A new determination of the primordial He abundance using the He I λ 10830 Å emission line: cosmological implications

We present near-infrared (NIR) spectroscopic observations of the high-intensity He I lambda 10830 angstrom 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 He I lambda 10830 angstrom 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 Ha equivalent width EW(Ha) =>= 150 angstrom, excitation parameter O-2+/O >= 0.8, and with helium mass fraction Y derived with an accuracy better than 3 per cent. With this final sample we derived a primordial He-4 mass fraction Y-p = 0.2551 +/- 0.0022. The derived value of Y-p 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) x 10(-5). and the.2 technique, we found that the best agreement between these light element abundances is achieved in a cosmological model with a baryon mass density Omega(b)h(2) = 0.0240 +/- 0.0017 (68 per cent confidence level, CL), +/- 0.0028 (95.4 per cent CL), +/- 0.0034 (99 per cent CL) and an effective number of neutrino species N-eff = 3.58 +/- 0.25 (68 per cent CL), +/- 0.40 (95.4 per cent CL), +/- 0.50 (99 per cent CL). A non-standard value of N-eff is preferred at the 99 per cent CL, implying the possible existence of additional types of neutrino species.