Primordial ⁴He abundance: a determination based on the largest sample of H II regions with a methodology tested on model H II regions^{⋆,⋆⋆,⋆⋆⋆}

¹ LUTH, Observatoire de Meudon, 92195 Meudon Cedex, France
e-mail: izotov@mao.kiev.ua
² Main Astronomical Observatory, Ukrainian National Academy of Sciences, Zabolotnoho 27, 03680 Kyiv, Ukraine
³ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 23 November 2012
Accepted: 4 August 2013

Abstract

We verified the validity of the empirical method to derive the ⁴He abundance used in our previous papers by applying it to CLOUDY (v13.01) models. Using newly published He i emissivities for which we present convenient fits as well as the output CLOUDY case B hydrogen and He i line intensities, we found that the empirical method is able to reproduce the input CLOUDY ⁴He abundance with an accuracy of better than 1%. The CLOUDY output data also allowed us to derive the non-recombination contribution to the intensities of the strongest Balmer hydrogen Hα, Hβ, Hγ, and Hδ emission lines and the ionisation correction factors for He. With these improvements we used our updated empirical method to derive the ⁴He abundances and to test corrections for several systematic effects in a sample of 1610 spectra of low-metallicity extragalactic H ii regions, the largest sample used so far. From this sample we extracted a subsample of 111 H ii regions with Hβ equivalent width EW(Hβ) ≥ 150 Å, with excitation parameter x = O²⁺/O ≥ 0.8, and with helium mass fraction Y derived with an accuracy better than 3%. With this subsample we derived the primordial ⁴He mass fraction Y_p = 0.254 ± 0.003 from linear regression Y – O/H. The derived value of Y_p is higher at the 68% confidence level (CL) than that predicted by the standard big bang nucleosynthesis (SBBN) model, possibly implying the existence of different types of neutrino species in addition to the three known types of active neutrinos. Using the most recently derived primordial abundances D/H = (2.60 ± 0.12) × 10^-5 and Y_p = 0.254 ± 0.003 and the χ² technique, we found that the best agreement between abundances of these light elements is achieved in a cosmological model with baryon mass density Ω_bh² = 0.0234 ± 0.0019 (68% CL) and an effective number of the neutrino species N_eff = 3.51 ± 0.35 (68% CL).