The Primordial Helium Abundance: Toward Understanding and Removing the Cosmic Scatter in thedY/dZRelation

We present results from photoionization models of low-metallicity HIIregions. These nebulae form the basis for measuring the primordial heliumabundance. Our models show that the helium ionization correction factor (ICF)can be non-negligible for nebulae excited by stars with effective temperatureslarger than 40,000 K. Furthermore, we find that when the effective temperaturerises to above 45,000 K, the ICF can be significantly negative. This result isindependent of the choice of stellar atmosphere. However, if an HII region hasan [O III] 5007/[O I] 6300 ratio greater than 300, then our models show that,regardless of its metallicity, it will have a negligibly small ICF. A similar,but metallicity dependent, result was found using the [O III] 5007/H$\beta$ratio. These two results can be used as selection criteria to remove nebulaewith potentially non-negligible ICFs. Using our metallicity independentcriterion on the data of Izotov & Thuan (1998) results in a 20% reduction ofthe rms scatter about the best fit $Y-Z$ line. A fit to the selected dataresults in a slight increase of the value of the primordial helium abundance.Comment: 10 pages, 5 figures, accepted by the Ap