April
2025
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2025ApJ...982...79B
Authors
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Burgasser, Adam J.
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Schneider, Adam C.
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Meisner, Aaron M.
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Caselden, Dan
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Hsu, Chih-Chun
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Gerasimov, Roman
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Aganze, Christian
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Softich, Emma
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Karpoor, Preethi
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Theissen, Christopher A.
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Brooks, Hunter
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Bickle, Thomas P.
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Gagné, Jonathan
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Artigau, Ètienne
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Marsset, Michaël
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Rothermich, Austin
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Faherty, Jacqueline K.
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Kirkpatrick, J. Davy
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Kuchner, Marc J.
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Andersen, Nikolaj Stevnbak
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Beaulieu, Paul
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Colin, Guillaume
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Gantier, Jean Marc
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Gramaize, Leopold
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Hamlet, Les
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Hinckley, Ken
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Kabatnik, Martin
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Kiwy, Frank
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Martin, David W.
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Massat, Diego H.
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Pendrill, William
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Sainio, Arttu
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Schümann, Jörg
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Thévenot, Melina
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Walla, Jim
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Wędracki, Zbigniew
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Backyard Worlds: Planet 9 Collaboration
Abstract
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We report the results of a spectroscopic survey of candidate T subdwarfs identified by the Backyard Worlds: Planet 9 program. Near-infrared spectra of 31 sources with red J ‑ W2 colors and large J-band reduced proper motions show varying signatures of subsolar metallicity, including strong collision-induced H2 absorption, obscured methane and water features, and weak K I absorption. These metallicity signatures are supported by spectral model fits and 3D velocities, indicating thick disk and halo population membership for several sources. We identify three new metal-poor T subdwarfs ([M/H] ≲ –0.5), CWISE J062316.19+071505.6, WISEA J152443.14‑262001.8, and CWISE J211250.11-052925.2; and 19 new "mild" subdwarfs with modest metal deficiency ([M/H] ≲ ‑0.25). We also identify three metal-rich brown dwarfs with thick disk kinematics. We provide kinematic evidence that the extreme L subdwarf 2MASS J053253.46+824646.5 and the mild T subdwarf CWISE J113010.07+313944.7 may be part of the Thamnos population, while the T subdwarf CWISE J155349.96+693355.2 may be part of the Helmi stream. We define a metallicity classification system for T dwarfs that adds mild subdwarfs (d/sdT), subdwarfs (sdT), and extreme subdwarfs (esdT) to the existing dwarf sequence. We also define a metallicity spectral index that correlates with metallicities inferred from spectral model fits and iron abundances from stellar primaries of benchmark T dwarf companions. This expansion of the T dwarf classification system supports investigations of ancient, metal-poor brown dwarfs now being uncovered in deep imaging and spectroscopic surveys.
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