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International audience; In this paper we compare observed, empirical, and modelled solar limb profiles and discuss their potential use to derive physical properties of the solar atmosphere. The PHOENIX, SolMod3D, and COSI radiative transfer codes as well as VAL-C models are used to calculate the solar limb shape under different assumptions. The main properties of each model are shown. The predicted limb shape as a function of wavelength for different features on the solar disk, such as quiet Sun, sunspots, and faculae, is investigated. These models provide overall consistent limb shapes with some discrepancies that are discussed here in terms of differences in solar atmosphere models, opacities, and the algorithms used to derive the solar limb profile. Our analysis confirms that the most common property of all models is limb shapes that are much steeper than what is observed, or predicted by the available empirical models. Furthermore, we have investigated the role of the Fraunhofer lines within the spectral domain used for the solar limb measurements. Our results show that the presence of the Fraunhofer lines significantly displaces the limb inflection point from its position estimated assuming only the photospheric continuum. The PICARD satellite, launched on 15 June 2010, will provide measurements of the limb shape at several wavelengths. This work shows that the precision of these measurements allows for discrimination among the available models.

Spectral energy distributions for models of arbitrarily rotating stars are computed using two-dimensional rotating stellar models, NLTE plane-parallel model atmospheres, and a code to integrate the appropriately weighted intensities over the visible surface of the stellar disk. The spectral energy distributions depend on the inclination angle between the observer and the rotation axis of the model. We use these curves to deduce what one would infer the model's luminosity and effective temperature to be assuming the object was nonrotating.

We present extensive NLTE calculations for a semi-empirical solar 1D chromosphere model based on the VAL C model. We report on a significant influence of nitrogen, oxygen, and sulfur on the emergent hydrogen emission with respect to LTE vs. NLTE calculations for these elements. Moreover, we present a model spectrum with 20 light and iron group elements computed in NLTE. We compare this to an observed solar spectrum and to a photospheric model spectrum. We find that the agreement for this model with the data is less good than the original VAL C model especially in the UV. This may imply the need of changes in the chromospheric temperature structure or may point to general problems of static 1D chromospheric models.

We present atmospheric models of red giant stars of various metallicities, including extremely metal poor (XMP, [Fe/H], Comment: 28 pages, 10 figures. Accepted for publication in April 2006 Astrophysical Journal

We present LTE and NLTE atmospheric models of a star with solar parameters, and study the effect of treating many thousands of Iron group lines out of LTE on the computed atmospheric structure, overall absolute flux distribution, and the moderately high resolution spectrum in the visible and near UV bands. Our NLTE modeling includes the first two or three ionization stages of 20 chemical elements, up to and including much of the Fe-group, and includes about 20000 Fe I and II lines. We investigate separately the effects of treating the light metals and the Fe-group elements in NLTE. Our main conclusions are that 1) NLTE line blanketed models with direct multi-level NLTE for many actual transitions gives qualitatively similar results as the more approximate treatment of Anderson (1989) for both the Fe statistical equilibrium and the atmospheric temperature structure, 2) models with many Fe lines in NLTE have a temperature structure that agrees more closely with LTE semi-empirical models based on center-to-limb variation and a wide variety of spectra lines, whereas LTE models agree more with semi-empirical models based only on an LTE calculation of the Fe I excitation equilibrium, 3) the NLTE effects of Fe-group elements on the model structure and flux distribution are much more important than the NLTE effects of all the light metals combined, and serve to substantially increases the violet and near UV flux level as a result of NLTE Fe over-ionization. These results suggest that there may still be important UV opacity missing from the models., Comment: Accepted for publication in The Astrophysical Journal

We present plane-parallel and spherical LTE and NLTE atmospheric models of a variety of stellar parameters of the red giant star Arcturus (alpha Boo, HD124897, HR5340) and study their ability to fit the measured absolute flux distribution. Our NLTE models include tens of thousands of the strongest lines in NLTE, and we investigate separately the effects of treating the light metals and the Fe group elements Fe and Ti in NLTE. We find that the NLTE effects of Fe group elements on the model structure and flux distribution are much more important than the NLTE effects of all the light metals combined, and serve to substantially increases the violet and near UV flux level as a result of NLTE Fe over-ionization. Both the LTE and NLTE models predict significantly more flux in the blue and UV bands than is observed. We find that within the moderately metal-poor metallicity range, the effect of NLTE on the overall UV flux level decreases with decreasing metallicity. These results suggest that there may still be important UV opacity missing from the models. We find that models of solar metallicity giants of similar spectral type to Arcturus fit well the observed flux distributions of those stars from the red to the near UV band. This suggests that the blue and near UV flux discrepancy is metallicity dependent, increasing with decreasing metallicity.

