SIMBAD references

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 δY_p≃0.002-0.004.