2014MNRAS.445..778I

We present near-infrared (NIR) spectroscopic observations of the high-intensity Hei λ10830 Å emission line in 45 low-metallicity Hii regions. We combined these NIR data with spectroscopic data in the optical range to derive the primordial He abundance. The use of the Hei λ10830 Å line, the intensity of which is very sensitive to the density of the Hii region, greatly improves the determination of the physical conditions in the He+ zone. This results in a considerably tighter Y-O/H linear regression compared to all previous studies. We extracted a final sample of 28 Hii regions with Hβ equivalent width EW(Hβ) ≥ 150 Å, excitation parameter O2 +/O ≥ 0.8, and with helium mass fraction Y derived with an accuracy better than 3 percent. With this final sample we derived a primordial 4He mass fraction Yp = 0.2551±0.0022. The derived value of Yp is higher than the one predicted by the standard big bang nucleosynthesis model. Using our derived Yp together with D/H = (2.53±0.04)x10^–5, and the χ2 technique, we found that the best agreement between these light element abundances is achieved in a cosmological model with a baryon mass density Ωbh2 = 0.0240±0.0017 (68 percent confidence level, CL),±0.0028 (95.4 percent CL),±0.0034 (99 percent CL) and an effective number of neutrino species Neff = 3.58±0.25 (68 percent CL),±0.40 (95.4 percent CL),±0.50 (99 percent CL). A non-standard value of Neff is preferred at the 99 percent CL, implying the possible existence of additional types of neutrino species.