VVV J165507.19-421755.5: A Nearby T Dwarf Hidden in the Galactic Plane

VVV J165507.19−421755.5 (J1655−4217) was initially discovered through "SMDET" machine learning analysis (Caselden et al. 2020) of unWISE coadds (Meisner et al. 2018). Although not immediately visible to the human eye in these coadds, J1655−4217 was visually confirmed to be a moving point source in imaging data from the Vista Variables in the Via Lactea survey (VVV; Minniti et al. 2010) over a 5.3 year baseline (Figure 1). Once its status as a candidate nearby source was confirmed, its spectrum was obtained using the Near-Infrared Echellette Spectrometer (NIRES; Wilson et al. 2004) on the Keck II 10 m Telescope.

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We obtained Keck/NIRES spectroscopy of J1655−4217 on the night of 2022 June 11 (UT) in clear and dry conditions with 05 seeing. NIRES is a cross-dispersed spectrometer, providing 1–2.4 μm spectroscopy at an average resolution of λ/Δλ ≈ 2700 for its 055 slit (Wilson et al. 2004). Four exposures of 250 s each were obtained, followed by an observation of the A0 V HD 154409 for flux and telluric calibration. Data were reduced using a modified version of the Spextool package (Cushing et al. 2004) using standard settings.

We analyzed a smoothed (30 pixels) and normalized version of the reduced NIRES spectrum using tools in the SPLAT Python library (Burgasser & Splat Development Team 2017). We compared the spectrum of J1655−4217 to T dwarf spectral standards (Burgasser et al. 2006; Theissen et al. 2022), and found a best overall fit (minimum χ²) to the T5 standard (Figure 1). This is also a good visual match, with no spectral peculiarities indicative of low surface gravity or unresolved multiplicity.

We obtained preliminary proper motion and parallax measurements from "VIRAC2", version 2 of the VVV Infrared Astrometric Catalogue (VIRAC; Smith et al. 2018). A total of 126 VVV epochal detections spanning a 9 yr time baseline were used for the astrometric fit. The VIRAC2 proper motion is (${\mu }_{\alpha }\cos (\delta )$, μ_δ ) = (−631.0 ± 1.3, −315.0 ± 1.4) mas yr⁻¹ and the corresponding trigonometric parallax measurement is π_abs = 66.0 ± 4.8 mas, corresponding to 15.2 ± 1.1 pc. The total proper motion is 705.3 ± 1.3 mas yr⁻¹ and the tangential velocity is 50.6 ± 3.7 km s⁻¹.

Using the individual VVV detections, we determined an average apparent K-band magnitude of 15.80 ± 0.07 mag (Vega). We then used the proper motion trajectory to identify J-band and H-band counterparts in the VVV data, and from these determined an average J-band (H-band) Vega apparent magnitude of 15.56 ± 0.01 (15.58 ± 0.02) mag. The implied J-band, H-band, and K-band absolute magnitudes (using the VIRAC2 trigonometric parallax) are all consistent with those of other field T5 dwarfs within 1σ (Dupuy & Liu 2012; Kirkpatrick et al. 2021). Note that the region surrounding J1655−4217 is too crowded in WISE (FWHM ≈ 6''; Wright et al. 2010) to extract accurate W1 or W2 flux information. This area was also imaged by Spitzer/GLIMPSE360 (Churchwell et al. 2009) in 2004, but J1655−4217 is badly contaminated by a similarly bright background source at that epoch.

We conclude that J1655−4217 is a new T5 brown dwarf member of the 20 pc solar neighborhood census (Kirkpatrick et al. 2021). Future studies can expand upon our measurements, including determination of its radial velocity for full kinematic analysis. J1655−4217 was likely overlooked in previous VVV astrometric surveys due to blending in several epochs. While its absolute magnitudes are consistent with a single source, J1655−4217's location in a crowded stellar field makes it an excellent adaptive optics target to search for fainter and cooler companions. Furthermore, the crowded field surrounding J1655−4217 and its accurately measured proper motion make this object a promising target for a future microlensing-based determination of its mass. The discovery of J1655−4217 reinforces the continued incompleteness of the brown dwarf census in the Galactic plane.

This work has been supported in part by the NASA Citizen Science Seed Funding Program, grant 80NSSC21K1485. This material is based upon work supported by the National Science Foundation under grant Nos. 2007068, 2009136, and 2009177.

This work used the high-performance computing facility of University of Hertfordshire. We gratefully acknowledge data from the ESO Public Survey program ID 179.B-2002 taken with the VISTA telescope, and products from the Cambridge Astronomical Survey Unit (CASU).

Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.

This publication makes use of data products from the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration. This publication also makes use of data products from NEOWISE, which is a project of the Jet Propulsion Laboratory/California Institute of Technology, funded by the Planetary Science Division of the National Aeronautics and Space Administration.

Facilities: Keck(NIRES), NEOWISE, Spitzer(IRAC), VISTA(VIRCAM), WISE.

Software: SMDET (Caselden et al. 2020), Spextool (Cushing et al. 2004), SPLAT (Burgasser & Splat Development Team 2017), WiseView (Caselden et al. 2018).