Calculated spectra for HeH+ and its effect on the opacity of cool metal poor stars

Abstract

The wavelength and Einstein A coefficient are calculated for all rotation-vibration transitions of ${}^4$He${}^1$H${}^{+}$, ${}^3$He${}^1$H${}^{+}$, ${}^4$He${}^2$H${}^{+}$ and ${}^3$He${}^2$H${}^{+}$, giving a complete line list and the partition function for ${}^4$HeH${}^{+}$ and its isotopologues. This opacity is included in the calculation of the total opacity of low-metallicity stars and its effect is analysed for different conditions of temperature, density and hydrogen number fraction. For a low helium number fraction (as in the Sun), it is found that HeH${}^{+}$ has a visible but small effect for very low densities ($\rho \le 10^{-10}$g cm${}^{-3}$), at temperatures around 3500 K. However, for high helium number fraction, the effect of HeH${}^{+}$ becomes important for higher densities ($\rho \le 10^{-6}$g cm${}^{-3}$), its effect being most important for a temperature around 3500 K. Synthetic spectra for a variety of different conditions are presented.

8 pages, 2 Figures Accepted for publication in Mon.Not.Roy.Astron.Soc