On the Dissociation Equilibrium of H2in Very Cool, Helium‐rich White Dwarf Atmospheres

Abstract

We investigate the dissociation equilibrium of H2 in very cool, helium-rich white dwarf atmospheres. We present the nonideal chemical equilibrium solution for the dissociation of molecular hydrogen in a medium of dense helium. We find that at the photosphere of cool white dwarfs of Teff = 4000 K, the nonideality 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 collision-induced absorption (CIA) opacity by an order of magnitude and will affect the determination of the abundance of hydrogen in very cool, helium-rich white dwarfs.