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

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.

accepted to ApJ Letters

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• The Bulge Radial Velocity Assay for RR Lyrae stars (BRAVA-RR) DR2: a Bimodal Bulge?
• A Dipole on the Sky: Predictions for Hypervelocity Stars from the Large Magellanic Cloud
• Evidence for Galactic disc RR~Lyrae stars in the Solar neighbourhood
• The Metallicity Distribution of the Milky Way Bulge
• The Controversial Star-Formation History and Helium Enrichment of the Milky Way Bulge
• Was the Milky Way Bulge Formed From The Buckling Disk Instability, Hierarchical Collapse, Accretion of Clumps, or All of the Above?
• Milky Way archaeology using RR Lyrae and type II Cepheids II. High velocity RR Lyrae stars, and mass of the Milky Way
• Chemical abundances in a high velocity RR Lyrae star near the bulge
• Relationship between the Elemental Abundances and the Kinematics of Galactic-Field RR Lyrae Stars
• On the Oosterhoff dichotomy in the Galactic bulge: II. kinematical distribution
• Impact of distance determinations on Galactic structure. II. Old tracers
• The Interstellar Extinction Toward the Milky Way Bulge with Planetary Nebulae, Red Clump, and RR Lyrae stars
• Radial velocities of RR Lyrae stars in and around NGC 6441
• Are the double-mode bulge RR Lyrae stars with identical period-ratios the relic of a disrupted stellar system?
• RR Lyrae stars as probes of the Milky Way structure and formation