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

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-1) 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 +/- 10 km s-1), and its estimated distance of 125 +/- 8 pc yields a speed of 456 +/- 27 km s-1 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 greater than or similar to 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.