On the Cosmic Origins of Carbon and Nitrogen

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 appropriate 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>8 M ⊙ ) dominate the 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 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)>8.3, secondary nitrogen becomes prominent, and nitrogen increases at a faster rate than oxygen -indeed the dependence is steeper than would be formally expected for a secondary element. The observed flat behavior of N/O versus O/H in metal-poor galaxies is explained by invoking low star formation rates which flatten the age-metallicity relation and allow N/O to rise

The ESO UVES advanced data products quasar sample – III. Evidence of bimodality in the [N/α] distribution

“…10, 11 and 12 and the lower scatter of these ratios are in line with this scenario. If massive stars produce primary N in the systems which lie in the lower plateau, then it is reasonable to expect that these systems should evolve, moving towards the upper plateau by increasing the N-abundance and then the N/O ratios, due to the delayed contribution of the IMS (Henry et al 2000). In this simplified scenario it is possible that DLAs in the transition zone are caught during the secondary N enrichment by IMS.…”

“…At low metallicities primary N production in IMS occurs from ⋆ e-mail:tzafar@eso.org the synthesis of carbon and oxygen freshly produced by the star in the He-burning shell. Thermal pulses occurring during the asymptotic giant branch (AGB) phase make possible the transport of the He-burning products to the H-burning shell producing primary N (Henry et al 2000;Marigo 2001). The signature of primary N production is expected to be an approximately constant N/O abundance with increasing O/H metallicity.…”

Massive star formation in Wolf-Rayet galaxies

“…These authors explained the constant N/O ratio in very low-metallicity objects assuming that the nitrogen is produced only as a primary element in massive, short-life stars. However, other authors have claimed that this may be not completely true (i.e., Henry et al 2000;Pilyugin et al 2003;Mollá et al 2006) because of the lack of a clear mechanism that produces N in massive stars besides the effect or the stellar rotation (Meynet & Maeder 2005). Furthermore, these galaxies already host old stellar populations, and hence low-and intermediate-mass stars should be also releasing N to the ISM.…”

CNO behaviour in planet-harbouring stars

“…If, instead, the carbon and oxygen are produced in the star itself and then used to produce nitrogen, then the nitrogen is called "primary". Several studies have proved that production of nitrogen at low metallicities comes from primary rather than secondary sources (Pagel & Edmunds 1981;Bessell & Norris 1982;Carbon et al 1987;Henry et al 2000;Israelian et al 2004). At higher metallicities, secondary processes dominate.…”