Modeling the progenitors of low-mass post-accretion binaries

TL;DR

This study models the formation of low-mass post-accretion binary systems involving AGB stars, revealing how different accretion scenarios explain observed abundance patterns in Ba, CH, and CEMP-s stars.

Contribution

It introduces a new grid of 2700 accretion models and applies maximum-likelihood analysis to match observational data, advancing understanding of binary star formation and element enrichment.

Findings

Consistent AGB donor masses of 2-3 solar masses across samples.

Weak Ba stars result from moderate mass transfer (~0.5 solar masses).

Strong Ba stars require larger accretion (>0.5 solar masses) but are not fully explained by models.

Abstract

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 ~1-6 $\rm{M_{\odot}}$. The s-process can be studied by modeling the accretion of material from AGB stars onto binary barium (Ba), CH, and carbon-enhanced metal-poor (CEMP)-s stars. Comparing observationally derived surface parameters and 1D-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. We compute a new grid of 2700 accretion models for low-mass post-accretion systems. A maximum-likelihood comparison determines the best fit models for observational samples of Ba, CH, and CEMP-s stars. We find consistent AGB donor masses in the mass range of 2-3 $\rm{M_{\odot}}$ across our sample of post-accretion binaries. We find the formation scenario for weak Ba stars is an AGB star transferring a moderate amount of mass ($\leq$0.5 $\rm{M_{\odot}}$) resulting in a ~2.0-2.5 $\rm{M_{\odot}}$ star. The strong Ba stars are best fit with lower final masses ~1.0-2.0 $\rm{M_{\odot}}$, and significant accreted mass ($\geq$0.5 $\rm{M_{\odot}}$). The CH and CEMP-s stars display lower final masses (~1.0 $\rm{M_{\odot}}$) and small amounts of transferred material (~0.1 $\rm{M_{\odot}}$). 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 $\rm{M_{\odot}}$ 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.