Very low-metallicity massive stars: Pre-SN evolution models and primary nitrogen production

Context. Precise measurements of the surface abundances of extremely low-metallicity stars have recently been obtained, and they provide new constraints for the stellar evolution models. Aims. Stellar evolution models were computed in order to explain the surface abundances observed, in particular of nitrogen. Methods. Two series of models were computed. The first series consists of 20 M-circle dot models with varying initial metallicity (Z = 0.02 down to Z = 10(-8)) and rotation (v(ini) = 0-600 km s(-1)). The second one consists of models with an initial metallicity of Z = 10-8, masses between 9 and 85 M-circle dot, and fast initial rotation velocities (v(ini) = 600-800 km s-1). Results. The most interesting models are those with Z = 10-8 ([Fe/H] similar to -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 CO core mass. Later in the evolution, the hydrogen shell deepens and produces a large amount of primary nitrogen. For the most massive models (M greater than or similar to 60 M-circle dot), 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 85 M-circle dot model ends up as a WO-type Wolf-Rayet star. Therefore the models predict SNe of type Ic and possibly long and soft GRBs at very low metallicities. The rotating 20 M-circle dot models can best reproduce the observed CNO abundances at the surface of extremely metal-poor (EMP) stars and the metallicity trends when their angular momentum content is the same as at solar metallicity (and therefore have an increasing surface velocity with decreasing metallicity). The wind of the massive-star models can reproduce the CNO abundances of the most metal-poor carbon-rich star known to date, HE1327-2326.