PopII 1/2 stars: very high 14N and low 16O yields

Abstract

Nine 20 Mo models were computed with metallicities ranging from solar, through $Z=10^{-5}$ ([Fe/H]=~-3.1) down to $Z=10^{-8}$ ([Fe/H]=~-6.1) and with initial rotational velocities between 0 and 600 km/s to study the impact of initial metallicity and rotational velocity. The very large amounts of N14 observed (~0.03 Mo) are only produced at $Z=10^{-8}$ (PopII 1/2). The strong dependence of the N14 yields on rotation and other parameters like the initial mass and metallicity may explain the large scatter in the observations of N14 abundance. The metallicity trends are best reproduced by the models with Omega_ini/Omega_c=~0.75, which is slightly above the mean observed value for OB solar metallicity stars. Indeed, in the model with Vini=600 km/s at $Z=10^{-8}$, the O16 yield is reduced due to strong mixing. This allows in particular to reproduce the upturn for C/O and a slightly decreasing [C/Fe], which are observed below [Fe/H]=~-3.