2005MNRAS.357..471E

The wavelength and Einstein A coefficient are calculated for all rotation-vibration transitions of ⁴He¹H⁺, ³He¹H⁺, ⁴He²H⁺ and ³ He²H⁺, giving a complete line list and the partition function for ⁴HeH⁺ 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 (ρ≤ 10^–10 g/cm3), at temperatures around 3500 K. However, for high helium number fraction, the effect of HeH⁺ becomes important for higher densities (ρ≤ 10^–6 g/cm3), its effect being most important for a temperature around 3500 K. Synthetic spectra for a variety of different conditions are presented.