Radiative C-13 burning in asymptotic giant branch stars and s-processing

Abstract

We present new evolutionary calculations for a 3 solar mass of Population I starting from zero-age main sequence and followed up to the 25th 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 show that any amount of C-13 possibly synthesized at the H/He interface during the interpulse period is completely burnt through (alpha, n) reactions before the next pulse develops. As a consequence, s-elements are produced in a radiative environment, at a neutron density of at most a few 107 n/cc. There are then engulfed and diluted into the next convective pulse, where a second (minor) neutron burst occurs, driven by the (22) Ne (alpha, n) (25) Mg reaction marginally activated near the end of the He-shell instability. Despite the differences of the present model with respect to past calculations, we predict s-process distributions very close to those computed by allowing C-13 to burn in convective pulses: C-13 burning and the ensuing s-processing are confirmed to account for the main observational and experimental constraints. The reasons for this are briefly discussed. Since no C-13 survives the interpulse phase, we conclude that no modifications of the pulse shape due to the energy feedback from the (13) C (alpha, n) (16) O reaction occur.