STELLAR EVOLUTION OF LOW AND INTERMEDIATE-MASS STARS .4. HYDRODYNAMICALLY-BASED OVERSHOOT AND NUCLEOSYNTHESIS IN AGB STARS

The focus of this study is on the treatment of those stellar regions i mmediately adjacent to convective zones. The results of hydrodynamical simulations by Freytag et al. (1996) show that the motion of convecti ve elements extends well beyond the boundary of the convectively unsta ble region. We have applied their parametrized description of the corr esponding velocities to the treatment of overshoot in stellar evolutio n calculations up to the AGE (Pop. I, M-ZAMS = 3M.). Our calculations show the 3(rd) dredge-up already at the 7(th) thermal pulse (TP), and the dredge-up parameter reaches lambda = 0.6 during the next five puls es. Accordingly, the amount of dredged up C-12 is up to 10(-3) M.. Our models develope a small so-called C-13-pocket consisting of a few 10( -7) M.. Finally, this treatment of boundaries of convective regions le ads to intershell abundances of typically (He-4/C-12/O-16)=(23/50/25) (compared to (70/26/1) in the standard treatment).