On the cosmic origins of carbon and nitrogen

The behaviour of C/O and N/O ratios as a function of metallicity in HII reg ions in galaxies is characterised, and used to derive rough values of chemi cal 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 pr oduction sites of carbon and nitrogen, with carbon coming from massive star s 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 expect ed to be small, but may be detectable for N/O. The small dispersion in valu es of N/O observed in low-metallicity blue compact dwarf galaxies may indic ate that these systems have fairly long-lived quiescent star formation. Bri ef consideration of the CNO cycles suggests that the most recent values for the O-17(p,alpha)N-14 cross-section are certainly better than older values .