Interpretation of CEMP(s) and CEMP(s + r) Stars with AGB Models

TL;DR

This paper compares AGB star models with observed CEMP stars to understand their s-process element enrichment, considering initial conditions and binary evolution, and discusses specific cases including r-process enrichment.

Contribution

It provides a detailed analysis of CEMP(s) and CEMP(s+r) stars using AGB models with varying parameters, offering new insights into their nucleosynthesis history and initial conditions.

Findings

AGB models can explain observed s-process distributions with different initial masses.

Na and Mg abundances serve as indicators of initial AGB mass.

Some CEMP(s+r) stars show evidence of pre-enrichment in r-process elements.

Abstract

Asymptotic Giant Branch (AGB) stars play a fundamental role in the s-process nucleosynthesis during their thermal pulsing phase. The theoretical predictions obtained by AGB models at different masses, s-process efficiencies, dilution factors and initial r-enrichment, are compared with spectroscopic observations of Carbon-Enhanced Metal-Poor stars enriched in s-process elements, CEMP(s), collected from the literature. We discuss here five stars as example, CS 22880-074, CS 22942-019, CS 29526-110, HE 0202-2204, and LP 625-44. All these objects lie on the main-sequence or on the giant phase, clearly before the TP-AGB stage: the hypothesis of mass transfer from an AGB companion, would explain the observed s-process enhancement. CS 29526-110 and LP 625-44 are CEMP(s+r) objects, and are interpreted assuming that the molecular cloud, from which the binary system formed, was already enriched in r-process elements by SNII pollution. In several cases, the observed s-process distribution may be accounted for AGB models of different initial masses with proper 13C-pocket efficiency and dilution factor. Na (and Mg), produced via the neutron capture chain starting from 22Ne, may provide an indicator of the initial AGB mass.