Some problems of radiation transfer in the atmospheres of hot stars

Abstract

The central stars of planetary nebulae have effective temperatures ranging from 3 × 104 °K to about 2 × 105 °K. Such stars may therefore be much hotter than the hottest main sequence stars which have temperatures of about 4 × 104 °K. All such determinations of temperature are based on the assumption that the central stars radiate as black bodies. To find out to what extent this is true, and to study the effects of a variation in surface gravity on the emergent flux, a number of model atmospheres has been computed. By virtue of their very high temperature these models have two important features: (1) The large contribution of electron scattering to the opacity; (2) The large effect of radiation pressure on the hydrostatic equilibrium. The atmospheres are assumed to be in radiative equilibrium, hydrostatic equilibrium and local thermodynamic equilibrium. The temperature distribution and radiation pressure gradient are approximated by that of the grey body, with a Rosseland mean absorption coefficient. Account is taken only of continuous absorption by hydrogen and helium. The results of these calculations show 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 revert into absorption. Comparisons with recent calculations by Böhm and Deinzer show that in denser stars, continuous absorption by higher ions of carbon, nitrogen, oxygen and neon will significantly reduce the stellar flux beyond the helium II ionization limit.