Characterizing the High-Velocity Stars of RAVE: The Discovery of a Metal-Rich Halo Star Born in the Galactic Disk

TL;DR

This study analyzes high-velocity stars in the solar neighborhood using RAVE data, discovering a metal-rich halo star likely ejected from the Galactic disk, shedding light on halo formation and stellar dynamics.

Contribution

It presents the first chemical and kinematic characterization of high-velocity stars from RAVE, including the discovery of a metal-rich halo star ejected from the disk.

Findings

High-velocity stars reach up to 13 kpc from the Galactic plane.

The metallicity distribution peaks at [M/H] = -1.2 dex, consistent with the inner halo.

A metal-rich halo star likely ejected from the disk was discovered.

Abstract

We aim to characterize high-velocity (HiVel) stars in the solar vicinity both chemically and kinematically using the fourth data release of the RAdial Velocity Experiment (RAVE). We used a sample of 57 HiVel stars with Galactic rest-frame velocities larger than 275 km s$^{-1}$. With 6D position and velocity information, we integrated the orbits of the HiVel stars and found that, on average, they reach out to 13 kpc from the Galactic plane and have relatively eccentric orbits consistent with the Galactic halo. Using the stellar parameters and [$\alpha$/Fe] estimates from RAVE, we found the metallicity distribution of the HiVel stars peak at [M/H] = -1.2 dex and is chemically consistent with the inner halo. There are a few notable exceptions that include a hypervelocity star (HVS) candidate, an extremely high-velocity bound halo star, and one star that is kinematically consistent with the halo but chemically consistent with the disk. High-resolution spectra were obtained for the metal-rich HiVel star candidate and the second highest velocity star in the sample. Using these high-resolution data, we report the discovery of a metal-rich halo star that has likely been dynamically ejected into the halo from the Galactic thick disk. This discovery could aid in explaining the assembly of the most metal-rich component of the Galactic halo.