Chemical compositions of thin-disk, high-metallicity red horizontal-branch field stars

TL;DR

This study analyzes the chemical compositions and evolutionary stages of 76 field red horizontal-branch stars, revealing a predominance of low-velocity, solar-metallicity RHB stars that challenge standard stellar evolution models.

Contribution

It provides detailed chemical abundance and kinematic data for a large sample of RHB stars, identifying a significant population of low-velocity, solar-metallicity RHB stars not previously emphasized.

Findings

Most RHB stars in the sample are low velocity and solar-metallicity.

The sample includes 18 probable RHB stars with detailed abundance analysis.

The existence of low-velocity, solar-metallicity RHB stars challenges standard stellar evolution expectations.

Abstract

We present a detailed abundance analysis and atmospheric parameters of 76 stars from a survey to identify field Galactic red horizontal-branch (RHB) stars. High-resolution echelle spectra (R\simeq60,000, S/N>=100) were obtained with 2.7 m Smith Telescope at McDonald Observatory. The target stars were selected only by color and parallax information. Overall metallicities and relative abundances of proton-capture elements (C I, N I, O I, Li I), alpha-elements (Ca I and Si I), and neutron-capture elements (Eu II and La II) were determined by either equivalent width or synthetic spectrum analyses. We used CN features at 7995-8040 {\AA} region in order to determine 12^C/13^C ratios of our targets. Investigation of the evolutionary stages, using spectroscopic T_eff and log g values along with derived 12^C/13^C ratios, revealed the presence of 18 probable RHB stars in our sample. We also derived kinematics of the stars with available distance information. Taking into account both the kinematics and probable evolutionary stages, we conclude that our sample contains five thick disk and 13 thin disk RHB stars. Up until now, RHB stars have been considered as members of the thick disk, and were expected to have large space velocities and sub-solar metallicities. However, our sample is dominated by low velocity solar-metallicity RHB stars; their existence cannot be easily explained with standard stellar evolution.