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Abstract

We report C, Si, and S isotope measurements on 34 presolar silicon carbide grains of Type AB, characterized by C-12/C-13 < 10. Nitrogen, Mg-Al-, and Ca-Ti-isotopic compositions were measured on a subset of these grains. Three grains show large S-32 excesses, a signature that has been previously observed for grains from supernovae (SNe). Enrichments in S-32 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 Si-32 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 Ti-44, relatively low Al-26/Al-27 ratios (a few times 10(-3)), and radiogenic S-32 along with low C-12/C-13 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 C-12/C-13 ratios and enhanced abundances of the light s-process elements, can produce Si-32, which makes such stars attractive sources for AB grains with S-32 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.