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.

arXiv comment12 pages, 4 figures. Accepted for publication in ApJ Letters

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• Ab initio calculation of the 66 low lying electronic states of HeH${}^{+}$: adiabatic and diabatic representations
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• Photodissociation and radiative association of HeH+ in the metastable triplet state
• Gas phase mean opacities for varying [M/H], N/O, and C/O
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• Astrophysics in 2005
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