Author name code: steffen ADS astronomy entries on 2022-09-14 author:"Steffen, Matthias" ------------------------------------------------------------------------ Title: Barium lines in high-quality spectra of two metal-poor giants in the Galactic halo Authors: Cescutti, G.; Morossi, C.; Franchini, M.; Di Marcantonio, P.; Chiappini, C.; Steffen, M.; Valentini, M.; François, P.; Christlieb, N.; Cortés, C.; Kobayashi, C.; Depagne, E. Bibcode: 2021A&A...654A.164C Altcode: 2021arXiv210900277C Context. Theoretical results showed the possibility that neutron capture elements were produced in the early Universe by two different sources: a frequent s-process source hosted by rotating massive stars, and a rare r-process source hosted most likely by neutron star mergers. The two sources produce barium with different isotopic compositions.
Aims: We aim to investigate the lines of barium in two halo stars, HD 6268 and HD 4306. The spectra present an exquisite quality, both in terms of resolution (R > 100 000) and signal-to-noise (~400). Due to hyperfine splitting (hfs) effects, barium lines are expected to show slightly different profiles depending on the barium isotopic fraction.
Methods: We applied a standard local thermodynamic equilibrium synthesis of the barium lines. We compared the synthetic results assuming an s-process isotopic pattern or an r-process isotopic pattern for the two barium lines for each star that exhibited hfs. We also applied a methodology, less dependent on the accuracy of the theoretical Ba hfs structure, that transforms the lines of HD 4306 into those we would observe if its atmospheric parameter values (i.e. Teff, log g, micro- and macro-turbulence, V sin i, and Ba abundance) were the same as those of HD 6268.
Results: With both methods, our results show that the barium lines with hfs effects of HD 4306 are in agreement with an s-process composition and the lines in HD 6268 have a different profile, which is most likely linked to the presence of an r-process isotopic pattern.
Conclusions: Two lines of barium of HD 6268 and HD 4306 seem to confirm the theoretical expectation that both r-process events and also s-process contribution by rotating massive stars have polluted the ancient halo of our Galaxy.

Based on observations made with the UVES at the ESO Very Large Telescope, Paranal Observatory, Chile (ID 098.B-0094(A); P.I. G. Cescutti). Title: The metal-poor end of the Spite plateau. II. Chemical and dynamical investigation Authors: Matas Pinto, A. M.; Spite, M.; Caffau, E.; Bonifacio, P.; Sbordone, L.; Sivarani, T.; Steffen, M.; Spite, F.; François, P.; Di Matteo, P. Bibcode: 2021A&A...654A.170M Altcode: 2021arXiv211000243M Context. The study of old, metal-poor stars deepens our knowledge on the early stages of the universe. In particular, the study of these stars gives us a valuable insight into the masses of the first massive stars and their emission of ionising photons.
Aims: We present a detailed chemical analysis and determination of the kinematic and orbital properties of a sample of 11 dwarf stars. These are metal-poor stars, and a few of them present a low lithium content. We inspected whether the other elements also present anomalies.
Methods: We analysed the high-resolution UVES spectra of a few metal-poor stars using the Turbospectrum code to synthesise spectral lines profiles. This allowed us to derive a detailed chemical analysis of Fe, C, Li, Na, Mg, Al, Si, CaI, CaII, ScII, TiII, Cr, Mn, Co, Ni, Sr, and Ba.
Results: We find excellent coherence with the reference metal-poor First Stars sample. The lithium-poor stars do not present any anomaly of the abundance of the elements other than lithium. Among the Li-poor stars, we show that CS 22882-027 is very probably a blue-straggler. The star CS 30302-145, which has a Li abundance compatible with the plateau, has a very low Si abundance and a high Mn abundance. In many aspects, it is similar to the α-poor star HE 1424-0241, but it is less extreme. It could have been formed in a satellite galaxy and later been accreted by our Galaxy. This hypothesis is also supported by its kinematics.

The table with equivalent widths discussed in this paper is 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/654/A170

Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere (Programmes 076.A-0463 PI(Lopez), 077.D-0299 PI(Bonifacio)), 086.D-0871(A) (PI Meléndez). Title: VizieR Online Data Catalog: Abundances of metal-poor stars (Matas Pinto+, 2021) Authors: Matas Pinto, A. M.; Spite, M.; Caffau, E.; Bonifacio, P.; Sbordone, L.; Sivarani, T.; Steffen, M.; Spite, F.; Francois, P.; Di Matteo, P. Bibcode: 2021yCat..36540170M Altcode: The observations of the stars we studied are described in detail in Paper I (Sbordone et al., 2010A&A...522A..26S, Cat. J/A+A/522/A26) (see their Table 1).

Briefly, observations were performed with the high-resolution spectrograph UVES at the ESO-VLT. The spectra have a resolving power R=~40000 and were centred at 390nm (spectral range: 330-451nm) and 580nm (spectral range: 479-680nm). For two stars (BS 17572-100 and HE 1413-1954) that were previously studied in the frame of the HERES program (Christlieb et al., 2004A&A...428.1027C; Barklem et al., 2005A&A...439..129B, Cat. J/A+A/439/129) from UVES spectra centred at 437nm (spectral range: 376-497nm), the blue spectra were centred at 346 nm (spectral range: 320-386nm). The S/N of the spectra at 400nm is only about half of the S/N measured at 670nm (see Table 1 in Paper I) and thus generally does not exceed 50. For two stars, CS 22188-033 and HE 0148-2611, new UVES spectra from the ESO archives, centred at 390 and 580nm, were also used, increasing the S/N ratio of the mean spectrum. The data were reduced using the standard UVES pipeline with the same procedures as used in Bonifacio et al. (2007A&A...462..851B).

Here we present the table with equivalent widths discussed in the paper.

(2 data files). 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: 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: 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 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: Probing the atmosphere of HD189733b with the Na I and K I lines Authors: Keles, E.; Kitzmann, D.; Mallonn, M.; Alexoudi, X.; Fossati, L.; Pino, L.; Seidel, J. V.; Carroll, T. A.; Steffen, M.; Ilyin, I.; Poppenhäger, K.; Strassmeier, K. G.; von Essen, C.; Nascimbeni, V.; Turner, J. D. Bibcode: 2020MNRAS.498.1023K Altcode: 2020MNRAS.tmp.2443K; 2020arXiv200804044K High spectral resolution transmission spectroscopy is a powerful tool to characterize exoplanet atmospheres. Especially for hot Jupiters, this technique is highly relevant, due to their high-altitude absorption, e.g. from resonant sodium (Na I) and potassium (K I) lines. We resolve the atmospheric K I absorption on HD189733b with the aim to compare the resolved K I line and previously obtained high-resolution Na I-D line observations with synthetic transmission spectra. The line profiles suggest atmospheric processes leading to a line broadening of the order of ∼10 km/s for the Na I-D lines and only a few km/s for the K I line. The investigation hints that either the atmosphere of HD189733b lacks a significant amount of K I or the alkali lines probe different atmospheric regions with different temperature, which could explain the differences we see in the resolved absorption lines. 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: 16th Potsdam Thinkshop: The rotation periods of cool stars: Measurements, uses, connections and prospects Authors: Barnes, S. A.; Steffen, M.; Strassmeier, K. G. Bibcode: 2020AN....341..485B Altcode: No abstract at ADS Title: Improving spectroscopic lithium abundances. Fitting functions for 3D non-LTE corrections in FGK stars of different metallicity Authors: Mott, A.; Steffen, M.; Caffau, E.; Strassmeier, K. G. Bibcode: 2020A&A...638A..58M Altcode: 2020arXiv200410803M Context. Accurate spectroscopic lithium abundances are essential in addressing a variety of open questions, such as the origin of a uniform lithium content in the atmospheres of metal-poor stars (Spite plateau) or the existence of a correlation between the properties of extrasolar planetary systems and the lithium abundance in the atmosphere of their host stars.
Aims: We have developed a tool that allows the user to improve the accuracy of standard lithium abundance determinations based on 1D model atmospheres and the assumption of local thermodynamic equilibrium (LTE) by applying corrections that account for hydrodynamic (3D) and non-LTE (NLTE) effects in FGK stars of different metallicity.
Methods: Based on a grid of CO5BOLD 3D models and associated 1D hydrostatic atmospheres, we computed three libraries of synthetic spectra of the lithium λ 670.8 nm line for a wide range of lithium abundances, accounting for detailed line formation in 3D NLTE, 1D NLTE, and 1D LTE, respectively. The resulting curves-of-growth were then used to derive 3D NLTE and 1D NLTE lithium abundance corrections.
Results: For all metallicities, the largest corrections are found at the coolest effective temperature, Teff = 5000 K. They are mostly positive, up to + 0.2 dex, for the weakest lines (lithium abundance A(Li)1DLTE = 1.0), whereas they become more negative towards lower metallicities, where they can reach - 0.4 dex for the strongest lines (A(Li)1DLTE = 3.0) at [Fe/H] = - 2.0. We demonstrate that 3D and NLTE effects are small for metal-poor stars on the Spite plateau, leading to errors of at most ± 0.05 dex if ignored. We present analytical functions evaluating the 3D NLTE and 1D NLTE corrections as a function of Teff [5000…6500 K], log g [3.5…4.5], and LTE lithium abundance A(Li) [1.0…3.0] for a fixed grid of metallicities [Fe/H] [ - 3.0…0.0]. In addition, we also provide analytical fitting functions for directly converting a given lithium abundance into an equivalent width, or vice versa, a given equivalent width (EW) into a lithium abundance. For convenience, a Python script is made available that evaluates all fitting functions for given Teff, log g, [Fe/H], and A(Li) or EW.
Conclusions: By means of the fitting functions developed in this work, the results of complex 3D and NLTE calculations are made readily accessible and quickly applicable to large samples of stars across a wide range of metallicities. Improving the accuracy of spectroscopic lithium abundance determinations will contribute to a better understanding of the open questions related to the lithium content in metal-poor and solar-like stellar atmospheres. 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: Confronting expansion distances of planetary nebulae with Gaia DR2 measurements Authors: Schönberner, D.; Steffen, M. Bibcode: 2019A&A...625A.137S Altcode: 2019arXiv190410721S Context. Individual distances to planetary nebulae are of the utmost relevance for our understanding of post-asymptotic giant-branch evolution because they allow a precise determination of stellar and nebular properties. Also, objects with individual distances serve as calibrators for the so-called statistical distances based on secondary nebular properties.
Aims: With independently known distances, it is possible to check empirically our understanding of the formation and evolution of planetary nebulae as suggested by existing hydrodynamical simulations.
Methods: We compared the expansion parallaxes that have recently been determined for a number of planetary nebulae with the trigonometric parallaxes provided by the Gaia Data Release 2.
Results: Except for two out of 11 nebulae, we found good agreement between the expansion and the Gaia trigonometric parallaxes without any systematic trend with distance. Therefore, the Gaia measurements also prove that the correction factors necessary to convert proper motions of shocks into Doppler velocities cannot be ignored. Rather, the size of these correction factors and their evolution with time as predicted by 1D hydrodynamical models of planetary nebulae is basically validated. These correction factors are generally greater than unity and are different for the outer shell and the inner bright rim of a planetary nebula. The Gaia measurements also confirm earlier findings that spectroscopic methods often lead to an overestimation of the distance. They also show that even modelling of the entire system of star and nebula by means of sophisticated photoionisation modelling may not always provide reliable results.
Conclusions: The Gaia measurements confirm the basic correctness of the present radiation-hydrodynamics models, which predict that both the shell and the rim of a planetary nebula are two independently expanding entities, created and driven by different physical processes, namely thermal pressure (shell) or wind interaction (rim), both of which vary differently with time.

This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. 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: Hot bubbles of planetary nebulae with hydrogen-deficient winds. II. Analytical approximations with application to BD + 30°3639 Authors: Heller, R.; Jacob, R.; Schönberner, D.; Steffen, M. Bibcode: 2018A&A...620A..98H Altcode: 2018arXiv180908305H Context. The first high-resolution X-ray spectroscopy of a planetary nebula, BD +30° 3639, opened the possibility to study plasma conditions and chemical compositions of X-ray emitting "hot" bubbles of planetary nebulae in much greater detail than before.
Aims: We investigate (i) how diagnostic line ratios are influenced by the bubble's thermal structure and chemical profile, (ii) whether the chemical composition inside the bubble of BD +30° 3639 is consistent with the hydrogen-poor composition of the stellar photosphere and wind, and (iii) whether hydrogen-rich nebular matter has already been added to the bubble of BD +30° 3639 by evaporation.
Methods: We applied an analytical, one-dimensional (1D) model for wind-blown bubbles with temperature and density profiles based on self-similar solutions including thermal conduction. We also constructed heat-conduction bubbles with a chemical stratification. The X-ray emission was computed using the well-documented CHIANTI code. These bubble models are used to re-analyse the high-resolution X-ray spectrum from the hot bubble of BD +30° 3639.
Results: We found that our 1D heat-conducting bubble models reproduce the observed line ratios much better than plasmas with single electron temperatures. In particular, all the temperature- and abundance-sensitive line ratios are consistent with BD +30° 3639 X-ray observations for (i) an intervening column density of neutral hydrogen, NH = 0.20-0.10+0.05 × 1022cm-2, (ii) a characteristic bubble X-ray temperature of TX = 1.8 ± 0.1 MK together with (iii) a very high neon mass fraction of about 0.05, virtually as high as that of oxygen. For lower values of NH, we cannot exclude the possibility that the hot bubble of BD +30° 3639 contains a small amount of "evaporated" (or mixed) hydrogen-rich nebular matter. Given the possible range of NH, the fraction of evaporated hydrogen-rich matter cannot exceed 3% of the bubble mass.
Conclusions: The diffuse X-ray emission from BD +30° 3639 can be well explained by models of wind-blown bubbles with thermal conduction and a chemical composition equal to that of the hydrogen-poor and carbon-, oxygen-, and neon-rich stellar surface. 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: 3D non-LTE corrections for Li abundance and 6Li/7Li isotopic ratio in solar-type stars. I. Application to HD 207129 and HD 95456 Authors: Harutyunyan, G.; Steffen, M.; Mott, A.; Caffau, E.; Israelian, G.; González Hernández, J. I.; Strassmeier, K. G. Bibcode: 2018A&A...618A..16H Altcode: 2018arXiv180704089H Context. Convective motions in solar-type stellar atmospheres induce Doppler shifts that affect the strengths and shapes of spectral absorption lines and create slightly asymmetric line profiles. One-dimensional (1D) local thermodynamic equilibrium (LTE) studies of elemental abundances are not able to reproduce this phenomenon, which becomes particularly important when modeling the impact of isotopic fine structure, like the subtle depression created by the 6Li isotope on the red wing of the Li I resonance doublet line.
Aims: The purpose of this work is to provide corrections for the lithium abundance, A(Li), and the 6Li/7Li isotopic ratio that can easily be applied to correct 1D LTE lithium abundances in G and F dwarf stars of approximately solar mass and metallicity for three-dimensional (3D) and non-LTE (NLTE) effects.
Methods: The corrections for A(Li) and 6Li/7Li are computed using grids of 3D NLTE and 1D LTE synthetic lithium line profiles, generated from 3D hydro-dynamical CO5BOLD and 1D hydrostatic model atmospheres, respectively. For comparative purposes, all calculations are performed for three different line lists representing the Li I λ670.8 nm spectral region. The 3D NLTE corrections are then approximated by analytical expressions as a function of the stellar parameters (Teff, log ℊ, [Fe/H], ν sin i, A(Li), 6Li/7Li). These are applied to adjust the 1D LTE isotopic lithium abundances in two solar-type stars, HD 207129 and HD 95456, for which high-quality HARPS observations are available.
Results: The derived 3D NLTE corrections range between -0.01 and +0.11 dex for A(Li), and between -4.9 and -0.4% for 6Li/7Li, depending on the adopted stellar parameters. We confirm that the inferred 6Li abundance depends critically on the strength of the Si I 670.8025 nm line. Our findings show a general consistency with recent works on lithium abundance corrections. After the application of such corrections, we do not find a significant amount of 6Li in any of the two target stars.
Conclusions: In the case of 6Li/7Li, our corrections are always negative, showing that 1D LTE analysis can significantly overestimate the presence of 6Li (up to 4.9% points) in the atmospheres of solar-like dwarf stars. These results emphasize the importance of reliable 3D model atmospheres combined with NLTE line formation for deriving precise isotopic lithium abundances. Although 3D NLTE spectral synthesis implies an extensive computational effort, the results can be made accessible with parametric tools like the ones presented in this work.

The table with the 3D NLTE corrections is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/618/A16 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: 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: VizieR Online Data Catalog: A(Li) and 6Li/7Li 3D NLTE corrections (Harutyunyan+, 2018) Authors: Harutyunyan, G.; Steffen, M.; Mott, A.; Caffau, E.; Israelian, G.; Gonzalez Hernandez, J. I.; Strassmeier, K. G. Bibcode: 2018yCat..36180016H Altcode: A grid of 3D non-LTE (NLTE) corrections for the lithium abundance, A(Li), and the 6Li/7Li isotopic ratio are presented. These corrections can be easily applied to correct 1D LTE lithium abundances in G and F dwarf stars of approximately solar mass and metallicity for 3D and NLTE effects. The stellar parameters defining the grid are effective temperatures, Teff (5900, 6300 and 6500K), surface gravity, logg (4.0 and 4.5), metallicity, [Fe/H] (-1.0, -0.5, 0.0, +0.5), 1D LTE lithium abundance, A(Li) (1.5, 2.0, 2.5), 1D LTE 6Li/7Li isotopic ratio (0, 5 and 10%), as well as projected rotational velocity, vsini (0, 2, 4 and 6km/s). Based on this table, a web page calculator was created that allows to compute the 3D NLTE corrections of A(Li) and 6Li/7Li ratio for a given combination of stellar parameters (https://pages.aip.de/li67nlte3d/).

(1 data file). Title: Simulation of the small-scale magnetism in main-sequence stellar atmospheres Authors: Salhab, R. G.; Steiner, O.; Berdyugina, S. V.; Freytag, B.; Rajaguru, S. P.; Steffen, M. Bibcode: 2018A&A...614A..78S Altcode: Context. Observations of the Sun tell us that its granular and subgranular small-scale magnetism has significant consequences for global quantities such as the total solar irradiance or convective blueshift of spectral lines.
Aims: In this paper, properties of the small-scale magnetism of four cool stellar atmospheres, including the Sun, are investigated, and in particular its effects on the radiative intensity and flux.
Methods: We carried out three-dimensional radiation magnetohydrodynamic simulations with the CO5BOLD code in two different settings: with and without a magnetic field. These are thought to represent states of high and low small-scale magnetic activity of a stellar magnetic cycle.
Results: We find that the presence of small-scale magnetism increases the bolometric intensity and flux in all investigated models. The surplus in radiative flux of the magnetic over the magnetic field-free atmosphere increases with increasing effective temperature, Teff, from 0.47% for spectral type K8V to 1.05% for the solar model, but decreases for higher effective temperatures than solar. The degree of evacuation of the magnetic flux concentrations monotonically increases with Teff as does their depression of the visible optical surface, that is the Wilson depression. Nevertheless, the strength of the field concentrations on this surface stays remarkably unchanged at ≈1560 G throughout the considered range of spectral types. With respect to the surrounding gas pressure, the field strength is close to (thermal) equipartition for the Sun and spectral type F5V but is clearly sub-equipartition for K2V and more so for K8V. The magnetic flux concentrations appear most conspicuous for model K2V owing to their high brightness contrast.
Conclusions: For mean magnetic flux densities of approximately 50 G, we expect the small-scale magnetism of stars in the spectral range from F5V to K8V to produce a positive contribution to their bolometric luminosity. The modulation seems to be most effective for early G-type stars. Title: 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: PEPSI deep spectra. I. The Sun-as-a-star Authors: Strassmeier, K. G.; Ilyin, I.; Steffen, M. Bibcode: 2018A&A...612A..44S Altcode: 2017arXiv171206960S Context. Full-disk solar flux spectra can be directly compared to stellar spectra and thereby serve as our most important reference source for, for example stellar chemical abundances, magnetic activity phenomena, radial-velocity signatures or global pulsations. Aim. As part of the first Potsdam Echelle Polarimetric and Spectroscopic Instrument (PEPSI) key-science project, we aim to provide well-exposed and average-combined (viz. deep) high-resolution spectra of representative stellar targets. Such deep spectra contain an overwhelming amount of information, typically much more than what could be analyzed and discussed within a single publication. Therefore, these spectra will be made available in form of (electronic) atlases. The first star in this series of papers is our Sun. It also acts as a system-performance cornerstone.
Methods: The Sun was monitored with PEPSI at the Large Binocular Telescope (LBT). Instead of the LBT we used a small robotic solar disk integration (SDI) telescope. The deep spectra in this paper are the results of combining up to ≈100 consecutive exposures per wavelength setting and are compared with other solar flux atlases.
Results: Our software for the optimal data extraction and reduction of PEPSI spectra is described and verified with the solar data. Three deep solar flux spectra with a spectral resolution of up to 270 000, a continuous wavelength coverage from 383 nm to 914 nm, and a photon signal to noise ratio (S/N) of between 2000-8000:1 depending on wavelength are presented. Additionally, a time-series of 996 high-cadence spectra in one cross disperser is used to search for intrinsic solar modulations. The wavelength calibration based on Th-Ar exposures and simultaneous Fabry-Pérot combs enables an absolute wavelength solution within 10 m s-1 (rms) with respect to the HARPS laser-comb solar atlas and a relative rms of 1.2 m s-1 for one day. For science demonstration, we redetermined the disk-average solar Li abundance to 1.09 ± 0.04 dex on the basis of 3D NLTE model atmospheres. We detected disk-averaged p-mode RV oscillations with a full amplitude of 47 cm s-1 at 5.5 min.
Conclusions: Comparisons with two solar FTS atlases, as well as with the HARPS solar atlas, validate the PEPSI data product. Now, PEPSI/SDI solar-flux spectra are being taken with a sampling of one deep spectrum per day, and are supposed to continue a full magnetic cycle of the Sun.

Based on data acquired with PEPSI fed by the solar disk integration (SDI) telescope operated by AIP at the Large Binocular Telescope Observatory (LBTO). The LBT is an international collaboration among institutions in the United States, Italy and Germany. LBT Corporation partners are: The University of Arizona on behalf of the Arizona Board of Regents; Istituto Nazionale di Astrofisica, Italy; LBT Beteiligungsgesellschaft, Germany, representing the Max-Planck Society, The Leibniz Institute for Astrophysics Potsdam (AIP), and Heidelberg University; The Ohio State University, and The Research Corporation, on behalf of The University of Notre Dame, University of Minnesota and University of Virginia. 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: 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: Analysis of the X-ray spectrum of the hot bubble of BD+30°3639 Authors: Schönberner, Detlef; Jacob, Ralf; Heller, René; Steffen, Matthias Bibcode: 2017IAUS..323..109S Altcode: 2016arXiv161201389S We developed a model for wind-blown bubbles with temperature and density profiles based on self-similar solutions including thermal conduction. We constructed also heat-conduction bubbles with chemical discontinuities. The X-ray emission is computed using the well-documented CHIANTI code (v6.0.1). These bubble models are used to (re)analyse the high-resolution X-ray spectrum of the hot bubble of BD+30°3639, and they appeared to be much superior to constant temperature approaches.

