Constraints on AGB models from the heavy-element composition of presolar SiC grains

Abstract. Presolar SiC grains formed around Asymptotic Giant Branch (AGB) stars during theircarbon-rich phase and contain heavy elements in trace amounts showing the signature of the slow neutron capture process ( s process). Thanks to recent advances in analysis techniques,SiC data now provide extremely precise information on neutron capture cross sections andAGB models. For example, high-precision data for Mo in single SiC grains indicate thata revision of the 95 Mo neutron capture cross section is needed, while data for Zr indicatesthat the 22 Ne( ,n) 25 Mg reaction cannot be a dominant neutron source for the s process inAGB stars. We present model predictions for the composition of Fe-peak elements in AGBstars. These elements could be analysed in the near future thus providing further stringentconstraints to our understanding of AGB stars. Key words. nuclear reactions, nucleosynthesis, abundances – stars: AGB 1. Introduction Among presolar meteoritic material, siliconcarbide (SiC) grains have isotopic composi-tions very di erent to those commonly mea-sured in the solar system. The majority of thesegrains are believed to have formed in the ex-tended envelopes of carbon-rich AsymptoticGiant Branch (AGB) stars. A strong indica-tion of this origin is the composition of ele-