THE CHEMICAL HISTORY OF THE METAL-POOR GALAXY IZW-18

A chemical evolution model following the evolution of the abundances o f He, C, N, O and Fe for the metal poor galaxy IZw 18 is presented. Th is model takes into account detailed nucleosynthesis and rates of supe rnovae of type II and I, and assumes that a mass outflow is developed as a consequence of supernova explosions. The star formation is assume d to have proceeded in short but intense bursts, so that supernovae of type II are the main responsible for the mass removal. In this framew ork we made the hypothesis that the star forming region mainly looses the nucleosynthesis products of supernovae of type II with the consequ ence 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 10-20 Myr, and that a substantial fraction of primary nitrogen should have b een produced by massive stars. By comparison with new HST data referri ng 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 r ecent starburst or with two starbursts separated by a gap of at least 1 Gyr. Predictions for the expected abundances of N, Fe and C in the H I region, which could be checked by looking at NI, FeII and CII, are g iven. Our results show that a negligible amount of stellar helium has been produced. We strengthen the importance of IZw 18 and more general ly extreme metal poor galaxies for the primordial He abundance determi nation.