Sulfur isotopic compositions of submicrometer SiC grains from the Murchison meteorite

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

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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 (τ 1/2 = 153 yr). Silicon-32 as well as 29Si and 30Si can be produced in SNe by short neutron bursts; evidence for initial 44Ti (τ 1/2 = 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
Original languageEnglish
Pages (from-to)1-23
Number of pages23
JournalThe Astrophysical Journal
Volume799
Issue number2
DOIs
Publication statusPublished - 2015

Keywords

  • astrochemistry
  • circumstellar matter
  • nuclear reactions, nucleosynthesis, abundances
  • supernovae: general
  • stars: AGB and post-AGB

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