Journal ArticleDiscovery of a Hypervelocity L Subdwarf at the Star/Brown Dwarf Mass LimitBurgasser, Adam J; Gerasimov, Roman; Kremer, Kyle; Brooks, Hunter; Alvarado, Efrain; Schneider, Adam C; Meisner, Aaron M; Theissen, Christopher A; Softich, Emma; Karpoor, Preethi; Bickle, Thomas P; Kabatnik, Martin; Rothermich, Austin; Caselden, Dan; Kirkpatrick, J Davy; Faherty, Jacqueline K; Casewell, Sarah L; Kuchner, Marc JThe_Backyard_Worlds_Planet_9_Collaboration<title>Abstract</title> <p>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 (<italic>μ</italic>= 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⁻¹in 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 ≳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.</p>IOP2024-08-0110533769The Astrophysical Journal Letters9711L252041-8205https://doi.org/10.3847/2041-8213/ad66072009136; 2238468National Science Foundation