We found for the X-ray emission of BD+30°3639 that temperature-sensitive and abundance-sensitive line ratios computed on the basis of heat-conducting wind-blown bubbles and with abundances as found in the stellar photosphere/wind can only be reconciled with the observations if the hot bubble of BD+30°3639 is chemically stratified, i.e. if it contains also a small mass fraction (~= 3 %) of hydrogen-rich matter immediately behind the conduction front. Neon appears to be strongly enriched, with a mass fraction of at least about 0.06. 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: Lithium abundance and 6Li/7Li ratio in the active giant HD 123351. I. A comparative analysis of 3D and 1D NLTE line-profile fits Authors: Mott, A.; Steffen, M.; Caffau, E.; Spada, F.; Strassmeier, K. G. Bibcode: 2017A&A...604A..44M Altcode: 2017arXiv170406460M Context. Current three-dimensional (3D) hydrodynamical model atmospheres together with detailed spectrum synthesis, accounting for departures from local thermodynamic equilibrium (LTE), permit to derive reliable atomic and isotopic chemical abundances from high-resolution stellar spectra. Not much is known about the presence of the fragile 6Li isotope in evolved solar-metallicity red giant branch (RGB) stars, not to mention its production in magnetically active targets like HD 123351.
Aims: A detailed spectroscopic investigation of the lithium resonance doublet in HD 123351 in terms of both abundance and isotopic ratio is presented. From fits of the observed spectrum, taken at the Canada-France-Hawaii telescope, with synthetic line profiles based on 1D and 3D model atmospheres, we seek to estimate the abundance of the 6Li isotope and to place constraints on its origin.
Methods: We derive the lithium abundance A(Li) and the 6Li/7Li isotopic ratio by fitting different synthetic spectra to the Li-line region of a high-resolution CFHT spectrum (R = 120 000, S/N = 400). The synthetic spectra are computed with four different line lists, using in parallel 3D hydrodynamical CO5BOLD and 1D LHD model atmospheres and treating the line formation of the lithium components in non-LTE (NLTE). The fitting procedure is repeated with different assumptions and wavelength ranges to obtain a reasonable estimate of the involved uncertainties.
Results: We find A(Li) = 1.69 ± 0.11 dex and 6Li/7Li = 8.0 ± 4.4% in 3D-NLTE, using the line list of Meléndez et al. (2012, A&A, 543, A29), updated with new atomic data for V I, which results in the best fit of the lithium line profile of HD 123351. Two other line lists lead to similar results but with inferior fit qualities.
Conclusions: Our 2σ detection of the 6Li isotope is the result of a careful statistical analysis and the visual inspection of each achieved fit. Since the presence of a significant amount of 6Li in the atmosphere of a cool evolved star is not expected in the framework of standard stellar evolution theory, non-standard, external lithium production mechanisms, possibly related to stellar activity or a recent accretion of rocky material, need to be invoked to explain the detection of 6Li in HD 123351. 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: 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: 3D non-LTE corrections for the 6Li/7Li isotopic ratio in solar-type stars Authors: Harutyunyan, G.; Steffen, M.; Mott, A.; Caffau, E.; Israelian, G.; González Hernández, J. I.; Strassmeier, K. G. Bibcode: 2017MmSAI..88...61H Altcode: Doppler shifts induced by convective motions in stellar atmospheres affect the shape of spectral absorption lines and create slightly asymmetric line profiles. It is important to take this effect into account in modeling the subtle depression created by the 6Li isotope which lies on the red wing of the Li I 670.8 nm resonance doublet line, since convective motions in stellar atmospheres can mimic a presence of 6Li when intrinsically symmetric theoretical line profiles are presumed for the analysis of the 7Li doublet \citep{cayrel2007}. Based on CO5BOLD hydrodynamical model atmospheres, we compute 3D non-local thermodynamic equilibrium (NLTE) corrections for the 6Li/7Li isotopic ratio by using a grid of 3D NLTE and 1D LTE synthetic spectra. These corrections must be added to the results of the 1D LTE analysis to correct them for the combined 3D non-LTE effects. As one would expect, the resulting corrections are always negative and they range between 0 and -5 %, depending on effective temperature, surface gravity, and metallicity. For each metallicity we derive an analytic expression approximating the 3D NLTE corrections as a function of effective temperature, surface gravity and projected rotational velocity. Title: Radiation transport in CO5BOLD. A short-characteristics module for local box models Authors: Steffen, M. Bibcode: 2017MmSAI..88...22S Altcode: We give a description of a new radiative transfer package based on an energy-conserving short-characteristics (SC) approach, developed for both CO5BOLD 3D radiation hydrodynamics local box simulations and NLTE3D line formation post processing. First test results for a realistic 3D solar atmosphere model indicate that the thermal structure obtained with the new radiation transport scheme closely matches the one found with the standard long-characteristics Feautrier (LCF) method that has served in CO5BOLD for many years. This is a very reassuring result, corroborating the validity of the CO5BOLD models computed previously with the LCF radiation transport. 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: Lithium in the active sub-giant HD123351. A quantitative analysis with 3D and 1D model atmospheres using different observed spectra Authors: Mott, A.; Steffen, M.; Caffau, E.; Strassmeier, K. G. Bibcode: 2017MmSAI..88...68M Altcode: Current 3D hydrodynamical model atmosphere simulations together with non-LTE spectrum synthesis calculations permit to determine reliable atomic and in particular isotopic chemical abundances. Although this approach is computationally time demanding, it became feasible in studying lithium in stellar spectra. In the literature not much is known about the presence of the more fragile {6Li} isotope in evolved metal-rich objects. In this case the analysis is complicated by the lack of a suitable list of atomic and molecular lines in the spectral region of the lithium resonance line at 670.8 nm.

Here we present a spectroscopic comparative analysis of the Li doublet region of HD 123351, an active sub-giant star of solar metallicity. We fit the Li profile in three observed spectra characterized by different qualities: two very-high resolution spectra (Gecko@CFHT, R=120 000, SNR=400 and PEPSI@LBT, R=150 000, SNR=663) and a high-resolution SOPHIE@OHP spectrum (R=40 000, SNR=300). We adopt a set of model atmospheres, both 3D and 1D, having different stellar parameters (T_{eff} and log g). The 3D models are taken from the CIFIST grid of COBOLD model atmospheres and departures from LTE are considered for the lithium components. For the blends other than the lithium in this wavelength region we adopt the linelist of \citet{melendez12}. We find consistent results for all three observations and an overall good fit with the selected list of atomic and molecular lines, indicating a high {6Li} content.

The presence of {6Li} is not expected in cool stellar atmospheres. Its detection is of crucial importance for understanding mixing processes in stars and external lithium production mechanisms, possibly related to stellar activity or planetray accretion of {6Li}-rich material. 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: 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: Reconstructing the Milky Way's history: spectroscopic surveys, asteroseismology, and chemodynamical models Authors: Chiappini, Cristina; Montalbán, Josefina; Steffen, Matthias Bibcode: 2016AN....337..773C Altcode: No abstract at ADS Title: Investigation of the lithium 670.7 nm wavelength range in the solar spectrum Authors: Caffau, Elisabetta; Mott, Alessandro; Harutyunyan, Gohar; Malherbe, Jean-Marie; Steffen, Matthias Bibcode: 2016cosp...41E.281C Altcode: Lithium is a key chemical element, with a chemical evolution that is different from that of most other elements. It is also very fragile, as it is destroyed by nuclear reactions with protons at temperatures higher than about 2.5 million K. According to standard Big Bang nucleosynthesis, only the isotope 7Li is produced in significant amounts, while the primordial abundance of the lighter isotope 6Li is negligible. Lithium is not produced by nucleosynthesis in normal stars, except in peculiar phases of stellar evolution (e.g. in AGB stars and Novae). Lithium may also be formed as a result of flares in the atmospheres of young, active stars. To investigate the history of Li production and depletion in the Galaxy, it is necessary to analyse stars of all ages, including those at solar metallicity. In this case, the spectroscopic determination of the Li abundance is complicated by the presence of other spectral lines overlapping with the Li doublet at 670.7 nm. The correct identification and knowledge of the atomic parameters of these blend lines is critical, especially if the 6LI/7Li isotopic ratio is to be derived. In this investigation, we consider several line lists of the blending components available in the literature and use them to compute synthetic spectra, performing the line formation computations both for the classical 1D Holweger-Mueller model and a CO5BOLD 3D hydrodynamical simulation of the solar atmosphere. The synthetic spectra are then compared to the solar spectrum observed at different limb angles. This allows us to check the quality of existing line lists, to find potentially misidentified blend lines, and to construct an optimized line list for solar-type stars. Title: The Central Role of FORS1/2 Spectropolarimetric Observations for the Progress of Stellar Magnetism Studies Authors: Schöller, M.; Hubrig, S.; Ilyin, I.; Steffen, M.; Briquet, M.; Kholtygin, A. F. Bibcode: 2016Msngr.163...21S Altcode: The spectropolarimetric mode of the FOcal Reducer and low dispersion Spectrographs (FORS), which was first implemented in FORS1, and then moved to FORS2 seven years ago, has made it possible to probe the presence of magnetic fields in stars of different spectral classes at almost all stages of stellar evolution. While in the early days of FORS1, many of the observations were related to magnetic Ap/Bp stars and their progenitor Herbig Ae/Be stars, recent spectropolarimetric studies with FORS2 have involved more challenging targets, such as massive O- and B-type stars in clusters and in the field, very fast rotating massive stars with magnetospheres, Wolf-Rayet stars and central stars of planetary nebulae. The role of FORS observations for stellar magnetic field measurements is summarised and improvements in the measurement technique are described. Title: Hot bubbles of planetary nebulae with hydrogen-deficient winds. I. Heat conduction in a chemically stratified plasma Authors: Sandin, C.; Steffen, M.; Schönberner, D.; Rühling, U. Bibcode: 2016A&A...586A..57S Altcode: 2015arXiv151204986S Heat conduction has been found a plausible solution to explain discrepancies between expected and measured temperatures in hot bubbles of planetary nebulae (PNe). While the heat conduction process depends on the chemical composition, to date it has been exclusively studied for pure hydrogen plasmas in PNe. A smaller population of PNe show hydrogen-deficient and helium- and carbon-enriched surfaces surrounded by bubbles of the same composition; considerable differences are expected in physical properties of these objects in comparison to the pure hydrogen case. The aim of this study is to explore how a chemistry-dependent formulation of the heat conduction affects physical properties and how it affects the X-ray emission from PN bubbles of hydrogen-deficient stars. We extend the description of heat conduction in our radiation hydrodynamics code to work with any chemical composition. We then compare the bubble-formation process with a representative PN model using both the new and the old descriptions. We also compare differences in the resulting X-ray temperature and luminosity observables of the two descriptions. The improved equations show that the heat conduction in our representative model of a hydrogen-deficient PN is nearly as efficient with the chemistry-dependent description; a lower value on the diffusion coefficient is compensated by a slightly steeper temperature gradient. The bubble becomes somewhat hotter with the improved equations, but differences are otherwise minute. The observable properties of the bubble in terms of the X-ray temperature and luminosity are seemingly unaffected. 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: Consistent metallicity scale for cool dwarfs and giants. A benchmark test using the Hyades Authors: Dutra-Ferreira, L.; Pasquini, L.; Smiljanic, R.; Porto de Mello, G. F.; Steffen, M. Bibcode: 2016A&A...585A..75D Altcode: 2015arXiv150907725D Context. In several instances chemical abundances of main-sequence and giant stars are used simultaneously under the assumption that they share the same abundance scale. This assumption, if wrong, might have important implications in different astrophysical contexts.
Aims: It is therefore crucial to understand whether the metallicity or abundance differences among dwarfs and giants are real or are produced by systematic errors in the analysis. We aim to ascertain a methodology capable of producing a consistent metallicity scale for giants and dwarfs.
Methods: To achieve that, we analyzed giants and dwarfs in the Hyades open cluster, under the assumption that they share the same chemical composition. All the stars in this cluster have archival high-resolution spectroscopic data obtained with HARPS and UVES. In addition, the giants have interferometric measurements of the angular diameters. We analyzed the sample with two methods. The first method constrains the atmospheric parameters independently from spectroscopic method. For that we present a novel calibration of microturbulence based on 3D model atmospheres. The second method is the classical spectroscopic analysis based on Fe lines. We also tested two different line lists in an attempt to minimize possible non-LTE effects and to optimize the treatment of the giants.
Results: We show that it is possible to obtain a consistent metallicity scale between dwarfs and giants. The preferred method should constrain the three parameters Teff, log g, and ξ independent of spectroscopy. A careful selection of Fe lines is also important. In particular, the lines should not be chosen based on the Sun or other dwarfs, but specifically to be free of blends in the spectra of giants. When attention is paid to the line list, the classical spectroscopic method can also produce consistent results. In our test, the metallicities derived with the well-constrained set of stellar parameters are consistent independent of the line list used. Therefore, for this cluster we favor the metallicity of +0.18 ± 0.03 dex obtained with this method. The classical spectroscopic analysis, using the line list optimized for the giants, provides a metallicity of +0.14 ± 0.03 dex, in agreement with previous works.

Based on data obtained from the ESO Science Archive Facility. The observations were made with ESO Telescopes at the La Silla and Paranal Observatories under programmes ID 070.D-0356, 088.C-0513 and 070.D-0421. Title: SSALMON - The Solar Simulations for the Atacama Large Millimeter Observatory Network Authors: Wedemeyer, S.; Bastian, T.; Brajša, R.; Barta, M.; Hudson, H.; Fleishman, G.; Loukitcheva, M.; Fleck, B.; Kontar, E.; De Pontieu, B.; Tiwari, S.; Kato, Y.; Soler, R.; Yagoubov, P.; Black, J. H.; Antolin, P.; Gunár, S.; Labrosse, N.; Benz, A. O.; Nindos, A.; Steffen, M.; Scullion, E.; Doyle, J. G.; Zaqarashvili, T.; Hanslmeier, A.; Nakariakov, V. M.; Heinzel, P.; Ayres, T.; Karlicky, M. Bibcode: 2015AdSpR..56.2679W Altcode: 2015arXiv150205601W The Solar Simulations for the Atacama Large Millimeter Observatory Network (SSALMON) was initiated in 2014 in connection with two ALMA development studies. The Atacama Large Millimeter/submillimeter Array (ALMA) is a powerful new tool, which can also observe the Sun at high spatial, temporal, and spectral resolution. The international SSALMONetwork aims at co-ordinating the further development of solar observing modes for ALMA and at promoting scientific opportunities for solar physics with particular focus on numerical simulations, which can provide important constraints for the observing modes and can aid the interpretation of future observations. The radiation detected by ALMA originates mostly in the solar chromosphere - a complex and dynamic layer between the photosphere and corona, which plays an important role in the transport of energy and matter and the heating of the outer layers of the solar atmosphere. Potential targets include active regions, prominences, quiet Sun regions, flares. Here, we give a brief overview over the network and potential science cases for future solar observations with ALMA. Title: Chemical composition of a sample of bright solar-metallicity stars Authors: Caffau, E.; Mott, A.; Steffen, M.; Bonifacio, P.; Strassmeier, K. G.; Gallagher, A.; Faraggiana, R.; Sbordone, L. Bibcode: 2015AN....336..968C Altcode: 2015arXiv151004269C We present a detailed analysis of seven young stars observed with the spectrograph SOPHIE at the Observatoire de Haute-Provence for which the chemical composition was incomplete or absent in the literature. For five stars, we derived the stellar parameters and chemical compositions using our automatic pipeline optimized for F, G, and K stars, while for the other two stars with high rotational velocity, we derived the stellar parameters by using other information (parallax), and performed a line-by-line analysis. Chromospheric emission-line fluxes from Ca II are obtained for all targets. The stellar parameters we derive are generally in good agreement with what is available in the literature. We provide a chemical analysis of two of the stars for the first time. The star HIP 80124 shows a strong Li feature at 670.8 nm implying a high lithium abundance. Its chemical pattern is not consistent with it being a solar sibling, as has been suggested.

Data obtained at Observatoire de Haute Provence, with the SOPHIE spectrograph. 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: On the Evolution of Magnetic White Dwarfs Authors: Tremblay, P. -E.; Fontaine, G.; Freytag, B.; Steiner, O.; Ludwig, H. -G.; Steffen, M.; Wedemeyer, S.; Brassard, P. Bibcode: 2015ApJ...812...19T Altcode: 2015arXiv150905398T We present the first radiation magnetohydrodynamic simulations of the atmosphere of white dwarf stars. We demonstrate that convective energy transfer is seriously impeded by magnetic fields when the plasma-β parameter, the thermal-to-magnetic-pressure ratio, becomes smaller than unity. The critical field strength that inhibits convection in the photosphere of white dwarfs is in the range B = 1-50 kG, which is much smaller than the typical 1-1000 MG field strengths observed in magnetic white dwarfs, implying that these objects have radiative atmospheres. We have employed evolutionary models to study the cooling process of high-field magnetic white dwarfs, where convection is entirely suppressed during the full evolution (B ≳ 10 MG). We find that the inhibition of convection has no effect on cooling rates until the effective temperature (Teff) reaches a value of around 5500 K. In this regime, the standard convective sequences start to deviate from the ones without convection due to the convective coupling between the outer layers and the degenerate reservoir of thermal energy. Since no magnetic white dwarfs are currently known at the low temperatures where this coupling significantly changes the evolution, the effects of magnetism on cooling rates are not expected to be observed. This result contrasts with a recent suggestion that magnetic white dwarfs with Teff ≲ 10,000 K cool significantly slower than non-magnetic degenerates. Title: 3D 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: 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: The Born-again Planetary Nebulae Abell 30 and Abell 78 Authors: Todt, H.; Guerrero, M. A.; Fang, X.; Toala, J. A.; Arthur, J. S.; Blair, W. P.; Chu, Y. -H.; Gruendl, R. A.; Hamann, W. -R.; Marquez-Lugo, R. A.; Oskinova, L.; Ruiz, N.; Steffen, M.; Schoenberner, D. Bibcode: 2015ASPC..493..141T Altcode: The planetary nebulae Abell 30 and Abell 78 are born-again nebulae, which are believed to have undergone a very late thermal pulse, resulting in the ejection of hydrogen-poor material. Born-again PNe are a rare phenomenon, only a few are known. It is really intriguing that in all cases the expansion of the hydrogen-poor ejecta is highly asymmetrical. Here we present new HST observations of the expansion of the ejecta in Abell 30 and Abell 78, and compare them to ≍20 yrs older HST images. 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: The Chandra Planetary Nebula Survey (ChanPlaNS). III. X-Ray Emission from the Central Stars of Planetary Nebulae Authors: Montez, R., Jr.; Kastner, J. H.; Balick, B.; Behar, E.; Blackman, E.; Bujarrabal, V.; Chu, Y. -H.; Corradi, R. L. M.; De Marco, O.; Frank, A.; Freeman, M.; Frew, D. J.; Guerrero, M. A.; Jones, D.; Lopez, J. A.; Miszalski, B.; Nordhaus, J.; Parker, Q. A.; Sahai, R.; Sandin, C.; Schonberner, D.; Soker, N.; Sokoloski, J. L.; Steffen, M.; Toalá, J. A.; Ueta, T.; Villaver, E.; Zijlstra, A. Bibcode: 2015ApJ...800....8M Altcode: 2014arXiv1412.2793M We present X-ray spectral analysis of 20 point-like X-ray sources detected in Chandra Planetary Nebula Survey observations of 59 planetary nebulae (PNe) in the solar neighborhood. Most of these 20 detections are associated with luminous central stars within relatively young, compact nebulae. The vast majority of these point-like X-ray-emitting sources at PN cores display relatively "hard" (>=0.5 keV) X-ray emission components that are unlikely to be due to photospheric emission from the hot central stars (CSPN). Instead, we demonstrate that these sources are well modeled by optically thin thermal plasmas. From the plasma properties, we identify two classes of CSPN X-ray emission: (1) high-temperature plasmas with X-ray luminosities, L X, that appear uncorrelated with the CSPN bolometric luminosity, L bol and (2) lower-temperature plasmas with L X/L bol ~ 10-7. We suggest these two classes correspond to the physical processes of magnetically active binary companions and self-shocking stellar winds, respectively. In many cases this conclusion is supported by corroborative multiwavelength evidence for the wind and binary properties of the PN central stars. By thus honing in on the origins of X-ray emission from PN central stars, we enhance the ability of CSPN X-ray sources to constrain models of PN shaping that invoke wind interactions and binarity. 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: Properties of small-scale magnetism of stellar atmospheres Authors: Steiner, Oskar; Salhab, René; Freytag, Bernd; Rajaguru, Paul; Schaffenberger, Werner; Steffen, Matthias Bibcode: 2014PASJ...66S...5S Altcode: 2014PASJ..tmp...95S The magnetic field outside of sunspots is concentrated in the intergranular space, where it forms a delicate filigree of bright ribbons and dots as seen on broad band images of the Sun. We expect this small-scale magnetic field to exhibit a similar behavior in stellar atmospheres. In order to find out more about it, we perform numerical simulations of the surface layers of stellar atmospheres. Here, we report on preliminary results from simulations in the range between 4000 K and 6500 K effective temperature with an initial vertical, homogeneous magnetic field of 50 G strength. We find that the field strength of the strongest magnetic flux concentrations increases with decreasing effective temperature at the height level where the average Rosseland optical depth is one. On the other hand, at the same level, the field is less strong than the thermal equipartition value in the coolest model but assumes superequipartition in the models hotter than 5000 K. While the Wilson depression of the strongest field concentrations is about one pressure scale height in the coolest model, it is more than four times the pressure scale height in the hottest one. We also find that the relative contribution of the bright filigree to the bolometric, vertically directed radiative intensity is most significant for the Teff = 5000 K model (0.6%-0.79%) and least significant for the hottest and coolest models (0.1%-0.46% and 0.14%-0.32%, respectively). This behavior suggests that the effect of the small-scale magnetic field on the photometric variability is more significant for K dwarf stars than for F-type and also M-type stars. Title: Detecting weak magnetic fields in the central stars of planetary nebulae Authors: Steffen, M.; Hubrig, S.; Todt, H.; Schöller, M.; Sandin, C.; Hamann, W. -R.; Schönberner, D. Bibcode: 2014psce.conf..351S Altcode: We have carried out low-resolution spectropolarimetric observations with FORS 2, installed on the VLT, ESO, of a representative sample of 12 bright central stars of Planetary Nebulae (PNe) with different morphology. Two of the sample are hydrogen-deficient (Wolf-Rayet type) stars. Our measurements rule out the existence of strong global magnetic fields of the order of kG in any of the PN central stars of our sample. Even so, our data may indicate the presence of weak mean longitudinal magnetic fields of a few hundred Gauss in the central stars of two elliptical nebulae, IC 418 and NGC 2392, and a very weak magnetic field of about 100 G in the Wolf-Rayet type central star Hen 2-113. However, the significance of these marginal detections depends on the method adopted for estimating the uncertainties in the magnetic-field measurements. Title: The Chandra Planetary Nebula Survey (ChanPlaNS). II. X-Ray Emission from Compact Planetary Nebulae Authors: Freeman, M.; Montez, R., Jr.; Kastner, J. H.; Balick, B.; Frew, D. J.; Jones, D.; Miszalski, B.; Sahai, R.; Blackman, E.; Chu, Y. -H.; De Marco, O.; Frank, A.; Guerrero, M. A.; Lopez, J. A.; Zijlstra, A.; Bujarrabal, V.; Corradi, R. L. M.; Nordhaus, J.; Parker, Q. A.; Sandin, C.; Schönberner, D.; Soker, N.; Sokoloski, J. L.; Steffen, M.; Toalá, J. A.; Ueta, T.; Villaver, E. Bibcode: 2014ApJ...794...99F Altcode: 2014arXiv1407.4141F We present results from the most recent set of observations obtained as part of the Chandra X-ray observatory Planetary Nebula Survey (ChanPlaNS), the first comprehensive X-ray survey of planetary nebulae (PNe) in the solar neighborhood (i.e., within ~1.5 kpc of the Sun). The survey is designed to place constraints on the frequency of appearance and range of X-ray spectral characteristics of X-ray-emitting PN central stars and the evolutionary timescales of wind-shock-heated bubbles within PNe. ChanPlaNS began with a combined Cycle 12 and archive Chandra survey of 35 PNe. ChanPlaNS continued via a Chandra Cycle 14 Large Program which targeted all (24) remaining known compact (R neb <~ 0.4 pc), young PNe that lie within ~1.5 kpc. Results from these Cycle 14 observations include first-time X-ray detections of hot bubbles within NGC 1501, 3918, 6153, and 6369, and point sources in HbDs 1, NGC 6337, and Sp 1. The addition of the Cycle 14 results brings the overall ChanPlaNS diffuse X-ray detection rate to ~27% and the point source detection rate to ~36%. It has become clearer that diffuse X-ray emission is associated with young (lsim 5 × 103 yr), and likewise compact (R neb <~ 0.15 pc), PNe with closed structures and high central electron densities (ne >~ 1000 cm-3), and is rarely associated with PNe that show H2 emission and/or pronounced butterfly structures. Hb 5 is one such exception of a PN with a butterfly structure that hosts diffuse X-ray emission. Additionally, two of the five new diffuse X-ray detections (NGC 1501 and NGC 6369) host [WR]-type central stars, supporting the hypothesis that PNe with central stars of [WR]-type are likely to display diffuse X-ray emission. 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: Weak magnetic fields in central stars of planetary nebulae? Authors: Steffen, M.; Hubrig, S.; Todt, H.; Schöller, M.; Hamann, W. -R.; Sandin, C.; Schönberner, D. Bibcode: 2014A&A...570A..88S Altcode: 2014arXiv1408.3000S Context. It is not yet clear whether magnetic fields play an essential role in shaping planetary nebulae (PNe), or whether stellar rotation alone and/or a close binary companion, stellar or substellar, can account for the variety of the observed nebular morphologies.
Aims: In a quest for empirical evidence verifying or disproving the role of magnetic fields in shaping planetary nebulae, we follow up on previous attempts to measure the magnetic field in a representative sample of PN central stars.
Methods: We obtained low-resolution polarimetric spectra with FORS 2 installed on the Antu telescope of the VLT for a sample of 12 bright central stars of PNe with different morphologies, including two round nebulae, seven elliptical nebulae, and three bipolar nebulae. Two targets are Wolf-Rayet type central stars.
Results: For the majority of the observed central stars, we do not find any significant evidence for the existence of surface magnetic fields. However, our measurements may indicate the presence of weak mean longitudinal magnetic fields of the order of 100 Gauss in the central star of the young elliptical planetary nebula IC 418 as well as in the Wolf-Rayet type central star of the bipolar nebula Hen 2-113 and the weak emission line central star of the elliptical nebula Hen 2-131. A clear detection of a 250 G mean longitudinal field is achieved for the A-type companion of the central star of NGC 1514. Some of the central stars show a moderate night-to-night spectrum variability, which may be the signature of a variable stellar wind and/or rotational modulation due to magnetic features.
Conclusions: Since our analysis indicates only weak fields, if any, in a few targets of our sample, we conclude that strong magnetic fields of the order of kG are not widespread among PNe central stars. Nevertheless, simple estimates based on a theoretical model of magnetized wind bubbles suggest that even weak magnetic fields below the current detection limit of the order of 100 G may well be sufficient to contribute to the shaping of the surrounding nebulae throughout their evolution. Our current sample is too small to draw conclusions about a correlation between nebular morphology and the presence of stellar magnetic fields.

