The Primordial Abundance of 4He: A Self-consistent Empirical Analysis of Systematic Effects in a Large Sample of Low-Metallicity H II Regions

We determine the primordial helium mass fraction Yp using 93 spectra of 86 low-metallicity extragalactic H II regions. This sample constitutes the largest and most homogeneous high-quality data set in existence for the determination of Yp. For comparison, and to improve the statistics in our investigation of systematic effects affecting the Yp determination, we have also considered a sample of 271 low-metallicity H II regions selected from Data Release 5 of the Sloan Digital Sky Survey. Although this larger sample shows more scatter, it gives results that are consistent at the 2 σ level with our original sample. We have considered known systematic effects that may affect the 4He abundance determination. They include different sets of He I line emissivities and reddening laws, collisional and fluorescent enhancements of He I recombination lines, underlying He I stellar absorption lines, collisional excitation of hydrogen lines, temperature and ionization structure of the H II region, and deviation of He I and H emission-line intensities from case B. However, the most likely value of Yp depends on the adopted set of He I line emissivities. Using Monte Carlo methods to solve simultaneously the above systematic effects, we find a primordial helium mass fraction of Yp = 0.2472 ± 0.0012 when using the He I emissivities from Benjamin and coworkers and 0.2516 ± 0.0011 when using those from Porter and coworkers. The first value agrees well with the value given by standard big bang nucleosynthesis (SBBN) theory, while the value obtained with the likely more accurate emissivities of Porter and coworkers is higher at the 2 σ level. This latter value, if confirmed, would imply slight deviations from SBBN.