JWST Observations of the Metal-poor T Dwarf WISEA J155349.96+693355.2: The First Brown Dwarf Likely Associated with the Gaia-Enceladus Milky Way Substructure

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2026
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Abstract
Abstract We present JWST NIRSpec and MIRI observations of the metal-poor T dwarf WISEA J155349.96+693355.2. The combined NIRSpec/prism plus MIRI/LRS spectrum (R ~100) provides 0.6–12 μm spectroscopic coverage that reveals the presence of H2O, CH4, and NH3 absorption features, enhanced collision-induced H2 absorption, and weak or absent CO and CO2 absorption, all indicators of a metal-poor low-temperature atmosphere. Comparison of the spectral data to a suite of atmosphere models yields an effective temperature Teff = 960$^{+28}_{-32}$ K and a solar-scaled metallicity of [M/H] = $-1.16^{+0.28}_{-0.25}$ dex. We find indications that the uncertain trigonometric parallax, likely subject to Lutz-Kelker bias, may be overestimated. Adopting the photometric distance yields a bolometric luminosity log10Lbol/L⊙ = −5.29 ± 0.10, from which we infer Teff = 974 ± 56 K, consistent with the temperature derived via our atmospheric model fits. From the moderate-resolution (R ~3, 000) NIRSpec/G395H spectrum, we measure a radial velocity of −163 ± 5 km s−1, which combined with the source’s measured proper motion and estimated distance indicates a highly eccentric (e ≥ 0.9) Galactic orbit consistent with an accreted halo object. The UVW kinematics of WISEA J155349.96+693355.2, its bulk metallicity, and its modest alpha enrichment ([α/Fe] = +0.15$^{+0.04}_{-0.06}$ dex) are all equivalent to stars identified in the Gaia-Enceladus Milky Way substructure, making it the first brown dwarf with kinematic and chemical evidence of association.
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Authors Aaron M. Meisner, Adam J. Burgasser, Chih-Chun Hsu, Sherelyn Alejandro Merchan, Jacqueline K. Faherty, Genaro Suárez, ZengHua Zhang, Roman Gerasimov, Christian Aganze, Nicolas Lodieu, Eileen C. Gonzales, Adam C. Schneider, Stanimir A. Metchev, Dan Caselden, Michael C. Cushing, Christopher A. Theissen, Jason J. Wang, Harshil Kothari
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Journal monthly notices of the royal astronomical society
Year 2026
DOI
10.1093/mnras/stag2133
URL
https://doi.org/10.1093/mnras/stag2133
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