On the Dissociation Equilibrium of H2 in Very Cool, Helium-Rich White Dwarf Atmospheres

Abstract

We investigate the dissociation equilibrium of ${\mathrm{H}}_2$ in very cool, helium-rich white dwarf atmospheres. We present the solution of the non-ideal chemical equilibrium for the dissociation of molecular hydrogen in a medium of dense helium. We find that at the photosphere of cool white dwarfs of $T_{\mathrm{eff}}=4000K$, the non-ideality results in an increase of the mole fraction of molecular hydrogen by up to a factor of $\sim 10$, compared to the equilibrium value for the ideal gas. This increases the ${\mathrm{H}}_2-\mathrm{He}$ CIA opacity by an order of magnitude and will affect the determination of the abundance of hydrogen in very cool, helium-rich white dwarfs.

9 pages, 5 figures, 1 table; Accepted for publication in The Astrophysical Journal