A high-velocity bulge RR Lyrae variable on a halo-like orbit

TL;DR

This paper reports on a high-velocity RR Lyrae star near the Galactic bulge, highlighting its likely halo origin and implications for bulge star studies, including potential contamination and extreme velocity scenarios.

Contribution

It identifies a rare high-velocity RR Lyrae star in the bulge region and discusses its implications for understanding bulge and halo star populations.

Findings

The star has an unusually high velocity of -372 km/s.

It is likely a halo interloper within the bulge region.

Halo contamination affects studies of metal-poor bulge stars.

Abstract

We report on the RR Lyrae variable star, MACHO 176.18833.411, located toward the Galactic bulge and observed within the data from the ongoing Bulge RR Lyrae Radial Velocity Assay (BRAVA-RR), which has the unusual radial velocity of -372 +- 8 km/s and true space velocity of -482 +- 22 km/s relative to the Galactic rest frame. Located less than 1 kpc from the Galactic center and toward a field at (l,b)=(3,-2.5), this pulsating star has properties suggesting it belongs to the bulge RR Lyrae star population yet a velocity indicating it is abnormal, at least with respect to bulge giants and red clump stars. We show that this star is most likely a halo interloper and therefore suggest that halo contamination is not insignificant when studying metal-poor stars found within the bulge area, even for stars within 1 kpc of the Galactic center. We discuss the possibility that MACHO 176.18833.411 is on the extreme edge of the bulge RR Lyrae radial velocity distribution, and also consider a more exotic scenario in which it is a runaway star moving through the Galaxy.