JADES: Spectroscopic Confirmation and Proper Motion for a T-Dwarf at 2 Kiloparsecs

TL;DR

This study confirms a distant T-dwarf star at 2 kiloparsecs using JWST spectroscopy, revealing its low metallicity, proper motion, and likely galactic halo or thick disk membership, advancing understanding of low-mass objects in the galaxy.

Contribution

First spectroscopic confirmation of a T-dwarf at kiloparsec distances, demonstrating JWST's capability to identify and analyze low-mass stellar objects in extragalactic fields.

Findings

Confirmed T-dwarf at 2 kpc with JWST/NIRSpec spectrum.

Measured proper motion of 20 ± 4 mas/yr.

Indicates low metallicity and likely halo or thick disk origin.

Abstract

Large area observations of extragalactic deep fields with the James Webb Space Telescope (JWST) have provided a wealth of candidate low-mass L- and T-class brown dwarfs. The existence of these sources, which are at derived distances of hundreds of parsecs to several kiloparsecs from the Sun, has strong implications for the low-mass end of the stellar initial mass function, and the link between stars and planets at low metallicities. In this letter, we present a JWST/NIRSpec PRISM spectrum of brown dwarf JADES-GS-BD-9, confirming its photometric selection from observations taken as part of the JWST Advanced Deep Extragalactic Survey (JADES) program. Fits to this spectrum indicate that the brown dwarf has an effective temperature of 800-900K (T5 - T6) at a distance of $1.8 - 2.3$kpc from the Sun, with evidence of the source being at low metallicity ([M/H] $\leq -0.5$). Finally, because of the cadence of JADES NIRCam observations of this source, we additionally uncover a proper motion between the 2022 and 2023 centroids, and we measure a proper motion of $20 \pm 4$ mas yr$^{-1}$ (a transverse velocity of 214 km s$^{-1}$ at 2.25 kpc). At this predicted metallicity, distance, and transverse velocity, it is likely that this source belongs either to the edge of the Milky Way thick disk or the galactic halo. This spectral confirmation demonstrates the efficacy of photometric selection of these important sources across deep extragalactic JWST imaging.