43 - Low temperature opacities

Summary

Abstract

The importance of low temperature opacities in stellar calibrations led us to compute new sets of Rosseland mean opacities for different Z-values. For the solar metallicity, these tables have been compared to those of Alexander (1975), Cox (1983), Sharp (1991) and Kurucz (1992).

Introduction

Opacities in the atmospheric layers are generally not considered of great importance in the calculation of theoretical evolutionary tracks since the atmosphere of a star only comprises a tiny part of its mass (see however, section 1.2).

Until recently, the most commonly used “atmospheric” or “low-T” opacity tables were those of Cox & Stewart (1970), Alexander (1975) and Cox (1983) but there are rather large discrepancies between these different tables for typical T and ρ ranges encountered in stellar atmospheres of solar type stars.

Furthermore, for pop I stars, low-T opacities are calculated for very few values of the metallicity, Z, and the solar chemical composition is generally used in the calculation of tracks, whatever the actual value of Z.

Low-T opacities and stellar calibrations

Theoretical evolutionary tracks depend on mass, age, chemical composition on the zero age main sequence and convection parameter, α (ratio of mixing length to pressure scale height in the convective layers). Calibrating a star consists in computing evolutionary models that reproduce, at given age, chemical composition on ZAMS and convection parameter, the observed values of the luminosity and effective temperature of the star.