SEMI-CONVECTION AND OVERSHOOTING IN INTERMEDIATE-MASS AND MASSIVE STARS

Two mixing schemes, overshooting and semi-convection, are studied in the framework of evolutionary models for Pop. I intermediate-mass and massive stars up to 20 M.. Overshooting is applied either to the core boundary, or to the boundaries of all convective zones. A generalized computational algorithm is proposed for semi-convection. The evolutionary models are computed from the quasi-hydrostatic Pre-Main-Sequence phase to the end of core helium burning. They are compared to models adopting the standard mixing scheme with no mixing beyond the Schwarzschild's limit. Overshooting, when applied to the boundaries of all convective zones, does not modifiy the evolutionary features of the stars. We find that semi-convection does not develop in intermediate-mass stars. In contrast, semi-convection and core overshooting completely modify the structural evolution of massive stars after central hydrogen exhaustion. The tracks followed in the H-R diagram and the times spent in different phases of evolution disfavour the models with core overshooting and favours the ones including semi-convection.