2013ApJ...776L..29F

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