Geneva Observatory, University of Geneva, Switzerland

More and more evidence lead to consider classical Be stars as stars rotating close to the critical velocity. If so, then the question which naturally arises is the origin of this high surface velocity. We determine which are the mechanisms accelerating the surface of single stars during the Main Sequence evolution. We study their dependence on the metallicity and derive the frequency of stars with different surface velocities in clusters of various ages and metallicities. We have computed 112 stellar models of four different initial masses between 3 and 60 M, at four different metallicities between 0 and 0.020, and with seven different values of the ratio Ω/Ω_crit between 0.1 and 0.99. For all the models, computations were performed until either the end of the Main Sequence evolution or the reaching of the critical limit. The evolution of surface velocities during the Main Sequence lifetime results from an interplay between meridional circulation (bringing angular momentum to the surface) and mass loss by stellar winds (removing it). The dependence on metallicity of these two mechanisms plays a key role in determining for each metallicity, a limiting range of initial masses (spectral types) for stars able to reach or at least approach the critical limit. Present models predict a higher frequency of fast rotating stars in clusters with ages between 10 and 25 Myr. This is the range of ages where most of Be stars are observed. To reproduce the observed frequencies of Be stars, it is necessary to assume first that the Be star phenomenon occurs already for stars with v/v_crit  0.7 and second, that the fraction of fast rotators on the Zero Age Main Sequence is higher at lower metallicities. Depending on the stage at which the star becomes a Be star, the star at this stage may present more or less important enrichments in nitrogen at the surface.

Accepted by A&A
Preprints from sylvia.ekstrom@obs.unige.ch
or on the web at http://arxiv.org/abs/0711.1735