The early star generations: the dominant effect of rotation on the CNO yields

Georges Meynet, Sylvia Ekstrom, Andre Maeder

Geneva Observatory

We examine the role of rotation on the evolution and chemical yields of very metal--poor stars. The models include the same physics, which was applied successfully at the solar $Z$ and for the SMC, in particular, shear diffusion, meridional circulation, horizontal turbulence, and rotationally enhanced mass loss. Models of very low $Z$ experience a much stronger internal mixing in all phases than at solar $Z$. Also, rotating models at very low $Z$, contrary to the usual considerations, show a large mass loss, which mainly results from the efficient mixing of the products of the 3$\alpha$ reaction into the H--burning shell. This mixing allows convective dredge--up to enrich the stellar surface in heavy elements during the red supergiant phase, which in turn favours a large loss of mass by stellar winds, especially as rotation also increases the duration of this phase. On the whole, the low $Z$ stars may lose about half of their mass. Massive stars initially rotating at half of their critical velocity are likely to avoid the pair--instability supernova. The chemical composition of the rotationally enhanced winds of very low $Z$ stars show large CNO enhancements by factors of $10^3$ to $10^7$, together with large excesses of $^{13}$C and $^{17}$O and moderate amounts of Na and Al. The excesses of primary N are particularly striking. When these ejecta from the rotationally enhanced winds are diluted with the supernova ejecta from the corresponding CO cores, we find [C/Fe], [N/Fe],[O/Fe abundance ratios that are very similar to those observed in the C--rich, extremely metal--poor stars (CEMP). We show that rotating AGB stars and rotating massive stars have about the same effects on the CNO enhancements. Abundances of s-process elements and the ${12}$C/$^{13}$C ratio could help us to distinguish between contributions from AGB and massive stars.

Reference: Astronomy and Astrophysics
Status: Manuscript has been accepted

Weblink: astro-ph/0510560