Stellar evolution with rotation: VIII. Models at Z= 10$^\mathsf{-5}$ and CNO yields for early galactic evolution

Abstract

We calculate a grid of star models with and without the effects of axial rotation for stars in the mass range between 2 and 60 for the metallicity . Star models with initial masses superior or equal to 9 were computed up to the end of the carbon–burning phase. Star models with masses between 2 and 7 were evolved beyond the end of the He–burning phase through a few thermal pulses during the AGB phase. Compared to models at , the low Z models show faster rotating cores and stronger internal Ω–gradients, which favour an important mixing of the chemical elements. The enhancement of N/C at the surface may reach 2 to 3 orders of magnitude for fast rotating stars. Surface enrichments may make the evolved stars less metal poor than they were initially. In very low Z models, primary nitrogen is produced during the He–burning phase by rotational diffusion of 12C into the H–burning shell. A large fraction of the primary 14N escapes further destruction and enters the envelope of AGB stars, being ejected during the TP–AGB phase and the formation of a planetary nebula. The intermediate mass stars of very low Z are the main producers of primary 14N, but massive stars also contribute to this production; no significant primary nitrogen is made in models at metallicity or above. We calculate the chemical yields in He, C, N, O and heavy elements and discuss the chemical evolution of the CNO elements at very low Z. Remarkably, the C/O vs. O/H diagram is mainly sensitive to the interval of stellar masses, while the N/O vs. O/H diagram is mainly sensitive to the average rotation of the stars contributing to the element synthesis. The presently available observations in these diagrams seem to favour contributions either from stars down to about 2 with normal rotation velocities or from stars above 8 but with very fast rotation.