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 K. The inclusion of HeH significantly reddens spectral energy distributions and T ! 8000 eff broadband color indices for models with K. This has serious implications for existing model atmoT ! 5500 eff spheres, synthetic spectra, and cooling curves for helium-rich white dwarfs. Subject headings: equation of state — stars: atmospheres — white dwarfs