EVIDENCE FOR RADIOGENIC SULFUR-32 IN TYPE AB PRESOLAR SILICON CARBIDE GRAINS?
- Max Planck Institute for Chemistry, Hahn-Meitner-Weg 1, D-55128 Mainz (Germany)
- Laboratory for Space Sciences and Physics Department, Campus Box 1105, Washington University, St. Louis, MO 63130 (United States)
- Department of Physics, University of Basel, Klingelbergstrasse 82, CH-4056 Basel (Switzerland)
- Department of Physics and Astronomy, University of Victoria, Canada, P.O. Box 3055, Victoria, BC V8W 3P6 (Canada)
We report C, Si, and S isotope measurements on 34 presolar silicon carbide grains of Type AB, characterized by {sup 12}C/{sup 13}C < 10. Nitrogen, Mg-Al-, and Ca-Ti-isotopic compositions were measured on a subset of these grains. Three grains show large {sup 32}S excesses, a signature that has been previously observed for grains from supernovae (SNe). Enrichments in {sup 32}S may be due to contributions from the Si/S zone and the result of S molecule chemistry in still unmixed SN ejecta or due to incorporation of radioactive {sup 32}Si from C-rich explosive He shell ejecta. However, a SN origin remains unlikely for the three AB grains considered here, because of missing evidence for {sup 44}Ti, relatively low {sup 26}Al/{sup 27}Al ratios (a few times 10{sup –3}), and radiogenic {sup 32}S along with low {sup 12}C/{sup 13}C ratios. Instead, we show that born-again asymptotic giant branch (AGB) stars that have undergone a very-late thermal pulse (VLTP), known to have low {sup 12}C/{sup 13}C ratios and enhanced abundances of the light s-process elements, can produce {sup 32}Si, which makes such stars attractive sources for AB grains with {sup 32}S excesses. This lends support to the proposal that at least some AB grains originate from born-again AGB stars, although uncertainties in the born-again AGB star models and possible variations of initial S-isotopic compositions in the parent stars of AB grains make it difficult to draw a definitive conclusion.
- OSTI ID:
- 22224069
- Journal Information:
- Astrophysical Journal Letters, Vol. 776, Issue 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 2041-8205
- Country of Publication:
- United States
- Language:
- English
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