines. In terms of abundances, the 2D results are systematically about 0.1 dex lower than for the 3D case for Fe i lines. Furthermore, the predicted line asymmetries and shifts are much inferior in 2D with discrepancies amounting to ~ 200 m s-1. Given these shortcomings and computing time considerations it is better to use 3D simulations of even modest resolution than high-resolution 2D simulations. Title: The atmospheric dynamics in 2D and 3D simulations of stellar surface convection Authors: Ludwig, Hans-Guenter; Nordlund, Ake Bibcode: 2000ASSL..254...37L Altcode: 2000stas.conf...37L 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: 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: Convection and the Eclipsing Binary AI Phoenicis: How Well Can We Constrain the Mixing-Length parameter from Stellar Modelling? Authors: Ludwig, H. -G.; Salaries, M. Bibcode: 1999ASPC..173..229L Altcode: 1999sstt.conf..229L No abstract at ADS Title: Balmer Line Formation in Convective Stellar Atmospheres Authors: Steffen, M.; Ludwig, H. -G. Bibcode: 1999ASPC..173..217S Altcode: 1999sstt.conf..217S No abstract at ADS Title: New peculiar CO data of the shell around IRC +10 216 Authors: Groenewegen, M. A. T.; Ludwig, H. -G. Bibcode: 1998A&A...339..489G Altcode: 1998astro.ph..9010G A CO(1-0) on-source spectrum of the well-known carbon star IRC +10 216 taken with the IRAM 30m telescope in June 1996 shows excess emission between -18.3 and -14.3 km s(-1) at the red wing of the underlying profile. The excess emission is confirmed in January 1997 but is absent in April 1997 and June 1998 IRAM spectra. Such a transient feature has not been seen before in this star or any other AGB star. In April 1997 we mapped the circumstellar shell out to 110\arcsec. Both the J = 1-0 and J = 2-1 spectra show ``spikes'' or components which vary in strength with position in the envelope. One of these components corresponds to the velocity interval mentioned above. An immediate conclusion is that the circumstellar shell is not spherically symmetric, contrary to what was believed based on lower spectral resolution data. We are probably seeing emission from a complex geometrical structure. Neither a disk structure nor a double-wind structure seem to be able to explain the observations. The on-source transient behaviour of the red excess emission can reasonably well be explained by a single large ( ~ 5 x 10(13) cm) blob, that expands due to internal motion. 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: 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: Is stellar granulation turbulence? Authors: Nordlund, A.; Spruit, H. C.; Ludwig, H. -G.; Trampedach, R. Bibcode: 1997A&A...328..229N Altcode: We show that power spectra of granulation images or velocity fields cannot be compared meaningfully with spectra from theoretical models based on turbulent cascades. The small scale power in these images is due almost entirely to the sharp edges between granules and intergranular lanes, not to turbulence in the usual sense. This is demonstrated with a number of experiments with result from numerical simulations, and with simpler synthetic data with power spectra similar to that of granulation. The reason for the seemingly laminar behavior of the granulation flow, in spite of the high Reynolds numbers involved, is the influence of stratification on the local ratio of turbulence to bulk flow. The rapid expansion of upflows, their deep origin and near-adiabatic stratification lead to low levels of turbulence in the overturning fluid at the surface. Higher levels of turbulence are expected in the converging flows near downdrafts, but mostly at scales that are below current observational resolution limits, and contributing relatively little to the total convective flux and to spectral line broadening. Title: A precision-controlled solar model with realistic subatmospheric stratification Authors: Schlattl, H.; Weiss, A.; Ludwig, H. -G.; Bernkopf, J. Bibcode: 1997ASSL..225...65S Altcode: 1997scor.proc...65S The Garching stellar evolution code has been modified to allow the calculation of an up-to-date solar model. The modifications are of numerical and physical nature. Concerning the first aspect, the spatial as well as the temporal discretization of the PDEs can now be done with a controlled and guaranteed accuracy of up to $\approx 10^{-5}$. With regard to the second aspect, latest input physics tables, diffusion and synthetic atmospheres used as outer