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
Year: 2026
Type: preprint
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 $μ$m spectroscopic coverage that reveals the presence of H$_2$O, 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 $T_{\rm eff}$ = 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$_{10}L_{bol}/L_{\odot}$ = -5.29 $\pm$ 0.10, from which we infer $T_{\rm eff}$ = 974 $\pm$ 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 $\pm$ 5 km/s, which combined with the source's measured proper motion and estimated distance indicates a highly eccentric (e $\geq$ 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.
Source: arXiv (Cornell University)
Authors: Meisner, Aaron M., Burgasser, Adam J., Hsu, Chih-Chun, Merchan, Sherelyn Alejandro, Faherty, Jacqueline K., Suárez, Genaro, Zhang, ZengHua, Gerasimov, Roman, Aganze, Christian, Lodieu, Nicolas, Gonzales, Eileen C., Schneider, Adam C., Metchev, Stanimir A., Caselden, Dan, Cushing, Michael C., Theissen, Christopher A., Wang, Jason J., Kothari, Harshil
Institutions: University of California San Diego, Northwestern University, Nanjing University, University of Notre Dame, Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, Universidad de La Laguna, Instituto de Astrofísica de Canarias, San Francisco State University, United States Naval Observatory, U.S. Naval Observatory Flagstaff Station, Western University, American Museum of Natural History, University of Toledo
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
Awards: G5070458386, G8266622939, G8920660035
OpenAlex ID: W7214341755