Based on observations obtained at the European Southern Observatory, Paranal, Chile (ESO program No. 088.D-0425(A)). 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: 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: A super lithium-rich red-clump star in the open cluster Trumpler 5 Authors: Monaco, L.; Boffin, H. M. J.; Bonifacio, P.; Villanova, S.; Carraro, G.; Caffau, E.; Steffen, M.; Ahumada, J. A.; Beletsky, Y.; Beccari, G. Bibcode: 2014A&A...564L...6M Altcode: 2014arXiv1403.6461M Context. The existence of lithium-rich low-mass red giant stars still represents a challenge for stellar evolution models. Stellar clusters are privileged environments for this kind of investigation.
Aims: To investigate the chemical abundance pattern of the old open cluster Trumpler 5, we observed a sample of four red-clump stars with high-resolution optical spectrographs. One of them (#3416) reveals extremely strong lithium lines in its spectrum.
Methods: One-dimensional, local thermodynamic equilibrium analysis was performed on the spectra of the observed stars. A 3D-NLTE analysis was performed to derive the lithium abundance of star #3416.
Results: Star #3416 is super Li-rich with A(Li) = 3.75 dex. The lack of 6Li enrichment (6Li/7Li < 2%), the low carbon isotopic ratio (12C/13C = 14 ± 3), and the lack of evidence for radial velocity variation or enhanced rotational velocity (vsini = 2.8 km s-1) all suggest that lithium production has occurred in this star through the Cameron & Fowler mechanism.
Conclusions: We identified a super Li-rich core helium-burning, red-clump star in an open cluster. Internal production is the most likely cause of the observed enrichment. Given the expected short duration of a star's Li-rich phase, enrichment is likely to have occurred at the red clump or in the immediately preceding phases, namely during the He-flash at the tip of the red giant branch (RGB) or while ascending the brightest portion of the RGB.

Based on observations made with ESO Telescopes at the La Silla Paranal Observatory under program ID 088.D-0045(A).Appendix A is available in electronic form at http://www.aanda.org Title: Detection of magnetic fields in central stars of planetary nebulae Authors: Todt, H.; Steffen, M.; Hubrig, S.; Schöller, M.; Hamann, W. -R.; Sandin, C.; Schönberner, D. Bibcode: 2014apn6.confE.103T Altcode: Magnetic fields may play an essential role in shaping planetary nebulae. However, convincing evidence for the presence of magnetic fields on their central stars is still missing. We obtained low-resolution polarimetric spectra with FORS2 for twelve central stars. Their nebulae cover different morphologies, comprising two round nebulae, seven elliptical nebulae, and three bipolar nebulae. For one central star, which has a bipolar nebula, we clearly detect a weak magnetic field of the order of 100 Gauss. Interestingly, the central star exhibits a hydrogen-free, Wolf-Rayet type spectrum. For a few further stars in our sample we find marginal evidence for weak magnetic fields. The detection of magnetic fields via polarimetry strongly depends on the quality of the observations, and on the type of the line spectrum. We perform careful estimates of the error margins, using statistical tests and Monte-Carlo simulations, which confirm the significance of the detection reported here. 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: A hydrodynamical study of multiple-shell planetary nebulae. III. Expansion properties and internal kinematics: Theory versus observation Authors: Schönberner, D.; Jacob, R.; Lehmann, H.; Hildebrandt, G.; Steffen, M.; Zwanzig, A.; Sandin, C.; Corradi, R. L. M. Bibcode: 2014AN....335..378S Altcode: 2014arXiv1404.0391S We present the result of a study on the expansion properties and internal kinematics of round/elliptical planetary nebulae of the Milky Way disk, the halo, and of the globular cluster M 15. The purpose of this study is to considerably enlarge the small sample of nebulae with precisely determined expansion properties (Schönberner et al. \cite{SJSPCA.05}). To this aim, we selected a representative sample of objects with different evolutionary stages and metallicities and conducted high-resolution échelle spectroscopy. In most cases we succeeded in detecting the weak signals from the outer nebular shell which are attached to the main line emission from the bright nebular rim. Next to the measurement of the motion of the rim gas by decomposition of the main line components into Gaussians, we were able to measure separately, for most objects for the first time, the gas velocity immediately behind the leading shock of the shell, i.e. the post-shock velocity. We more than doubled the number of objects for which the velocities of both rim and shell are known and confirm that the overall expansion of planetary nebulae is accelerating with time. There are, however, differences between the expansion behaviour of the shell and the rim: The post-shock velocity is starting at values as low as around 20 km s-1 for the youngest nebulae, just above the AGB wind velocity of ∼ 10-15 km s-1, and is reaching values of about 40 km s-1 for the nebulae around hotter central stars. Contrarily, the rim matter is at first decelerated below the typical AGB-wind velocity and remains at about 5-10 km s-1 for a while until finally a typical flow velocity of up to 30 km s-1 is reached. This observed distinct velocity evolution of both rim and shell is explained by radiation-hydrodynamics simulations, at least qualitatively: It is due to the ever changing stellar radiation field and wind-wind interaction together with the varying density profile ahead of the leading shock during the progress of evolution. The wind-wind interaction works on the rim dynamics while the radiation field and upstream density gradient is responsible for the shell dynamics. Because of these time-dependent boundary conditions, a planetary nebula will never evolve into a simple self-similar expansion. Also the metal-poor objects behave as theory predicts: The post-shock velocities are higher and the rim flow velocities are equal or even lower compared to disk objects at similar evolutionary stage. The old nebula around low-luminosity central stars contained in our sample expand still fast and are dominated by reionisation. We detected, for the first time, in some objects an asymmetric expansion behaviour: The relative expansions between rim and shell appear to be different for the receding and approaching parts of the nebular envelope.

Based partly on observations obtained at the European Southern Observatory, Paranal, Chile (ESO programme No. 077.D-0652). 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: 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: 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: The evolution of planetary nebulae. VIII. True expansion rates and visibility times Authors: Jacob, R.; Schönberner, D.; Steffen, M. Bibcode: 2013A&A...558A..78J Altcode: 2013arXiv1307.6189J Context. The visibility time of planetary nebulae (PNe) in stellar systems is an essential quantity for estimating the size of a PN population in the context of general population studies. For instance, it enters directly into the PN death rate determination.
Aims: The basic ingredient for determining visibility times is the typical nebular expansion velocity, as a suited average over all PN sizes of a PN population within a certain volume or stellar system. The true expansion speed of the outer nebular edge of a PN is, however, not accessible by spectroscopy - a difficulty that we surmount by radiation-hydrodynamics modelling.
Methods: We first discuss the definition of the PN radius and possible differences between the observable PN radius and its physical counterpart, the position of the leading shock of the nebular shell. We also compare the Hα surface-brightness evolution predicted by our radiation-hydrodynamics models with the recent Hα surface-brightness radius calibration of Frew (2008, Ph.D. Thesis, Macquarie University, Australia) and find excellent agreement. We then carefully investigate the existing spectroscopic data on nebular expansion velocities for a local PN sample with objects up to a distance of 2 kpc with well-defined round/elliptical shapes. We evaluate, by means of our radiation-hydrodynamics models, how these observed expansion velocities must be corrected in order to get the true expansion speed of the outer nebular edge.
Results: We find a mean true expansion velocity of 42 km s-1, i.e. nearly twice as high as the commonly adopted value to date. Accordingly, the time for a PN to expand to a radius of, say 0.9 pc, is only 21 000 ± 5000 years. This visibility time of a PN holds for all central star masses since a nebula does not become extinct as the central star fades. There is, however, a dependence on metallicity in the sense that the visibility time becomes shorter for lower nebular metal content.
Conclusions: These statements on the visibility time only hold for volume-limited samples. Extragalactic samples that contain spatially unresolved nebulae are flux limited, and in this case the visibility time directly depends on the limiting magnitude of the survey. To reach a visibility time of 21 000 years, the survey must reach about 7 mag below the bright cut-off of the planetary nebula luminosity function. With the higher expansion rate of PNe derived here we determined their local death-rate density as (1.4 ± 0.5) × 10-12 PN pc-3 yr-1, using the local PN density advocated by Frew (2008).

Appendix A is available in electronic form at http://www.aanda.orgDedicated to the memory of Volker Weidemann who died on March 14, 2012, at the age of 87 years. 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: Evaluating local correlation tracking using CO5BOLD simulations of solar granulation Authors: Verma, M.; Steffen, M.; Denker, C. Bibcode: 2013A&A...555A.136V Altcode: 2013arXiv1305.6033V Context. Flows on the solar surface are intimately linked to solar activity, and local correlation tracking (LCT) is one of the standard techniques for capturing the dynamics of these processes by cross-correlating solar images. However, the link between contrast variations in successive images to the underlying plasma motions has to be quantitatively confirmed.
Aims: Radiation hydrodynamics simulations of solar granulation (e.g., CO5BOLD) provide access to both the wavelength-integrated, emergent continuum intensity and the three-dimensional velocity field at various heights in the solar atmosphere. Thus, applying LCT to continuum images yields horizontal proper motions, which are then compared to the velocity field of the simulated (non-magnetic) granulation. In this study, we evaluate the performance of an LCT algorithm previously developed for bulk-processing Hinode G-band images, establish it as a quantitative tool for measuring horizontal proper motions, and clearly work out the limitations of LCT or similar techniques designed to track optical flows.
Methods: Horizontal flow maps and frequency distributions of the flow speed were computed for a variety of LCT input parameters including the spatial resolution, the width of the sampling window, the time cadence of successive images, and the averaging time used to determine persistent flow properties. Smoothed velocity fields from the hydrodynamics simulation at three atmospheric layers (log τ = -1, 0, and +1) served as a point of reference for the LCT results.
Results: LCT recovers many of the granulation properties, e.g., the shape of the flow speed distributions, the relationship between mean flow speed and averaging time, and also - with significant smoothing of the simulated velocity field - morphological features of the flow and divergence maps. However, the horizontal proper motions are grossly underestimated by as much as a factor of three. The LCT flows match best the flows deeper in the atmosphere at log τ = +1.
Conclusions: Despite the limitations of optical flow techniques, they are a valuable tool in describing horizontal proper motions on the Sun, as long as the results are not taken at face value but with a proper understanding of the input parameter space and the limitations inherent to the algorithm.

Movies are available in electronic form at http://www.aanda.org Title: The Intensity-Velocity Phase Spectra of Evanescent Oscillations and Acoustic Sources Authors: Severino, G.; Straus, T.; Oliviero, M.; Steffen, M.; Fleck, B. Bibcode: 2013SoPh..284..297S Altcode: 2012SoPh..tmp..295S There are three major issues in modeling solar evanescent oscillations: the variation of the intensity [I]-velocity [V] phase difference of p-modes close to the base of photosphere; the existence of a plateau of negative I-V phase differences below and between the ridges of the low-frequency p-modes; the explanation of the I-V cross-spectra of the evanescent oscillations. We present new interpretations for the first two issues, based on modeling intensity fluctuations taking steep temperature gradients, opacity, and non-adiabatic cooling into account. 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: 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: Detection of Diffuse X-Ray Emission from Planetary Nebulae with Nebular O VI Authors: Ruiz, N.; Chu, Y. -H.; Gruendl, R. A.; Guerrero, M. A.; Jacob, R.; Schönberner, D.; Steffen, M. Bibcode: 2013ApJ...767...35R Altcode: 2013arXiv1302.3886R The presence of O VI ions can be indicative of plasma temperatures of a few ×105 K that are expected in heat conduction layers between the hot shocked stellar wind gas at several 106 K and the cooler (104 K) nebular gas of planetary nebulae (PNe). We have used FUSE observations of PNe to search for nebular O VI emission or absorption as a diagnostic of the conduction layer to ensure the presence of hot interior gas. Three PNe showing nebular O VI, namely IC 418, NGC 2392, and NGC 6826, have been selected for Chandra observations and diffuse X-ray emission is indeed detected in each of these PNe. Among the three, NGC 2392 has peculiarly high diffuse X-ray luminosity and plasma temperature compared with those expected from its stellar wind's mechanical luminosity and terminal velocity. The limited effects of heat conduction on the plasma temperature of a hot bubble at the low terminal velocity of the stellar wind of NGC 2392 may partially account for its high plasma temperature, but the high X-ray luminosity needs to be powered by processes other than the observed stellar wind, probably the presence of an unseen binary companion of the central star of the PN (CSPN) of NGC 2392. We have compiled relevant information on the X-ray, stellar, and nebular properties of PNe with a bubble morphology and found that the expectations of bubble models including heat conduction compare favorably with the present X-ray observations of hot bubbles around H-rich CSPNe, but have notable discrepancies for those around H-poor [WR] CSPNe. We note that PNe with more massive central stars can produce hotter plasma and higher X-ray surface brightness inside central hot bubbles. 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: First steps with CO5BOLD using HLLMHD and PP reconstruction . Authors: Steiner, O.; Rajaguru, S. P.; Vigeesh, G.; Steffen, M.; Schaffenberger, W.; Freytag, B. Bibcode: 2013MSAIS..24..100S Altcode: We report on first experiences with real-life applications using the MHD-module of CO5BOLD together with the piecewise parabolic reconstruction scheme and present preliminary results of stellar magnetic models with Teff = 4000 K to Teff = 5770 K. Title: Granulation in DA white dwarfs from CO5BOLD 3D model atmospheres Authors: Tremblay, P. -E.; Ludwig, H. -G.; Freytag, B.; Steffen, M. Bibcode: 2013MSAIS..24...61T Altcode: Time-dependent 3D simulations of pure-hydrogen DA white dwarf atmospheres have been computed in recent years. Synthetic Balmer lines spectra drawn from these radiation-hydrodynamics (RHD) simulations have been shown to predict surface gravities significantly lower than the standard 1D models, in much better agreement with the expectation that white dwarfs cool at constant mass. We have now computed a grid of CO5BOLD pure-hydrogen 3D model atmospheres for surface gravities from log g = 7 to log g = 8.5 and effective temperatures from 6000 to 13,000 K. Over this range, we observe a significant variation of the intensity contrast of the surface granulation patterns, which indicates the strength of the 3D effects. Furthermore, the size and appearance of granules are also varying considerably. An explanation of these behaviours can lead to a better understanding of the physical processes responsible for the energy transfer in white dwarf atmospheres. Title: 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: Spectral line asymmetries in the metal-poor red giant HD 122563: CO5BOLD predictions versus observations Authors: Klevas, J.; Ludwig, A. Kučinskas H. -G.; Bonifacio, P.; Steffen, M. Bibcode: 2013MSAIS..24...78K Altcode: 2013arXiv1303.6838K We study the influence of convection on the asymmetries and Doppler shifts of Fe I spectral lines in the metal-poor red giant HD 122563. To this end, we compute theoretical Fe I line shifts and line bisectors using 3D hydrodynamical model atmosphere of HD 122563 calculated with the CO5BOLD code. We then make a detailed comparison of the theoretical line shifts and bisectors with those derived from the high quality HARPS spectrum of HD 122563 taken from the ESO Science Archive Facility (R=115 000, average signal-to-noise ratio, S/N ≈ 310). In general, we find a good agreement between the theoretically predicted and observed Doppler shifts of Fe I line cores, with somewhat larger discrepancies seen in the case of weaker (equivalent width W<5 pm) and stronger lines (W>11 pm). Both observed and theoretical coreshifts cover a range between 0 and -1 km/s, with increasingly stronger blueshifts for weaker lines and slight hints of a coreshift dependence on wavelength. Theoretical bisectors reproduce the observed ones reasonably well too, however, theoretical bisectors of the weak red (lambda > 600 nm) Fe I lines have blueshifts that are by up to ∼200 m/s larger than observed. The obtained results therefore suggest that the current CO5BOLD models are capable of reproducing the large-scale velocity fields in the atmosphere of HD 122563 sufficiently well. Nevertheless, further efforts are needed in order to understand the physical reasons behind the discrepancies in theoretical predictions and observed properties of the weakest and strongest Fe I lines. 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: 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: Magnetic field detection in the bright A0-type supergiant HD 92207 Authors: Hubrig, S.; Schöller, M.; Kholtygin, A. F.; González, J. F.; Kharchenko, N. V.; Steffen, M. Bibcode: 2012A&A...546L...6H Altcode: Context. Recent developments in observational techniques and theories revealed the potential significance of magnetic fields for stellar structure, evolution, and circumstellar environment. At present, the distribution of magnetic field strengths in massive stars from the zero-age main sequence to more evolved stages, which would shed light on the origin of the magnetic field, has not been studied.
Aims: We searched for the presence of a magnetic field in the visually brightest early A-type supergiant HD 92207.
Methods: Observations were obtained using the low-resolution spectropolarimetric mode of FORS 2 (FOcal Reducer low dispersion Spectrograph) mounted on the 8 m Antu telescope of the VLT. For the mean longitudinal magnetic field measurements, we applied a linear regression analysis in two ways: using only the absorption hydrogen Balmer lines or using the entire spectrum including all available absorption lines.
Results: A mean longitudinal magnetic field at a significance level of more than 3σ was detected in two out of three observations distributed over about one year. It is one of the rare cases where a field of about a few hundred Gauss is detected in an early A-type supergiant. All line profiles in the spectra of HD 92207 undergo distinct variations in radial velocities and intensities, probably caused by previously detected non-radial pulsations.

