The Evolution of Helium and Hydrogen Ionization Corrections as H II Regions Age

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. When abundances are derived from observations, the correction for unseen neutral helium or hydrogen is usually assumed to be absent; i.e., the ionization correction factor (icf) is taken to be unity (icf ≡ 1). In a previous paper, we revisited the question of the icf for H II regions ionized by clusters of young, hot, metal-poor stars, confirming earlier work which had demonstrated a "reverse" ionization correction: icf < 1. In that work, the icf was calculated using more nearly realistic models of inhomogeneous H II regions, revealing that for those H II regions ionized by young stars with "hard" radiation spectra the icf is reduced even farther below unity compared with homogeneous models. Based on these results, our previous paper suggested that the published values of YP needed to be reduced by an amount of order 0.003. As star clusters age, their stellar spectra evolve and so, too, will their icf's. Here the evolution of the icf is studied, along with that of two alternate measures of the "hardness" of the radiation spectrum. The differences between the icf for radiation-bounded and matter-bounded models are also explored, along with the effect on the icf of the He/H ratio (since He and H compete for some of the same ionizing photons). Particular attention is paid to the amount of doubly ionized helium predicted, leading us to suggest that observations of, or bounds to, He++ may help to discriminate among models of H II regions ionized by starbursts of different ages and spectra. We apply our analysis to the Izotov & Thuan data set, utilizing the radiation softness parameter, the [O III]/[O I] ratio, and the presence or absence of He++, to find 0.95 ≲ icf ≲ 0.99. This suggests that the Izotov & Thuan estimate of the primordial helium abundance should be reduced by ΔY ≈ 0.006 ± 0.002, from 0.244 ± 0.002 to 0.238 ± 0.003.