New Quests in Stellar Astrophysics II Ultraviolet Properties of Evol
This book presents an up-to-date collection of reviews and contributed articles in the field of ultraviolet astronomy. Its content has been mainly motivated by the recent access to the rest frame UV light of distant red galaxies, gained through large opti
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Steward Observatory, University of Arizona, Tucson, AZ 85721 [email protected] Department of Astronomy, University of Maryland, College Park, MD 20740 [email protected]
Summary. We briefly review recent progress in modeling stellar atmospheres. We first summarize various levels of approximations being used in constructing the models, and present a brief overview of existing model atmosphere codes. We then concentrate on a description of our universal computer program Tlusty, and our recent grids of NLTE metal line blanketed model photospheres of O-stars – OSTAR2002 – and early B-stars – BSTAR2006.
1 Introduction and Motivation Model stellar atmospheres are one of the building blocks of our understanding of the radiating objects in the Universe. Indeed, most of our knowledge of the physical and chemical state of stars, interstellar medium, galaxies, but also quasars, GRB’s, and supernovae comes from analyzing their spectra. Modeling stellar atmospheres is one of the most mature fields that deals with analysis and spectroscopic diagnostics of astronomical bodies. Yet, the last decade brought a renewed interest in modeling stellar atmospheres. There are several reasons for that. The first, more or less obvious, reason is a significantly increased quality and quantity of observational data that bring new challenges for modelers. Thanks to Hipparcos, the accurate distances are now known for many stars, which means that the normalization factor to convert the predicted fluxes at the stellar surface to observed fluxes at the Earth is no longer a free parameter. Similarly, recent interferometric observations revealed a highly non-spherical shape of certain stars. The most interesting example to date is provided by Vega, the main photometric standard star, which exhibits a highly distorted shape and a range of effective temperatures between 7900 and 10,150 K (Aufdenberg et al. 2006). The last decade also brought an introduction, after almost a century, of completely new stellar spectral types - L and T dwarfs (sometimes referred M. Ch´ avez Dagostino et al. (eds.), New Quests in Stellar Astrophysics II, Astrophysics and Space Science Proceedings, DOI 10.1007/978-0-387-87621-4 30, c Springer Science+Business Media, LLC 2009
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Ivan Hubeny and Thierry Lanz
to as brown dwarfs). At present, there is a vigorous debate about the name for a spectroscopic class of objects cooler than the coolest T-dwarfs; likely candidate being “Y-dwarfs”, essentially because there are not many more remaining letters of the alphabet available. The second reason, or motivation, for a progress in modeling stellar atmospheres is an ever-increasing computer power, both the memory available, as well as the computer speed. Consequently, much more sophisticated model atmospheres of unprecedented degree of realism may now be constructed in a reasonable amount of computer time. However, even with most powerful computers and largest memory chips available, one would still not be able to compute sophisticated models without employing clever and
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