THE PRIMORDIAL HELIUM ABUNDANCE AS DETERMINED FROM CHEMICAL EVOLUTION OF IRREGULAR GALAXIES

In a previous study we explored the effects of Galactic chemical evolution on the determination of the primordial He-4 abundance, Y(P), from the zero metallicity intercept of the observed Y versus Z correlations. In the present paper we derive formulae in which one of the approximations made in that study is removed in the fit to the Y versus Z correlation data. We also investigate the effects on the Y(P) value of various approaches to fitting the data and find that a simultaneous fitting of all the data is the best approach to determining Y(P). We examine the recently suggested method of using N/O versus O/H as a means to determine the primary nitrogen abundance at low metallicity. As in the previous work it is found that the Y versus N/H data require a combination of primary and secondary nitrogen in all fits. We determine that Y(P) = 0.227 +/- 0.006 (1 sigma). This value is approximately 1.5 sigma below the minimum value allowed by the standard homogeneous big bang model with three light neutrino species.