The intermediate neutron capture process

VI. Proton ingestion and i-process in rotating magnetic asymptotic giant branch stars

¹ Institut d’Astronomie et d’Astrophysique, Université Libre de Bruxelles, CP 226 B-1050 Brussels, Belgium
² BLU-ULB, Brussels Laboratory of the Universe, blu.ulb.be Brussels, Belgium
³ Département d’Astronomie, Université de Genève Chemin Pegasi 51 1290 Versoix, Switzerland
⁴ Yunnan Observatories, Chinese Academy of Sciences Kunming 650216, China

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 10 October 2025
Accepted: 20 December 2025

Abstract

Context. The intermediate neutron-capture process (i-process) can occur during proton ingestion events (PIEs), which may take place in the early evolutionary phases of asymptotic giant branch (AGB) stars.

Aims. We investigate the impact of rotational and magnetic mixing on i-process nucleosynthesis in low-metallicity, low-mass AGB stars.

Methods. We computed AGB models with [Fe/H] = −2.5 and −1.7 and initial masses of 1 and 1.5 M_⊙ using the STAREVOL code, including a network of 1160 nuclei coupled to transport equations. Rotating models incorporate a calibrated Tayler-Spruit (TS) dynamo to account for core rotation rates inferred from asteroseismic observations of solar-metallicity sub-giants and giants. Initial rotation velocities of 0, 30, and 90 km s⁻¹ were considered, along with varying assumptions for magnetic mixing.

Results. Rotation without magnetic fields strongly suppresses the i-process due to the production of primary ¹⁴N, which is subsequently converted into ²²Ne – a potent neutron poison during the PIE. Including magnetic fields via the TS dynamo restores the models close to their non-rotating counterparts: strong core-envelope coupling suppresses shear mixing and prevents primary ¹⁴N synthesis, yielding i-process nucleosynthesis similar to non-rotating models. We also find that rotational mixing during the AGB phase is insufficient to affect the occurrence of PIEs.

Conclusions. Proton ingestion event-driven nucleosynthesis proceeds similarly in asteroseismic-calibrated magnetic rotating AGB stars and non-rotating stars, producing identical abundance patterns.