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 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 (p ≤ 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 (p ≤ 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.