Based on observations obtained at the European Southern Observatory (Prgs. 074.D-0008(B), 078.D-0330(A), 087.D-0049(A), and 088.D-0425(A)). Title: The role of heat conduction to the formation of [WC]-type planetary nebulae Authors: Sandin, Christer; Steffen, Matthias; Jacob, Ralf; Schönberner, Detlef; Rühling, Ute; Hamann, Wolf-Rainer; Todt, Helge Bibcode: 2012IAUS..283..494S Altcode: X-ray observations of young Planetary Nebulæ (PNe) have revealed diffuse emission in extended regions around both H-rich and H-deficient central stars. In order to also reproduce physical properties of H-deficient objects, we have, at first, extended our time-dependent radiation-hydrodynamic models with heat conduction for such conditions. Here we present some of the important physical concepts, which determine how and when a hot wind-blown bubble forms. In this study we have had to consider the, largely unknown, evolution of the CSPN, the slow (AGB) wind, the fast hot-CSPN wind, and the chemical composition. The main conclusion of our work is that heat conduction is needed to explain X-ray properties of wind-blown bubbles also in H-deficient objects. Title: Planetary nebula abundance determinations: A view from 1D-RHD simulations Authors: Jacob, Ralf; Sandin, Christer; Schönberner, Detlef; Steffen, Matthias Bibcode: 2012IAUS..283..398J Altcode: In the last years (metallicity-dependent) radiation-hydrodynamics simulations have become a powerful tool to understand the formation and evolution of PNe in terms of simple morphologies and kinematics. Contrary to photoionization models, with their ad-hoc assumptions on structure and physics, the RHD models are self-consistent with respect to their density distribution, velocity field, chemical composition, and stellar evolution. We use our models as simple proxies for real PNe and investigate the reliability of common abundance determination methods, which are based on either plasma diagnostics or static photoionization (PI) models. Title: Modeling the diffuse X-ray emission of planetary nebulae with different chemical composition Authors: Steffen, Matthias; Sandin, Christer; Jacob, Ralf; Schönberner, Detlef Bibcode: 2012IAUS..283..215S Altcode: 2011arXiv1109.6355S Based on time-dependent radiation-hydrodynamics simulations of the evolution of Planetary Nebulae (PNe), we have carried out a systematic parameter study to address the non-trivial question of how the diffuse X-ray emission of PNe with closed central cavities is expected to depend on the evolutionary state of the nebula, the mass of the central star, and the metallicity of stellar wind and circumstellar matter. We have also investigated how the model predictions depend on the treatment of thermal conduction at the interface between the central `hot bubble' and the `cool' inner nebula, and compare the results with recent X-ray observations. Our study includes models whose properties resemble the extreme case of PNe with Wolf-Rayet type central stars. Indeed, such models are found to produce the highest X-ray luminosities. Title: The Chandra X-Ray Survey of Planetary Nebulae (ChanPlaNS): Probing Binarity, Magnetic Fields, and Wind Collisions Authors: Kastner, J. H.; Montez, R., Jr.; Balick, B.; Frew, D. J.; Miszalski, B.; Sahai, R.; Blackman, E.; Chu, Y. -H.; De Marco, O.; Frank, A.; Guerrero, M. A.; Lopez, J. A.; Rapson, V.; Zijlstra, A.; Behar, E.; Bujarrabal, V.; Corradi, R. L. M.; Nordhaus, J.; Parker, Q. A.; Sandin, C.; Schönberner, D.; Soker, N.; Sokoloski, J. L.; Steffen, M.; Ueta, T.; Villaver, E. Bibcode: 2012AJ....144...58K Altcode: 2012arXiv1204.6055K We present an overview of the initial results from the Chandra Planetary Nebula Survey (ChanPlaNS), the first systematic (volume-limited) Chandra X-Ray Observatory survey of planetary nebulae (PNe) in the solar neighborhood. The first phase of ChanPlaNS targeted 21 mostly high-excitation PNe within ~1.5 kpc of Earth, yielding four detections of diffuse X-ray emission and nine detections of X-ray-luminous point sources at the central stars (CSPNe) of these objects. Combining these results with those obtained from Chandra archival data for all (14) other PNe within ~1.5 kpc that have been observed to date, we find an overall X-ray detection rate of ~70% for the 35 sample objects. Roughly 50% of the PNe observed by Chandra harbor X-ray-luminous CSPNe, while soft, diffuse X-ray emission tracing shocks—in most cases, "hot bubbles"—formed by energetic wind collisions is detected in ~30%; five objects display both diffuse and point-like emission components. The presence (or absence) of X-ray sources appears correlated with PN density structure, in that molecule-poor, elliptical nebulae are more likely to display X-ray emission (either point-like or diffuse) than molecule-rich, bipolar, or Ring-like nebulae. All but one of the point-like CSPNe X-ray sources display X-ray spectra that are harder than expected from hot (~100 kK) central stars emitting as simple blackbodies; the lone apparent exception is the central star of the Dumbbell nebula, NGC 6853. These hard X-ray excesses may suggest a high frequency of binary companions to CSPNe. Other potential explanations include self-shocking winds or PN mass fallback. Most PNe detected as diffuse X-ray sources are elliptical nebulae that display a nested shell/halo structure and bright ansae; the diffuse X-ray emission regions are confined within inner, sharp-rimmed shells. All sample PNe that display diffuse X-ray emission have inner shell dynamical ages <~ 5 × 103 yr, placing firm constraints on the timescale for strong shocks due to wind interactions in PNe. The high-energy emission arising in such wind shocks may contribute to the high excitation states of certain archetypical "hot bubble" nebulae (e.g., NGC 2392, 3242, 6826, and 7009). Title: Ablation and Wind Mass-Loading in the Born-Again Planetary Nebula A 30 Authors: Guerrero, Martín A.; Chu, You-Hua; Hamann, Wolf-Rainer; Oskinova, Lidia; Schönberner, Detlef; Todt, Helge; Steffen, Matthias; Ruiz, Nieves; Gruendl, Robert A.; Blair, William P. Bibcode: 2012IAUS..283..378G Altcode: We present XMM-Newton and Chandra observations of the born-again planetary nebula A 30. These X-ray observations reveal a bright unresolved source at the position of the central star whose X-ray luminosity exceeds by far the model expectations for photospheric emission and for shocks within the stellar wind. We suggest that a ``born-again hot bubble'' may be responsible for this X-ray emission. Diffuse X-ray emission associated with the petal-like features and one of the H-poor knots seen in the optical is also found. The weakened emission of carbon lines in the spectrum of the diffuse emission can be interpreted as the dilution of stellar wind by mass-loading or as the detection of material ejected during a very late thermal pulse. Title: X-ray spectra of wind-driven bubbles with chemical gradients Authors: Jacob, Ralf; Sandin, Christer; Schönberner, Detlef; Steffen, Matthias Bibcode: 2012IAUS..283..396J Altcode: As a tool helping to interpret diffuse X-ray emission of PNe, and as a supplement to our RHD simulations, we have started to construct a grid of theoretical X-ray spectra of wind-blown bubbles with temperature and density profiles according to thermal conduction theory. We investigate how the X-ray spectra depend on chemical composition (e.g. H-rich vs. H-deficient) and how temperature and abundance determinations reflect gradients of temperature and chemical composition within the bubbles. These synthetic models shall allow to quickly perform detailed parameter studies without the need for dedicated hydrodynamical simulations. We report on ideas and goals. Title: On the internal kinematics of PNe Authors: Jacob, Ralf; Schönberner, Detlef; Lehmann, Holger; Zwanzig, Alena; Sandin, Christer; Steffen, Matthias Bibcode: 2012IAUS..283..400J Altcode: Our sample of round/elliptical double-shell PNe around central stars (CSs) with H-rich surface chemistry covers all evolutionary phases across the HRD. By means of high-resolution and high-S/N spectra we determine bulk matter velocities of the inner wind-driven rims and the maximum (= post-shock) gas velocities of the surrounding thermally expanding shells. Studying the details of the internal kinematics allows a look at processes of PN formation and at PN expansion history helping, for instance, to determine expansion distances. Title: Rebirth of X-Ray Emission from the Born-again Planetary Nebula A30 Authors: Guerrero, M. A.; Ruiz, N.; Hamann, W. -R.; Chu, Y. -H.; Todt, H.; Schönberner, D.; Oskinova, L.; Gruendl, R. A.; Steffen, M.; Blair, W. P.; Toalá, J. A. Bibcode: 2012ApJ...755..129G Altcode: 2012arXiv1202.4463G The planetary nebula A30 is believed to have undergone a very late thermal pulse resulting in the ejection of knots of hydrogen-poor material. Using multi-epoch Hubble Space Telescope images, we have detected the angular expansion of these knots and derived an age of 850+280 - 150 yr. To investigate the spectral and spatial properties of the soft X-ray emission detected by ROSAT, we have obtained Chandra and XMM-Newton deep observations of A30. The X-ray emission from A30 can be separated into two components: a point source at the central star and diffuse emission associated with the hydrogen-poor knots and the cloverleaf structure inside the nebular shell. To help us assess the role of the current stellar wind in powering this X-ray emission, we have determined the stellar parameters and wind properties of the central star of A30 using a non-LTE model fit to its optical and UV spectra. The spatial distribution and spectral properties of the diffuse X-ray emission are highly suggestive that it is generated by the post-born-again and present fast stellar winds interacting with the hydrogen-poor ejecta of the born-again event. This emission can be attributed to shock-heated plasma, as the hydrogen-poor knots are ablated by the stellar winds, under which circumstances the efficient mass loading of the present fast stellar wind raises its density and damps its velocity to produce the observed diffuse soft X-rays. Charge transfer reactions between the ions of the stellar winds and material of the born-again ejecta have also been considered as a possible mechanism for the production of diffuse X-ray emission, and upper limits on the expected X-ray production by this mechanism have been derived. The origin of the X-ray emission from the central star of A30 is puzzling: shocks in the present fast stellar wind and photospheric emission can be ruled out, while the development of a new, compact hot bubble confining the fast stellar wind seems implausible. 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: Excitation of Slow-Modes in Network Magnetic Elements Authors: Kato, Y.; Steiner, O.; Steffen, M.; Suematsu, Y. Bibcode: 2012ASPC..455..237K Altcode: From radiation magnetohydrodynamic (RMHD) simulations of the solar atmosphere we have found a new mechanism for the excitation of longitudinal slow modes within magnetic flux concentrations. It is found that the convective downdrafts in the immediate surroundings of magnetic elements are responsible for the excitation of slow modes. The coupling between the external downdraft and the plasma motion internal to the flux concentration is mediated by the inertial forces of the downdraft that act on the magnetic flux concentration. These forces pump the internal atmosphere in the downward direction, which entails a fast downflow in the photospheric and chromospheric layers of the magnetic element. Subsequent to the transient pumping phase, the atmosphere rebounds, causing a slow mode traveling along the magnetic flux concentration in the upward direction and developing into a shock wave in chromospheric heights, possibly capable of producing some kind of dynamic fibril. This event occurs recurrently. We compare the power spectra of the temperature and velocity of the flux-sheet atmosphere to the corresponding spectra of the unmagnetized atmosphere. Title: Using Hubble Space Telescope Images to Test Theoretical Models of Planetary Nebulae Authors: Braxton, Kelsey M.; Balick, B.; Jacob, R.; Steffen, M.; Schonberner, D. Bibcode: 2012AAS...22043110B Altcode: Classical ionization models that predict the ionization structure of planetary nebulae generally assume constant gas density (or a central void and constant density) in the shell. More recently, Steffen, Jacob, Schoenberner (2005, A&A, 441, 573) have computed fully consistent one-dimensional radiation hydrodynamic ionization models (RHI models) with realistic stellar winds and evolving central stars. Their numerical results offer concrete predictions concerning the projected three-dimensional shapes of simple round and elliptical PNe. A first glance at their models shows them to be a superior way of modeling ionization structures in realistic PNe. We present high-spatial resolution and carefully flux calibrated images of PNe selected in Halpha, [OIII], and [NII] as well as the ratios [OIII]/Halpha and [NII]/Halpha in order to test the predictions of the RHI models. These were obtained uniformly and at the same epoch using the WFPC2 camera on the Hubble Space Telescope. Some of the targets are ionization bounded and others are density bounded. In the future these images will be compared to the models in detail. At this point we note many of the common features--some of them unexpected--that the models must explain. 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: 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: Simulations of the solar near-surface layers with the CO5BOLD, MURaM, and Stagger codes Authors: Beeck, B.; Collet, R.; Steffen, M.; Asplund, M.; Cameron, R. H.; Freytag, B.; Hayek, W.; Ludwig, H. -G.; Schüssler, M. Bibcode: 2012A&A...539A.121B Altcode: 2012arXiv1201.1103B Context. Radiative hydrodynamic simulations of solar and stellar surface convection have become an important tool for exploring the structure and gas dynamics in the envelopes and atmospheres of late-type stars and for improving our understanding of the formation of stellar spectra.
Aims: We quantitatively compare results from three-dimensional, radiative hydrodynamic simulations of convection near the solar surface generated with three numerical codes (CO5BOLD, MURaM, and Stagger) and different simulation setups in order to investigate the level of similarity and to cross-validate the simulations.
Methods: For all three simulations, we considered the average stratifications of various quantities (temperature, pressure, flow velocity, etc.) on surfaces of constant geometrical or optical depth, as well as their temporal and spatial fluctuations. We also compared observables, such as the spatially resolved patterns of the emerging intensity and of the vertical velocity at the solar optical surface as well as the center-to-limb variation of the continuum intensity at various wavelengths.
Results: The depth profiles of the thermodynamical quantities and of the convective velocities as well as their spatial fluctuations agree quite well. Slight deviations can be understood in terms of differences in box size, spatial resolution and in the treatment of non-gray radiative transfer between the simulations.
Conclusions: The results give confidence in the reliability of the results from comprehensive radiative hydrodynamic simulations. Title: Simulations of stellar convection with CO5BOLD Authors: Freytag, B.; Steffen, M.; Ludwig, H. -G.; Wedemeyer-Böhm, S.; Schaffenberger, W.; Steiner, O. Bibcode: 2012JCoPh.231..919F Altcode: 2011arXiv1110.6844F High-resolution images of the solar surface show a granulation pattern of hot rising and cooler downward-sinking material - the top of the deep-reaching solar convection zone. Convection plays a role for the thermal structure of the solar interior and the dynamo acting there, for the stratification of the photosphere, where most of the visible light is emitted, as well as for the energy budget of the spectacular processes in the chromosphere and corona. Convective stellar atmospheres can be modeled by numerically solving the coupled equations of (magneto)hydrodynamics and non-local radiation transport in the presence of a gravity field. The CO5BOLD code described in this article is designed for so-called "realistic" simulations that take into account the detailed microphysics under the conditions in solar or stellar surface layers (equation-of-state and optical properties of the matter). These simulations indeed deserve the label "realistic" because they reproduce the various observables very well - with only minor differences between different implementations. The agreement with observations has improved over time and the simulations are now well-established and have been performed for a number of stars. Still, severe challenges are encountered when it comes to extending these simulations to include ideally the entire star or substellar object: the strong stratification leads to completely different conditions in the interior, the photosphere, and the corona. Simulations have to cover spatial scales from the sub-granular level to the stellar diameter and time scales from photospheric wave travel times to stellar rotation or dynamo cycle periods. Various non-equilibrium processes have to be taken into account. Last but not least, realistic simulations are based on detailed microphysics and depend on the quality of the input data, which can be the actual accuracy limiter. This article provides an overview of the physical problem and the numerical solution and the capabilities of CO5BOLD, illustrated with a number of applications. Title: 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: 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: The Galactic evolution of phosphorus Authors: Caffau, E.; Bonifacio, P.; Faraggiana, R.; Steffen, M. Bibcode: 2011A&A...532A..98C Altcode: 2011arXiv1107.2657C Context. As a galaxy evolves, its chemical composition changes and the abundance ratios of different elements are powerful probes of the underlying evolutionary processes. Phosphorous is an element whose evolution has remained quite elusive until now, because it is difficult to detect in cool stars. The infrared weak P i lines of the multiplet 1, at 1050-1082 nm, are the most reliable indicators of the presence of phosphorus. The availability of CRIRES at VLT has permitted access to this wavelength range in stellar spectra.
Aims: We attempt to measure the phosphorus abundance of twenty cool stars in the Galactic disk.
Methods: The spectra are analysed with one-dimensional model-atmospheres computed in local thermodynamic equilibrium (LTE). The line formation computations are performed assuming LTE.
Results: The ratio of phosphorus to iron behaves similarly to sulphur, increasing towards lower metallicity stars. Its ratio with respect to sulphur is roughly constant and slightly larger than solar, [P/S] = 0.10 ± 0.10.
Conclusions: We succeed in taking an important step towards the understanding of the chemical evolution of phosphorus in the Galaxy. However, the observed rise in the P/Fe abundance ratio is steeper than predicted by Galactic chemical evolution model developed by Kobayashi and collaborators. Phosphorus appears to evolve differently from the light odd-Z elements sodium and aluminium. The constant value of [P/S] with metallicity implies that P production is insensitive to the neutron excess, thus processes other than neutron captures operate. We suggest that proton captures on 30Si and α captures on 27Al are possibilities to investigate. We see no clear distinction between our results for stars with planets and stars without any detected planet.

Based on observations obtained with the CRIRES spectrograph at ESO-VLT Antu 8.2 m telescope at Paranal, Programme 386.D-0130, P.I. E. Caffau. Title: Excitation of magneto-acoustic waves in network magnetic elements Authors: Kato, Yoshiaki; Steiner, Oskar; Steffen, Matthias; Suematsu, Yoshinori Bibcode: 2011IAUS..273..442K Altcode: From radiation magnetohydrodynamic (RMHD) simulations we track the temporal evolution of a vertical magnetic flux sheet embedded in a two-dimensional non-stationary atmosphere that reaches all the way from the upper convection zone to the low chromosphere. Examining its temporal behavior near the interface between the convection zone and the photosphere, we describe the excitation of propagating longitudinal waves within the magnetic element as a result of convective motion in its surroundings. 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: Excitation of Slow Modes in Network Magnetic Elements Through Magnetic Pumping Authors: Kato, Yoshiaki; Steiner, Oskar; Steffen, Matthias; Suematsu, Yoshinori Bibcode: 2011ApJ...730L..24K Altcode: 2011arXiv1102.5164K From radiation magnetohydrodynamic simulations of the solar atmosphere, we find a new mechanism for the excitation of longitudinal slow modes within magnetic flux concentrations. We find that the convective downdrafts in the immediate surroundings of magnetic elements are responsible for the excitation of slow modes. The coupling between the external downdraft and the plasma motion internal to the flux concentration is mediated by the inertial forces of the downdraft that act on the magnetic flux concentration. These forces, in conjunction with the downward movement, pump the internal atmosphere in the downward direction, which entails a fast downdraft in the photospheric and chromospheric layers of the magnetic element. Subsequent to the transient pumping phase, the atmosphere rebounds, causing a slow mode traveling along the magnetic flux concentration in the upward direction. It develops into a shock wave in chromospheric heights, possibly capable of producing some kind of dynamic fibril. We propose an observational detection of this process. 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: 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: Watching planetary nebulae grow with HST Authors: Balick, B.; Huehnerhoff, J.; Steffen, M.; Schoenberner, D.; Hajian, A.; Behr, B. B. Bibcode: 2011apn5.confP..43B Altcode: 2011apn5.procA..43B HST/WFPC2/PC images of PNe spanning over a decade allow secular changes in their sharp-edged structures to monitored and compared to the predictions of hydrodynamical interacting winds models. We selected a sample of 18 simple, bright, and relatively nearby targets for second- or third-epoch observations in 2008 using the F502N ([OIII]) and F658N ([NII]) filters. All data sets were identically calibrated and compared using difference images. About 75% showed signs of expansion (by up to 1%) and changes in surface brightness (by up to ±5%). In most cases the expansions were largely uniform. Exceptions tend to be found in ionization fronts at the perimeter of the ionized gas, though NGC6543 shows signs of nonuniform expansion in its interior as well. Results for NGC 2392, 3132, 3242, 3918, and 6543 were presented. Title: On the chemical composition of the metal-poor planetary nebula PNG135.9+55.9 Authors: Sandin, C.; Jacob, R.; Schoenberner, D.; Steffen, M.; Roth, M. M. Bibcode: 2011apn5.confP..51S Altcode: 2011apn5.procA..51S The metal-poor planetary nebula PN G135.9+55.9 has a particularly-low oxygen abundance, that is a matter of an ongoing discussion. We report on our recent results of both new accurate observations by means of integral field spectroscopy with PMAS, and on the outcome of new radiation hydrodynamics models. Our goal with these studies was to calculate new abundance estimates. We find that expansion cooling, and deviations from thermal equilibrium, become increasingly important to the physical structure at metalicities that are as low as in this object. The resulting low electron temperatures cause substantial deviations in the estimated abundances compared to an approach using standard hydrostatic photo-ionization models. Title: The role of thermal conduction in WR-type planetary nebulae Authors: Sandin, C.; Steffen, M.; Schoenberner, D.; Ruehling, U.; Hamann, W. R. Bibcode: 2011apn5.confP..53S Altcode: 2011apn5.procA..53S Thermal conduction plays an important role to the interpretation of X-ray spectra of planetary nebulae. Models including this effect so far only used a theoretical formulation that assumes a pure hydrogen composition. To permit modeling of objects with other compositions, such as Wolf-Rayet stars, we have now extended the thermal-conduction description in our models. We will present the outcome of our study in terms of how these changes affect the new models and the predicted X-ray emission spectra. Title: What does the Sun suggest about global oscillation amplitudes in solar-like stars? Authors: Severino, G.; Straus, Th.; Oliviero, M.; Steffen, M. Bibcode: 2010AN....331..896S Altcode: Motivated by the results of CoRoT and based on the solar experience, we develop a new stellar scaling law for the intensity-velocity amplitude ratio (gain) of resonant oscillations, which is a quantity independent of the excitation model. The comparison of our approach with observations gives new interesting results for the Sun as a star. Moreover, for a sample of three solar-like stars observed by CoRoT, we found that our scaling law provides an explanation of the low observed luminosity amplitudes which is an alternative to non-adiabatic effects. 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: Diffuse X-rays from PNe with WR-type central stars Authors: Rühling, U.; Sandin, C.; Steffen, M.; Schönberner, D.; Hamann, W. -R.; Todt, H. Bibcode: 2010AIPC.1273..213R Altcode: With the help of detailed nebula modeling and X-ray observations, we want to shed light on the enigmatic origin of Wolf-Rayet type central stars of planetary nebulae. This method allows us to assign observed [WC] stars to one of the proposed evolutionary scenarios, attributing the loss of hydrogen to a ``late'', ``very late'', or an ``AGB final'' thermal pulse (LTP, VLTP and AFTP, respectively). We conclude that BD+30° 3639 evolved through an AFTP. Title: The evolution of planetary nebulae. VII. Modelling planetary nebulae of distant stellar systems Authors: Schönberner, D.; Jacob, R.; Sandin, C.; Steffen, M. Bibcode: 2010A&A...523A..86S Altcode: 2010arXiv1009.0999S
Aims: By means of hydrodynamical models we do the first investigations of how the properties of planetary nebulae are affected by their metal content and what can be learned from spatially unresolved spectrograms of planetary nebulae in distant stellar systems.
Methods: We computed a new series of 1D radiation-hydrodynamics planetary nebulae model sequences with central stars of 0.595 M⊙ surrounded by initial envelope structures that differ only by their metal content. At selected phases along the evolutionary path, the hydrodynamic terms were switched off, allowing the models to relax for fixed radial structure and radiation field into their equilibrium state with respect to energy and ionisation. The analyses of the line spectra emitted from both the dynamical and static models enabled us to systematically study the influence of hydrodynamics as a function of metallicity and evolution. We also recomputed selected sequences already used in previous publications, but now with different metal abundances. These sequences were used to study the expansion properties of planetary nebulae close to the bright cut-off of the planetary nebula luminosity function.
Results: Our simulations show that the metal content strongly influences the expansion of planetary nebulae: the lower the metal content, the weaker the pressure of the stellar wind bubble, but the faster the expansion of the outer shell because of the higher electron temperature. This is in variance with the predictions of the interacting-stellar-winds model (or its variants) according to which only the central-star wind is thought to be responsible for driving the expansion of a planetary nebula. Metal-poor objects around slowly evolving central stars become very dilute and are prone to depart from thermal equilibrium because then adiabatic expansion contributes to gas cooling. We find indications that photoheating and line cooling are not fully balanced in the evolved planetary nebulae of the Galactic halo. Expansion rates based on widths of volume-integrated line profiles computed from our radiation-hydrodynamics models compare very well with observations of distant stellar system. Objects close to the bright cut-off of the planetary nebula luminosity function consist of rather massive central stars (>0.6 M⊙) with optically thick (or nearly thick) nebular shells. The half-width-half-maximum velocity during this bright phase is virtually independent of metallicity, as observed, but somewhat depends on the final AGB-wind parameters.
Conclusions: The observed expansion properties of planetary nebulae in distant stellar systems with different metallicities are explained very well by our 1D radiation-hydrodynamics models. This result demonstrates convincingly that the formation and acceleration of a planetary nebula occurs mainly because of ionisation and heating of the circumstellar matter by the stellar radiation field, and that the pressure exerted by the shocked stellar wind is less important. Determinations of nebular abundances by means of photoionisation modelling may become problematic for those cases where expansion cooling must be considered. 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: 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: 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: 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: 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 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: The evolution of planetary nebulae. VI. On the chemical composition of the metal-poor PN G135.9+55.9 Authors: Sandin, C.; Jacob, R.; Schönberner, D.; Steffen, M.; Roth, M. M. Bibcode: 2010A&A...512A..18S Altcode: 2009arXiv0912.5430S The actual value of the oxygen abundance of the metal-poor planetary nebula PN G135.9+55.9 has frequently been debated in the literature. We wanted to clarify the situation by making an improved abundance determination based on a study that includes both new accurate observations and new models. We made observations using the method of integral field spectroscopy with the PMAS instrument, and also used ultraviolet observations that were measured with HST-STIS. In our interpretation of the reduced and calibrated spectrum we used for the first time, recent radiation hydrodynamic models, which were calculated with several setups of scaled values of mean Galactic disk planetary nebula metallicities. For evolved planetary nebulae, such as PN G135.9+55.9, it turns out that departures from thermal equilibrium can be significant, leading to much lower electron temperatures, hence weaker emission in collisionally excited lines. Based on our time-dependent hydrodynamic models and the observed emission line [O iii] λ5007, we found a very low oxygen content of about 1/80 of the mean Galactic disk value. This result is consistent with emission line measurements in the ultraviolet wavelength range. The C/O and Ne/O ratios are unusually high and similar to those of another halo object, BoBn-1.

