Models and observations of the s process in AGB stars

Maria Lugaro, Amanda I. Karakas, Sara Bisterzo

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Approximately half of the solar abundances of nuclei heavier than iron are created in the deep layers of asymptotic giant branch (AGB) stars via slow neutron captures (the s process). Freshly made heavy elements, such as Zr, Ba, and Pb, are carried to the stellar surface by recurrent mixing episodes and shed into the interstellar medium via strong stellar winds, thus contributing to the chemical evolution of galaxies. In the past few years several new modelling tools and observational constraints have added to our understanding of how the s process operates in AGB stars of different initial masses and metallicities. For AGB stars of low masses (≥4 M), the 13C(α n)16O reaction is the main neutron source. The exact mixing mechanism leading to the formation of 13C is still unknown and multidimensional hydrodynamic models are needed to address this point. Recent stellar population modelling including s-process nucleosynthesis indicate that the spread in the efficiency of the 13C neutron source is limited to a factor of two of the value obtained when reasonable basic assumptions are applied to the mixing mechanism. This is confirmed by new refined measurements of the isotopic composition of heavy elements in meteoritic silicon carbide grains. On the other hand, observations of the Rb and Zr abundances in massive AGB stars (>4 M) indicate that the main neutron source in these stars is the 22Ne(α n) 25Mg reaction. The increasing number of observations becoming available for the abundances of heavy elements in post-AGB stars and planetary nebulae, the progeny of AGB stars, can also be used to test the ideas above. At low metallicity, the main constraints for the s process come from observations of s-process-enriched carbon-enhanced metal-poor (CEMP) stars. In particular, the origin of CEMP stars enriched in both slow and rapid neutron-capture elements represent a challenge to our understanding of the origin of heavy elements.

Original languageEnglish
JournalProceedings of Science
Publication statusPublished - 1 Dec 2008
EventInternational Symposium on Nuclei in the Cosmos 2008 - Mackinac Island, United States of America
Duration: 27 Jul 20081 Aug 2008
Conference number: 10th

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