Ionization Corrections for Low-Metallicity H II Regions and the Primordial Helium Abundance

Helium and hydrogen recombination lines observed in low-metallicity, extragalactic H II regions provide the data used to infer the primordial helium mass fraction, YP. The ionization corrections for unseen neutral helium (or hydrogen) are usually assumed to be absent; i.e., the ionization correction factor (ICF) is taken to be unity (ICF ≡ 1). In this paper we revisit the question of the ICF for H II regions ionized by clusters of young, hot, metal-poor stars. Our key result is that for the H II regions used in the determination of YP, 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, with "hard" radiation spectra, the ICF is reduced further below unity. In Monte Carlos using H II region data from the literature (Izotov & Thuan) we estimate a reduction in the published value of YP of order 0.003, which is roughly twice as large as the quoted statistical error in the YP determination.