1995A&A...297..634K

A chemical evolution model following the evolution of the abundances of He, C, N, O and Fe for the metal poor galaxy IZw 18 is presented. This model takes into account detailed nucleosynthesis and rates of supernovae of type II and I, and assumes that a mass outflow is developed as a consequence of supernova explosions. The star formation is assumed to have proceeded in short but intense bursts, so that supernovae of type II are the main responsible for the mass removal. In this framework we made the hypothesis that the star forming region mainly looses the nucleosynthesis products of supernovae of type II with the consequence of removing a part of the newly produced oxygen but not the newly produced He and N. By comparison with the spectroscopic properties of the HII region we conclude that IZw 18 must have experienced not more than two bursts of star formation with duration no longer than 10-20Myr, and that a substantial fraction of primary nitrogen should have been produced by massive stars. By comparison with new HST data referring to the HI region, suggesting an oxygen abundance lower by a factor of 30 than in the HII region, we conclude that the observed abundance is compatible, inside the observational errors, either with only one recent starburst or with two starbursts separated by a gap of at least 1Gyr. Predictions for the expected abundances of N, Fe and C in the HI region, which could be checked by looking at NI, FeII and CII, are given. Our results show that a negligible amount of stellar helium has been produced. We strengthen the importance of IZw 18 and more generally extreme metal poor galaxies for the primordial He abundance determination.