Mixing and Rotation

Abstract

The inclusion of rotation in massive star models improves the agreement between theory and observations on at leastthree important points: 1) rotational mixing allows to produce variations of the surface abundances already during the Main-Sequence phase as is observed. The changes of the surface abundances are more important when, for a giveninitial velocity, the initial masses are larger, and/or the metallicities are lower; 2) the observed numberof red supergiants at the metallicity of the Small Magellanic Cloud (SMC) can be accounted for; 3) the observed variation of the number ratioof Wolf-Rayet to O-type stars as a function of the metallicity can be reproduced. For all these comparisonsnon-rotating models give unsatisfactory fits. Rotating models results also give interestinginsights on questions such as the origin of Be stars, the mechanisms responsible forthe huge mass loss rates undergone by the Luminous Blue Variables, the rotation ratesof pulsars, the progenitors of collapsarsand the sources of primary nitrogen at low metallicity.