The intermediate neutron capture process. III. The i-process in AGB stars of different masses and metallicities without overshoot

TL;DR

This study models the intermediate neutron capture (i-process) in AGB stars of different masses and metallicities, revealing how proton ingestion events influence nucleosynthesis without extra mixing processes.

Contribution

It provides detailed i-process yields from AGB star models with varied initial conditions, highlighting the effects of proton ingestion events on stellar evolution and element synthesis.

Findings

Proton ingestion occurs in half of the models, leading to neutron densities of 10^{14}-10^{15} cm^{-3}.

Surface enrichment from PIEs is consistent across models and persists without extra mixing.

Models can produce heavy elements up to lead (Pb) without overshoot or a $^{13}$C-pocket.

Abstract

Alongside the slow (s) and rapid (r) neutron capture processes, an intermediate neutron capture process (i-process) is thought to exist. It happens when protons are mixed in a convective helium-burning zone, and is referred to as proton ingestion event (PIE). A possible astrophysical site is the asymptotic giant branch (AGB) phase of low-mass low-metallicity stars. We provide i-process yields of a grid of AGB stars experiencing PIEs. We computed 12 models with initial masses of 1, 2, and 3 $M_{\odot}$ and metallicities of [Fe/H] $=-3.0$, $-2.5$ $-2.3,$ and $-2.0, $ with the stellar evolution code STAREVOL. We used a nuclear network of 1160 species at maximum, coupled to the chemical transport equations. These simulations do not include any extra mixing process. Proton ingestion takes place in six out of our 12 AGB models. These models experience i-process nucleosynthesis characterized by neutron densities of $\simeq 10^{14} -10^{15}$ cm$^{-3}$. Depending on the PIE properties two different evolution paths follow: either the stellar envelope is quickly lost and no more thermal pulses develop or the AGB phase resumes with additional thermal pulses. This behaviour critically depends on the pulse number when the PIE occurs, the mass of the ingested protons, and the extent to which the pulse material is diluted in the convective envelope. The surface enrichment after a PIE is a robust feature of our models and it persists under various convective assumptions. Our models can synthesise heavy elements up to Pb without any parametrized extra mixing process such as overshoot or inclusion of a $^{13}$C-pocket. Nevertheless, it remains to be explored how the i-process depends on mixing processes, such as overshoot, thermohaline, or rotation.