Lead: Asymptotic giant branch production and galactic chemical evolution

The enrichment of Pb in the Galaxy is followed in the framework of a detail ed model of Galactic chemical evolution that already proved adequate to rep roduce the chemical enrichment of O and of the elements from Ba to Eu. The stellar yields are computed through nucleosynthesis calculations in the asy mptotic giant branch (AGB) phase of low- and intermediate-mass stars coveri ng a wide range of metallicities. The physical parameters of the stellar st ructure were derived from full stellar evolutionary models computed previou sly. We show that low-mass AGB stars are the main producers of Pb in the Ga laxy, with a complex dependence on metallicity and a maximum efficiency at [Fe/H] similar to -1. Our calculations succeed in reproducing the abundance s 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 Pb-208, is actually played by the high production of Pb in low-mass and low-metallicity AGB stars. We then follow the Galactic chem ical evolution of Pb isotopes and give our expectations on the s-process co ntribution to each of them at the epoch of the solar system formation. Fina lly, we present new spectroscopic estimates of Pb abundance on a sample of field stars and compare them, together with a few other determinations avai lable, with the predicted trend of [Pb/Fe] in the Galaxy.