Ba-enhanced dwarf and subgiant stars in the LAMOST Galactic surveys

TL;DR

This study analyzes the origins and characteristics of Ba-enhanced dwarf and subgiant stars in the LAMOST survey, revealing their diverse formation pathways and a notable scarcity of high-[$oldsymbol{ extalpha}$/Fe] Ba-enhanced stars.

Contribution

It provides a comprehensive analysis of the chemical and kinematic properties of Ba-enhanced stars, highlighting their formation mechanisms and the existence of a high-$ extalpha$ desert in the Galactic context.

Findings

One third of Ba-enhanced stars show C and N enhancement, indicating binary evolution.

Most Ba-enhanced stars with normal C and N are warm stars likely affected by internal elemental transport.

A significant lack of high-[$ extalpha$/Fe] Ba-enhanced stars in the [Fe/H]--[$ extalpha$/Fe] plane, termed as high-$ extalpha$ desert.

Abstract

Ba-enhanced stars are interesting probes of stellar astrophysics and Galactic formation history. In this work, we investigate the chemistry and kinematics for a large sample of Ba-enhanced ([Ba/Fe]$>$1.0) dwarf and subgiant stars with $5000 < T_{\rm eff }< 6700$\,K from LAMOST. We find that both stellar internal evolution process and external mass exchange due to binary evolution are responsible for the origins of the Ba-enhancement of our sample stars. About one third of them exhibit C and N enhancement and ultraviolet brightness excess, indicating they are products of binary evolution. The remaining Ba-enhanced stars with normal C and N abundances are mostly warm stars with $T_{\rm eff} > 6000$\,K. They are likely consequences of stellar internal elemental transport processes, but they show very different elemental patterns to the hotter Am/Fm stars. Our results reveal a substantially lack of high-[$\alpha$/Fe] Ba-enhanced stars in the [Fe/H]--[$\alpha$/Fe] plane, which we dub as a {\em high-$\alpha$ desert}. We suggest it is due to a lower efficiency for producing Ba-enhanced stars by low-mass AGB progenitors in binary systems. Our results call for detailed modellings for these Ba-enhanced stellar peculiars, in the context of both stellar internal elemental transport and external mass accretion.