Isotopic Signatures of Supernova Nucleosynthesis in Presolar Silicon Carbide Grains of Type AB with Supersolar 14N/15N Ratios

We report high-resolution C, N, Al, Si, and S isotope data of 38 presolar SiC grains of type AB. Seventeen of these grains are of subtype AB1 (14N/15N < 440 = solar) and 20 of subtype AB2 (14N/15N ≥ 440), previously proposed to be mainly from supernovae (AB1) and J-type carbon stars (AB2), respectively. Our data are compatible with previously obtained isotope data of AB grains, except that 26Al/27Al ratios of AB1 grains span a narrower range. The data are compared with predictions from supernova models that consider H ingestion into the He shell during the pre-supernova phase. In these models a mixture of explosive H and He burning occurs at the bottom of the He shell during passage of the supernova shock, forming the so-called O/nova zone. Mixing matter from the O/nova zone with matter from the overlying He/C zone and the stellar envelope shows that the isotopic compositions and trends of both AB1 and AB2 grains can be matched within the model uncertainties. This demonstrates that supernovae should be considered as potential sources of AB2 grains, in addition to J-type carbon stars and born-again asymptotic giant branch stars, as previously proposed