  Rotational mixing in massive stars is a widely applied concept, with far
reaching consequences for stellar evolution. Nitrogen surface abundances for a
large and homogeneous sample of massive B-type stars in the LMC were obtained
by the VLT-FLAMES Survey of Massive Stars. This sample is the first covering a
broad range of projected stellar rotational velocities, with a large enough
sample of high quality data to allow for a statistically significant analysis.
We use the sample to provide the first rigorous test of the theory of
rotational mixing in massive stars. We calculated a grid of stellar evolution
models, using the FLAMES sample to calibrate some of the uncertain mixing
processes. We developed a new population-synthesis code, which uses this grid
to simulate a large population of stars with masses, ages and rotational
velocity distributions consistent with those from the FLAMES sample. The
synthesized population is then filtered by the selection effects in the
observed sample, to enable a direct comparison between the empirical results
and theoretical predictions. Our simulations reproduce the fraction of stars
without significant nitrogen enrichment. The predicted number of rapid rotators
with enhanced nitrogen is about twice as large as found observationally.
Furthermore, a group of stars consisting of slowly rotating, nitrogen-enriched
objects and another consisting of rapidly rotating un-enriched objects can not
be reproduced by our single-star population synthesis. Additional physical
processes appear to be required to understand the population of massive
main-sequence stars from the FLAMES sample.We discuss the possible role of
binary stars and magnetic fields in the interpretation of our results. We find
that the population of slowly rotating nitrogen-enriched stars is unlikely
produced via mass transfer and subsequent tidal spin-down in close binary
systems

Solar and Stellar Astrophysics