Sulfur isotopic compositions of submicrometer SiC grains from the Murchison meteorite

Xu, Yuchen; Zinner, Ernst; Gallino, Roberto; Heger, Alexander; Pignatari, Marco; Lin, Yangting

Abstract

We report C, Si, N, S, Mg-Al, and Ca-Ti isotopic compositions of presolar silicon carbide (SiC) grains from the SiC-rich KJE size fraction (0.5-0.8 μm) of the Murchison meteorite. One thousand one hundred thirteen SiC grains were identified based on their C and Si isotopic ratios. Mainstream, AB, C, X, Y, and Z subtypes of SiC, and X-type silicon nitride (Si₃N₄) account for 81.4%, 5.7%, 0.1%, 1.5%, 5.8%, 4.9%, and 0.4%, respectively. Twenty-five grains with unusual Si isotopic ratios, including one C grain, 16 X grains, 1 Y grain, 5 Z grains, and 2 X-type Si₃N₄ grains were selected for N, S, Mg-Al, and Ca-Ti isotopic analysis. The C grain is highly enriched in ²⁹Si and ³⁰Si (δ²⁹Si = 1345‰ ± 19‰, δ³⁰Si = 1272‰ ± 19‰). It has a huge ³²S excess, larger than any seen before, and larger than that predicted for the Si/S supernova (SN) zone, providing evidence against the elemental fractionation model by Hoppe et al. Two SN models investigated here present a more satisfying explanation in terms of a radiogenic origin of ³²S from the decay of short-lived ³²Si (τ1/2 = 153 yr). Silicon-32 as well as ²⁹Si and ³⁰Si can be produced in SNe by short neutron bursts; evidence for initial 44Ti (τ1/2 = 60 yr) in the C grain is additional evidence for an SN origin. The X grains have marginal ³²S excesses, much smaller than expected from their large ²⁸Si excesses. Similarly, the Y and Z grains do not show the S-isotopic anomalies expected from their large Si isotopic anomalies. Low intrinsic S contents and contamination with isotopically normal S are the most likely explanations.

Citation

Xu, Y., Zinner, E., Gallino, R., Heger, A., Pignatari, M., & Lin, Y. (2015). Sulfur isotopic compositions of submicrometer SiC grains from the Murchison meteorite. The Astrophysical journal, 799(2), 156. https://doi.org/10.1088/0004-637x/799/2/156

• Journal Article Type: Article
• Acceptance Date: Nov 19, 2014
• Online Publication Date: Jan 27, 2015
• Publication Date: Feb 1, 2015
• Deposit Date: Jan 4, 2016
• Publicly Available Date: Nov 23, 2017
• Journal: Astrophysical journal
• Print ISSN: 0004-637X
• Publisher: American Astronomical Society
• Peer Reviewed: Peer Reviewed
• Volume: 799
• Issue: 2
• Pages: 156
• DOI: https://doi.org/10.1088/0004-637x/799/2/156
• Keywords: Astrochemistry, Circumstellar matter, Nuclear reactions, nucleosynthesis, abundances, Supernovae: general, Stars: AGB and post-AGB
• Public URL: https://hull-repository.worktribe.com/output/383703
• Publisher URL: http://iopscience.iop.org/article/10.1088/0004-637X/799/2/156/meta;jsessionid=CC1F5BF90428A89C0F85784F8540446E.c2.iopscience.cld.iop.org
• Additional Information: Copy of article published in: Astrophysical journal, 2015, v.799, issue 2
• Contract Date: Nov 23, 2017