Letter to the Editor

Proton ingestion in asymptotic giant branch stars as a possible explanation for J-type stars and AB2 grains

Institut d’Astronomie et d’Astrophysique, Université Libre de Bruxelles, CP 226, B-1050 Brussels, Belgium

^⋆ Corresponding author; arthur.choplin@ulb.be

Received: 6 June 2024
Accepted: 8 October 2024

Abstract

Context. J-type stars are a subclass of carbon stars that are generally Li-rich, not enriched in s-elements, and have low ¹²C/¹³C ratios. They were suggested to be the manufacturers of the pre-solar grains of type AB2 (having low ¹²C/¹³C and supersolar ¹⁴N/¹⁵N).

Aims. In this Letter, we investigate the possibility that J-type stars are early asymptotic giant branch (AGB) stars that experienced a proton ingestion event (PIE).

Methods. We used the stellar evolution code STAREVOL to compute AGB stellar models with initial masses of 1, 2, and 3M_⊙ and metallicities [Fe/H] = − 0.5 and 0.0. We included overshooting above the thermal pulse and used a network of 1160 nuclei coupled to the transport equations. The outputs of these models were compared to observations of J-type stars and AB2 grains.

Results. In solar-metallicity AGB stars, PIEs can be triggered if a sufficiently high overshoot is considered. These events lead to low ¹²C/¹³C ratios, high Li abundances, and no enrichment in s-elements. We find that the 2 − 3 M_⊙ AGB models experiencing a PIE can account for most of the observational features of J-type stars and AB2 grains. The remaining tensions between models and observations are (1) the low ¹⁴N/¹⁵N ratio of some AB2 grains and of 2 out of 13 J-type stars, (2) the high ²⁶Al/²⁷Al of some AB2 grains, and (3) the J-type stars with A(Li) < 2. Extra mixing mechanisms can alleviate some of these tensions, such as thermohaline or rotation.

Conclusions. This work highlights a possible match between AGB stellar models that undergo a PIE and J-type stars and AB2 grains. To account for other types of carbon stars, such as N-type stars, PIEs should only develop in a fraction of solar-metallicity AGB stars. Additional work is needed to assess how the occurrence of PIEs depends on mixing parameters and initial conditions, and therefore to further confirm or exclude the proposed scenario.