Evolution towards the critical limit and the origin of Be stars
S. Ekstr"om, G. Meynet, A. Maeder & F. Barblan
Geneva Observatora, 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_sun, at four different metallicities between 0 and 0.020, and with seven different values of the ratio Omega/Omega_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 ge 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.
Reference: Astronomy and Astrophysics
Status: Manuscript has been accepted