We present a solution for the long standing problem concerning the \chromospheric" behaviour of the Mn i 5395/5432 A lines in the solar spectrum using multi-line/multi-species NLTE modelling. Using comprehensive spectral line formation modelling, we show that the Mn i lines are very sensitive to optical pumping in a transition which overlaps with Mg ii k. It therefore follows that one has to be careful with the choice of lines as temperature indicators and for the determination of the Mn abundances although on the other hand, due to the formation process of these lines they may be useful as a solar and stellar activity diagnostic.

We present a grid of nova models that have an extremely large number of species treated in non-LTE, and apply it to the analysis of an extensive time series of ultraviolet spectroscopic data for Nova Cygni 1992. We use ultraviolet colors to derive the time development of the effective temperature of the expanding atmosphere during the fireball phase and the first ten days of the optically thick wind phase. We find that the nova has a pure optically thick wind spectrum until about 10 days after the explosion. During this interval, we find that synthetic spectra based on our derived temperature sequence agree very well with the observed spectra. We find that a sequence of hydrogen deficient models provides an equally good fit providing the model effective temperature is shifted upwards by ~1000 K. We find that high resolution UV spectra of the optically thick wind phase are fit moderately well by the models. We find that a high resolution spectrum of the fireball phase is better fit by a model with a steep density gradient, similar to that of a supernova, than by a nova model., Comment: 24 pages, 17 figures, Accepted by Astrophysical Journal

Astronomy and Astrophysics 547 (2012): A108 reproduced with permission from Astronomy & Astrophysics, Context. Our ability to extract information from the spectra of stars depends on reliable models of stellar atmospheres and appropriate techniques for spectral synthesis. Various model codes and strategies for the analysis of stellar spectra are available today. Aims. We aim to compare the results of deriving stellar parameters using different atmosphere models and different analysis strategies. The focus is set on high-resolution spectroscopy of cool giant stars. Methods. Spectra representing four cool giant stars were made available to various groups and individuals working in the area of spectral synthesis, asking them to derive stellar parameters from the data provided. The results were discussed at a workshop in Vienna in 2010. Most of the major codes currently used in the astronomical community for analyses of stellar spectra were included in this experiment. Results. We present the results from the different groups, as well as an additional experiment comparing the synthetic spectra produced by various codes for a given set of stellar parameters. Similarities and differences of the results are discussed. Conclusions. Several valid approaches to analyze a given spectrum of a star result in quite a wide range of solutions. The main causes for the differences in parameters derived by different groups seem to lie in the physical input data and in the details of the analysis method. This clearly shows how far from a definitive abundance analysis we still are, The workshop on which this paper is based was kindly supported by the ESF through the GREAT initiative, by the Robert F.Wing Support Fund at Ohio State University, and the Department of Astronomy at the University of Vienna. Funded by the Austrian Science Fund FWF projects P20046, P21988, and P23737. U.H. Support from the Swedish National Space Board. C.A. Partial support by the Spanish grant YA2008-08013-C03-03. H.N. Financial support from the Alexander von Humboldt foundation. K.E. Support from The Swedish Research Council. C.W. Financial support of CNES, OCA and ESO. Financial support of CNES and CNRS. Support by the Austrian Science Fund FWF under project number P23006

The characteristics of SRC variables are summarized from a comprehensive survey of the Galactic population based on new and archival spectroscopic (Moncrieff, Ph.D. Thesis, Saint Mary’s University, 2011) and spectrophotometric (White and Wing, Astrophys. J. 222:209–219, 1978) observations, and brightness estimates from Harvard plates and AAVSO visual archives, with attention on Berkeley 87 member BC Cyg. New spectroscopic observations of BC Cyg phased using a 692d.8 period, found by a new analysis tied to times of light maximum, are consistent with radial pulsation, similar to that in Cepheids. Results for other SRCs are more ambiguous, with starspot activity or supergranulation cycles constituting a complication for small-amplitude stars displaying longer term trends.

The spectral energy distribution (SED) of a non-spherical star could differ significantly from the SED of a spherical star with the same average temperature and luminosity. Calculation of the SED of a deformed star is often approximated as a composite of several spectra, each produced by a plane parallel model of given effective temperature and gravity. The weighting of these spectra over the stellar surface, and hence the inferred effective temperature and luminosity, will be dependent on the inclination of the rotation axis of the star with respect to the observer, as well as the temperature and gravity distribution on the stellar surface. Here we calculate the surface conditions of rapidly rotating stars with a 2D stellar structure and evolution code and compare the effective temperature distribution to that predicted by von Zeipel's law. We calculate the composite spectrum for a deformed star by interpolating within a grid of intensity spectra of plane parallel model atmospheres and integrating over the surface of the star. Using this method, we find that the deduced variation of effective temperature with inclination can be as much as 3000 K for an early B star, depending on the details of the underlying model., Comment: 38 pages, 9 figures (AAStex preprint format). Accepted for publication in the ApJ