boundary conditions at $\tau \approx 20$ have been implemented. In addition, a spatially variable mixing length has been employed to reproduce the T-P-stratification of the outer convective layers as calculated by 2D-hydrodynamical models. We find that the changes in the subatmospheric regions lead to a further improvement of the solar model. Title: Two mass-losing carbon stars in the Galactic halo Authors: Groenewegen, M. A. T.; Oudmaijer, R. D.; Ludwig, H. -G. Bibcode: 1997MNRAS.292..686G Altcode: Observations of two mass-losing carbon stars in the Galactic halo, IRAS08546+1732 and 12560+1656, are presented. These objects were discovered serendipitously, and stand out from the usual carbon stars at high galactic latitudes in that they have optical and IRAS colours consistent with current mass-loss. New optical spectra, CO observations, and modelling of the spectral energy distribution and of the CO lines are presented. Luminosities are derived using a period-luminosity relation. From the dust modelling IRAS08546+1732 is found to be at 20 kpc from the Sun (11.3 kpc from the Galactic plane) and has a mass-loss rate of 3.3x10^-6 Msolar yr^-1. From the CO non-detection we deduce that it is probably oxygen-deficient, corroborating earlier work. IRAS12560+1656 is found to be at 8.0 kpc from the Sun (7.8 kpc from the Galactic plane) and has a mass-loss rate of 1.3x10^-6 Msolar yr^-1. The detection of the ^12CO J=2-1 transition in the spectrum of IRAS12560+1656 after an integration time of 10h makes it probably the longest ^12CO integration on a stellar object. The detection itself makes the star one of the most distant stellar objects detected in the CO line. The outflow velocity of 3.2 km s^-1 is very low, and the stellar velocity is +88 km s^-1 with respect to the LSR. Modelling of the CO line implies an oxygen abundance of 0.7 dex below solar. We examine existing data on the `faint high-latitude carbon stars' and identify two additional distant, mass-losing, N-type AGB stars. The nature of halo carbon stars is discussed, and suggestions on how to find more mass-losing halo AGB stars are presented. Title: A solar model with improved subatmospheric stratification. Authors: Schlattl, H.; Weiss, A.; Ludwig, H. -G. Bibcode: 1997A&A...322..646S Altcode: The calculated intermediate and high degree p-mode frequencies of standard solar models show greater disagreement with the observations than low degree modes. This leads to the conclusion that the subatmospheric structure of the models has to be improved. We are presenting solar models with up-to-date input physics and controlled numerical accuracy of 10^-5^. For the outer boundary condition we use synthetic atmospheres fitted to the interior solution at τ=~20. In addition, a spatially variable mixing length parameter is employed to reproduce the pressure-temperature stratification of the outer convective zone calculated by 2D-hydrodynamical models we used for comparison. With this changed subatmospheric structure we could improve the agreement between predicted and observed p-mode frequencies in our solar models. 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: 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: Hydrodynamical model atmospheres: convection and line formation in the Sun Authors: Ludwig, H. -G.; Steffen, M. Bibcode: 1995IAUS..176P.235L Altcode: No abstract at ADS 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: Spectroscopic Effects of T-Inhomogeneities in the Atmospheres of DA White Dwarfs Authors: Ludwig, Hans-Günter; Steffen, Matthias Bibcode: 1995LNP...443..128L Altcode: 1995whdw.conf..128L No abstract at ADS Title: Grasping at the Hot End of ZZ Ceti Variability Authors: Gautschy, Alfred; Ludwig, Hans-Günter Bibcode: 1995LNP...443..295G Altcode: 1995whdw.conf..295G No abstract at ADS Title: Numerical simulations of convection at the surface of a ZZ Ceti white dwarf. Authors: Ludwig, H. -G.; Jordan, S.; Steffen, M. Bibcode: 1994A&A...284..105L Altcode: We applied two-dimensional hydrodynamics and non-grey radiative transfer calculations to the surface layers of a hydrogen-rich white dwarf (spectral type DA) with T_eff_=12600K and log(g)=8.0, corresponding to a position in the HR-diagram slightly cooler than the hot boundary of the ZZ Ceti instability strip. In our simulations the entire convection zone including the overshoot layers is embedded in the computational box so that we obtain a complete and detailed model of convection for this