Based in part on observations collected at the Centro Astronómico Hispano Alemán (CAHA), operated jointly by the Max-Planck-Institut für Astronomie and the Instituto de Astrofísica de Andalucia (CSIC). 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: 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: 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: On the Role of Acoustic-Gravity Waves in the Energetics of the Solar Atmosphere Authors: Straus, T.; Fleck, B.; Jefferies, S. M.; McIntosh, S. W.; Severino, G.; Steffen, M.; Tarbell, T. D. Bibcode: 2009ASPC..415...95S Altcode: 2010arXiv1003.3773S In a recent paper (Straus et al. 2008) we determined the energy flux of internal gravity waves in the lower solar atmosphere using a combination of 3D numerical simulations and observations obtained with the IBIS instrument operated at the Dunn Solar Telescope and the Michelson Doppler Imager (MDI) on SOHO. In this paper we extend these studies using coordinated observations from SOT/NFI and SOT/SP on Hinode and MDI. The new measurements confirm that gravity waves are the dominant phenomenon in the quiet middle/upper photosphere and that they transport more mechanical energy than the high-frequency (> 5 mHz) acoustic waves, even though we find an acoustic flux 3-5 times larger than the upper limit estimate of Fossum & Carlsson (2006). It therefore appears justified to reconsider the significance of (non-M)HD waves for the energy balance of the solar chromosphere. 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: 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: 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: 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: 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: 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 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: 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: Solar Photosphere and Chromosphere Authors: Steffen, Matthias Bibcode: 2009LanB...4B...28S Altcode: 2009LanB...4B.4114S No abstract at ADS Title: Monitoring mass motions of Betelgeuse's photosphere using robotic telescopes . Authors: Weber, M.; Carroll, T.; Granzer, T.; Steffen, M.; Strassmeier, K. G. Bibcode: 2009MmSAI..80..743W Altcode: We started monitoring Betelgeuse using STELLA/SES, the STELLA échelle spectrograph fed by a robotic 1.2 m telescope on Tenerife, and the automatic photometric telescope (APT) T7 in Arizona in fall 2008. In this first observing season, we have collected 67 high resolution spectra from 390 to 900 nm at a resolution of 50,000 and a S/N between 100 and 300, and a comparable number of photometric observations in the Halpha filter. In this presentation, we report on the initial findings based on this first data set: Radial velocities, effective temperature (along with surface gravity and metallicity) are automatically computed by the STELLA/SES data reduction & analysis pipeline. We compare these global measurements and the photometric brightness with velocities and temperature indicators derived from individual spectral lines, to bring these values in line with recently published observations. Furthermore we compute synthetic line profiles from state-of-the-art 3D stellar convection models, and compare the line-profiles, their shapes and positions to our observations. The final aim of the observing program is to find out if the spectral line variations can be explained using these non-magnetic convection models. Title: Models of surface convection and dust clouds in brown dwarfs Authors: Freytag, B.; Allard, F.; Ludwig, H. -G.; Homeier, D.; Steffen, M. Bibcode: 2008PhST..133a4005F Altcode: The influence of dust grains on the atmospheres of brown dwarfs is visible in observed spectra. To investigate what prevents the dust grains from falling down, or how fresh condensable material is mixed up in the atmosphere to allow new grains to form, we performed 2D radiation-hydrodynamics simulations with CO5BOLD of the upper part of the convection zone and the atmosphere containing the dust cloud layers. We find that unlike in models of Cepheids, the convective overshoot does not play a major role. Instead, the mixing in the dust clouds is controlled by gravity waves. Title: he models comprise the upper part of the convection zone and the atmosphere with %the dust cloud layers. We find that direct convective overshoot does not play a major role. Instead, the mixing in the clouds is controlled by gravity waves. Authors: Freytag, B.; Allard, F.; Ludwig, H. -G.; Homeier, D.; Steffen, M.; Sharp, C. Bibcode: 2008sf2a.conf..469F Altcode: To investigate the mechanism that controls the formation and gravitational settling of dust grains as well as the mixing of fresh condensable material into the atmosphere of brown dwarfs, we performed 2D radiation-hydrodynamics simulations with CO5BOLD. Title: The evolution of planetary nebulae. V. The diffuse X-ray emission Authors: Steffen, M.; Schönberner, D.; Warmuth, A. Bibcode: 2008A&A...489..173S Altcode: 2008arXiv0807.3290S Context: Observations with space-borne X-ray telescopes revealed the existence of soft, diffuse X-ray emission from the inner regions of planetary nebulae. Although the existing images support the idea that this emission arises from the hot shocked central-star wind which fills the inner cavity of a planetary nebula, existing models have difficulties to explain the observations consistently.
Aims: We investigate how the inclusion of thermal conduction changes the physical parameters of the hot shocked wind gas and the amount of X-ray emission predicted by time-dependent hydrodynamical models of planetary nebulae with central stars of normal, hydrogen-rich surface composition.
Methods: We upgraded our 1D hydrodynamics code NEBEL by to account for energy transfer due to heat conduction, which is of importance at the interface separating the hot shocked wind gas (“hot bubble”) from the much cooler nebular material. With this new version of NEBEL we recomputed a selection of our already existing hydrodynamical sequences and obtained synthetic X-ray spectra for representative models along the evolutionary tracks by means of the freely available CHIANTI package.
Results: Heat conduction leads to lower temperatures and higher densities within a bubble and brings the physical properties of the X-ray emitting domain into close agreement with the values derived from observations. The amount of X-rays emitted during the course of evolution depends on the energy dumped into the bubble by the fast stellar wind, on the efficiency of “evaporating” cool nebular gas via heat conduction, and on the bubble's expansion rate. We find from our models that the X-ray luminosity of a planetary nebula increases during its evolution across the HR diagram until stellar luminosity and wind power decline. Depending on the central-star mass and the evolutionary phase, our models predict X-ray [ 0.45-2.5 keV] luminosities between 10-8 and 10-4 of the stellar bolometric luminosities, in good agreement with the observations. Less than 1% of the wind power is radiated away in this X-ray band. Although temperature, density, and also the mass of the hot bubble is significantly altered by heat conduction, the dynamics of the whole system remains practically the same.
Conclusions: Heat conduction allows the construction of nebular models which predict the correct amount of X-ray emission and at the same time are fully consistent with the observed mass-loss rate and wind speed. Thermal conduction must be considered as a viable physical process for explaining the diffuse X-ray emission from planetary nebulae with closed inner cavities. Magnetic fields must then be absent or extremely weak.

Dedicated to the memory of M. Perinotto, a dear friend and esteemed colleague who died unexpectedly and much too

early on August 15, 2007. Title: On the Role of Acoustic-gravity Waves in the Energetics of the Solar Atmosphere Authors: Straus, T.; Fleck, B.; Jefferies, S. M.; Cauzzi, G.; McIntosh, S. W.; Reardon, K.; Severino, G.; Steffen, M.; Suter, M.; Tarbell, T. D. Bibcode: 2008ESPM...12.2.11S Altcode: We revisit the dynamics and energetics of the solar atmosphere, using a combination of high-quality observations and 3D numerical simulations of the overshoot region of compressible convection into the stable photosphere. We discuss the contribution of acoustic-gravity waves to the energy balance of the photosphere and low chromosphere. We demonstrate the presence of propagating internal gravity waves at low frequencies (< 5mHz). Surprisingly, these waves are found to be the dominant phenomenon in the quiet middle/upper photosphere and to transport a significant amount of mechanical energy into the atmosphere outweighing the contribution of high-frequency (> 5mHz) acoustic waves by more than an order of magnitude. We compare the properties of high-frequency waves in the simulations with results of recent high cadence, high resolution Doppler velocity measurements obtained with SOT/SP and SOT/NFI on Hinode. Our results seem to be in conflict with the simple picture of upward propagating sound waves. We discuss the implications of our findings on the energy flux estimate at high-frequencies. 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: Velocity and Intensity Power and Cross Spectra in Numerical Simulations of Solar Convection Authors: Severino, G.; Straus, T.; Steffen, M. Bibcode: 2008SoPh..251..549S Altcode: 2008SoPh..tmp...54S Fitting observed power and cross spectra of medium-degree p modes in velocity (V) and intensity (I) has been widely used for getting information about the p-mode excitation process and, in particular, for trying to determine the type and location of the acoustic sources. Numerical simulations of solar convection allow one to "observe" velocity and temperature (T, used as proxy for I) fluctuations in different reference frames. Sampling the oscillations on planes of constant optical depth (τ-frame) closely corresponds to the observer's point of view, whereas sampling the oscillations at constant geometrical height (z-frame) is more appropriate for comparison with predictions from theoretical models based on Eulerian hydrodynamics. The results of the analysis in the two frames show significant differences. Considering the effects introduced on oscillations by the steep temperature gradient of the photosphere and by the temperature- and pressure-dependent continuum opacity, we develop a new model for fitting the simulated V and T power and cross spectra both in the τ- and z-frames and discuss its merits and limitations. 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: Spatially resolved spectroscopy of planetary nebulae and their halos. I. Five galactic disk objects Authors: Sandin, C.; Schönberner, D.; Roth, M. M.; Steffen, M.; Böhm, P.; Monreal-Ibero, A. Bibcode: 2008A&A...486..545S Altcode: 2008arXiv0802.3813S Strong mass loss off stars at the tip of the asymptotic giant branch (AGB) profoundly affects properties of these stars and their surroundings, including the subsequent planetary nebula (PN) stage. With this study we wanted to determine physical properties of mass loss by studying weakly emitting halos, focusing on objects in the galactic disk. Halos surround the, up to several thousand times, brighter central regions of PNe. Young halos, specifically, still contain information of the preceeding final mass loss stage on the AGB. In the observations we used the method of integral field spectroscopy with the PMAS instrument. This is the first committed study of halos of PNe that uses this technique. We improved our data analysis by a number of steps. In a study of the influence of scattered light we found that a moderate fraction of intensities in the inner halo originate in adjacent regions. As we combine line intensities of distant wavelengths, and because radial intensity gradients are steep, we corrected for effects of differential atmospheric refraction. In order to increase the signal-to-noise of weak emission lines we introduced a dedicated method to bin spectra of individual spatial elements. We also developed a general technique to part the temperature-sensitive oxygen line [O III] λ4363 from the adjacent telluric mercury line Hg λ4358 - without using separate sky exposures. By these steps we avoided introducing errors of several thousand Kelvin to our temperature measurements in the halo. For IC 3568 we detected a halo. For M 2-2 we found a halo radius that is 2.5 times larger than reported earlier. We derived radially densely sampled temperature and density structures for four nebulae, which all extend from the central regions and out into the halo. NGC 7662, IC 3568, and NGC 6826 show steep radially increasing temperatures and a hot halo, indicating that the gas in the halo is not in thermal equilibrium. M 2-2 shows a larger temperature in the central region and an otherwise constant value. From the density structures we made estimates of core and halo masses and - for the first time reliable - mass loss rates at the tip of the AGB. All four objects show inwards radially increasing mass loss rate structures, which represent a rise by a factor of about 4-7, during the final mass loss phase, that covers a time period of approximately 104 years. Within a factor of two, the average of the maximum mass loss rates, which are distance dependent, is dot{M}max≃10-4 {M}⊙ yr-1.

Based on observations collected at the Centro Astronómico Hispano Alemán (CAHA), operated jointly by the Max-Planck Institut für Astronomie and the Instituto de Astrofísica de Andalucia (CSIC).

Figures 24-29 are only available in electronic form at http://www.aanda.org Title: The Energy Flux of Internal Gravity Waves in the Lower Solar Atmosphere Authors: Straus, Thomas; Fleck, Bernhard; Jefferies, Stuart M.; Cauzzi, Gianna; McIntosh, Scott W.; Reardon, Kevin; Severino, Giuseppe; Steffen, Matthias Bibcode: 2008ApJ...681L.125S Altcode: Stably stratified fluids, such as stellar and planetary atmospheres, can support and propagate gravity waves. On Earth these waves, which can transport energy and momentum over large distances and can trigger convection, contribute to the formation of our weather and global climate. Gravity waves also play a pivotal role in planetary sciences and modern stellar physics. They have also been proposed as an agent for the heating of stellar atmospheres and coronae, the exact mechanism behind which is one of the outstanding puzzles in solar and stellar physics. Using a combination of high-quality observations and 3D numerical simulations we have the first unambiguous detection of propagating gravity waves in the Sun's (and hence a stellar) atmosphere. Moreover, we are able to determine the height dependence of their energy flux and find that at the base of the Sun's chromosphere it is around 5 kW m-2. This amount of energy is comparable to the radiative losses of the entire chromosphere and points to internal gravity waves as a key mediator of energy into the solar atmosphere. Title: Toward Better Simulations of Planetary Nebulae Luminosity Functions Authors: Méndez, R. H.; Teodorescu, A. M.; Schönberner, D.; Jacob, R.; Steffen, M. Bibcode: 2008ApJ...681..325M Altcode: 2008arXiv0804.0826M We describe a procedure for the numerical simulation of the planetary nebulae luminosity function (PNLF), improving on previous work. Earlier PNLF simulations were based on an imitation of the observed distribution of the intensities of [O III] λ5007 relative to Hβ, generated predominantly using random numbers. We are now able to replace this by a distribution derived from the predictions of hydrodynamical PN models (Schönberner et al. 2007), which are made to evolve as the central star moves across the HR diagram, using proper initial and boundary conditions. In this way we move one step closer to a physically consistent procedure for the generation of a PNLF. As an example of these new simulations, we have been able to reproduce the observed PNLF in the Small Magellanic Cloud. 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: 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: 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: 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: Rotating `star-in-a-box' experiments Authors: Steffen, M.; Freytag, B. Bibcode: 2007AN....328.1054S Altcode: Using the radiation hydrodynamics code CO5BOLD in its `star-in-a-box' setup, we have performed exploratory simulations of global convection in a rotating reference frame. The goal is to study the interaction of convection and rotation by direct numerical simulation. For these first experiments, we chose an idealized configuration (a scaled-down, fast rotating Sun) whose properties resemble those of red supergiants in some respect. We describe the setup and time evolution of these models, and discuss the particular problems we have encountered. Finally, we derive the resulting differential rotation pattern and meridional flow field by temporal and azimuthal averaging of the simulation data. We find anti-solar differential rotation for all cases studied so far.

Movies are available via http://www.aip.de/AN/movies Title: On the Mass-Loss History at the Tip of the AGB Authors: Schönberner, D.; Steffen, M. Bibcode: 2007ASPC..378..343S Altcode: We demonstrate by means of radiation-hydrodynamics simulations how the spectral energy distribution (SED) of post-AGB objects depends on the mass-loss properties during evolution off the tip of the AGB. Title: A hydrodynamical study of multiple-shell planetary nebulae. II. Measuring the post-shock velocities in the shells Authors: Corradi, R. L. M.; Steffen, M.; Schönberner, D.; Jacob, R. Bibcode: 2007A&A...474..529C Altcode: Context: Planetary Nebulae (PNe) display complex radial surface brightness profiles whose understanding is crucial for a correct interpretation of their formation and evolution. In particular, the intermediate shells commonly observed around the bright rims of PNe contain important information for the discussion of the mass loss rate at the end of the AGB, for the determination of the post-AGB age of the central stars and the distance via the expansion parallax method, and for the analysis of line profiles for unresolved, faraway objects like extragalactic PNe.
Aims: This paper describes the dynamical properties of PNe shells and presents a new practical method to properly determine their expansion velocity.
Methods: The analysis is guided by realistic radiative-hydrodynamical simulations which allow to test and verify different methods for the kinematical study of PNe shells using high-resolution long-slit spectra.
Results: We show that the use of the derivative of the line profile allows us to determine accurately the post-shock gas velocity in the shells. This method is generally superior to other techniques previously used. In addition, if applied to long-slit data, it allows to determine the velocities of the shells even when they are very close to those of the PNe rims. Its application to 10 real PNe confirms the model predictions and previous conclusions about the substantial mass loss increase during the latest AGB evolution.
Conclusions: The method and discussion presented in this paper are meant to be used as a guide for a correct determination of some basic kinematic and evolutionary properties of Galactic and extragalactic PNe, including their expansion parallaxes.

Based on observations obtained at the 3.5 m NTT and CAT telescopes of the European Southern Observatory in Chile, and at the 2.6 m NOT telescope operated by NOTSA in the Spanish Observatorio del Roque de Los Muchachos of the Instituto de Astrofísica de Canarias. 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: The evolution of planetary nebulae. IV. On the physics of the luminosity function Authors: Schönberner, D.; Jacob, R.; Steffen, M.; Sandin, C. Bibcode: 2007A&A...473..467S Altcode: 2007arXiv0708.4292S Context: The luminosity function of planetary nebulae, in use for about two decades in extragalactic distance determinations, is still subject to controversial interpretations.
Aims: The physical basis of the luminosity function is investigated by means of several evolutionary sequences of model planetary nebulae computed with a 1D radiation-hydrodynamics code.
Methods: The nebular evolution is followed from the vicinity of the asymptotic-giant branch across the Hertzsprung-Russell diagram until the white-dwarf domain is reached, using various central-star models coupled to different initial envelope configurations. Along each sequence the relevant line emissions of the nebulae are computed and analysed.
Results: Maximum line luminosities in Hβ and [O iii] 5007 Å are achieved at stellar effective temperatures of about 65 000 K and 95 000...100 000 K, respectively, provided the nebula remains optically thick for ionising photons. In the optically thin case, the maximum line emission occurs at or shortly after the thick/thin transition. Our models suggest that most planetary nebulae with hotter (⪆ 45 000 K) central stars are optically thin in the Lyman continuum, and that their [O iii] 5007 Å emission fails to explain the bright end of the observed planetary nebulae luminosity function. However, sequences with central stars of ⪆0.6 M⊙ and rather dense initial envelopes remain virtually optically thick and are able to populate the bright end of the luminosity function. Individual luminosity functions depend strongly on the central-star mass and on the variation of the nebular optical depth with time.
Conclusions: Hydrodynamical simulations of planetary nebulae are essential for any understanding of the basic physics behind their observed luminosity function. In particular, our models do not support the claim of Marigo et al. (2004, A&A, 423, 995) according to which the maximum 5007 Å luminosity occurs during the recombination phase well beyond 100 000 K when the stellar luminosity declines and the nebular models become, at least partially, optically thick. Consequently, there is no need to invoke relatively massive central stars of, say > 0.7 M⊙, to account for the bright end of the luminosity function.

