ELEMENT ABUNDANCES IN BLUE COMPACT GALAXIES

Models of the chemical evolution of blue compact galaxies, where the star formation is assumed to proceed in short, intense bursts of activity, have been computed, with special attention to the evolution of helium, nitrogen, oxygen and iron. The contribution to the chemical enrichment by supernovae (SNe) of the various types (Ia, Ib, II) is taken into account, following the most up-to-date prescriptions. Galactic winds powered by SNe are also considered and confirmed as being able to explain the observed element distributions. In particular, winds releasing metals in different proportions can explain the high ratio of DELTA(He/H)/DELTA(O/H) and the distribution of N/O with O/H observed in extragalactic H II regions. No anomalous assumption on the initial mass function (IMF) slope and/or mass cut-offs seems to be required to obtain model predictions in agreement with the data. In particular, Salpeter's initial mass function is applicable to all these galaxies, although some slight differences cannot be excluded. The [O/Fe] ratios resulting from these models are always positive, except when many star formation episodes and differential galactic winds are assumed, thus suggesting the latter scenario for systems like the Magellanic Clouds, where the observed [O/Fe] ratios are negative.