CHEMICAL EVOLUTION IN A MULTIPLE STARBURST ENVIRONMENT - ODD ABUNDANCE RATIOS IN THE LIGHT OF THE FIRST GENERATION OF STARS

The cumulative temporal evolution of carbon, nitrogen and oxygen in th e interstellar gas of star-forming regions has been studied using a nu merical model of chemical evolution. In the context of searching for g enuinely young galaxies it is shown that such an object would have, be sides low elemental abundances, abnormal abundance ratios of(N/O) and (C/N) during the first few 10(7) years after the onset of its first bu rst of star formation. These ratios are much lower and higher, respect ively, than what is observed in a sample of nearby blue compact and dw arf irregular galaxy youth candidates. This result is independent of t he initial abundances of the cloud of gas from which the first stars a re formed. It is shown that the chemical abundances and their ratios a re extremely sensitive to the initial mass function and that the obser vations are well represented by a model with a solar neighbourhood ini tial mass function in a medium which has undergone several short burst s of star formation. The selected sample of blue compact and dwarf irr egular galaxies could in fact appear as pristine objects if they lack stars less massive than similar to 2-3 M. Or if they formed out of gas already enriched in heavy elements. It is shown that the different be haviour of (N/O) as a function of (O/H), often interpreted as due to v arious contributions of primary and secondary nitrogen, in fact could be an effect of stellar mass-loss as a function of metallicity.