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 Su
rvey 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
We present a dense grid of evolutionary tracks and isochrones of rotating massive main-sequence stars. We provide three grids with different initial compositions tailored to compare with early OB stars in the Small and Large Magellanic Clouds and in
the Galaxy. Each grid covers masses ranging from 5 to 60 Msun and initial rotation rates between 0 and about 600 km/s. To calibrate our models we used the results of the VLT-FLAMES Survey of Massive Stars. We determine the amount of convective overshooting by using the observed drop in rotation rates for stars with surface gravities log g < 3.2 to determine the width of the main sequence. We calibrate the efficiency of rotationally induced mixing using the nitrogen abundance determinations for B stars in the Large Magellanic cloud. We describe and provide evolutionary tracks and the evolution of the central and surface abundances. In particular, we discuss the occurrence of quasi-chemically homogeneous evolution, i.e. the severe effects of efficient mixing of the stellar interior found for the most massive fast rotators. We provide a detailed set of isochrones for rotating stars. Rotation as an initial parameter leads to a degeneracy between the age and the mass of massive main sequence stars if determined from its observed location in the Hertzsprung-Russell diagram. We show that the consideration of surface abundances can resolve this degeneracy.
