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Cosmic-Ray Exposure Ages of Large Presolar SiC Grains

Published online by Cambridge University Press:  05 March 2013

Frank Gyngard
Affiliation:
Laboratory for Space Sciences and the Physics Department, Washington University, Box 1105, One Brookings Drive, St. Louis, MO 63130, USA
Sachiko Amari
Affiliation:
Laboratory for Space Sciences and the Physics Department, Washington University, Box 1105, One Brookings Drive, St. Louis, MO 63130, USA
Ernst Zinner*
Affiliation:
Laboratory for Space Sciences and the Physics Department, Washington University, Box 1105, One Brookings Drive, St. Louis, MO 63130, USA
Ulrich Ott
Affiliation:
Max-Planck-Institut für Chemie, Becherweg 27, D-55128, Mainz, Germany
*
CCorresponding author. Email: [email protected]
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Abstract

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Among presolar SiC grains found in the Murchison carbonaceous meteorites (average size less than 0.5 μm) are very large grains, ranging in size up to 50 μm. We interpret 6Li excesses measured in eight of these grains as being the result of spallation reactions by Galactic cosmic rays during the time the grains spent in the interstellar medium before their incorporation into the meteorite. Derived interstellar exposure ages range from 40 My to 1 Gy, the highest values being consistent with theoretical expectations of interstellar grain lifetimes. Although six grains have almost identical C and Si isotopic compositions, their exposure ages are very different. This observation, combined with low trace element contents, and unusual grain sizes, raises fundamental questions about their stellar sources.

Type
Grains
Copyright
Copyright © Astronomical Society of Australia 2009

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