The impact of overshoot on the i-process in AGB stars

¹ Max-Planck Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85741 Garching, Germany
e-mail: brempl@mpa-garching.mpg.de
² Ludwig-Maximillians-Universität München, Geschwister-Scholl-Platz 1, 80539 Munich, Germany
³ Excellence Cluster ORIGINS, Boltzmannstrasse 2, 85748 Garching, Germany

Received: 29 February 2024
Accepted: 12 April 2024

Abstract

Context. The production of neutron-rich elements at neutron densities intermediate to those of the s- and r-processes, the so-called i-process, has been identified as possibly being responsible for the observed abundance pattern found in certain carbon-enhanced metal-poor (CEMP) stars. The production site may be low-metallicity stars on the asymptotic giant branch (AGB) where the physical processes during the thermal pulses are not well known.

Aims. We investigate the impact of overshoot from various convective boundaries during the AGB phase on proton ingestion events (PIEs) and the neutron densities as a necessary precondition for the i-process as well as on the structure and continued evolution of the models.

Methods. We therefore analyzed models of a 1.2 M_⊙, Z = 5 × 10⁻⁵ star. A fiducial model without overshoot on the AGB (overshoot was applied during the pre-AGB evolution) serves as a reference. The same model was then run with various overshoot values and the resulting models were compared to one another. Light element nucleosynthesis is also discussed. Additionally, we introduce a new timescale argument to predict PIE occurrence to discriminate between a physical and a numerical reason for a nonoccurrence. A comparison to observations as well as previous studies was conducted before finally presenting the most promising choice of overshoot parameters for the occurrence of the i-process in low-mass, low-metallicity models.

Results. The fiducial model reveals high neutron densities and a persistent split of the pulse-driven convection zone (PDCZ). Overshoot from the PDCZ results in either temporary or permanent remerging of the split PDCZ, influencing the star’s structure and evolution. While both overshoot and non-overshoot models exhibit PIEs generating neutron densities suitable for the i-process, they lead to varied C/O and N/O ratios and notable Li enhancements. Comparison with previous studies and observations of CEMP-r/s stars suggests that while surface enhancements in our models may be exaggerated, abundance ratios align well. Though, for high values of overshoot from the PDCZ the agreement becomes worse.