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

TL;DR

This paper models explosive helium shell nucleosynthesis in massive stars with hydrogen ingestion, explaining isotopic anomalies in presolar grains and contributing to understanding nitrogen isotopic ratios in the galaxy.

Contribution

It introduces new supernova explosion models considering hydrogen ingestion in the helium shell, reproducing isotopic signatures in presolar grains and addressing nitrogen isotope puzzles.

Findings

Models reproduce observed 12C/13C and 14N/15N ratios in grains.

Enhanced production of 13C, 15N, 22Na, and 26Al in models.

Potential explanation for Ne-E(L) component in graphite grains.

Abstract

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 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 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 production 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 alternative 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.