Isotope Anomalies in the Fe-group Elements in Meteorites and Connections to Nucleosynthesis in AGB Stars

We study the effects of neutron captures in AGB stars on “Fe-group” elements, with an emphasis on Cr, Fe, and Ni. These elements show anomalies in Cr, Fe, and Ni in solar-system materials, which are commonly attributed to SNe. However, as large fractions of the interstellar medium (ISM) were reprocessed in AGB stars, these elements were reprocessed, too. We calculate the effects of such reprocessing on Cr, Fe, and Ni through 1.5M⊙ and 3M⊙ AGB models, adopting solar and 1/3 solar metallicities. All cases produce excesses of Cr, Fe, and Ni, while the other isotopes are little altered; hence, the observations may be explained by AGB processing. The results are robust and not dependent on the detailed initial isotopic composition. Consequences for other “Fe group” elements are then explored. They include Ti excesses, and some production of Ti. In many circumstellar condensates, Ti quantitatively reflects these effects of AGB neutron captures. Scatter in the data results from small variations (granularity) in the isotopic composition of the local ISM. For Si, the main effects are instead due to variations in the local ISM from different SNe sources. The problem of Ca is discussed, particularly with regard to Ca. The measured data are usually represented assuming terrestrial values for Ca/Ca. Materials processed in AGB stars or sources with variable initial Ca/Ca ratios can give apparent Ca excesses/deficiencies, attributed to SNe. The broader issue of Galactic Chemical Evolution is also discussed in view of the isotopic granularity in the ISM. Subject headings: stars: AGB and post-AGB Solar system: meteorites: Solar system: presolar grains ISM: abundances Nuclear reactions, nucleosynthesis, abundances