The intermediate neutron capture process

V. The i-process in AGB stars with overshoot

Institut d’Astronomie et d’Astrophysique, Université Libre de Bruxelles, CP 226, 1050 Brussels, Belgium
e-mail: arthur.choplin@ulb.be

Received: 14 December 2023
Accepted: 9 February 2024

Abstract

Context. The intermediate neutron capture process (i-process) can develop during proton ingestion events (PIE), potentially during the early stages of low-mass low-metallicity asymptotic giant branch (AGB) stars.

Aims. We examine the impact of overshoot mixing on the triggering and development of i-process nucleosynthesis in AGB stars of various initial masses and metallicities.

Methods. We computed AGB stellar models, with initial masses of 1, 2, 3, and 4 M_⊙ and metallicities in the −2.5 ≤ [Fe/H] ≤ 0 range, using the stellar evolution code STAREVOL with a network of 1160 nuclei coupled to the transport equations. We considered different overshooting profiles below and above the thermal pulses, and below the convective envelope.

Results. The occurrence of PIEs is found to be primarily governed by the amount of overshooting at the top of pulse (f_top) and to increase with rising f_top. For f_top = 0, 0.02, 0.04, and 0.1, we find that 0%, 6%, 24%, and 86% of our 21 AGB models with −2 < [Fe/H] < 0 experience a PIE, respectively. Variations of the overshooting parameters during a PIE leads to a scatter on abundances of 0.5 − 1 dex on elements, with 36 < Z < 56; however, this barely impacts the production of elements with 56 < Z < 80, which therefore appear to be a reliable prediction of our models. Actinides are only produced if the overshooting at the top of pulse is small enough. We also find that PIEs leave a ¹³C-pocket at the bottom of the pulse that can give rise to an additional radiative s-process nucleosynthesis. In the case of the 2 M_⊙ models with [Fe/H] = −1 and −0.5, it produces a noticeable mixed i + s chemical signature at the surface. Finally, the chemical abundance patterns of 22 observed r/s-stars candidates (18 dwarfs or giants and 4 post-AGB) with −2 < [Fe/H] < −1 are found to be in reasonable agreement with our AGB model predictions. The binary status of the dwarfs/giants being unclear, we suggest that these stars have acquired their chemical pattern either from the mass transfer of a now-extinct AGB companion or from an early generation AGB star that polluted the natal cloud.

Conclusions. The occurrence of PIEs and the development of i-process nucleosynthesis in AGB stars remains sensitive to the overshooting parametrization. A high (yet realistic) f_top value triggers PIEs at (almost) all metallicities. The existence of r/s-stars at [Fe/H] ≃ −1 is in favour of an i-process operating in AGB stars up to this metallicity. Stricter constraints from multi-dimensional hydrodynamical models on overshoot coefficients could deliver new insights into the contribution of AGB stars to heavy elements in the Universe.