Rotating massive stars @ very low Z: high C and N production

Abstract

Two series of models and their yields are presented in this paper. The first series consists of 20 Mo models with varying initial metallicity (solar down to $Z=10^{-8}$) and rotation (Vini=0-600 km/s). The second one consists of models with an initial metallicity of $Z=10^{-8}$, masses between 20 and 85 Mo and average rotation velocities at these metallicities (Vini=600-800 km/s). The most interesting models are the models with $Z=10^{-8}$ ([Fe/H]=~-6.6). In the course of helium burning, carbon and oxygen are mixed into the hydrogen burning shell. This boosts the importance of the shell and causes a reduction of the size of the CO core. Later in the evolution, the hydrogen shell deepens and produces large amount of primary nitrogen. For the most massive models (M>~60 Mo), significant mass loss occurs during the red supergiant stage. This mass loss is due to the surface enrichment in CNO elements via rotational and convective mixing. The yields of the fast rotating 20 Mo models can best reproduce (within our study) the observed abundances at the surface of extremely metal poor (EMP) stars. The wind of the massive models can reproduce the CNO abundances of the carbon--rich UMPs, in particular for the most metal poor star known to date, HE1327-2326.