Based in parts on observations

made with the NASA/ESA Hubble Space Telescope, obtained at the

Space Science Institute, which is operated by the Association of the

Universities for Research in Astronomy, Inc., under NASA contract

NAS 5-26555. The data are retrieved from the ESO/ST-ECF Science Archive

Facility. Title: The PEPSI "deep spectrum" project Authors: Steffen, Matthias; Strassmeier, Klaus G. Bibcode: 2007AN....328..632S Altcode: No abstract at ADS 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: Spatially resolved spectroscopic studies of planetary nebulae and their halos Authors: Sandin, C.; Schönberner, D.; Roth, M. M.; Steffen, M.; Böhm, P.; Monreal-Iber, A. Bibcode: 2007apn4.confE...6S Altcode: No abstract at ADS Title: Internal Gravity Waves and their Role in the Energetics of the Solar Atmosphere Authors: Fleck, Bernard; Straus, T.; Jefferies, S.; McIntosh, S. W.; Severino, G.; Steffen, M. Bibcode: 2007AAS...210.2410F Altcode: 2007BAAS...39..130F Internal gravity waves are believed to be excited by convective overshoot in the solar atmosphere. We compare the results from numerical simulations of the overshoot region of compressible convection into a stable photosphere, with observations of the velocity field at several heights in the solar atmosphere. We find a consistent picture for the quiet middle/upper photosphere in which internal gravity waves are the dominant phenomenon at low frequencies (< 2.5 mHz). We estimate the contribution of these waves to the energy balance in the photosphere and low chromosphere. Title: 3-D hydrodynamic simulations of convection in A stars Authors: Kochukhov, O.; Freytag, B.; Piskunov, N.; Steffen, M. Bibcode: 2007IAUS..239...68K Altcode: 2006astro.ph.10111K Broadening and asymmetry of spectral lines in slowly rotating late A-type stars provide evidence for high-amplitude convective motions. The properties of turbulence observed in the A-star atmospheres are not understood theoretically and contradict results of previous numerical simulations of convection. Here we describe an ongoing effort to understand the puzzling convection signatures of A stars with the help of 3-D hydrodynamic simulations. Our approach combines realistic spectrum synthesis and non-grey hydrodynamic models computed with the CO5BOLD code. We discuss these theoretical predictions and confront them with high-resolution spectra of A stars. Our models have, for the first time, succeeded in reproducing the observed profiles of weak spectral lines without introducing fudge broadening parameters. Title: Radiative hydrodynamics models of stellar convection Authors: Steffen, Matthias Bibcode: 2007IAUS..239...36S Altcode: No abstract at ADS Title: Carbon monoxide in the solar atmosphere. II. Radiative cooling by CO lines Authors: Wedemeyer-Böhm, S.; Steffen, M. Bibcode: 2007A&A...462L..31W Altcode: 2006astro.ph.12197W Aims: The role of carbon monoxide as a cooling agent for the thermal structure of the mid-photospheric to low-chromospheric layers of the solar atmosphere in internetwork regions is investigated.
Methods: The treatment of radiative cooling via spectral lines of carbon monoxide (CO) has been added to the radiation chemo-hydrodynamics code CO5BOLD. The radiation transport has now been solved in a continuum band with Rosseland mean opacity and an additional band with CO opacity. The latter is calculated as a Planck mean over the CO band between 4.4 and 6.2 μm. The time-dependent CO number density is derived from the solution of a chemical reaction network.
Results: The CO opacity indeed causes additional cooling at the fronts of propagating shock waves in the chromosphere. There, the time-dependent approach results in a higher CO number density compared to the equilibrium case and hence in a larger net radiative cooling rate. The average gas temperature stratification of the model atmosphere, however, is only reduced by roughly 100 K. Also the temperature fluctuations and the CO number density are only affected to small extent. A numerical experiment without dynamics shows that the CO cooling process works in principle and drives the atmosphere to a cool radiative equilibrium state. At chromospheric heights, the radiative relaxation of the atmosphere to a cool state takes several 1000 s. The CO cooling process thus would seem to be too slow compared to atmospheric dynamics to be responsible for the very cool temperature regions observed in the solar atmosphere.
Conclusions: . The hydrodynamical timescales in our solar atmosphere model are much too short to allow for the radiative relaxation to a cool state, thus suppressing the potential thermal instability due to carbon monoxide as a cooling agent. Apparently, the thermal structure and dynamics of the outer model atmosphere are instead determined primarily by shock waves. Title: 3D-Spectroscopy of Extragalactic Planetary Nebulae as Diagnostic Probes for Galaxy Evolution Authors: Kelz, A.; Monreal-Ibero, A.; Roth, M. M.; Sandin, C.; Schönberner, D.; Steffen, M. Bibcode: 2007spts.conf..339K Altcode: 2006astro.ph..7498K In addition to study extragalactic stellar populations in their integrated light, the detailed analysis of individual resolved objects has become feasible, mainly for luminous giant stars and for extragalactic planetary nebulae (XPNe) in nearby galaxies. A recently started project at the Astrophysical Institute Potsdam (AIP), called ``XPN--Physics'', aims to verify if XPNe are useful probes to measure the chemical abundances of their parent stellar population. The project involves theoretical and observational work packages. Title: A First Three-Dimensional Model for the Carbon Monoxide Concentration in the Solar Atmosphere Authors: Wedemeyer-Böhm, S.; Kamp, I.; Freytag, B.; Bruls, J.; Steffen, M. Bibcode: 2006ASPC..354..301W Altcode: The time-dependent and self-consistent treatment of carbon monoxide (CO) has been added to the radiation chemo-hydrodynamics code CO5BOLD. It includes the solution of a chemical reaction network and the advection of the resulting particle densities with the hydrodynamic flow field. Here we present a first 3D simulation of the non-magnetic solar photosphere and low chromosphere, calculated with the upgraded code. In the resulting model, the highest amount of CO is located in the cool regions of the reversed granulation pattern in the middle photosphere. A large fraction of carbon is bound by CO throughout the chromosphere with exception of hot shock waves where the CO concentration is strongly reduced. The distribution of carbon monoxide is very inhomogeneous due to co-existing regions of hot and cool gas caused by the hydrodynamic flow. High-resolution observations of CO could thus provide important constraints for the thermal structure of the solar photosphere and chromosphere. Title: Detection of a Far-Infrared Bow Shock Nebula around R Hya: The First MIRIAD Results Authors: Ueta, T.; Speck, A. K.; Stencel, R. E.; Herwig, F.; Gehrz, R. D.; Szczerba, R.; Izumiura, H.; Zijlstra, A. A.; Latter, W. B.; Matsuura, M.; Meixner, M.; Steffen, M.; Elitzur, M. Bibcode: 2006ApJ...648L..39U Altcode: 2006astro.ph..7303U We present the first results of the MIRIAD (MIPS InfraRed Imaging of AGB Dust shells) project using the Spitzer Space Telescope. The primary aim of the project is to probe the material distribution in the extended circumstellar envelopes (CSEs) of evolved stars and recover the fossil record of their mass-loss history. Hence, we must map the whole of the CSEs plus the surrounding sky for background subtraction while avoiding the central star that is brighter than the detector saturation limit. With our unique mapping strategy, we have achieved better than 1 MJy sr-1 sensitivity in 3 hr of integration and successfully detected a faint (<5 MJy sr-1), extended (~400") far-infrared nebula around the asymptotic giant branch (AGB) star R Hya. Based on the parabolic structure of the nebula, the direction of the space motion of the star with respect to the nebula shape, and the presence of extended Hα emission cospatial to the nebula, we suggest that the detected far-IR nebula is due to a bow shock at the interface of the interstellar medium and the AGB wind of this moving star. This is the first detection of the stellar-wind bow shock interaction for an AGB star and exemplifies the potential of Spitzer as a tool to examine the detailed structure of extended far-IR nebulae around bright central sources. Title: Resonant Oscillation Modes and Background in Realistic Hydrodynamical Simulations of Solar Surface Convection Authors: Straus, T.; Severino, G.; Steffen, M. Bibcode: 2006ESASP.617E...4S Altcode: 2006soho...17E...4S No abstract at ADS Title: Working with VIMOS-IFU data: Searching and characterizing the faint haloes of planetary nebulae Authors: Monreal-Ibero, A.; Roth, M. M.; Schönberner, D.; Steffen, M.; Böhm, P. Bibcode: 2006NewAR..50..426M Altcode: In this contribution, we present the VIMOS-IFU data analysis of two planetary nebulae: NGC 3242 and NGC 4361. Due to the technical nature of the workshop, we emphasize some specific problems of data reduction and analysis: accuracy in flux calibration, importance of scattered light and improvement of the S/N ratio. We estimate that accuracy of our flux calibration is ∼15% in the spectral range 4200-6100 Å. At ∼10″ from the center of a point source, the intensity is ≲0.02% of the peak value, indicating that scattered light is negligible. Some examples of science that we are doing are shown. In particular, we have established an apparent temperature gradient across the halo of NGC NGC3242. Title: Nod & Shuffle 3D Spectroscopy Authors: Roth, Martin M.; Cardiel, Nicolas; Cenarro, Javier; Schönberner, Detlef; Steffen, Matthias Bibcode: 2006ASSL..336...99R Altcode: 2006sda..conf...99R No abstract at ADS Title: The modelling of the X-ray emission of planetary nebulae Authors: Schönberner, D.; Steffen, M.; Warmuth, A. Bibcode: 2006IAUS..234..161S Altcode: We report on our recent efforts to compute the X-ray emission from the hot, shocked stellar wind gas filling the inner cavity of planetary nebulae. To this end, we updated our 1D hydrodynamics code NEBEL by including a module that computes the heat transfer by thermal conduction across the contact discontinuity separating the hot shocked wind gas from the much cooler nebular material. Given the temperature and density structure of the hot bubble the X-ray emission is computed by means of the CHIANTI code. We find a reasonably close agreement of the computed X-ray luminosities with recent observations of Newton-XMM and Chandra. Our simulations also predict how the X-ray emission depends on the wind luminosity and the stellar parameters. Title: 3D Spectroscopy — a powerful new tool for PN research Authors: Roth, Martin M.; Schönberner, D.; Steffen, M.; Monreal, A.; Sandin, C. Bibcode: 2006IAUS..234...17R Altcode: Historically, technological progress with detectors and instrumentation has been essential for advances in any field of observational astronomy, e.g. the advent of CCDs being crucial for high dynamic range imaging and quantitative spectroscopy of galactic PNe, faint object spectrophotometry for the discovery of extragalactic PNe to distances as far as 100 Mpc, etc. The emerging technique of integral field ("3D") spectroscopy, which is being applied quite successfully to extragalactic astronomy, has unfortunately hardly been used so far for the study of PNe. However, 3D spectroscopy has an enormous potential for various observational problems, ranging from high spatial resolution emission line mapping in different wavelengths simultaneously, over extremely high sensitivity spectroscopy of low surface brightness objects like e.g. PN haloes, to accurate 3D spectrophotometry of extragalactic PNe, and many others. As an attempt to encourage PN researchers to make better use of these new opportunities, the presently existing suite of 3D instruments on 4-8m class telescopes is reviewed, highlighting some examples of successful 3D observations for the study of PNe. Title: Hydrodynamical interpretation of basic nebular structures Authors: Steffen, M.; Schönberner, D. Bibcode: 2006IAUS..234..285S Altcode: Based on existing hydrodynamical simulations, we review our present understanding of the formation and evolution of planetary nebulae (PNe) and discuss the relevant processes responsible for the development of the basic (1D) nebular structures. Title: New observations of the halo radial temperature structure in NGC 7662 Authors: Sandin, C.; Schönberner, D.; Roth, M. M.; Steffen, M.; Monreal-Ibero, A.; Böhm, P.; Tripphahn, U. Bibcode: 2006IAUS..234..501S Altcode: 2006astro.ph..9279S We report on our studies of the physical structure of the planetary nebula (PN) NGC 7662. Using (3D) Integral Field Spectroscopy we have been able to measure the electron temperature more accurately and at a larger number of radial locations than before. Here we briefly present our method by which we find a strong positive temperature gradient with increasing radius. According to hydrodynamic models a hot halo, when compared to the central star, can be the product of the passage of an ionization front (e.g. Marten 1993). Such a gradient is not found in equilibrium models, and this finding - when confirmed for other objects - strongly advocates the use of hydrodynamic models when modeling PN halos. Title: Crowded Field 3D Spectrophotometry of Extragalactic Planetary Nebulae Authors: Roth, Martin M.; Becker, Thomas; Böhm, Petra; Schönberner, Detlef; Steffen, Matthias; Exter, Katrina Bibcode: 2006pnbm.conf..239R Altcode: No abstract at ADS Title: On the luminosity function of planetary nebulae Authors: Schönberner, D.; Jacob, R.; Steffen, M. Bibcode: 2006IAUS..234..505S Altcode: A new assessment of the physics of the luminosity function of planetary nebulae is presented, based on our grid of nebular evolutionary sequences computed with a 1D radiation-hydrodynamics code. The nebular evolution is followed from the formation stage in the vicinity of the asymptotic-giant branch across the Hertzsprung-Russell diagram until the white-dwarf domain is reached, for various central-star models coupled to different initial envelope configurations. Along each sequence the relevant line luminosities of the nebulae are computed and analyzed. Our models predict that reasonably dense initial circumstellar envelopes with central stars of slightly above 0.6 M⊙ will remain nearly optically thick and are able to provide the observed 5007 Å cutoff luminosity. We cannot support the claim of Marigo et al. (2004) according to which only planetary nebulae with central stars of > 0.7 M⊙ are able to provide sufficient 5007 Å line emission to account for the bright end of the luminosity function. Title: Simulations of Magnetohydrodynamics and CO Formation from the Convection Zone to the Chromosphere Authors: Wedemeyer-Böhm, S.; Schaffenberger, W.; Steiner, O.; Steffen, M.; Freytag, B.; Kamp, I. Bibcode: 2005ESASP.596E..16W Altcode: 2005ccmf.confE..16W No abstract at ADS Title: On the Reliability of Planetary Nebulae as Extragalactic Probes Authors: Schönberner, D.; Jacob, R.; Steffen, M.; Roth, M. M. Bibcode: 2005AIPC..804..269S Altcode: We report on the first 1D radiation-hydrodynamics simulations aimed at exploring the metallicity dependance of the evolution of planetary nebulae. Using scaled mean abundances, based on Galactic disk objects, we found that for lower metallicities the nebulae become more dilute and prone to departures from thermal equilibrium because of the reduced wind power of the central star and the reduced cooling efficiency of the nebular gas. At selected evolutionary phases we `switched off' the hydrodynamic terms and computed equilibrium models for the fixed density structure and radiation field. This enabled us to study how the electron temperature and the emission of important diagnostic lines is influenced by hydrodynamics. Title: 3D Spectroscopy of Planetary Nebulae - Diagnostic Tools from the Milky Way to Nearby Galaxies and Beyond Authors: Roth, Martin M.; Schönberner, Detlef; Steffen, Matthias; Monreal, Ana Bibcode: 2005AIPC..804...20R Altcode: Integral field (``3D'') spectroscopy is an emerging observing technique in the process of becoming a common user tool, with a variety of powerful instruments becoming available at most major observatories. The application of this new method has a number of advantages over conventional techniques for both extended and point sources. We describe our experience with 3D spectroscopy applied to extremely low surface brightness regions in haloes of galactic planetary nebulae, and to extragalactic planetary nebulae in local group galaxies. Title: Searching and Characterizing the Faint Haloes of Planetary Nebulae: A Study Case for Integral Field Spectroscopy Authors: Monreal-Ibero, A.; Roth, M. M.; Schönberner, D.; Steffen, M.; Böhm, P. Bibcode: 2005AIPC..804..155M Altcode: We present here the first results of a research project devoted to the characterization of the physical properties of the planetary nebulae haloes. We succeeded in determining some properties in the halo of NGC 3242, but we did not detect any halo feature down to the detection limit in NGC 4361. Title: Modeling X-ray Emission from Planetary Nebulae Authors: Steffen, M.; Schönberner, D.; Warmuth, A.; Schwope, A.; Landi, E.; Perinotto, M.; Bucciantini, N. Bibcode: 2005AIPC..804..161S Altcode: Recent observations using the Chandra X-ray Observatory and XXM Newton revealed without doubt diffuse X-ray emission from the shock-heated wind gas in planetary nebula (PN) interiors. Typical properties of the emitting gas are temperatures of a few 106 K and electron densities of the order of 100 cm-3. According to current hydrodynamical models, the shocked gas becomes too hot (T ~ 107-108 K) and too tenuous (ne ~ 1 cm-3) to produce the observed X-ray emission. However, the hot gas is confined by the rather cool (T ~ 10 000K) nebular gas, and thermal heat conduction across the interface between the hot and cool gas becomes important. It changes the contact discontinuity into a more extended transition layer covering the temperature range where the observed X-ray emission is thought to arise. To date, only similarity solutions for the hydrodynamical problem of PN evolution with heat conduction have been derived (Zhekov & Perinotto 1996). We present first results from new numerical simulations of the PN evolution including thermal conduction by electrons. We confront the X-ray luminosities predicted by these models with those derived from XMM/Chandra observations. Title: The evolution of planetary nebulae. III. Internal kinematics and expansion parallaxes Authors: Schönberner, D.; Jacob, R.; Steffen, M. Bibcode: 2005A&A...441..573S Altcode: A detailed theoretical study of the basic internal kinematics of planetary nebulae is presented, based on 1D radiation-hydrodynamics simulations of circumstellar envelopes around central stars of 0.595 and 0.696 M⊙. By means of observable quantities like radial surface-brightness distributions and emission-line profiles computed from the models, a comparison with real objects was performed and revealed a reasonable agreement. This allowed to draw important conclusions by investigating the kinematics of these models in detail. Firstly, it is shown that the determination of kinematical ages, normally considered to be simple if size and expansion rate of an object are given, can seriously be flawed. Secondly, the expansion law of a planetary nebula is different from what is assumed for deriving spatio-kinematical models. Thirdly and most importantly, our hydrodynamical models help to correctly use existing angular expansion measurements for distance determinations. The mere combination of the angular expansion rates with the spectroscopic expansion velocities leads always to a serious underestimate of the distance, the degree of which depends on the evolutionary state of the object. The necessary correction factor varies between 3 and 1.3. Individual correction factors can be estimated with an accuracy of about 10% by matching our hydrodynamical models to real objects. As a result, revised distances for a few objects with reliable angular expansion rates are presented. But even these corrected distances are not always satisfying: they still appear to be inconsistent with other distance determinations and, even more disturbing, with the accepted theory of post-asymptotic giant branch evolution. As a byproduct of the angular expansion measurements, the transition times from the vicinity of the asymptotic giant branch to the planetary-nebula regime could be estimated. They appear to be shorter than assumed in the present evolutionary calculations. Title: Integral Field Spectroscopy of Faint Halos of Planetary Nebulae Authors: Monreal-Ibero, A.; Roth, M. M.; Schönberner, D.; Steffen, M.; Böhm, P. Bibcode: 2005ApJ...628L.139M Altcode: 2005astro.ph..6300M We present the first integral field spectroscopy observations of the two planetary nebulae NGC 3242 and NGC 4361 with the VIMOS instrument attached to VLT-UT3. By co-adding a large number of spaxels, we reach an emission-line detection limit of 5×10-18 ergs cm-2 s-1 arcsec-2. In the case of NGC 3242, we succeed in determining some properties of the halo. The radial surface brightness profile in [O III] implies increasing mass loss before the formation of the planetary nebula. Traces of the mysterious ``rings'' are clearly visible. We find for the first time an apparent temperature gradient across a halo: from about 16,000 K close to the shell/halo transition to 20,000 K at the halo's outer edge. No line emission is seen in the suspected halo region of NGC 4361 down to the sensitivity limit. Title: MIPS Infrared Imaging of AGB Dustshells (MIRIAD): tracing mass-loss histories in the extremely large shells around evolved stars Authors: Speck, Angela; Elitzur, Moshe; Gehrz, Robert; Herwig, Falk; Izumiura, Hideyuki; Latter, William; Matsuura, Mikako; Meixner, Margaret; Steffen, Matthias; Stencel, Robert; Szczerba, Ryszard; Ueta, Toshiya; Zijlstra, Albert Bibcode: 2005sptz.prop20258S Altcode: Evolved intermediate mass stars are major contributors to the interstellar medium. However, the mechanisms by which they do this are not well understood. The circumstellar shells of evolved stars (AGB and post-AGB stars) contain the fossil record of their mass loss, and therefore have the potential to verify many aspects of stellar evolution. IRAS and ISO data indicate that huge dust shells exist around many such objects, extending several parsecs from the central star. Furthermore, these large dust shells show evidence for mass-loss variations that correlate with evolutionary changes in the star itself. Previous observations lacked the sensitivity and spatial resolution to investigate the full extent and detailed structure of these large dust shells. Using Spitzer/MIPS's unique sensitivity and mapping capabilities, we propose to produce far-IR images of the parsec-sized dust shells around four carefully selected evolved stars in order to determine the distribution of material in these circumstellar envelopes. These maps will be the deepest yet (sensitivity 1MJy/sr) and have the most complete spatial coverage to date. Crucially, mapping in this level of detail will allow us to: (a) constrain the masses of the progenitor stars; (b) test theories of stellar evolution and mass-loss mechanisms; (c) determine the effect of dust chemistry on mass loss (and therefore on stellar evolution); (d) determine when the aspherical structures so prevalent in planetary nebulae actually develop and thus constrain the cause. Title: Science with the STELLA robotic observatory Authors: Weber, M.; Strassmeier, K. G.; Bartus, J.; Korhonen, H.; Kővári, Zs.; Oláh, K.; Schwope, A.; Staude, A.; Steffen, M. Bibcode: 2005ESASP.560.1025W Altcode: 2005csss...13.1025W No abstract at ADS 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: The evolution of planetary nebulae. II. Circumstellar environment and expansion properties Authors: Schönberner, D.; Jacob, R.; Steffen, M.; Perinotto, M.; Corradi, R. L. M.; Acker, A. Bibcode: 2005A&A...431..963S Altcode: We investigate and discuss the expansion properties of planetary nebulae by means of 1D radiation-hydrodynamics models computed for different initial envelope configurations and central star evolutionary tracks. In particular, we study how the expansion depends on the initial density gradient of the circumstellar envelope and show that it is possible to derive information on the very last mass-loss episodes during the star's final evolution along and off the asymptotic giant branch. To facilitate the comparison of the models with real objects, we have also computed observable quantities like surface brightness and emission-line profiles. With the help of newly acquired high-resolution emission-line profiles for a sample of planetary nebulae we show that models with initial envelopes based on the assumption of a stationary wind outflow fail to explain the observed expansion speeds of virtually all of the observed planetary nebulae. Instead it must be assumed that during the very last phase of evolution along the final asymptotic giant branch evolution the mass-loss rate increases in strength, resulting in a much steeper slope of the circumstellar radial density distribution. Under these conditions, the expansion properties of the nebular gas differ considerably from the self-similar solutions found for isothermal conditions. Furthermore, the mass loss must remain at a rather high level until the stellar remnant begins to evolve quickly towards the central star regime. Current theoretical computations of dust-driven mass-loss which are restricted to rather low temperatures cannot be applied during the star's departure from the asymptotic giant branch.

Based on observations obtained at the 3.5 μm NTT and the 1.2 μm CAT telescope of the European Southern Observatory, La Silla, and at the 2.6 μm NOT telescope operated on the island of La Palma by NOTSA, in the Spanish Observatorio del Roque de Los Muchachos of the Institutode Astrofísica de Canarias.

