EDP Sciences Journals List
Advanced Search
Subscriber Authentication Point
Home All issues Volume 390/2 Article
Free access article

Issue A&A
Volume 390, Number 2, August I 2002
Page(s) 561 - 583
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20020755



A&A 390, 561-583 (2002)
DOI: 10.1051/0004-6361:20020755

Stellar evolution with rotation

VIII. Models at Z = 10 $^\mathsf{-5}$ and CNO yields for early galactic evolution
G. Meynet and A. Maeder

Geneva Observatory, 1290 Sauverny, Switzerland
    e-mail: Andre.Maeder@obs.unige.ch

(Received 21 December 2001 / Accepted 14 May 2002 )

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  $M_{\odot}$ for the metallicity Z = 10-5. Star models with initial masses superior or equal to 9 $M_{\odot}$ were computed up to the end of the carbon-burning phase. Star models with masses between 2 and 7 $M_{\odot}$ 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  $M_{\odot}$ with normal rotation velocities or from stars above 8  $M_{\odot}$ but with very fast rotation.


Key words: nuclear reactions, nucleosynthesis, abundances -- stars: interiors -- stars: evolution -- stars: rotation -- stars: early-types -- stars: AGB and post-AGB

Offprint request: G. Meynet, Georges.Meynet@obs.unige.ch

SIMBAD Objects



© ESO 2002


What is OpenURL?

The OpenURL standard is a protocol for transmission of metadata describing the resource that you wish to access. An OpenURL link contains article metadata and directs it to the OpenURL server of your choice. The OpenURL server can provide access to the resource and also offer complementary services (specific search engine, export of references...). The OpenURL link can be generated by different means.
  • If your librarian has set up your subscription with an OpenURL resolver, OpenURL links appear automatically on the abstract pages.
  • You can define your own OpenURL resolver with your EDPS Account. In this case your choice will be given priority over that of your library.
  • You can use an add-on for your browser (Firefox or I.E.) to display OpenURL links on a page (see http://www.openly.com/openurlref/). You should disable this module if you wish to use the OpenURL server that you or your library have defined.