Silicon Carbide Grains of Type X and Supernova Nucleosynthesis

Silicon carbide grains of type X constitute ~1% of all presolar SiC grains found in primitive meteorites. These grains have N and Si excesses, mostly light C, high inferred Al/Al ratios, and evidence (from Ca and Ti excesses) for the initial presence of the short-lived radioisotopes Ti and V. These isotopic signatures clearly indicate an origin in the ejecta of Type II supernovae (SNe) and material from different SN zones had to contribute to the mix from which the grains formed: He/N (Al), He/C (C and N), and the Si/S zone (Si, Ti, V). However, in detail there are many discrepancies between grain data and theoretical predictions from SN models. One major discrepancy concerns the distribution of the Si isotopic ratios. In a Si 3-isotope plot, most grains plot along a line, which points to a primary isotopic composition with Si/Si ! 1/3"solar and almost no Si. However, such a composition is not produced by any of the SN models. Another set of discrepancies involves the correlation between C, N and Al isotopic ratios: C/C and N/N ratios are high in the He/C and low in the He/N zone, whereas the Al/Al ratios show the opposite behavior. As a consequence, mixing between these zones results in negative correlations between these ratios, in contrast to the grain data. Another example is the lack of large Fe excesses in X grains. While mixing with material from the Si/S zone, rich in Si, is necessary to explain the Si excesses in these grains, the Fe, also abundant in this zone, must have been separated from the Si before grain condensation by a process still not understood.