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

Abstract

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 ^(54)Cr, ^(58)Fe, and ^(64)Ni in solar system materials, which are commonly attributed to supernovae (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 1.5 M⊙ and 3 M⊙ AGB models, adopting solar and 1/3 solar metallicities. All cases produce excesses of ^(54)Cr, ^(58)Fe, and ^(64)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 ^(50)Ti excesses and some production of ^(46,47,49)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 SN sources. The problem of Ca is discussed, particularly with regard to ^(48)Ca. The measured data are usually represented assuming terrestrial values for ^(42)Ca/^(44)Ca. Materials processed in AGB stars or sources with variable initial ^(42)Ca/^(44)Ca ratios can give apparent ^(48)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.

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