Dedicated to Prof. V. Weidemann on the occasion of his 80th birthday, October 3, 2004. Title: Numerical simulations of convection in A-stars Authors: Freytag, Bernd; Steffen, Matthias Bibcode: 2004IAUS..224..139F Altcode: Radiation hydrodynamics simulations have been used to produce numerical models of the convective surface layers of a number of stars, including the Sun and other stars on or above the main-sequence, white dwarfs of type DA, and red supergiants. Title: Ionization and its Structural Impacts on the Evolution of Planetary Nebulae Authors: Schönberner, D.; Steffen, M.; Jacob, R. Bibcode: 2004ASPC..313..283S Altcode: 2004apnw.conf..283S We review our present knowledge about the formation and evolution of planetary nebulae and evaluate the relative importance of photoionization and wind interaction. It turns out that heating by photoionization drives the expansion of a planetary nebula during its entire life, while wind interaction accelerates and shapes the inner regions only during the later stages of evolution. We found observational evidence that the transition from spherical AGB-wind structures to more aspherical ones must occur when the star begins to evolve slowly off the AGB. Title: The evolution of planetary nebulae. I. A radiation-hydrodynamics parameter study Authors: Perinotto, M.; Schönberner, D.; Steffen, M.; Calonaci, C. Bibcode: 2004A&A...414..993P Altcode: We follow hydrodynamically the evolution of spherimodel planetary nebulae subject to different initial conditions and with various central stars, investigating how combinations of central-star mass and asymptotic giant branch mass-loss rate determine the shape and kinematics of a planetary nebula. With this approach we aim at constituting a framework useful for the interpretation of the evolutionary status and previous mass-loss history of observed individual nebulae, making use of their kinematiproperties and surface brightness characteristics. In particular, the models are compared with the observed morphologies and kinematics of double shell nebulae. The dynamical structure of all the models is characterized by a more or less complicated shock wave pattern set up by ionization and wind interaction whose combined action results in general in a typical double-shell structure. We have found that models with simple initial structures based on a constant AGB mass-loss rate fail to comply with observed shell morphologies and surface-brightness distributions. A reasonable agreement with the observations is only found for a model where the mass-loss rate is strongly increasing towards the end of the asymptotic giant-branch evolution. Depending on the central star's evolutionary speed and the density of the cool wind expelled along the asymptotic giant-branch, planetary nebulae may never get optically thin. This is primarily the case for the more massive central stars, and this fact offers a rather natural explanation for the long standing problem of the very existence of molecular hydrogen in the immediate vicinity of hot central stars. We also show that distances to planetary nebulae based on expansion parallaxes are systematically too small by a significant amount. 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: On the expansion parallaxes of planetary nebulae. Authors: Schoenberner, D.; Jacob, R.; Steffen, M. Bibcode: 2004ANS...325..105S Altcode: 2004ANS...325a.105S; 2004ANS...325..P37S No abstract at ADS Title: Spectroscopy of extragalactic planetary nebulae as tracers of intermediate age and old stellar populations. Authors: Roth, M. M.; Schoenberner, D.; Steffen, M.; Becker, T. Bibcode: 2004ANS...325...46R Altcode: 2004ANS...325..F09R; 2004ANS...325a..46R No abstract at ADS Title: Probing the mass-loss history at the tip of the AGB by planetary nebulae. Authors: Schoenberner, D.; Jacob, R.; Hildebrandt, G.; Steffen, M.; Lehmann, H.; Corradi, R.; Acker, A. Bibcode: 2004ANS...325..104S Altcode: 2004ANS...325..P36S; 2004ANS...325a.104S No abstract at ADS 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: 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: Long Term Modeling of Mass Loss on the Late AGB Authors: Simis, Yvonne; Schonberner, Detlef; Steffen, Matthias Bibcode: 2003ANS...324...18S Altcode: 2003ANS...324..D03S; 2003ANS...324b..18S No abstract at ADS Title: Ionized haloes in planetary nebulae: new discoveries, literature compilation and basic statistical properties Authors: Corradi, R. L. M.; Schönberner, D.; Steffen, M.; Perinotto, M. Bibcode: 2003MNRAS.340..417C Altcode: We present a comprehensive observational study of haloes around planetary nebulae (PNe). Deep Hα+[NII] and/or [OIII] narrow-band images have been obtained for 35 PNe, and faint extended haloes have been newly discovered in the following 10 objects: Cn 1-5, IC 2165, IC 2553, NGC 2792, NGC 2867, NGC 3918, NGC 5979, NGC 6578, PB 4, and possibly IC 1747. New deep images have also been obtained of other known or suspected haloes, including the huge extended emission around NGC 3242 and Sh 2-200. In addition, the literature was searched, and together with the new observations an improved data base containing some 50 PN haloes has been compiled.

The halo sample is illustrated in an image atlas contained in this paper, and the original images are made available for use by the scientific community at <externallink type="url">http://www.ing. iac.es/~rcorradi/HALOES/</A>.

The haloes have been classified following the predictions of modern radiation-hydrodynamical simulations that describe the formation and evolution of ionized multiple shells and haloes around PNe. According to the models, the observed haloes have been divided into the following groups: (i) circular or slightly elliptical asymptotic giant branch (AGB) haloes, which contain the signature of the last thermal pulse on the AGB; (ii) highly asymmetrical AGB haloes; (iii) candidate recombination haloes, i.e. limb-brightened extended shells that are expected to be produced by recombination during the late post-AGB evolution, when the luminosity of the central star drops rapidly by a significant factor; (iv) uncertain cases which deserve further study for a reliable classification; (v) non-detections, i.e. PNe in which no halo is found to a level of <~10-3 the peak surface brightness of the inner nebulae.

We discuss the properties of the haloes: detection rate, morphology, location of the central stars in the Hertzsprung-Russell diagram, sizes, surface brightness profiles, and kinematical ages. Among the most notable results, we find that, as predicted by models, ionized AGB haloes are a quite common phenomenon in PNe, having been found in 60 per cent of elliptical PNe for which adequately deep images exist. Another 10 per cent show possible recombination haloes. In addition, using the kinematical ages of the haloes and inner nebulae, we conclude that most of the PNe with observed AGB haloes have left the AGB far from a thermal pulse, at a phase when hydrogen burning is the dominant energy source. We find no significant differences between the AGB haloes of hydrogen-poor and hydrogen-rich central stars. Title: Formation and Evolution of Planetary Nebulae: A Radiation Hydrodynamics Study Authors: Perinotto, M.; Calonaci, C.; Schönberner, D.; Steffen, M.; Blöcker, T. Bibcode: 2003IAUS..209..157P Altcode: No abstract at ADS Title: AGB Mass-Loss History and Haloes Around Planetary Nebulae Authors: Corradi, R. L. M.; Steffen, M.; Schönberner, D.; Perinotto, M. Bibcode: 2003IAUS..209..455C Altcode: No abstract at ADS Title: From Red Giants to White Dwarfs -- A Radiation-Hydrodynamics Simulation of the Planetary Nebula Stage Authors: Schönberner, D.; Steffen, M. Bibcode: 2003ASIB..105...19S Altcode: 2003whdw.conf...19S; 2003whsw.conf...19S Based on 1D radiation-hydrodynamics simulations along the upper AGB we followed the evolution of AGB wind envelopes towards the white dwarf domain. Taking into account photoionization and wind interaction in a realistic manner, the hydrodynamics of the post-AGB phase leads, if coupled to an evolving 0.6 Modot central star model, to morphologies very similar to what is observed for spherical/elliptical planetary nebulae. Title: Structure and Evolution of Planetary Nebula Haloes Authors: Steffen, M.; Schönberner, D. Bibcode: 2003IAUS..209..439S Altcode: No abstract at ADS 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: The Formation and Evolution of Planetary Nebulae (invited review) Authors: Schönberner, D.; Steffen, M. Bibcode: 2003IAUS..209..147S Altcode: No abstract at ADS Title: Granulation Abundance Corrections from Hydrodynamical Convection Simulations Authors: Steffen, M.; Holweger, H. Bibcode: 2003IAUS..210P.D15S Altcode: No abstract at ADS Title: Spots on the surface of Betelgeuse -- Results from new 3D stellar convection models Authors: Freytag, B.; Steffen, M.; Dorch, B. Bibcode: 2002AN....323..213F Altcode: The observed irregular brightness fluctuations of the well-known red supergiant Betelgeuse (alpha Ori, M2 Iab) have been attributed by M. Schwarzschild (1975) to the changing granulation pattern formed by only a few giant convection cells covering the surface of this giant star. The surface structure revealed by modern interferometric methods appears to be generally consistent with the explanation as large-scale granular intensity fluctuations. The interferometric data can be modeled equally well by assuming the presence of a few (up to 3) unresolved hot or cool spots on a limb-darkened disk. In an effort to improve our theoretical understanding of the Betelgeuse phenomena, we have applied a new radiation hydrodynamics code (CO5BOLD) to the problem of global convection in giant stars. For this purpose, the "local box" setup usually employed for the simulation of solar-type surface convection cannot be used. Rather, we have chosen a radically different approach: the whole star is enclosed in a cube ("star-in-a-box" setup). The properties of the stellar model are defined by the prescribed gravitational central potential and by a special inner boundary condition which replaces the unresolved core, including the source of nuclear energy production. We present current results obtained from this novel generation of 3D stellar convection simulations, proceeding from a toy model ("Mini-Sun") towards the numerically more demanding supergiant regime. We discuss the basic observational properties of Betelgeuse in the light of our best model obtained so far (T_eff = 3300 K, log g = -0.4). Finally, we describe a first attempt to investigate the interaction of the global convective flows with magnetic fields based on the kinematic approximation. Title: Line formation in convective stellar atmospheres. I. Granulation corrections for solar photospheric abundances Authors: Steffen, M.; Holweger, H. Bibcode: 2002A&A...387..258S Altcode: 2002astro.ph..3127S In an effort to estimate the largely unknown effects of photospheric temperature fluctuations on spectroscopic abundance determinations, we have studied the problem of LTE line formation in the inhomogeneous solar photosphere based on detailed 2-dimensional radiation hydrodynamics simulations of the convective surface layers of the Sun. By means of a strictly differential 1D/2D comparison of the emergent equivalent widths, we have derived ``granulation abundance corrections'' for individual lines, which have to be applied to standard abundance determinations based on homogeneous 1D model atmospheres in order to correct for the influence of the photospheric temperature fluctuations. In general, we find a line strengthening in the presence of temperature inhomogeneities as a consequence of the non-linear temperature dependence of the line opacity. The resulting corrections are negligible for lines with an excitation potential around Ei=5 eV, regardless of element and ionization stage. Moderate granulation effects (Deltagran ~ -0.1 dex) are obtained for weak, high-excitation lines (Ei >~ 10 eV) of C I, N I, O I as well as Mg II and Si II. The largest corrections are found for ground state lines (Ei=0 eV) of neutral atoms with an ionization potential between 6 and 8 eV like Mg I, Ca I, Ti I, Fe I, amounting to Delta_gran ~ -0.3 dex in the case of Ti I. For many lines of practical relevance, the magnitude of the abundance correction may be estimated from interpolation in the tables and graphs provided with this paper. The application of abundance corrections may often be an acceptable alternative to a detailed fitting of individual line profiles based on hydrodynamical simulations. The present study should be helpful in providing upper bounds for possible errors of spectroscopic abundance analyses, and for identifying spectral lines which are least sensitive to the influence of photospheric temperature inhomogeneities. Title: Planetary Nebulae with Double Shells and Haloes: Insights from Hydrodynamical Simulations Authors: Schönberner, D.; Steffen, M. Bibcode: 2002RMxAC..12..144S Altcode: We combined hydrodynamical simulations with stellar evolutionary calculations based on prescribed mass-loss rates in order to get a better understanding of how structure and kinematics of circumstellar matter develop with time during the late AGB and the following post-AGB and Planetary Nebula (PN) evolution. Title: Habitable zones in extrasolar planetary systems Authors: Franck, Siegfried; von Bloh, Werner; Bounama, Christine; Steffen, Matthias; Schönberner, Detlef; Schellnhuber, Hans-Joachim Bibcode: 2002abqc.book...47F Altcode: Summarizing we can state that the habitable zone (HZ) is the range of orbital distances from a star, in which a planet can maintain liquid water and biological productivity on its surface. The HZ can be calculated with the help of climatological approaches or within the framework of Earth system science. According to our model, the HZ for the present solar system extends between about 0.95 AU and about 1.2 AU and was broader in the past. For extrasolar systems we can postulate a distinct HZ for young central stars in the mass range between about 0.4 and 2 solar masses. The next two parts describe model calculations for the Sun and for other single main sequence stars, respectively. In the final we give our main conclusions and point out several areas for future work. Title: Habitable Zones and the Number of Gaia's Sisters Authors: Franck, S.; von Bloh, W.; Bounama, C.; Steffen, M.; Schönberner, D.; Schellnhuber, H. -J. Bibcode: 2002ASPC..269..261F Altcode: 2002esip.conf..261F No abstract at ADS Title: Spectroscopic influence of temperature inhomogeneities Authors: Cayrel, Roger; Steffen, Matthias Bibcode: 2002HiA....12..423C Altcode: The effect of temperature inhomogeneities on the formation of two oxygen lines and one Fe II line has been investigated based on 2-D radiation hydrodynamics simulations of convection in the atmospheres of metal-poor stars. It was expected that the IR O I triplet lines would be enhanced by the temperature inhomogeneities, more than the forbidden [O I] line or Fe II lines. The actual computations done on two snapshots under the assumption of LTE have not confirmed this expectation, instead suggesting that the main difference between "classical" and "dynamical" atmospheres of metal-poor dwarfs is the significantly cooler mean temperature of the dynamical upper photosphere (τRoss<~0.01). Title: The Evolution of AGB Stars Towards Planetary Nebulae Authors: Schönberner, Detlef; Steffen, Matthias Bibcode: 2001ASSL..265...85S Altcode: 2001pao..conf...85S We combined hydrodynamical simulations with stellar evolutionary calculations based on prescribed mass-loss rates in order to get a better understanding of how structure and kinematics of circumstellar matter develop with time during the late AGB and the following post-AGB evolution. We show (i) that the final density profile of the circumstellar shells produced by AGB mass loss falls off steeper than ρ ~r-2, (ii) that stages of high mass-loss rates are periodically interrupted by shorter phases of much lower outflow rates and speeds due to the luminosity dips associated with thermal pulses, and (iii) that during the post-AGB evolution ionization destroys the initial density and velocity distribution set up at the end of the AGB. Title: Evolution of Thin Gas Shells Along the AGB and Beyond Authors: Steffen, Matthias; Schönberner, Detlef Bibcode: 2001ASSL..265..131S Altcode: 2001pao..conf..131S Based on numerical simulations with two different computer codes, we have studied the time-dependent hydrodynamics of circumstellar gas/dust shells of AGB stars in their final stages of evolution. We discuss two different mechanisms capable of producing very thin shells of greatly enhanced gas density in the dusty outflows from these stars and illustrate their observable signatures. Our model calculations demonstrate that the thin shells produced on the AGB survive the transition to the post-AGB phase and can still be detected in the haloes of some Planetary Nebulae. 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: Limits of photosynthesis in extrasolar planetary systems for earth-like planets Authors: Franck, S.; von Bloh, W.; Bounama, C.; Steffen, M.; Schönberner, D.; Schellnhuber, H. J. Bibcode: 2001AdSpR..28..695F Altcode: We present a general modeling scheme for investigating the possibility of photosynthesis-based life on extrasolar planets. The scheme focuses on the identification of the habitable zone in main-sequence-star planetary systems with planets of Earth mass and size. Our definition of habitability is based on the long-term possibility of photosynthetic biomass production as a function of mean planetary surface temperature and atmospheric CO 2-content. All the astrophysical, climatological, biogeochemical, and geodynamic key processes involved in the generation of photosynthesis-driven life conditions are taken into account. Implicitly, a co-genetic origin of the central star and the orbiting planet is assumed. The numerical solution of an advanced geodynamic model yields realistic look-up diagrams for determining the limits of photosynthesis in extrasolar planetary systems, assuming minimum CO 2 levels set by the demand of C 4 photosynthesis. Title: Extrasolar Planetary Habitable Zones and the Number of Gaias Authors: von Bloh, W.; Franck, S.; Bounama, C.; Steffen, M.; Schönberner, D.; Schellnhuber, H. -J. Bibcode: 2001AGM....18..P35V Altcode: We present a general modelling scheme for assessing the suitability for life on any Earth-like extrasolar planet by calculating the habitable zone (HZ) in main-sequence-star planetary systems. Our approach is based on an integrated Earth system analysis that relates the boundaries of the HZ to the limits of C4-photosynthesis processes. Within this model, the evolution of the HZ for any main-sequence-star planetary system can be calculated straightforwardly, and a convenient filter can be constructed that picks the candidates for photosynthesis-based life from all the extrasolar planets discovered by novel observational methods. These results can be used to determine the average number of planets per planetary system that are within the HZ. With the help of a segment of the Drake equation, the number of ``Gaias'' (i.e., extrasolar terrestrial planets with a globally acting biosphere) can be estimated. Our calculation gives about half a million Gaias in the Milky Way. Title: Numerical Simulation of Stellar Convection: Comparison with Mixing-length Theory Authors: Steffen, M.; Jordan, S. Bibcode: 2000eaa..bookE5198S Altcode: The energy released by nuclear fusion in the STELLAR INTERIOR is carried to the surface by two different transport mechanisms, radiation and convection. In the presence of a temperature gradient, there is always a net radiative energy flux since more photons travel from hot to cool regions than in the other direction. If the temperature gradient exceeds a certain t... Title: On the origin of thin detached gas shells around AGB stars. Insights from time-dependent hydrodynamical simulations Authors: Steffen, M.; Schönberner, D. Bibcode: 2000A&A...357..180S Altcode: We have applied two different computer codes to study the time-dependent hydrodynamics of circumstellar gas/dust shells of AGB stars in their final stages of evolution. A two-component radiation hydrodynamics code is designed to model a stellar wind driven by radiation pressure on dust grains. Combined with detailed stellar evolution calculations, this approach allows us to simulate the dynamical response of the AGB wind envelope and the emergent spectral energy distribution to temporal changes of the stellar luminosity and mass loss rate. A completely independent one-component, Godunov-type hydrodynamics code, which is particularly well suited to resolve shock fronts, is used to check the results obtained with the numerically more diffusive two-component code. First, we verify that a presumed short episode of high mass loss translates into a correspondingly narrow, high-density shell moving through the circumstellar envelope, provided that the mass loss rate, and hence the outflow velocity, is essentially constant during the mass ejection. In principle, this scenario remains a viable explanation for the existence of the very thin molecular shells recently detected around some carbon-rich AGB stars. Second, we discovered that an alternative mechanism producing very thin shells of greatly enhanced gas density can operate in the dusty outflows from AGB stars: the interaction of a faster inner wind running into a slower outer wind, sweeping up matter at the interface between both type of winds. Based on different numerical simulations and on a simple analytical model, we show that this mechanism easily leads to the formation of very thin shells without the need to invoke large variations of the mass loss rate on very short time scales. Finally, we demonstrate that a typical helium-shell flash induces both a mass loss `eruption' and a two-wind interaction due to the increased outflow velocity during the high mass loss phase, leading to the formation of a thin compressed gas shell. Very likely, this mechanism is responsible for the origin of the CO shells found around some semiregular, optically visible carbon stars, the most prominent example being TT Cygni. Title: 2D Numerical Simulation of Stellar Convection Authors: Steffen, Matthias Bibcode: 2000astro.ph..3074S Altcode: The dynamics and thermal structure of the surface layers of stars with outer convection zones can be studied in some detail by means of numerical simulations of time-dependent compressible convection. In an effort to investigate the properties of ``stellar granulation'' as a function of spectral type, we have carried out elaborate 2-dimensional radiation hydrodynamics calculations of surface convection for a variety of stellar parameters. The main features of these simulations are reviewed, with particular reference to standard mixing length models. Title: A hydrodynamical study of multiple-shell planetaries . I. NGC 2438 Authors: Corradi, R. L. M.; Schönberner, D.; Steffen, M.; Perinotto, M. Bibcode: 2000A&A...354.1071C Altcode: We obtained deep imaging and high-resolution spectroscopy of the planetary nebula NGC 2438. In addition to the bright inner rim and the faint halo-like shell already known from previous imagery, our observations also highlight the existence of an even fainter large halo of nearly spherical shape. Analysis of the surface-brightness profiles and the line splitting indicates a clear ionization stratification within the bright rim, with the flow velocities increasing with distance from the central star, reaching 37 km s-1. We compared the surface-brightness profiles and kinematical data of NGC 2438 with state-of-the-art radiation-hydrodynamics models, and obtained a surprisingly good qualitative agreement. By means of this modelling we show that the faint, halo-like shell has formed by recombination from a once well-developed photoionized shell in response to a fast luminosity drop of the central star. The large, very faint halo is interpreted as being the relic of the now fully ionized AGB wind. Also the velocity/ionization stratification and the thickness of the bright rim are fully consistent with the prediction of the models for such an evolved planetary nebula. The properties of NGC 2438 can be explained by assuming that the planetary nebula formation started about 45 000 years after a thermal pulse on the AGB, and that the remnant star continued to burn hydrogen in a shell until burning has stopped quite recently. Based on observations obtained at the 3.5m~NTT telescope of the European Southern Observatory Title: On Carbon Star Evolution in the IRAS Two-Color Diagram Authors: Szczerba, Ryszard; Steffen, Matthias; Volk, Kevin Bibcode: 2000IAUS..177..581S Altcode: No abstract at ADS Title: Effects of Photospheric Temperature Inhomogeneities on Lithium abundance Determinations (2D) (Invited Paper) Authors: Cayrel, R.; Steffen, M. Bibcode: 2000IAUS..198..437C Altcode: 2000astro.ph..3075C Based on detailed 2D radiation hydrodynamics (RHD) simulations, we have investigated the effects of photospheric temperature inhomogeneities induced by convection on spectroscopic determinations of the lithium abundance. Computations have been performed both for the solar case and for a metal-poor dwarf. NLTE effects are taken into account, using a five-level atomic model for LiI. Comparisons are presented with traditional 1D models having the same effective temperature and gravity. The net result is that, while LTE results differ dramatically between 1D and 2D models, especially in the metal-poor case, this does not remain true when NLTE effects are included: 1D/2D differences in the inferred NLTE Li abundance are always well below 0.1 dex. The present computations still assume LTE in the continuum. New computations removing this assumption are planned for the near future. Title: Radiation Hydrodynamics Simulations of the Solar Chromosphere Authors: Wedemeyer, Sven; Freytag, Bernd; Steffen, Matthias; Holweger, Hartmut Bibcode: 2000AGM....17..P01W Altcode: While heating of the solar corona is commonly attributed to reconnection of magnetic field lines, the mechanism responsible for heating the chromosphere of the quiet Sun, away from active regions, is still under debate1,2. The basic question which we will address in this contribution i s: Can generation of acoustic waves by turbulent convection in photospheric and subphotospheric layers explain the chromospheric emission of the quiet Sun? With a new 3D radiation hydrodynamics code3 we are able to compute models ex tending from the upper convection zone to the middle chromosphere. The code can handle shocks with a minimum of numerical dissipation. Therefore generation and propagation of acoustic waves can be investigated, permitting the evaluation of wave dissipation in the chromosphere in a physically consistent manner. We present first results and discuss the principal problems and future prospects. Title: On the Transition from AGB Stars to Planetaries: The Spherical Case Authors: Schönberner, Detlef; Steffen, Matthias Bibcode: 2000ASPC..199...59S Altcode: 2000apn..conf...59S; 1999astro.ph.12357S We discuss the basic physical model and the relevant processes responsible for creating and shaping planetary nebulae out of a cool AGB wind envelope. We show that a hydrodynamical treatment along the upper AGB leads quite naturally to more realistic starting configurations for planetaries with density slopes steeper than r-2. Taking into account photoionization and wind interaction in a realistic manner, the hydrodynamics of post-AGB wind envelopes leads to density structures and velocity fields in close resemblance to observations of spherical or elliptical planetary nebulae. Title: Determination of habitable zones in extrasolar planetary systems: Where are Gaia's sisters? Authors: Franck, Siegfried; von Bloh, Werner; Bounama, Christine; Steffen, Matthias; Schönberner, Detlef; Schellnhuber, Hans-Joachim Bibcode: 2000JGR...105.1651F Altcode: A general modeling scheme for assessing the suitability for life of extrasolar planets is presented. The scheme focuses on the identification of the ``habitable zone'' in main sequence star planetary systems accommodating Earth-like components. Our definition of habitability is based on the long-term possibility of photosynthetic biomass production under geodynamic conditions. Therefore all the pertinent astrophysical, climatological, biogeochemical, and geodynamic processes involved in the generation of photosynthesis-driven life conditions are taken into account. Implicitly, a cogenetic origin of the central star and the orbiting planet is assumed. A geostatic model version is developed and investigated in parallel for demonstration purposes. The numerical solution of the advanced geodynamic model yields realistic lookup diagrams for convenient habitability determination. As an illustration, the MACHO-98-BLG-35 event is scrutinized. It is shown that this event is definitely not tantamount to the discovery of one of Gaia's sisters. Title: Carbon- and Oxygen-Rich Stars in the IRAS Two-Color Diagram: Results from Hydrodynamical Models of AGB Winds Authors: Steffen, M.; Szczerba, R.; Men'shchikov, A.; Schönberner, D. Bibcode: 2000IAUS..177..579S Altcode: No abstract at ADS Title: From the Tip of the AGB Towards a Planetary: A Hydrodynamical Simulation Authors: Schönberner, D.; Steffen, M.; Stahlberg, J.; Kifonidis, K.; Blöcker, T. Bibcode: 2000IAUS..177..469S Altcode: No abstract at ADS Title: The Solar p-Mode Background: Observations and Hydrodynamical Models Authors: Straus, Th.; Steffen, M.; Severino, G.; Freytag, B. Bibcode: 1999ESASP.448..203S Altcode: 1999mfsp.conf..203S; 1999ESPM....9..203S No abstract at ADS Title: A calibration of the mixing-length for solar-type stars based on hydrodynamical simulations. I. Methodical aspects and results for solar metallicity Authors: Ludwig, Hans-Günter; Freytag, Bernd; Steffen, Matthias Bibcode: 1999A&A...346..111L Altcode: 1998astro.ph.11179L Based on detailed 2D numerical radiation hydrodynamics (RHD) calculations of time-dependent compressible convection, we have studied the dynamics and thermal structure of the convective surface layers of solar-type stars. The RHD models provide information about the convective efficiency in the superadiabatic region at the top of convective envelopes and predict the asymptotic value of the entropy of the deep, adiabatically stratified layers (Fig. \ref{f:sstarhd}). This information is translated into an effective mixing-length parameter \alphaMLT suitable to construct standard stellar structure models. We validate the approach by a detailed comparison to helioseismic data. The grid of RHD models for solar metallicity comprises 58 simulation runs with a helium abundance of Y=0.28 in the range of effective temperatures 4300pun {K}<=Teff<= 7100pun {K} and gravities 2.54<={log g}<= 4.74. We find a moderate, nevertheless significant variation of \alphaMLT between about 1.3 for F-dwarfs and 1.75 for K-subgiants with a dominant dependence on Teff (Fig. \ref{f:mlp}). In the close neighbourhood of the Sun we find a plateau where \alphaMLT remains almost constant. The internal accuracy of the calibration of \alphaMLT is estimated to be +/- 0.05 with a possible systematic bias towards lower values. An analogous calibration of the convection theory of Canuto &\ Mazzitelli (1991, 1992; CMT) gives a different temperature dependence but a similar variation of the free parameter (Fig. \ref{f:mlpcm}). For the first time, values for the gravity-darkening exponent beta are derived independently of mixing-length theory: beta = 0.07... 0.10. We show that our findings are consistent with constraints from stellar stability considerations and provide compact fitting formulae for the calibrations. Title: Long-term evolution of AGB wind envelopes: Insights from hydrodynamical models Authors: Steffen, M.; Schönberner, D.; Szczerba, R. Bibcode: 1999IAUS..191..379S Altcode: Up to now, hydrodynamical models of dust-driven AGB winds do not generally take into account the `long-term' changes of the stellar parameters (on stellar evolution time scales of 103 to 105 yrs), although it is well known that the luminosity and (very likely) the mass loss rate undergo significant variations when so called `thermal pulses' occur on the upper AGB. In this review we demonstrate that time-dependent radiation hydrodynamics calculations are needed to understand the formation, structure, and spectral energy distribution of detached dust shells detected by IRAS and ISO. Combined with appropriate models, these observations can reveal part of the previous mass loss history on the AGB and allow an empirical check of presently adopted mass loss laws. Based on insights from hydrodynamical simulations, we discuss the two competing scenarios that have been put forward to explain the origin of the very thin molecular shells recently discovered around some carbon stars. We find that the signature of a short mass loss `eruption' broadens quickly with time due to the related velocity gradient across the shell. Hence, this scenario is not considered a likely explanation of detached CO shells. On the other hand, the alternative mechanism, interaction of winds, is shown to be capable of producing very thin shells of greatly enhanced gas density in the dusty outflows from AGB stars by sweeping up matter at the interface between both type of winds. 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: On the origin of thin detached gas shells around AGB stars: Insights from time-dependent hydrodynamical simulations Authors: Steffen, Matthias; Schönberner, Detlef Bibcode: 1999AGAb...15..109S Altcode: We have applied two different computer codes to study the time-dependent hydrodynamics of circumstellar gas/dust shells of AGB stars in their final stages of evolution. We verify that a presumed short episode of high mass loss translates into a correspondingly narrow, high-density shell moving through the circumstellar envelope at supersonic speed, provided that the mass loss rate, and hence the outflow velocity, is essentially constant during the mass loss `eruption'. In principle, this scenario remains a viable explanation for the existence of the very thin molecular shells recently detected around some carbon-rich AGB stars. We discovered that an alternative mechanism producing very thin shells of greatly enhanced gas density can operate in the dusty outflows from carbon-rich AGB stars: the interaction of a faster inner wind running into a slower outer wind, sweeping up matter at the interface between both type of winds. We show that this mechanism easily leads to the formation of very thin shells without the need to invoke large variations of the mass loss rate on very short time scales. Finally, we demonstrate that a typical He-shell flash produces both a mass loss `eruption' and a two-wind interaction due to the increased outflow velocity during the high mass loss phase, leading to the formation of a thin compressed gas shell. We propose that this mechanism is responsible for the origin of the observed very thin CO shell around the optically visible carbon star TT Cygni. Title: Silicon as a cosmic reference element: a reassessment of the solar SI abundance Authors: Wedemeyer, Sven; Holweger, Hartmut; Steffen, Matthias Bibcode: 1999AGAb...15..113V Altcode: 1999AGM....15..P53V; 1999AGM....15..P54W Silicon is an important reference elements for comparing various types of cosmic matter with the Sun. The most widely used sources of solar (photospheric) abundances, the compilation by Anders & Grevesse (1989) and its updates (e.g. Grevesse & Sauval 1998), are based on standard abundance analyses employing 1D solar models and, in most cases, assuming LTE. We report NLTE calculations for Si and a first attempt to determine the effect of horizontal temperature inhomogeneities associated with convection on the photospheric abundance of Si. We combine the result with that obtained previously for O and Fe (Aellig et al. 1999; Schnabel et al. 1999) and compare the photospheric Si/Fe, Si/O and Si/H abundance ratios with literature data for meteorites, the corona and solar wind, energetic particles and galactic B stars and H ii regions. References: Aellig M.R., Holweger H., Bochsler P., et al., 1999, Solar Wind Nine, AIP Conf. Proc. Vol. 471, 255 Anders E., Grevesse N., 1989, Geochim. Comochim. Acta 53, 197 Grevesse N., Sauval A.J., 1998, Space Sci. Rev. 85, 161 Schnabel R., Kock M., Holweger H., 1999, A&A 342, 610 Title: Line Formation in Convective Stellar Atmospheres Authors: Steffen, Matthias; Holweger, Hartmut Bibcode: 1999AGb....15..107S Altcode: 1999AGM....15..P40S Convection affects the temperature structure of a stellar atmosphere in a twofold way: it influences the mean vertical stratification and introduces horizontal inhomogeneities. This poses several questions: (i) What is the ``right'' alpha_MLT to produce the correct mean temperature stratification in the framework of mixing-length theory ? (ii) What errors are introduced, in the context of spectroscopic abundance determinations, by representing the horizontally averaged spectrum of an inhomogeneous atmosphere by the spectrum of a plane-parallel mean stratification? (iii) Is there something like a spectroscopically equivalent mean stratification of an inhomogeneous atmosphere? In order to address these questions, we have computed synthetic line profiles for a sample of fictitious spectral lines of different chemical elements, based on both mixing-length atmospheres and 2D hydrodynamical solar granulation models. Through a differential comparison of the equivalent widths computed from the inhomogeneous atmospheres and different 1D mean models, respectively, we derive correction factors to be applied to standard 1D spectroscopic abundance determinations in order to correct for the effects of stellar `granulation'. We find that the abundance correction depends sensitively on the ionization stage of the chemical element and on the excitation potential of the spectral line considered. In some cases the `granulation correction' can amount to -0.2 dex. 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: Formation and Evolution of Planetary Nebulae Authors: Schönberner, D.; Steffen, M. Bibcode: 1999ASPC..188..281S Altcode: 1999oisc.conf..281S No abstract at ADS Title: Hydrodynamical models and synthetic spectra of circumstellar dust shells around AGB stars. II. Time-dependent simulations Authors: Steffen, Matthias; Szczerba, Ryszard; Schoenberner, Detlef Bibcode: 1998A&A...337..149S Altcode: We have employed time-dependent two-component hydrodynamics/radiative transfer calculations to investigate the structure, dynamics and emergent spectral energy distribution of dusty circumstellar shells around carbon and oxygen stars in the final stages of their AGB evolution. These internally consistent, physical models describe a stellar wind driven by radiation pressure on dust grains and subsequent momentum transfer to the gas component via collisions. Detailed stellar evolution calculations, with a prescribed mass loss rate that is a function of the fundamental stellar parameters, have been used as a time-dependent inner boundary condition for the numerical solution of the coupled equations of hydrodynamics and frequency-dependent radiative transfer governing the structure and temporal evolution of the circumstellar dust/gas shell. The calculations are based on one particular evolutionary track for an initial stellar mass M_i=3.0 Msun\ and a final mass M_f=0.605 Msun, but for different assumptions concerning the composition of the dust grains: amorphous carbon or ``astronomical'' silicates. Using our hydrodynamics code to simulate the dynamical response of the circumstellar wind shell to the evolutionary changes of the stellar parameters, we find that the large temporal variations of stellar luminosity and mass loss rate associated with the final thermal pulses near the end of the AGB evolution lead to characteristic, time-dependent signatures in the density structure and emergent energy distribution of the circumstellar dust shell. We present the resulting ``loops'' in the IRAS two-color-diagram, which we find to extend to regions quite remote from the simple color-color relation defined by steady state models. These time-dependent hydrodynamical models explain the existence of carbon and oxygen stars with excess emission near lambda 60 and 100 mu m as a natural consequence of the sharp decrease of the mass loss rate following a thermal pulse, leading to the development of a detached dust shell. As an illustration, we present a series of synthetic spectra and corresponding 100 mu m surface brightness distributions showing the time-evolution of the circumstellar dust emission during a thermal pulse cycle, both for a carbon-rich and an oxygen-rich dust shell. We demonstrate that it is unrealistic to assume a fixed velocity profile which is independent of mass loss rate: to a first approximation, the gas velocity is a bimodal function of the mass loss rate. A short event of high mass loss does not simply translate into a correspondingly narrow, high-density shell moving through the circumstellar envelope. Rather, the signature of a short mass loss peak broadens due to velocity gradients as it moves towards the outer regions of the wind. Hence, this is hardly a viable scenario to explain the existence of very thin molecular shells that have recently been detected around some carbon stars. Our simulations suggest a more promising mechanism producing thin shells of enhanced gas density in the outer regions of carbon-rich AGB shells: interaction of winds of different speed and density. 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: IR-Colors for Models of Post-AGB Evolution Authors: Szczerba, R.; Steffen, M. Bibcode: 1998Ap&SS.262..187S Altcode: 1999Ap&SS.262..187S No abstract at ADS 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: Hydrodynamical Modelling of the Evolution of Dusty Outflows from AGB-Stars Authors: Schönberner, D.; Steffen, M.; Szczerba, R. Bibcode: 1998Ap&SS.255..459S Altcode: 1997Ap&SS.255..459S 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: Hydrodynamical models and synthetic spectra of circumstellar dust shells around AGB stars Authors: Steffen, M.; Szczerba, R.; Men'shchikov, A.; Schoenberner, D. Bibcode: 1997A&AS..126...39S Altcode: We present a sample of hydrodynamical steady state models of circumstellar gas/dust shells around late type giants together with computed spectral energy distributions (SEDs). In these models, the stellar wind is driven by radiation pressure on dust grains and subsequent momentum transfer to the gas molecules via collisions. Given the fundamental stellar parameters (M_*, L_*, T_eff), the mass loss rate (M_dot), and the dust properties, a self-consistent physical model of the circumstellar gas/dust shell is obtained from the numerical solution of the coupled equations of hydrodynamics and radiative transfer. The computed outflow velocities and infrared fluxes of the circumstellar envelopes can be compared directly with the observed properties of stars on asymptotic giant branch. Plotting the positions of our steady state models in different IRAS two-color-diagrams, we confirm that, for fixed dust properties, all models fall on a simple color-color relation with M_dot (or optical depth) as the only parameter. Surprisingly, we find a good agreement between the synthetic spectra resulting from the self-consistent hydrodynamical approach and those obtained from much simpler models based on a constant outflow velocity and ignoring drift of dust relative to the gas. Our models are compared with the results of similar calculations by \cite[Netzer & Elitzur (1993).]{Net93} We find significant differences which are probably the result of some unrealistic approximations in the treatment of radiative transfer underlying the model calculations of Netzer & Elitzur. Moreover, our results demonstrate that, in general, gas pressure cannot be neglected for winds with relatively low expansion velocities (u_e < 30 km/s). For given stellar parameters and dust properties, the theoretical minimum (maximum) mass loss rate decreases (increases) significantly when gas pressure is taken into account. Title: Numerical simulations of stellar surface convection. Authors: Freytag, B.; Steffen, M. Bibcode: 1997AGAb...13..176F Altcode: No abstract at ADS Title: Modeling the Long-Term Evolution of Dusty Outflow Outflows of AGB-Stars Authors: Steffen, M.; Szczerba, R. Bibcode: 1997Ap&SS.251..131S Altcode: No abstract at ADS Title: IR-Colors for Models of Post-AGB Evolution Authors: Szczerba, R.; Steffen, M. Bibcode: 1997Ap&SS.251..149S Altcode: No abstract at ADS Title: VizieR Online Data Catalog: Models of circumstellar dust shells (Steffen+ 1997) Authors: Steffen, M.; Szczerba, R.; Men'shchikov, A.; Schoenberner, D. Bibcode: 1997yCat..41260039S Altcode: Main properties of the steady state hydrodynamical models of circumstellar gas/dust shells around late type giants, computed for different stellar parameters (mass, luminosity, effective temperature) and dust composition (astronomical silicates, graphite, or amorphous carbon, with properties given in Tab.1 and Fig.1). Each table lists the results as a function of the adopted mass loss rate for fixed stellar parameters and dust properties. Tables 2 - 11 are related to Figures 2 - 11: Each of the figures shows the velocity structure and the resulting spectral energy distribution for a subset of models listed in the respective table.

