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ABSTRACT We report isotope data for C, N, Al, Si, and S of 33 presolar SiC and Si3N4 grains (0.3–1.6 $$\mu$$m) of Type X, C, D, and N from the Murchison CM2 meteorite of likely core-collapse supernova (CCSN) origin which we discuss together with data of six SiC X grains from an earlier study. The isotope data are discussed in the context of hydrogen ingestion supernova (SN) models. We have modified previously used ad-hoc mixing schemes in that we considered (i) heterogeneous H ingestion into the He shell of the pre-SN star, (ii) a variable C-N fractionation for the condensation of SiC grains in the SN ejecta, and (iii) smaller mass units for better fine-tuning. With our modified ad-hoc mixing approach over small scales (0.2–0.4 M⊙), with major contributions from the O-rich O/nova zone, we find remarkably good fits (within a few per cent) for 12C/13C, 26Al/27Al, and 29Si/28Si ratios. The 14N/15N ratio of SiC grains can be well matched if variable C-N fractionation is considered. However, the Si3N4 isotope data point to overproduction of 15N in hydrogen ingestion CCSN models and lower C-N fractionation during SiC condensation than applied here. Our ad-hoc mixing approach based on current CCSN models suggests that the O-rich O/nova zone, which uniquely combines explosive H- and He-burning signatures, is favourable for SiC and Si3N4 formation. The effective range of C/O abundance variations in the He shell triggered by H ingestion events in the massive star progenitor is currently not well constrained and needs further investigation. 

Abstract We report C, N, Mg-Al, Si, and S isotope data of six 1–3μm-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 monoisotopic²⁶Mg which gives clear evidence for the decay of radioactive²⁶Al. Inferred initial²⁶Al/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 high²⁶Al/²⁷Al ratios observed here may suggest that the lower²⁶Al/²⁷Al ratios of many X grains from the literature are the result of significant Al contamination and in part also of an improper quantification of²⁶Al. The real dispersion of²⁶Al/²⁷Al ratios in X grains needs to be explored by future studies. The high observed²⁶Al/²⁷Al ratios in this work provide a crucial constraint for the production of²⁶Al 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 high²⁶Al/²⁷Al ratios observed here; in contrast, H ingestion models are able to reproduce the²⁶Al/²⁷Al ratios along with C, N, and Si isotopic ratios reasonably well.