representative object. We address the important question to what extent models based on mixing length theory (MLT) are able to predict the physical properties of convection. We find a rapidly (timescale ~100ms) evolving flow pattern with fast concentrated downdrafts surrounded by slow broad upflows of warmer material. Convection carries up to 30% of the total flux and excites internal gravity waves by dynamical processes associated with the merging of downdrafts. The mean entropy gradient is reversed with respect to MLT predictions in the deeper layers of the convection zone. Strong overshoot occurs at its upper and lower boundary. A synthetic spectrum calculated from the mean photospheric temperature stratification can be fitted satisfactorily with a MLT model adopting α=1.5. At greater depth the temperature profile approaches a model with α=4. The total depth of the convective layers is rather small compared to values suggested by studies of the excitation mechanism for the pulsations of DAs. Title: Numerical simulation of overshoot at the base of the solar convection zone Authors: Ludwig, H. -G. Bibcode: 1994smf..conf..149L Altcode: No abstract at ADS Title: Numerical simulations of convection at the surface of a ZZ Ceti white dwarf. Authors: Ludwig, H. -G.; Jordan, S.; Steffen, M. Bibcode: 1993AGAb....9..147L Altcode: No abstract at ADS Title: First numerical simulations of convection at the surface of a ZZ Ceti white dwarf Authors: Ludwig, H. -G.; Jordan, S.; Steffen, M. Bibcode: 1993ASIC..403..471L Altcode: 1993wdao.conf..471L No abstract at ADS Title: Results from 2-D Numerical Simulations of Solar Granules Authors: Steffen, M.; Gigas, D.; Holweger, H.; Krüss, A.; Ludwig, H. -G. Bibcode: 1990IAUS..138..213S Altcode: No abstract at ADS Title: Non-grey radiative transfer in numerical simulations of photospheric convection. Authors: Ludwig, H. -G.; Steffen, M.; Rauch, T. Bibcode: 1990AGAb....5...38L Altcode: No abstract at ADS Title: A numerical simulation study of solar granular convection in cells of different horizontal dimension Authors: Steffen, M.; Ludwig, H. -G.; Kruess, A. Bibcode: 1989A&A...213..371S Altcode: The properties of a series of granular convection cells of different horizontal scale, obtained from two-dimensional numerical simulations of the solar granulation, are examined. For model diameters below approximately 2000 km, steady state solutions are obtained. It is shown that the properties of granular convection systematically depend on the horizontal scale of the granules. Results indicate that the larger granular structures have the strongest influence on the visible layers of the solar photosphere, while the smaller granules become increasingly more difficult to observe as their activity progressively disappears below the surface. It is demonstrated that details of numerical results can be largely understood in terms of 'buoyancy braking' and radiative transfer. Title: Results from 2-D Numerical Simulations of the Solar Granulation Authors: Steffen, M.; Gigas, D.; Holweger, H.; Krüß, A.; Ludwig, H. -G. Bibcode: 1989AGAb....3...12S Altcode: No abstract at ADS Title: Struktur solarer Konvektionszellen unterschiedlicher Durchmesser. Authors: Steffen, M.; Ludwig, H. G.; Krüss, A. Bibcode: 1988AGAb....1...10S Altcode: No abstract at ADS Title: Bestimmung von Stationskoordinaten aus der Analyse langer Bahnen. Authors: Reigber, C.; Ludwig, H. Bibcode: 1976VeBKI..35..126R Altcode: No abstract at ADS Title: Die Arbeiten des Sonderforschungsbereiches 78 Satellitengeodäsie der Technischen Universität München im Jahre 1974. Authors: Sigl, R.; Kaniuth, K.; Bauch, A.; Dichtl, G.; Nagel, E.; Schmidt, H. F.; Ilk, K. H.; Ludwig, H.; Schneider, M.; Reigber, C. Bibcode: 1975VeBKI..33.....S Altcode: No abstract at ADS Title: Einige Bemerkungen zur Bestimmung und Verwendung geozentrischer Koordinaten. Authors: Sigl, R.; Ludwig, H. Bibcode: 1973VeBKI..31..149S Altcode: No abstract at ADS Title: Die Arbeiten des Sonderforschungsbereiches 78 Satellitengeodäsie im Jahre 1972. Authors: Sigl, R.; Dichtl, G.; Ilk, K. H.; Lelgemann, D.; Wilson, P.; Ludwig, H.; Nagel, E.; Reigber, C.; Schneider, M.; Schmidt, F. Bibcode: 1973VeBKI..30.....S Altcode: No abstract at ADS Title: Anwendung der Hammersteinschen Methode der unendlich vielen Variablen auf Probleme der Satellitengeodäsie und Himmelsmechanik. Authors: Sigl, R.; Schneider, M.; Reigber, C.; Ludwig, H. Bibcode: 1970adhm.book.....S Altcode: No abstract at ADS