On the Dissociation Equilibrium of H2 in Very Cool, Helium-Rich White Dwarf Atmospheres
Abstract
We investigate the dissociation equilibrium of
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
eff
=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
H
2
−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.