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Astron. Astrophys. 318, 841-869 (1997)

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1. Introduction

Stellar observations are very often interpreted by means of the stellar model atmospheres, so that it is very important to know which is the physics used for the model calculations. In particular, cool stars are affected by convection and the more the stars are metal deficient the more the convection plays an important role in their atmospheres.

Because Kurucz model atmospheres, together with the corresponding fluxes and colours, are widely used in several fields of the astrophysical research, we estimated it worthwhile to discuss how convection is handled in the ATLAS9 codes stored on the CD-ROM No. 13 (Kurucz, 1993a) and in the successive update versions, and which were used for computing the grids of models, fluxes, and colours stored on the CD-ROMs No. 13 (Kurucz, 1993a), No. 19 (Kurucz, 1994), and successive releases (Kurucz, 1995).

The basic convection theory in ATLAS9 is the mixing-length theory as described in Kurucz (1970), but some modifications have been added after the year 1970, the most important being related to (1) the optical thickness of the convective elements, (2) the horizontal averaging of the opacity, and (3) the inclusion of an approximate overshooting. The first two modifications are discussed in Lester, Lane, and Kurucz (1982). The first one was incorporated in the several ATLAS versions successive to ATLAS5 (Kurucz, 1970), while the second one was finally included in ATLAS9.

In this paper we recall the formulas on which convection in ATLAS9 is based, and we discuss in particular the third modification and its effects on the ([FORMULA]), ([FORMULA]), ([FORMULA]), [FORMULA] color indices and on the Balmer profiles. We also discuss the ATLAS9 model for the Sun. Finally, Procyon (HR 2943 = HD 61421= [FORMULA]  CMi, F5 IV-V) is used as example to show how the different convection options in the models affect the value of the effective temperature [FORMULA], derived from the several methods which compare observed and computed quantities.

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998