The primordial abundance of ~4 He from a large sample of low-metallicity H ii regions

摘要

We determine the primordial helium mass fraction Y_p using 1700 spectra of lowmetallicity extragalactic H II regions. This sample is selected from the Data Release 7 of the Sloan Digital Sky Survey, from European Southern Observatory archival data and from our own observations. We have considered known systematic effects which may affect the ~4He abundance determination. They include collisional and fluorescent enhancements of He I recombination lines, underlying He i and hydrogen stellar absorption lines, collisional excitation of hydrogen lines, temperature and ionization structure of the H II region. Monte Carlo methods are used to solve simultaneously the above systematic effects. We find a primordial helium mass fraction Y_p = 0.2512 ± 0.0006(stat.) ± 0.0020 (syst.). This value is higher than the value given by Standard Big Bang Nucleosynthesis (SBBN) theory. If confirmed, it would imply slight deviations from SBBN.