Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Evolution of Helium and Hydrogen Ionization Corrections as HII
  Regions Age   Helium and hydrogen recombination lines observed in low-metallicity,
extragalactic, HII regions provide the data used to infer the primordial helium
mass fraction, Y_P. In deriving abundances from observations, the correction
for 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 previous
paper (VGS), we revisited the question of the icf, confirming a "reverse"
ionization correction: icf < 1. In VGS the icf was calculated using more nearly
realistic models of inhomogeneous HII regions, suggesting that the published
values of Y_P needed to be reduced by an amount of order 0.003. As star
clusters age, their stellar spectra evolve and so, too, will their icfs. 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 HII regions ionized
by starbursts of different ages and spectra. We apply our analysis to the
Izotov & 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 abundance
should be reduced by Delta-Y = 0.006 +- 0.002, from 0.244 +- 0.002 to 0.238 +-
0.003.

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The categories of this paper are: supr-con
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The categories of this paper are: funct-an
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The categories of this paper are: cmp-lg
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The categories of this paper are: math
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The categories of this paper are: stat
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The categories of this paper are: comp-gas
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The categories of this paper are: dg-ga
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The categories of this paper are: plasm-ph
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The categories of this paper are: nucl-ex
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The categories of this paper are: patt-sol
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The categories of this paper are: q-alg
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The categories of this paper are: bayes-an
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The categories of this paper are: q-fin
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The categories of this paper are: math-ph
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The categories of this paper are: nucl-th
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The categories of this paper are: gr-qc