Stellar Origin of N-15-rich Presolar SiC Grains of Type AB: Supernovae with Explosive Hydrogen Burning

摘要

We report C, N, and Si isotopic data for 59 highly C-13-enriched presolar submicron- to micron-sized SiC grains from the Murchison meteorite, including eight putative nova grains (PNGs) and 29 N-15-rich (N-14/N-15 <= solar) AB grains, and their Mg-Al, S, and Ca-Ti isotope data when available. These 37 grains are enriched in C-13, N-15, and Al-26 with the PNGs showing more extreme enhancements. The N-15-rich AB grains show systematically higher Al-26 and Si-30 excesses than the N-14-rich AB grains. Thus, we propose to divide the AB grains into groups 1 (N-14/N-15 < solar) and 2 (N-14/N-15 >= solar). For the first time, we have obtained both S and Ti isotopic data for five AB1 grains and one PNG and found S-32 and/or Ti-50 enhancements. Interestingly, one AB1 grain had the largest S-32 and Ti-50 excesses, strongly suggesting a neutron- capture nucleosynthetic origin of the S-32 excess and thus the initial presence of radiogenic Si-32 (t(1/2) = 153 years). More importantly, we found that the N-15 and Al-26 excesses of AB1 grains form a trend that extends to the region in the N-Al isotope plot occupied by C-2 grains, strongly indicating a common stellar origin for both AB1 and C-2 grains. Comparison of supernova models with the AB1 and C-2 grain data indicates that these grains came from supernovae that experienced H ingestion into the He/C zones of their progenitors.