The origin of nitrogen: the implications of very metal poor stars

Abstract

Our understanding on the nitrogen origin has recently greatly changed. New data on nitrogen abundances in very metal-poor stars ($-4<[Fe/H]<-3$) show a quite surprising result: a high N/O ratio suggestive of high levels of production of primary nitrogen in massive stars. Moreover, none of the stars measured so far has N/O ratios as low as the ones observed in Damped Lyman-α systems (DLAs) (currently the systems showing the lowest N/O ratios in the universe). We studied the implications of the above new data set for our understanding on the nitrogen enrichment in the Milky Way. We find that, to explain the new observations, we need to adopt stellar yields computed with stellar models in which rotation is taken into account and assume that stars born at Z <10⁻⁵ contribute a lot more N than the most recent calculations available in the literature for Z= 10⁻⁵. The implications of our findings for our understanding of the nature of Damped Lyman-α systems (DLAs) are also briefly discussed.