Density Effects on the Opacity of Cool Helium White Dwarf Atmospheres

Abstract

Density effects on two important processes contributing to the opacity of a weakly ionized helium gas are considered. The correction to free-free absorption by the negative helium ion is treated in the Born approximation using analytical approximations for the electron-atom interaction. The correction to Rayleigh scattering uses well-known formulae obtained from density fluctuation theory. In both processes, the main density effect appears through the static structure factor, which accounts for spatial correlations between the helium atoms. These correlations significantly reduce the cross sections at the high densities encountered in cool, He white dwarf atmospheres.