Letter to the Editor Stellar evolution of low and intermediate-mass stars IV. Hydrodynamically-based overshoot and nucleosynthesis in AGB stars

The focus of this study is on the treatment of those stellar regions immediately adjacent to convective zones. The results of hydrodynamical simulations by Freytag et al. (1996) show that the motion of convective elements extends well beyond the boundary of the convectively unstable region.We have applied their parametrized description of the corresponding velocities to the treatment of overshoot in stellar evolution calculations up to the AGB (Pop. I,MZAMS = 3M ). Our calculations show the 3rd dredge-up already at the 7th thermal pulse (TP), and the dredge-up parameter reaches λ = 0.6 during the next five pulses. Accordingly, the amount of dredged up 12C is up to 10−3M . Our models develope a small so-called 13C-pocket consisting of a few 10−7M . Finally, this treatment of boundaries of convective regions leads to intershell abundances of typically (4He/12C/16O)=(23/50/25) (compared to (70/26/1) in the standard treatment).