On The Cosmic Origins Of Carbon And Nitrogen
R.B.C.Henry, M.G.Edmunds, J.Köppen
ABSTRACT We analyze the behavior of N/O and C/O abundance ratios as a function of metallicity 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 approximate yield sets based upon analytical chemical evolution models fitted to the abundance data. We then use these yields to compute numerical chemical evolution models which satisfactorily reproduce the observed abundance trends and thereby identify the most likely production sites for carbon and nitrogen. Our results suggest that carbon and nitrogen originate from separate production sites and are decoupled from one another; massive stars (M .gt. 8 M_sun) dominate the production of carbon, while intermediate-mass stars betwen 4 and 8 M_sun dominate nitrogen production. Carbon production is positively sensitive to metallicity through mass loss processes in massive stars and has a pseudo-secondary character. Nitrogen production in intermediate mass stars is primary at low metallicity, but when 12+log(O/H) .gt. 8.3, secondary nitrogen becomes prominent, and nitrogen increases at a rate faster than oxygen - indedd the dependence is steeper than would be formally expected for a secondary element. Constant N/O values observed in metal-poor galaxies are explained by invoking low star formation rates that allow nitrogen ejection by slower-evolving intermediate-mass stars, which lag behind coeval massive stars by 250 Myr, to raise N/O to observed values as overall metallicity rises. The observed scatter and distribution of data points for N/O challenge the popular idea that observed intermittenr polluting by oxygen is occurring from massive stars following star bursts. Rather, we find most points cluster at relatively low N/O values, indicating that scatter is due to intermittent increases in nitrogen due to local contamination by Wolf-Rayet stars or lumninous blue variables. In addition, the effect of inflow of gas into galactic systems on secondary production of nitrogen from carbon may introduce some scatter into N/O ratios at high metallicities.