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Massive Stars: Chemical Abundances and Yields in Models with Rotation and Magnetic Fields André Maeder Abstract The abundances of CNO elements are major tests of stellar evolution. We compare the results of models with rotation and magnetic fields to the observations of OB stars, supergiants and WR stars. Rotation produces internal mixing and mass loss enhancements which allows us to account for the observed N/H and He enhancements. Interestingly enough, the effects of mixing are found larger at lower metallicities. The stellar populations (relative numbers of blue to red supergiants, WR stars, etc.) and the chemical yields are also influenced by rotation, in particular the yields in heavy elements are increased. The case of N-production is discussed. The role of the magnetic field, although not negligible, is more limited. |