CHEMICAL EVOLUTION OF IRREGULAR GALAXIES AND THE PRIMORDIAL HE-4 ABUNDANCE

We study several models for the origin and chemical evolution of compact irregular galaxies in order to determine the primordial He-4 abundance, Y(p), from the zero metallicity intercept of the observed Y versus Z correlations. This study confirms the suggestion that a straight-line fit to the observations does not necessarily give the correct primordial He-4 abundance. This is especially true for the extrapolation of the Y versus N/H data which depends upon the relative contributions from secondary and primary nitrogen in low metallicity stars. The extrapolation of the O/H data is also slightly nonlinear even for primary oxygen in a closed-box model with instantaneous recycling due to the time dependence of the hydrogen mass fraction, the breakdown of the instantaneous recycling approximation, the production of helium, nitrogen, and oxygen in stars of different mass, and ejection of metal-poor material from low-mass stars. Our best fits to the data, even after excluding possible contamination of H n regions from neighboring Wolf-Rayet stars, gives Y(p) = 0.228 +/- 0.005 for O/H and Y(p) = 0.223 +/- 0.006 for N/H. These primordial helium abundances are as much as 2 sigma below the minimum helium abundance which can be produced in the standard homogeneous big bang model with three light neutrino flavors. This discrepancy may be due to shortcomings of the chemical evolution models, additional systematic errors in the determination of the helium and/or metal abundances in extragalactic H II regions, or effects of nonstandard primordial nucleosynthesis.