The effects of He I λ10830 on helium abundance determinations

Abstract

Abstract. Observations of helium and hydrogen emission lines from metal-poor extragalactic H II regions, combined with estimates of metallicity, 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 I λ10830 infrared emission line in helium abundance determinations. The strong electron density dependence of He I λ10830 makes it ideal for better constraining density, potentially breaking the degeneracy with temperature. We revisit our analysis of the dataset published by Izotov, Thuan, & Stasińska (2007) and incorporate the newly available observations of He I λ10830 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 I λ10830 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 I λ10830 strongly supports the case for simultaneous (thus not requiring scaling) observations of visible and infrared helium emission line spectra.

THE PRIMORDIAL DEUTERIUM ABUNDANCE OF THE MOST METAL-POOR DAMPED Lyα SYSTEM∗

“…Using the Planck CMB observations, 9 the predicted Standard Model abundances of the primordial elements are (68% confidence limits; see Section 5) low-metallicity H II regions in nearby star-forming galaxies. Two analyses of the latest measurements, including an infrared transition that was not previously used, find Y P =0.2551±0.0022 (Izotov et al 2014) and Y P =0.2449±0.0040 (Aver et al 2015). These are mutually inconsistent, presumably due to some underlying difference between the analysis methods.…”

Breaking Be: a sterile neutrino solution to the cosmological lithium problem

“…[2] for deuterium and Ref. [3] for helium) and the corresponding predictions based on the standard models of cosmology and particle physics. In fact, in this framework (that we shall refer to as "standard BBN") primordial abundances only depend on the baryon-to-photon ratio η ≡ n b /n γ or, equivalently, on the baryon density ω b ≡ Ω b h 2 .…”

Introduction to big bang nucleosynthesis and modern cosmology

“…However, the extraction of the final helium abundance has been fraught with uncertainties due to correlations among errors in the neutral hydrogen determination and the inferred helium abundance. In 30,31 it was demonstrated that updated emissivities and the neutral hydrogen corrections generally increase the inferred abundance, while the correlated uncertainties increase the uncertainty in the final extracted helium abundance. Therefore, we adopt the value and uncertainty from 31 of Y p = 0.2449 ± 0.0040, (35) which is in general agreement with the predicted value from standard BBN when η 10 is fixed from the Planck analysis.…”