Lead: Asymptotic Giant Branch production and Galactic Chemical Evolution

The enrichment of Pb in the Galaxy is followed in the framework of a detailed model of Galactic chemical evolution that already proved adequate to reproduce the chemical enrichment of O and of the elements from Ba to Eu. The stellar yields are computed through nucleosynthesis calculations in the Asymptotic Giant Branch (AGB) phase of low- and intermediate-mass stars, covering a wide range of metallicities. The physical parameters of the stellar structure were derived from full stellar evolutionary models previously computed. We show that low-mass AGB stars are the main producers of Pb in the Galaxy, with a complex dependence on metallicity and a maximum efficiency at [Fe/H] ~ -1. Our calculations succeed in reproducing the abundances of Pb isotopes in the solar system: the role attributed by the classical analysis of the s-process to the 'strong component', in order to explain more than 50% of solar 208Pb, is actually played by the high production of Pb in low-mass and low-metallicity AGB stars. We then follow the Galactic chemical evolution of Pb isotopes and give our expectations on the s-process contribution to each of them at the epoch of the solar system formation. Finally, we present new spectroscopic estimates of Pb abundance on a sample of field stars and compare them, together with a few other determinations available, with the predicted trend of [Pb/Fe] in the Galaxy.