The primordial helium abundance:: Toward understanding and removing the cosmic scatter in the dY/dZ relation

We present results from photoionization models of low-metallicity H II regions. These nebulae form the basis for measuring the primordial helium abundance. Our models show that the helium ionization correction factor (ICF) can be nonnegligible for nebulae excited by stars with effective temperatures larger than 40,000 K. Furthermore, we find that when the effective temperature rises to above 45,000 K, the ICF can be significantly negative. This result is independent of the choice of stellar atmosphere. However, if an H II region has an [O III] lambda 5007/[O I] lambda 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] lambda 5007/H beta ratio. These two results can be used as selection criteria to remove nebulae with potentially nonnegligible ICFs. Use of our metallicity-independent criterion on the data of Izotov & Thuan results in a 20% reduction of the rms scatter about the best-fit Y-Z line. A fit to the selected data results in a slight increase of the value of the primordial helium abundance.