The primordial abundance of⁴He: An update

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 (Y_P). 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 Y_p, owing to the lack of sufficiently low-metallicity points. We use the extant data in a semiempirical approach to bounding the size of possible systematic uncertainties in the determination of Y_P. Our best estimate for the primordial abundance of ⁴He assuming a linear relation between ⁴He and O/H is Y_p = 0.230 ± 0.002(stat) based on the subset of H II regions with the lowest metallicity; for our full data set we find Y_P = 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.