Revised Primordial Helium Abundance Based on New Atomic Data

Abstract

We have derived a primordial helium abundance of Yp = 0.2477 +- 0.0029, based on new atomic physics computations of the recombination coefficients of He I and of the collisional excitation of the H I Balmer lines together with observations and photoionization models of metal-poor extragalactic H II regions. The new atomic data increase our previous determination of Yp by 0.0086, a very significant amount. By combining our Yp result with the predictions made by the standard Big Bang nucleosynthesis model, we find a baryon-to-photon ratio, η, in excellent agreement both with the ηvalue derived by the primordial deuterium abundance value observed in damped Lyman-αsystems and with the one obtained from the WMAP observations.

35 pages, 1 figure. AAS LaTeX. Accepted for publication in the Astrophysical Journal. The accepted version is more explicit in the description of the procedure and in the treatment of errors with respect to the first version, and the final numerical result is slightly changed. Scheduled for ApJ, Vol. 667, Sept 20, 2007