The Single Star Path to Be Stars

Be stars are rapidly rotating B main sequence stars, which show line emission due to an outflowing disc. By studying the evolution of rotating single star models, we can assess their contribution to the observed Be star populations. We identify the main effects which are responsible for single stars to approach critical rotation as functions of initial mass and metallicity, and predict the properties of populations of rotating single stars. We perform population synthesis with single star models of initial masses ranging between 3 and 30 solar masses, initial equatorial rotation velocities between 0 and 600 kms$^{-1}$ at compositions representing the Milky Way, Large and Small Magellanic Clouds. These models include efficient core-envelope coupling mediated by internal magnetic fields and correspond to the maximum efficiency of Be star production. We predict Be star fractions and the positions of fast rotating stars in the colour-magnitude diagram. We identify stellar wind mass-loss and the convective core mass fraction as the key parameters which determine the time dependance of the stellar rotation rates. Using empirical distributions of initial rotational velocities,our single star models can reproduce the trends observed in Be star fractions with mass and metallicity. However,they fail to produce a significant number of stars rotating very close to critical. We also find that rapidly rotating Be stars in the Magellanic Clouds should have significant surface nitrogen enrichments, which may be in conflict with abundance determinations of Be stars. Single star evolution may explain the high number of Be stars if 70 to 80% of critical rotationwould be sufficient to produce the Be phenomenon. However even in this case, the unexplained presence of many Be stars far below the cluster turn-off indicates the importance of the binary channel for Be star production.