Chemical Evolution of CNO Abundances

Abstract

New low and intermediate mass star yields recalculated by Buell (1987) are evaluated by using them in a Galactic Chemical Evolution model. We analyze their effects on CNO elemental abundances IntroductionStellar yields for low and intermediate mass (LIM) stars have been recalculated by Buell (1997), who, taking into account the effects of the convective dredge up and the Hot Bottom Burning (HBB), incorporates the most recent improvements in the TP-AGB processes.The aim of this job is to evaluate these new yields for LIM stars by using them as input in a Galactic Chemical Evolution (GCE) model. In particular, we analyze if the contribution of these stars is enough to justify the observed behavior of nitrogen vs. oxygen. 2.Stellar structure for LIM stars during the TP-AGB phaseThe evolution of a LIM star during the TP-AGB phase is dominated mainly by two processes: The third dredge up events and the Hot Bottom Burning (HBB). The first ones affect stars with masses between 2 M ⊙ and 4 M ⊙ , by increasing the abundance of carbon. In stars more massive than 4 M ⊙ , the HBB produces an abrupt increase of nitrogen and a similar decrease of carbon. In Figure 1 we show the ejected masses of 12 C and 14 N as a function of the stellar mass M * , for Z = Z ⊙ , compared with those from RV, where we see that the described behaviour for both elements does not appear since those authors had not taken into account the transformation from carbon in nitrogen during the HBB phase. In fact, nitrogen shows a variation around 4 M ⊙ for RV but it does not increase for larger stellar masses.