New Constraints for Supernova Models from Presolar Silicon Carbide X Grains with Very High ²⁶Al/²⁷Al Ratios

We report C, N, Mg-Al, Si, and S isotope data of six 1-3 μm-sized SiC grains of Type X from the Murchison CM2 chondrite, believed to have formed in the ejecta of core-collapse supernova (CCSN) explosions. Their C, N, and Si isotopic compositions are fully compatible with previously studied X grains. Magnesium is essentially monoisotopic ²⁶Mg which gives clear evidence for the decay of radioactive ²⁶Al. Inferred initial ²⁶Al/²⁷Al ratios are between 0.6 and 0.78 which is at the upper end of previously observed ratios of X grains. Contamination with terrestrial or solar system Al apparently is low or absent, which makes the X grains from this study particularly interesting and useful for a quantitative comparison of Al isotope data with predictions from supernova models. The consistently high ²⁶Al/²⁷Al ratios observed here may suggest that the lower ²⁶Al/²⁷Al ratios of many X grains from the literature are the result of significant Al contamination and in part also of an improper quantification of ²⁶Al. The real dispersion of ²⁶Al/²⁷Al ratios in X grains needs to be explored by future studies. The high observed ²⁶Al/²⁷Al ratios in this work provide a crucial constraint for the production of ²⁶Al in CCSN models. We explored different CCSN models, including both “classical” and H ingestion CCSN models. It is found that the classical models cannot account for the high ²⁶Al/²⁷Al ratios observed here; in contrast, H ingestion models are able to reproduce the ²⁶Al/²⁷Al ratios along with C, N, and Si isotopic ratios reasonably well.