Nova nucleosynthesis and Galactic evolution of the CNO isotopes

Abstract

In this paper we study the role played both by novae and single stars in enriching the interstellar medium of the Galaxy with CNO group nuclei, in the framework of a detailed successful model for the chemical evolution of both the Galactic halo and disc. First, we consider only the nucleosynthesis from single low-mass, intermediate-mass and massive stars. In particular, the nucleosynthesis prescriptions in the framework of the adopted model are such that (i) low- and intermediate-mass stars are responsible for the production of most of the Galactic 12C and 14N; (ii) massive stars produce the bulk of the Galactic 16O; (iii) 13C and 17O originate mostly in intermediate-mass stars, with only a minor contribution from low-mass and massive stars. In this context, we show that the behaviour of the 12C/13C, 14N/15N and 16O/17O isotopic ratios, as inferred from observations, can be explained only allowing for a substantial revision of the available stellar yields. On the other hand, the introduction of nova nucleosynthesis allows us to better explain the temporal evolution of the CNO isotopic ratios in the solar neighbourhood as well as their trends across the Galactic disc. Once all the nucleosynthesis sources of CNO elements are taken into account, we conclude that 13C, 15N and 17O are likely to have both a primary and a secondary origin, in contrast to previous beliefs. We show that, when adopting the most recent 17O yields from intermediate-mass stars published in the literature so far, we still get a too large solar abundance for this element, a problem already encountered in the past by other authors using different yield sets. Therefore, we conclude that in computing the 17O yields from intermediate-mass stars some considerable sink of 17O is probably neglected. The situation for 15N is less clear than that for 13C and 17O, mainly due to contradictory observational findings. However, a stellar factory restoring 15N on quite long time-scales seems to be needed in order to reproduce the observed positive gradient of 14N/15N across the disc, and novae are, at present, the best candidates for this factory. Given the uncertainties still present in the computation of theoretical stellar yields, our results can be used to put constraints on stellar evolution and nucleosynthesis models.