1995A&A...293..652O

The cumulative temporal evolution of carbon, nitrogen and oxygen in the interstellar gas of star-forming regions has been studied using a numerical model of chemical evolution. In the context of searching for genuinely young galaxies it is shown that such an object would have, besides low elemental abundances, abnormal abundance ratios of (N/O) and (C/N) during the first few 10⁷years after the onset of its first burst of star formation. These ratios are much lower and higher, respectively, than what is observed in a sample of nearby blue compact and dwarf irregular galaxy youth candidates. This result is independent of the initial abundances of the cloud of gas from which the first stars are formed. It is shown that the chemical abundances and their ratios are extremely sensitive to the initial mass function and that the observations are well represented by a model with a solar neighbourhood initial mass function in a medium which has undergone several short bursts of star formation. The selected sample of blue compact and dwarf irregular galaxies could in fact appear as pristine objects if they lack stars less massive than ∼2-3M_☉ or if they formed out of gas already enriched in heavy elements. It is shown that the different behaviour of (N/O) as a function of (O/H), often interpreted as due to various contributions of primary and secondary nitrogen, in fact could be an effect of stellar mass-loss as a function of metallicity.