(2 data files). 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: Through the upper AGB towards a planetary: a hydrodynamical simulation. Authors: Schönberner, D.; Steffen, M.; Stahlberg, J.; Kifonidis, K.; Blöcker, T. Bibcode: 1997seas.conf..146S Altcode: 1997ase..work..146S The authors present an exploratory investigation of the dynamical evolution of a dusty stellar wind envelope along the upper AGB and its transformation into a planetary nebula. They find the existence of AGB stars with detached shells to be a natural consequence of the mass loss variations during a thermal pulse. It is also shown that due to the large dynamical effects caused by the ionizing radiation field and the fast wind of the central star, it is impossible to deduce the AGB mass loss history from the planetary's density and velocity distribution. The structure of the halo, however, is still determined by the AGB mass loss history. The rapid decline of mass loss expected in the aftermath of thermal pulses leads to extended shells of low densities and explains halos with sharp boundaries. The density structure and velocity field of the model planetary closely resemble those of observed planetary nebulae. Title: Time-dependent hydrodynamical models of circumstellar dust shells around carbon- and oxygen-rich AGB stars. Authors: Steffen, M.; Szczerba, R.; Men'shchikov, A.; Schönberner, D. Bibcode: 1997seas.conf..154S Altcode: 1997ase..work..154S The authors have employed time-dependent two component hydrodynamics - radiative transfer calculations to investigate the structure, dynamics and emergent spectral energy distribution of dusty circumstellar shells around carbon and oxygen stars in the final stages of their AGB evolution. In these models, the wind is driven by radiation pressure on dust grains and subsequent momentum transfer to the gas molecules by collisions. Detailed stellar evolution calculations with a prescribed mass loss rate that varies as a function of the fundamental stellar parameters provide the time-dependent inner boundary conditions for the numerical solution of the coupled equations of hydrodynamics and radiative transfer which describe a consistent physical model of the evolution of the circumstellar shell. Using the hydrodynamical code to simulate the dynamical response of the circumstellar wind shell to the evolutionary changes of the stellar parameters, the authors find that the large temporal variations of L and M˙ associated with the final thermal pulses near the end of the AGB evolution lead to characteristic, time-dependent signatures in the density structure and in the infrared emission of the circumstellar dust. Title: Through the AGB towards a Planetary: A hydrodynamical simulation Authors: Steffen, M.; Schonberner, D.; Kifonidis, K.; Stahlberg, J. Bibcode: 1997IAUS..180..368S 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: On the detection of shocks in the solar granulation. Authors: Solanki, S. K.; Rueedi, I.; Bianda, M.; Steffen, M. Bibcode: 1996A&A...308..623S Altcode: We investigate the spectral signature of a hydrodynamic simulation of solar granulation and compare it with high resolution observations. The model gives the correct qualitative trend of increasing line width with decreasing continuum intensity seen by Nesis et al. (1992) and interpreted by them as a sign of post-shock turbulence. We find, however, that the profiles in the dark downflow lanes are broader even when there is no horizontal transonic flow or shock in the vicinity. We conclude that the observations of Nesis et al. do not provide any firm evidence for the presence of granular transonic flows. Nevertheless, the simulation predicts a promising diagnostic of shocked horizontal flows. We find that at the locations of the shocks the line profiles are particularly broad, especially near the solar limb. We present observations of quiet solar regions that show this specific signature of shocks and thus support the theoretical prediction of transonic granular flows. Title: Lyapunov exponents for solar surface convection. Authors: Steffen, M.; Freytag, B. Bibcode: 1995CSF.....5.1965S Altcode: The authors have carried out detailed 2D numerical radiation hydrodynamics calculations, specifically designed to model time-dependent, compressible convection in the surface layers of the Sun. These simulations, which take into account a realistic equation of state and use an elaborate scheme to describe multi-dimensional, non-local, frequency-dependent radiative transfer, allow a direct comparison with observed photometric and spectroscopic properties of solar granulation. Their purpose is to enhance the understanding of the dynamics and thermal structure of convective stellar atmospheres, and to investigate the generation of acoustic energy by turbulent convection. The authors briefly present some of the main properties of their solar convection models. In particular, the authors demonstrate the chaotic behaviour of solar surface convection, estimating the magnitude of the two largest Lyapunov exponents, λ1 and λ2, by analysing the time evolution of three simulations with slightly different initial conditions. The authors find that both λ1 and λ2 are positive and of similar magnitude as expected for a chaotic system of high dimension. The corresponding characteristic time scale λ1-1 of approximately 320 s is comparable to the convective turnover time. Title: Synthetic spectra computed from hydrodynamical model atmospheres of DA white dwarfs. Authors: Steffen, M.; Ludwig, H. -G.; Freytag, B. Bibcode: 1995A&A...300..473S Altcode: From detailed 2-dimensional numerical radiation hydrodynamics calculations of time-dependent compressible convection we have obtained the thermal structure of the convective surface layers of DA white dwarfs with effective temperatures near the blue edge of the ZZ Ceti instability strip. Synthetic line profiles of Hbeta_ and the red wing of Lalpha_ (including the very temperature sensitive satellite absorption features) computed from two representative inhomogeneous hydrodynamical models (T_eff_=12200K, 12600K; log g=8.0) are compared with the spectra resulting from different plane-parallel model atmospheres. We find that it is possible to represent a given inhomogeneous atmosphere by a spectroscopically equivalent 1D model, constructed to have the same frequency-integrated radiative flux as the respective 2D hydrodynamical model at all depths. Synthetic spectra computed from this representative 1D model are almost indistinguishable from the horizontally averaged 2D synthetic spectra of the corresponding inhomogeneous model. We conclude that in the investigated range of effective temperature (probably even for the whole range of convective DAs), spectroscopic analysis based on appropriate 1D atmospheres is almost unaffected by systematic errors associated with non-linear flux variations due to the substantial thermal inhomogeneities generated by photospheric convection in these stars. This work provides the basis for a well defined comparison between 2D or 3D hydrodynamical convection models and 1D standard mixing length models. Title: 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: Numerical simulations of surface convection in solar-type stars Authors: Freytag, B.; Steffen, M. Bibcode: 1995IAUS..176P.111F 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: 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: Acoustic flux generation in the solar convection zone Authors: Steffen, M. Bibcode: 1994smf..conf..294S Altcode: No abstract at ADS Title: Shocks in the solar photosphere and their spectroscopic signature Authors: Steffen, M.; Freytag, B.; Holweger, H. Bibcode: 1994smf..conf..298S Altcode: No abstract at ADS Title: 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: Acoustic flux generation in the solar convection zone. Authors: Steffen, M. Bibcode: 1993AGAb....9..118S Altcode: No abstract at ADS Title: The Depth of the Solar Convection Zone Inferred from Hydrodynamical Models of the Surface Layers Authors: Steffen, M. Bibcode: 1993ASPC...40..300S Altcode: 1993ist..proc..300S 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