On the cosmic origins of carbon and nitrogen

We analyze the behavior of N/O and C/O abundance ratios as a function of me tallicity as gauged by O/H in large, extant Galactic and extragalactic H II region abundance samples. We compile and compare published yields of C, N, and O for intermediate mass and massive stars and choose appropriate yield sets based on analytical chemical evolution models fitted to the abundance data. We then use these yields to compute numerical chemical evolution mod els that satisfactorily reproduce the observed abundance trends and thereby identify the most likely production sites for carbon and nitrogen. Our res ults suggest that carbon and nitrogen originate from separate production si tes and are decoupled from one another. Massive stars (M > 8 M.) dominate t he production of carbon, while intermediate-mass stars between 4 and 8 M., with a characteristic lag time of roughly 250 Myr following their formation , dominate nitrogen production. Carbon production is positively sensitive t o metallicity through mass-loss processes in massive stars and has a pseudo -secondary character. Nitrogen production in intermediate mass stars is pri mary at low metallicity, but when 12 + log (O/H) > 8.3, secondary nitrogen becomes prominent, and nitrogen increases at a faster rate than oxygen-inde ed, the dependence is steeper than would be formally expected for a seconda ry element. The observed fiat behavior of N/O versus O/H in metal-poor gala xies is explained by invoking low star formation rates that flatten the age -metallicity relation and allow N/O to rise to observed levels at low metal licities. The observed scatter and distribution of data points for N/O chal lenge the popular idea that observed intermittent polluting by oxygen is oc curring from massive stars following star bursts. Rather. we find most poin ts cluster at relatively low N/O values, indicating that scatter is caused by intermittent increases in nitrogen caused by local contamination by Wolf -Rayet stars or luminous blue variables. In addition, the effect of inflow of gas into galactic systems on secondary production of nitrogen from carbo n may introduce some scatter into N/O ratios at high metallicities.