The effects of He I 10830 on helium abundance determinations
  Observations of helium and hydrogen emission lines from metal-poor
extragalactic H II regions provide an independent method for determining the
primordial helium abundance, Y_p. Traditionally, the emission lines employed
are in the visible wavelength range, and the number of suitable lines is
limited. Furthermore, when using these lines, large systematic uncertainties in
helium abundance determinations arise due to the degeneracy of physical
parameters, such as temperature and density. Recently, Izotov, Thuan, & Guseva
(2014) have pioneered adding the He 10830 infrared emission line in helium
abundance determinations. The strong electron density dependence of He 10830
makes it ideal for better constraining density, potentially breaking the
degeneracy with temperature. We revisit our analysis of the dataset published
by Izotov, Thuan, & Stasinska (2007) and incorporate the newly available
observations of He 10830 by scaling them using the observed-to-theoretical
Paschen-gamma ratio. The solutions are better constrained, in particular for
electron density, temperature, and the neutral hydrogen fraction, improving the
model fit to data, with the result that more spectra now pass screening for
quality and reliability, in addition to a standard 95% confidence level cut.
Furthermore, the addition of He 10830 decreases the uncertainty on the helium
abundance for all galaxies, with reductions in the uncertainty ranging from
10-80%. Overall, we find a reduction in the uncertainty on Y_p by over 50%.
From a regression to zero metallicity, we determine Y_p = 0.2449 +/- 0.0040,
consistent with the BBN result, Y_p = 0.2470 +/- 0.0002, based on the Planck
determination of the baryon density. The dramatic improvement in the
uncertainty from incorporating He 10830 strongly supports the case for
simultaneous (thus not requiring scaling) observations of visible and infrared
helium emission line spectra.
