THE IMPACT OF SEMI-CONVECTION AND OVERSHOOTING ON THE SURFACE ABUNDANCES IN MASSIVE STARS

We analyse the effects of semi-convection and overshooting on the pred icted surface abundances after the first and second dredge-ups in 15 a nd 20 M. Pop. I stars. Overshooting is applied either to the core boun dary or to the boundaries of all convective zones. It is shown that th e surface abundances are sensitive to the mixing scheme adopted in the interior. The models including semi-convection lead to lower C-12/C-1 3 ratios than the other mixing schemes, while models with overshooting predict higher enhancements of sodium at the surface.