Evidence for Radiogenic Sulfur-32 in Type Ab Presolar Silicon Carbide Grains?

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