Stellar evolution with rotation. VIII. Models at Z = 10-5 and CNO yields for early galactic evolution

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 M_sun for the metallicity Z = 10^-5. Star models with initial masses superior or equal to 9 M_sun were computed up to the end of the carbon-burning phase. Star models with masses between 2 and 7 M_sun were evolved beyond the end of the He-burning phase through a few thermal pulses during the AGB phase. Compared to models at Z=0.02, the low Z models show faster rotating cores and stronger internal Omega -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 Z=0.004 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 M_sun with normal rotation velocities or from stars above 8 M_sun but with very fast rotation.