On the evolution of helium in blue compact galaxies

We discuss the chemical evolution of dwarf irregular and blue compact galax ies in light of recent data, new stellar yields, and chemical evolution mod els. We examine the abundance data for evidence of H II region self-enrichm ent effects, which would lead to correlations in the scatter of helium, nit rogen, and oxygen abundances around their mean trends. The observed helium abundance trends show no such correlations, although the nitrogen-oxygen tr end does show strong evidence for real scatter beyond observational error. We construct simple models for the chemical evolution of these galaxies, us ing the most recent yields of He-4, C, N, and O in intermediate- and high-m ass stars. The effects of galactic outflows, which can arise both from bulk heating and evaporation of the interstellar medium and from the partial es cape of enriched supernova ejecta are included. In agreement with other stu dies, we find that supernova-enriched outflows can roughly reproduce the ob served He, C, N, and O trends; however, in models that fit N versus O, the slopes Delta Y/Delta O and Delta Y/Delta N consistently fall more than 2 si gma below the fit to observations. We discuss the role of the models and th eir uncertainties in the extrapolation of primordial helium from the data. We also explore the model dependence arising from nucleosynthesis uncertain ties associated with nitrogen yields in intermediate-mass stars, the fate o f 8-11 M. stars, and massive star winds.