On the Cosmic Origins of Carbon and Nitrogen

The behaviour of C/O and N/O ratios as a function of metallicity in HII regions in galaxies is characterised, and used to derive rough values of chemical yields from analytic models. These `analytic yields' are used to select the best available numerical yields from published stellar nucleosynthesis calculations. This gives a reasonably coherent picture of the important production sites of carbon and nitrogen, with carbon coming from massive stars and nitrogen from intermediate mass stars. The effects of gas inflow and stellar nucleosynthesis time-delay on element ratios C/O and N/O are expected to be small, but may be detectable for N/O. The small dispersion in values of N/O observed in low-metallicity blue compact dwarf galaxies may indicate that these systems have fairly long-lived quiescent star formation. Brief consideration of the CNO cycles suggests that the most recent values for the 17O(p,α)14N cross-section are certainly better than older values.