On the Evolution of Helium in Blue Compact Galaxies

We discuss the chemical evolution of dwarf irregular and blue compactgalaxies in light of recent data, new stellar yields and chemical evolutionmodels. We examine the abundance data for evidence of HII regionself-enrichment effects, which would lead to correlations in the scatter ofhelium, nitrogen, and oxygen abundances around their mean trends. The observedhelium abundance trends show no such correlations, though the nitrogen--oxygentrend does show strong evidence for real scatter beyond observational error. Weconstruct simple models for the chemical evolution of these galaxies, using themost recent yields of \he4, C, N and O in intermediate- and high-mass stars.The effects of galactic outflows, which can arise both from bulk heating andevaporation of the ISM, and from the partial escape of enriched supernovaejecta are included. In agreement with other studies, we find thatsupernova-enriched outflows can roughly reproduce the observed He, C, N, and Otrends; however, in models that fit N versus O, the slopes $\Delta Y/\Delta$Oand $\Delta Y/\Delta$N consistently fall more than $2\sigma$ below the fit toobservations. We discuss the role of the models and their uncertainties in theextrapolation of primordial helium from the data. We also explore the modeldependence arising nucleosynthesis uncertainties associated with nitrogenyields in intermediate mass stars, the fate of $8-11 \msol$ stars, and massivestar winds.Comment: 27 pages, latex, 7 ps figure