How plausible are the proposed formation scenarios of CEMP-r/s stars?

TL;DR

This paper evaluates the plausibility of various formation scenarios for CEMP-$r/s$ stars by comparing predicted and observed frequencies, highlighting discrepancies and suggesting the $i$-process as a potential explanation.

Contribution

It assesses the likelihood of proposed formation scenarios for CEMP-$r/s$ stars and explores the $i$-process as a possible origin, providing a comparison with observational data.

Findings

Most scenarios underestimate the observed CEMP-$r/s$ frequency by at least a factor of 5.

The independent enrichment model predicts about 22% CEMP-$r/s$ to CEMP-$s$ ratio, aligning with the lowest observed estimates.

Stars with properties predicted by the model are at least ten times rarer in observations.

Abstract

CEMP-$r/s$ stars are metal-poor stars with enhanced abundances of carbon and heavy elements associated with the slow ($s$-) and rapid ($r$-) neutron-capture process. It is believed that carbon and $s$-elements were accreted from the wind of an AGB primary star, a scenario that is generally accepted to explain the formation of CEMP stars that are only enhanced in $s$-elements (CEMP-$s$ stars). The origin of $r$-element-enrichment in CEMP-$r/s$ stars is debated and many formation scenarios have been put forward. We aim to determine the likelihood of the scenarios proposed to explain the formation of CEMP-$r/s$ stars. We calculate the frequency of CEMP-$r/s$ stars among CEMP-$s$ stars for a variety of scenarios, and we compare it with that determined from an observed sample of CEMP-$r/s$ stars collected from the literature. The theoretical frequency of CEMP-$r/s$ stars predicted in most scenarios underestimates the observed ratio by at least a factor of 5. If the enrichments in $s$- and $r$-elements are independent, the model ratio of CEMP-$r/s$ to CEMP-$s$ stars is about 22%, that is approximately consistent with the lowest estimate of the observed ratio. However, this model predicts that about one third of all carbon-normal stars have [Ba/Fe] and [Eu/Fe] higher than 1, and that 40% of all CEMP stars have [Ba/Eu]$\le0$. Stars with these properties are at least ten times rarer in our observed sample. The $intermediate$ or $i$-process, which is supposedly active in some circumstances during the AGB phase, could provide an explanation of the origin of CEMP-$r/s$ stars, similar to that of CEMP-$s$ stars, in the context of wind mass accretion in binary systems. Further calculations of the nucleosynthesis of the $i$-process and of the detailed evolution of late AGB stars are needed to investigate if this scenario predicts a CEMP-$r/s$ star frequency consistent with the observations.