Nitrogen isotopes in AGB carbon stars and presolar SiC grains: a challenge for stellar nucleosynthesis

TL;DR

This study links isotopic ratios in presolar SiC grains to specific types of carbon stars, revealing challenges in current stellar nucleosynthesis models and suggesting some grains originate from stars previously not associated with their isotopic signatures.

Contribution

First spectroscopic measurements of 14N/15N ratios in Galactic carbon stars, connecting them to presolar SiC grain origins and highlighting discrepancies in existing nucleosynthesis models.

Findings

J- and some SC-type stars may produce A+B grains.

Most mainstream grains are compatible with N-type stars.

Current models cannot explain all observed isotopic ratios.

Abstract

Isotopic ratios of C, N, Si, and trace heavy elements in presolar SiC grains from meteorites provide crucial constraints to nucleosynthesis. A long-debated issue is the origin of the so-called A+B grains, as for them no stellar progenitor has so far been clearly identified on observational grounds. We report the first spectroscopic measurements of 14N/15N ratios in Galactic carbon stars of different spectral types and show that J- and some SC-type stars might produce A+B grains, even for 15N enrichments previously attributed to novae. We also show that most mainstream (MS) grains are compatible with the composition of N-type stars, but might also descend, in some cases, from SC stars. From the theoretical point of view, no astrophysical scenario can explain the C and N isotopic ratios of SC, J and N-type carbon stars together, as well as those of many grains produced by them. This poses urgent questions to stellar physics.