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

We investigate the dissociation equilibrium of H2 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 Teff = 4000K, the non-ideality results in an increase of the mole fraction of molecular hydrogen by up to a factor of ∼ 10, compared to the equilibrium value for the ideal gas. This increases the H2 −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. Subject headings: dense matter – stars – atmospheres – stars: white dwarfs