Carbon-rich presolar grains from massive stars: subsolar 12C/13C and 14N/15N ratios and the mystery of 15N

Pignatari, Marco and Zinner, E. and Hoppe, P. and Jordan, C. J. and Gibson, B. K. (2015) Carbon-rich presolar grains from massive stars: subsolar 12C/13C and 14N/15N ratios and the mystery of 15N. ASTROPHYSICAL JOURNAL LETTERS, 808 (2). ISSN 2041-8205

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Abstract

Carbon-rich grains with isotopic anomalies compared to the Sun are found in primi- tive meteorites. They were made by stars, and carry the original stellar nucleosynthesis signature. Silicon carbide grains of Type X and C, and low-density graphites condensed in the ejecta of core-collapse supernovae. We present a new set of models for the explo- sive He shell and compare them with the grains showing 12C/13C and 14N/15N 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 SN 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 13C and 15N. The short-lived radionuclides 22Na and 26Al are increased by orders of magnitude. The pro- duction of radiogenic 22Ne from the decay of 22Na in the He shell might solve the puzzle of the Ne-E(L) component in low-density graphite grains. This scenario is attractive for the SiC grains of type AB with 14N/15N ratios lower than solar, and provides an alter- native solution for SiC grains originally classified as nova grains. Finally, this process may contribute to the production of 14N and 15N in the Galaxy, helping to produce the 14N/15N ratio in the solar system.

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