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Title: New Cold Subdwarf Discoveries from Backyard Worlds and a Metallicity Classification System for T Subdwarfs
Abstract 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 redJ − W2 colors and largeJ-band reduced proper motions show varying signatures of subsolar metallicity, including strong collision-induced H2absorption, obscured methane and water features, and weak Kiabsorption. 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.  more » « less
Award ID(s):
2238468
PAR ID:
10674136
Author(s) / Creator(s):
; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; more » ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; « less
Corporate Creator(s):
Publisher / Repository:
The Astrophysical Journal
Date Published:
Journal Name:
The Astrophysical Journal
Volume:
982
Issue:
2
ISSN:
0004-637X
Page Range / eLocation ID:
79
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
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