Model Atmospheres for Central Stars of Planetary Nebulae

Abstract

The central stars of planetary nebulae have effective temperatures ranging from 3 × 10 4 °K to about 2 × 10 5 °K. To get a better understanding of the nature of very hot stars, nine model atmospheres have been computed for stars of effective temperature between 1 × 10 5 °K and 2.5 × 10 5 °K and differing values of surface gravity. All the models have the same chemical composition, the ratio of helium to hydrogen being 0.176 by numbers of atoms. The temperature distribution and radiation pressure gradient are approximated by that of a grey body, and a Rosseland mean absorption coefficient is used. A new method for deriving the source function of very hot stars is given together with an outline of the present numerical calculations. It is shown that in hot stars of low density, where electron scattering is the dominant source of opacity, emission features are to be expected, but that with increasing density, these will reverse into absorption.