A Spectroscopic Survey of Field Red Horizontal-Branch Stars

TL;DR

This spectroscopic survey of 340 field red horizontal-branch stars reveals their chemical compositions and kinematics, confirming their Galactic population memberships and providing insights into their physical properties and origins.

Contribution

The study provides detailed chemical and kinematic data for a large sample of RHB stars, confirming their classification and distinguishing thin and thick disk populations.

Findings

Half of the candidates are confirmed as true RHB stars.

Clear chemical and kinematic distinctions between thin and thick disk RHB stars.

Derived stellar parameters support the classification and population analysis.

Abstract

A metallicity, chemical composition, and kinematic survey has been conducted for a sample of 340 candidate field red horizontal branch stars. High resolution, high signal-to-noise spectra were gathered with the McDonald Observatory 2.7m Tull and the Hobby-Eberly Telescope echelle spectrographs, and were used to determine effective temperatures, surface gravities, microturbulent velocities, [Fe/H] metallicities, and abundance ratios [X/Fe] for seven $\alpha$ and Fe-group species. The derived temperatures and gravities confirm that at least half of the candidates are true RHB stars, with (average) parameters Teff$\sim$5000 K, log g$\sim$2.5. From the $\alpha$ abundances alone the thin and thick Galactic populations are apparent in our sample. Space motions for 90% of the program stars were computed from Hipparcos and Gaia parallaxes and proper motions. Correlations between chemical compositions and Galactic kinematics clearly indicate the existence of both thin and thick disk RHB stars.