  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.

Solar and Stellar Astrophysics