Physics – Condensed Matter – Materials Science
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p43a1647s&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P43A-1647
Physics
Condensed Matter
Materials Science
[6213] Planetary Sciences: Solar System Objects / Dust, [6240] Planetary Sciences: Solar System Objects / Meteorites And Tektites
Scientific paper
Most presolar SiC stardust grains derive from Asymptotic Giant Branch (AGB) stars, which are the main source of s-process nuclides in the galaxy. s-Process nucleosynthesis is not a major contributor to iron peak nuclides. The most interesting of these isotopes from an AGB nucleosynthetic standpoint are the trace isotopes 54Cr and 58Fe, which are predicted to have deviations from average Solar System ratios of ~100% and >200%, respectively. The other isotopes of Cr and Fe are essentially unchanged by AGB stars, and are representative of the isotopic composition of the protostellar cloud from which the SiC grains formed. They are thus tracers of Galactic Chemical Evolution, and provide a benchmark for the isotopic composition of the galaxy some five to seven billion years ago. SiC grains were isolated from the Murchison meteorite using high purity reagents and techniques specifically designed to prevent contamination with terrestrial Cr and Fe. Grains from the 2-4 μm size fraction were pressed into a high purity gold foil. Chromium and iron isotopic compositions were measured by Resonance Ionization Mass Spectrometry (RIMS) using new techniques specifically developed for high precision isotopic analysis of iron-peak elements. The Cr isotopic compositions of 19 grains form a distinct group. Several grains had resolvable (>2σ) 50Cr and 53Cr deficits ranging as low as -178%. The 54Cr/ 52Cr δ-values were all within 2σ of the Solar System value, though most were slightly higher. The average δ-values for the grains were -45±31% for 50Cr, -37±21% for 53Cr, and +25±26% for 54Cr. Iron results are pending. Given that AGB stars change most Fe and Cr isotope ratios very little, and that these grains' progenitor stars formed from a few hundred million to about three billion years before the Solar System formed (assuming their initial masses were 1.5 - 3 solar masses), and that grain interstellar residence times are likely less than ~100 - 200 million years, the Fe and Cr isotopes in mainstream grains largely retain the composition of the interstellar medium from about 5 to 7.5 billion years ago. Comparing our data to predictions for Cr synthesis in Type II and Type Ia supernovae (the main producers of Cr) shows deviations from simple mixing lines (assuming appropriate metallicities and initial mass functions). The mixing model predicts greater deficits in 50Cr and/or 53Cr than is seen in the grains. In the case of 54Cr, a 100% enhancement due to AGB nucleosynthesis nearly accounts for the deviation from the mixing line. Our work is supported by the NASA Cosmochemistry program, through grants to Argonne National Laboratory and the University of Chicago, and by the US Dept. of Energy, BES-Materials Sciences, under contract DEAC02-06CH11357.
Dauphas Nicolas
Davis Aileen M.
Levine Jerome
Pellin Michael
Savina M.
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