RADIATIVE C-13 BURNING IN ASYMPTOTIC GIANT BRANCH STARS AND S-PROCESSING

We present new evolutionary calculations for a 3 M. star of Population I starting from the zero-age main sequence and followed up to the 25t h thermal pulse of the asymptotic giant branch. The third dredge-up is found to occur in a self-consistent way from the 14th pulse on. We sh ow that any amount of C-13 possibly synthesized at the H/He interface during the interpulse period is completely burnt through (alpha, n) re actions before the next pulse develops. As a consequence, s-elements a re produced in a radiative environment, at a neutron density of at mos t a few 10(7) n cm(-3). They are then engulfed and diluted into the ne xt convective pulse, where a second (minor) neutron burst occurs, driv en by the Ne-22(alpha, n)Mg-25 reaction marginally activated near the end of the He-shell instability. Despite the differences of the presen t model with respect to past calculations, we predict s-process distri butions very close to those computed by allowing C-13 to burn in conve ctive pulses: C-13 burning and the ensuing s-processing are confirmed to account for the main observational and experimental constraints. Th e reasons for this are briefly discussed. Since no C-13 survives the i nterpulse phase, we conclude that no modifications of the pulse shape due to the energy feedback from the C-13(alpha, n)O-16 reaction occur.