Non-Ideal Equation of State, Refraction and Opacities in Very Cool, Helium-Rich White Dwarf Atmospheres

The atmospheres of cool, helium-rich white dwarfs constitute an exotic and poorly explored physical regime of stellar atmospheres. Under physical conditions where the temperature varies from $1000\mathrm{K}$ to $10000\mathrm{K}$, the density can reach values as large as $2\mathrm{g}\mathrm{/}\mathrm{c}{\mathrm{m}}^3$, and the pressure is as high as $1\mathrm{M}\mathrm{b}\mathrm{a}\mathrm{r}$, the atmosphere is no longer an ideal gas and must be treated as a dense fluid. Helium atoms become strongly correlated and refraction effects are present. Opacity sources such as $\mathrm{H}{\mathrm{e}}^{-}$ free-free absorption must be calculated with a formalism that has never been applied to astrophysical opacities. These effects have been ignored in previous models of cool white dwarf atmospheres.