The Role of HeH+ in Cool Helium-rich White Dwarfs

HeH+ is found to be the dominant positive ion over a wide range of temperatures and densities relevant to helium-rich white dwarfs. The inclusion of HeH+ in ionization equilibrium computations increases the abundance of free electrons by a significant factor. For temperatures below 8000 K, He- free-free absorption is increased by up to a factor of 5 by the inclusion of HeH+. Illustrative model atmospheres and spectral energy distributions are computed, which show that HeH+ has a strong effect on the density and pressure structure of helium-rich white dwarfs with Teff < 8000 K. The inclusion of HeH+ significantly reddens spectral energy distributions and broadband color indices for models with Teff < 5500 K. This has serious implications for existing model atmospheres, synthetic spectra, and cooling curves for helium-rich white dwarfs.