The role of HeH+ in cool helium rich white dwarfs

Abstract

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 upon the density and pressure structure of helium rich white dwarfs with teff < 8000 K. The inclusion of HeH${}^{+}$ significantly reddens spectral energy distributions and broad band 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.

12 pages, 4 figures. Accepted for publication in ApJ Letters