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.
Highlights
Primitive meteorites are carriers of several types of dust of presolar origin, coming from different stellar sources
Presolar C-rich grains that condensed in core-collapse supernovae (CCSNe), are low-density (LD) graphite grains (carrier of the Ne-E(L) component, Amari et al 1990), nano-diamonds, and silicon carbides (SiC) of Type X and of Type C (0.1%–0.2% of all presolar SiC grains; e.g., Pignatari et al 2013b)
In this work we show that the entrainment of H-rich material into the He shell before the SN explosion allows the coproduction of 13C, 15N and 26Al, accounting for isotopic signatures of presolar SiC grains with low 12C/13C ratios
Summary
Primitive meteorites are carriers of several types of dust of presolar origin, coming from different stellar sources. Probably linked to TiC subgrains, in LD graphite grains (Groopman et al 2012) The combination of both 14N/15N and 12C/13C ratios lower than solar observed in a fraction of C-rich grains is a challenge to theoretical CCSNe models, assuming mixing between different CCSNe layers and maintaining C/O > 1 (e.g., Travaglio et al 1999). In this work we show that the entrainment of H-rich material into the He shell before the SN explosion allows the coproduction of 13C, 15N and 26Al, accounting for isotopic signatures of presolar SiC grains with low 12C/13C ratios. This provides a new production scenario for SiC grains classified as nova and AB grains.
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