Modeling the progenitors of low-mass post-accretion binaries

¹ Goethe Universität Frankfurt Institut für Angewandte Physik, Max-von-Laue-Str. 1, 60438 Frankfurt am Main, Germany
² Max Planck Institute für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
³ H.H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol, BS8 1TL UK

^⋆ Corresponding author: dimoff@mpia.de

Received: 28 May 2025
Accepted: 22 July 2025

Abstract

Context. About half of the mass of all heavy elements with mass number A > 90 is formed through the slow neutron capture process (s-process), occurring in evolved asymptotic giant branch (AGB) stars with masses of ∼1 − 6 M_⊙. The s-process can be studied by modeling the evolution of barium (Ba), CH, and carbon-enhanced metal-poor (CEMP)-s stars.

Aims. Comparing observationally derived surface parameters and 1D local thermodynamic equilibrium (LTE) abundance patterns of s-process elements to theoretical binary accretion models, we aim to understand the formation of post-accretion systems. We explore the extent of dilution of the accreted material and describe the impact of convective mixing on the observed surface abundances.

Methods. We computed a new grid of 2691 stellar evolution models for low-mass post-accretion systems, including accretion from an AGB companion. A maximum-likelihood comparison to surface parameters and derived abundances determines the best fit models for a large observational sample of Ba, CH, and CEMP-s stars.

Results. We find consistent AGB donor masses in the mass range of 2 − 3 M_⊙ across our sample of post-accretion stars. We find the formation scenario for weak Ba stars is an AGB star transferring a moderate amount of mass (≤0.5 M_⊙) resulting in a ∼2.0 − 2.5 M_⊙ Ba star. The strong Ba stars are best fit with lower final masses ∼1.0 − 2.0 M_⊙ and significant accreted mass (≥0.5 M_⊙). The CH and CEMP-s stars display lower final masses (∼1.0 M_⊙) and small amounts of transferred material (∼0.1 M_⊙).

Conclusions. We find that Ba stars generally accrete more material than CEMP-s and CH stars. We also find that strong Ba stars must accrete more than 0.50 M_⊙ to explain their abundance patterns, and in this limit we are unable to reproduce the observed mass distribution of strong Ba stars. The mass distributions of the weak Ba stars, CEMP-s, and CH stars are well reproduced in our modeling.