New models of massive and Wolf-Rayet stars with mass loss and rotation

Abstract

We give results of models of massive stars which have a detailed physical treatment of rotation, including structural equations for shellular rotation, new treatments of shears in differentially rotating stars and of meridional circulation, together with mass loss rates depending on rotation. For a 20 M⊙ star, He- and N-enrichments at the stellar surface already occur during the MS phase for moderately low rotational velocities, thus most supergiants are enriched in helium and nitrogen. A long B- and A-supergiant phase results from rotational mixing, with some primary nitrogen formed at this stage. For the most massive stars, rotation makes the star to enter the WR stage during the MS phase thus avoiding the LBV and red supergiant stage.The WR life-times are considerably increased by rotation and the minimum mass for forming WR stars is lowered. Interestingly enough, the increase of the WN life-time is larger than for WC stars, so that rotation leads to a decrease of the WC/WN number ratio. Also, the fraction of transition WN/WC stars is much larger at higher rotation.Finally, on the basis of clusters in the SMC, in the LMC and towards the galactic interior and exterior, we show that for clusters with ages between about 1 and 3 x 107 yr the fraction of Be stars with respect to normal B stars is larger at lower metallicities. This may suggest a higher rotation at lower metallicities for massive stars, due to a different history of star formation.