The Primordial Abundance of4He: An Update

Abstract

We include new data in an updated analysis of helium in low-metallicity extragalactic H II regions with the goal of deriving the primordial abundance of 4He (YP). We show that the new observations of Izotov et al. are consistent with previous data. However, they should not be taken in isolation to determine YP, owing to the lack of sufficiently low-metallicity points. We use the extant data in a semi-empirical approach to bounding the size of possible systematic uncertainties in the determination of YP. Our best estimate for the primordial abundance of 4He assuming a linear relation between 4He and O/H is YP = 0.230 ? 0.003(stat) based on the subset of H II regions with the lowest metallicity; for our full data set we find YP = 0.234 ? 0.002(stat). Both values are entirely consistent with our previous results. We discuss the implications of these values for standard big bang nucleosynthesis (SBBN), particularly in the context of recent measurements of deuterium in high-redshift, low-metallicity QSO absorption-line systems.