CARBON-RICH PRESOLAR GRAINS FROM MASSIVE STARS: SUBSOLAR C-12/C-13 AND N-14/N-15 RATIOS AND THE MYSTERY OF N-15

摘要

Carbon-rich grains with isotopic anomalies compared to the Sun are found in primitive meteorites. They were made by stars, and carry the original stellar nucleosynthesis signature. Silicon carbide grains of Type X and C and low-density (LD) graphites condensed in the ejecta of core-collapse supernovae. We present a new set of models for the explosive He shell and compare them with the grains showing C-12/C-13 and N-14/N-15 ratios lower than solar. In the stellar progenitor H was ingested into the He shell and not fully destroyed before the explosion. Different explosion energies and H concentrations are considered. If the supernova shock hits the He-shell region with some H still present, the models can reproduce the C and N isotopic signatures in C-rich grains. Hot-CNO cycle isotopic signatures are obtained, including a large production of C-13 and N-15. The short-lived radionuclides Na-22 and Al-26 are increased by orders of magnitude. The production of radiogenic Ne-22 from the decay of Na-22 in the He shell might solve the puzzle of the Ne-E(L) component in LD graphite grains. This scenario is attractive for the SiC grains of type AB with N-14/N-15 ratios lower than solar, and provides an alternative solution for SiC grains originally classified as nova grains. Finally, this process may contribute to the production of N-14 and N-15 in the Galaxy, helping to produce the N-14/N-15 ratio in the solar system.