The Evolution of Helium and Hydrogen Ionization Corrections as HiiRegions Age

Helium and hydrogen recombination lines observed in low-metallicity,extragalactic, HII regions provide the data used to infer the primordial heliummass fraction, Y_P. In deriving abundances from observations, the correctionfor unseen neutral helium or hydrogen is usually assumed to be absent; i.e.,the ionization correction factor is taken to be unity (icf = 1). In a previouspaper (VGS), we revisited the question of the icf, confirming a "reverse"ionization correction: icf < 1. In VGS the icf was calculated using more nearlyrealistic models of inhomogeneous HII regions, suggesting that the publishedvalues of Y_P needed to be reduced by an amount of order 0.003. As starclusters age, their stellar spectra evolve and so, too, will their icfs. Herethe evolution of the icf is studied, along with that of two, alternate,measures of the "hardness" of the radiation spectrum. The differences betweenthe 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 forsome of the same ionizing photons). Particular attention is paid to the amountof doubly-ionized helium predicted, leading us to suggest that observations of,or bounds to, He++ may help to discriminate among models of HII regions ionizedby starbursts of different ages and spectra. We apply our analysis to theIzotov & Thuan (IT) data set utilizing the radiation softness parameter, the[OIII]/[OI] ratio, and the presence or absence of He++ to find 0.95 < icf <0.99. This suggests that the IT estimate of the primordial helium abundanceshould be reduced by Delta-Y = 0.006 +- 0.002, from 0.244 +- 0.002 to 0.238 +-0.003.Comment: 27 double-spaced pages, 11 figures, 5 equations; revised to match the version accepted for publication in the Ap