Journal ArticleNew Cold Subdwarf Discoveries from Backyard Worlds and a Metallicity Classification System for T SubdwarfsBurgasser, Adam J (ORCID:0000000265239536); Schneider, Adam C (ORCID:0000000262945937); Meisner, Aaron M (ORCID:0000000211257384); Caselden, Dan (ORCID:0000000178965791); Hsu, Chih-Chun (ORCID:0000000253707494); Gerasimov, Roman (ORCID:000000030398639X); Aganze, Christian (ORCID:0000000320949128); Softich, Emma (ORCID:0000000214201837); Karpoor, Preethi (ORCID:0000000214809041); Theissen, Christopher A (ORCID:0000000298075435); Brooks, Hunter (ORCID:0000000252530383); Bickle, Thomas P (ORCID:000000032235761X); Gagné, Jonathan (ORCID:0000000225929612); Artigau, Ètienne (ORCID:0000000335065667); Marsset, Michaël (ORCID:0000000186172425); Rothermich, Austin (ORCID:0000000340839962); Faherty, Jacqueline K (ORCID:0000000162510573); Kirkpatrick, J Davy (ORCID:000000034269260X); Kuchner, Marc J (ORCID:0000000223875489); Andersen, Nikolaj Stevnbak (ORCID:0000000347143829); Beaulieu, Paul; Colin, Guillaume (ORCID:0000000276301243); Gantier, Jean Marc (ORCID:0000000210441112); Gramaize, Leopold (ORCID:0000000289604964); Hamlet, Les (ORCID:0000000273892092); Hinckley, Ken (ORCID:0000000247334927); Kabatnik, Martin (ORCID:0000000349051370); Kiwy, Frank (ORCID:0000000186621622); Martin, David W; Massat, Diego H; Pendrill, William; Sainio, Arttu (ORCID:0000000348645484); Schümann, Jörg (ORCID:0000000275877195); Thévenot, Melina (ORCID:0000000152849231); Walla, Jim; Wędracki, ZbigniewThe_Backyard_Worlds_Planet_9_Collaboration<title>Abstract</title> <p>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<italic>J</italic> − W2 colors and large<italic>J</italic>-band reduced proper motions show varying signatures of subsolar metallicity, including strong collision-induced H₂absorption, obscured methane and water features, and weak K<sc>i</sc>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.</p>The Astrophysical Journal2025-03-2010674136The Astrophysical Journal9822790004-637Xhttps://doi.org/10.3847/1538-4357/adb39f2238468National Science Foundation