Ionization corrections for low-metallicity HII regions and the primordial helium abundance

Helium and hydrogen recombination lines observed in low-metallicity, extrag alactic H II regions provide the data used to infer the primordial helium m ass fraction, Y-p. The ionization corrections for unseen neutral helium (or hydrogen) are usually assumed to be absent; i.e., the ionization correctio n factor (ICF) is taken to be unity (ICF equivalent to 1). In this paper we revisit the question of the ICF for H II regions ionized by clusters of yo ung, hot, metal-poor stars. Our key result is that for the H II regions use d in the determination of Y-p, there is a "reverse" ionization correction: ICF < 1. We explore the effect on the ICF of more realistic inhomogeneous H II region models and find that for those regions ionized by young stars, w ith "hard" radiation spectra, the ICF is reduced further below unity. In Mo nte Carlos using H II region data from the literature (Izotov & Thuan) we e stimate a reduction in the published value of Y-p of order 0.003, which is roughly twice as large as the quoted statistical error in the Y-p determina tion.