The chemical evolution of gas-rich dwarf galaxies

In an effort to understand the evolution of N, O and He abundances in gas-r ich dwarf galaxies, we investigate the dispersion and mixing of supernova e jecta in relation to HII region evolution and develop a numerical model of chemical evolution based on a double-bursting mode of star formation (with an interval of the order of 3 x 10(7) yr between bursts of a pair) which ha s been designed to account for the existence of significant scatter in the N/O-O/H relation. The dependence of the abundances on gas fraction is explored on the basis o f this and similar models, in combination with various hypotheses concernin g inflow and selective and non-selective outflow. The gas fractions are unc ertain within wide limits for blue compact galaxies, but are more well defi ned for some dwarf irregulars. Selective winds do not give a good fit to N/ O, while closed models and models with non-selective winds with or without inflow are all found to be viable.