Discovery of a Hypervelocity L Subdwarf at the Star/Brown Dwarf Mass Limit

TL;DR

This paper reports the discovery of the first hypervelocity low-mass star or brown dwarf, suggesting a new class of high-velocity objects possibly ejected through extreme astrophysical events.

Contribution

It presents the identification and characterization of a hypervelocity L subdwarf, expanding the known population of high-velocity low-mass stars and brown dwarfs.

Findings

CWISE J124909.08+362116.0 is a metal-poor early L subdwarf.

It has a Galactic rest frame speed of approximately 456 km/s.

This object may be ejected from the Galactic center or other extreme scenarios.

Abstract

We report the discovery of a high velocity, very low-mass star or brown dwarf whose kinematics suggest it is unbound to the Milky Way. CWISE J124909.08+362116.0 was identified by citizen scientists in the Backyard Worlds: Planet 9 program as a high proper motion ($\mu$ $=$ 0''9/yr) faint red source. Moderate resolution spectroscopy with Keck/NIRES reveals it to be a metal-poor early L subdwarf with a large radial velocity ($-$103$\pm$10 km/s), and its estimated distance of 125$\pm$8 pc yields a speed of 456$\pm$27 km/s in the Galactic rest frame, near the local escape velocity for the Milky Way. We explore several potential scenarios for the origin of this source, including ejection from the Galactic center $\gtrsim$3 Gyr in the past, survival as the mass donor companion to an exploded white dwarf. acceleration through a three-body interaction with a black hole binary in a globular cluster, and accretion from a Milky Way satellite system. CWISE J1249+3621 is the first hypervelocity very low mass star or brown dwarf to be found, and the nearest of all such systems. It may represent a broader population of very high velocity, low-mass objects that have undergone extreme accelerations.