A High-Temperature Partition Function for [FORMULA][F][RM]H[/RM][SUP]+[/SUP][INF]3[/INF][/F][/FORMULA]

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Abstract
H3+ is the main supplier of electrons to metal-free stars below ~8000 K. To describe H3+ properly it is important to know its partition function. Previous determinations of this function are based on extrapolations from lower temperatures. A new partition function is presented that is calculated by explicit ab initio computation of levels lying up to 15,000 cm-1 above the ground state and with J ≤ 20. The effect of all other bound levels of H3+, which is significant at higher temperatures, is included using a simple model based on Padé approximants. The new partition function is very significantly larger (up to a factor of 100 in some cases) than previous estimates above 3000 K.