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

Nine 20M⊙ models were computed with metallicities ranging from solar, through Z = 10 ([Fe/H]∼-3.1) down to Z = 10 ([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 Hirschi (2006). The very large amounts of N observed (∼0.03M⊙) are only produced at Z = 10 (PopII 1/2). The strong dependence of the N yields on rotation and other parameters like the initial mass and metallicity may explain the large scatter in the observations of N abundance. The metallicity trends are best reproduced by the models with υini/υc ∼ 0.75, which is slightly above the mean observed value for OB solar metallicity stars. Indeed, in the model with υini=600 kms −1 at Z = 10, the O 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.