Primordial Helium Abundance: A Reanalysis of the Izotov‐Thuan Spectroscopic Sample

A reanalysis is made for the helium abundance determination for the Izotov-Thuan spectroscopic sample of extragalactic H II regions. We find that the effect of underlying stellar absorption of the He I lines, which is more important for metal-poor systems, affects significantly the inferred primordial helium abundance Y p obtained in the zero-metallicity limit and the slope of linear extrapolation, dY / dZ . This brings Y p from 0.234 ± 0.004 to 0.250 ± 0.004 and dY / dZ = 4.7 ± 1.0 to 1.1 ± 1.4. Conservatively, this indicates the importance of the proper understanding of underlying stellar absorption for accurate determinations of the primordial helium abundance to the error of δ Y p ≃ 0.002-0.004.