JWST Observations of the Metal-poor T Dwarf WISEA J155349.96+693355.2: The First Brown Dwarf Associated with the Gaia-Enceladus Milky Way Substructure
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, 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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BibTeX
@misc{meisner2026jwst,
title = {{JWST Observations of the Metal-poor T Dwarf WISEA J155349.96+693355.2: The First Brown Dwarf Associated with the Gaia-Enceladus Milky Way Substructure}},
author = {Aaron M. Meisner and Adam J. Burgasser and Chih-Chun Hsu and Sherelyn Alejandro Merchan and Jacqueline K. Faherty and Genaro Suárez and ZengHua Zhang and Roman Gerasimov and Christian Aganze and Nicolas Lodieu and Eileen C. Gonzales and Adam C. Schneider and Stanimir A. Metchev and Dan Caselden and Michael C. Cushing and Christopher A. Theissen and Jason J. Wang and Harshil Kothari},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.29365},
eprint = {2609.29365},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.29365},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - JWST Observations of the Metal-poor T Dwarf WISEA J155349.96+693355.2: The First Brown Dwarf Associated with the Gaia-Enceladus Milky Way Substructure
AU - Aaron M. Meisner
AU - Adam J. Burgasser
AU - Chih-Chun Hsu
AU - Sherelyn Alejandro Merchan
AU - Jacqueline K. Faherty
AU - Genaro Suárez
AU - ZengHua Zhang
AU - Roman Gerasimov
AU - Christian Aganze
AU - Nicolas Lodieu
AU - Eileen C. Gonzales
AU - Adam C. Schneider
AU - Stanimir A. Metchev
AU - Dan Caselden
AU - Michael C. Cushing
AU - Christopher A. Theissen
AU - Jason J. Wang
AU - Harshil Kothari
PY - 2026
DA - 2026/09/26
PB - arXiv
DO - 10.48550/arxiv.2609.29365
UR - https://arxiv.org/abs/2609.29365
AB - 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 H_2O, CH_4, and NH_3 absorption features, enhanced collision-induced H_2 absorption, and weak or absent CO and CO_2 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 T_eff = 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 log_10L_bol/L_⊙ = -5.29 ± 0.10, from which we infer T_eff = 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, 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.
ER -