The pressure ionization of metals in cool, helium-rich white dwarfs

The Hummer-Mihalas (1988) equation of state is used to calculate the ionization equilibrium of metals in the atmosphere of van Maanen 2. The results are compared with the predictions of the standard Saha ionization equations. The Hummer-Mihalas equation of state predicts a much greater degree of ionization due to pressure effects. It shows that the pressure ionization of metals is already important and influences the predicted line spectra in non-DA atmospheres of T(eff) of about 5800 K. This effect must be taken into account when constructing model atmospheres for cool, helium-rich white dwarfs. 15 refs.