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  1. Abstract

    Y dwarfs, the coolest known spectral class of brown dwarfs, overlap in mass and temperature with giant exoplanets, providing unique laboratories for studying low-temperature atmospheres. However, only a fraction of Y dwarf candidates have been spectroscopically confirmed. We present Keck/NIRES near-infrared spectroscopy of the nearby (d≈ 6–8 pc) brown dwarf CWISE J105512.11+544328.3. Although its near-infrared spectrum aligns best with the Y0 standard in theJband, no standard matches well across the fullYJHKwavelength range. The CWISE J105512.11+544328.3 NH3-H= 0.427 ± 0.0012 and CH4-J= 0.0385 ± 0.0007 absorption indices and absolute Spitzer [4.5] magnitude of 15.18 ± 0.22 are also indicative of an early-Y dwarf rather than a late-T dwarf. CWISE J105512.11+544328.3 additionally exhibits the bluest Spitzer [3.6]−[4.5] color among all spectroscopically confirmed Y dwarfs. Despite this anomalously blue Spitzer color given its low luminosity, CWISE J105512.11+544328.3 does not show other clear kinematic or spectral indications of low metallicity. Atmospheric model comparisons yield a log(g) ≤ 4.5 andTeff≈ 500 ± 150 K for this source. We classify CWISE J105512.11+544328.3 as a Y0 (pec) dwarf, adding to the remarkable diversity of the Y-type population. JWST spectroscopy would be crucial to understanding the origin of this Y dwarf’s unusual preference for low-gravity models and blue 3–5μm color.

     
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  2. Abstract

    We present spectroscopic confirmation of a nearby L dwarf pair, CWISE J061741.79+194512.8AB. Keck/NIRES near-infrared spectroscopy shows that the pair is composed of an L2 dwarf primary and an L4 dwarf secondary. High resolution spectroscopy of the combined light system with Keck/NIRSPEC yields a radial velocity of 29.2 ± 0.3 km s−1and a projected rotational velocityvsini=41.62.6+2.7km s−1. Our spectrophotometric distance estimate places the system at 28.2 ± 5.7 pc, significantly more distant than originally estimated in Kirkpatrick et al. The angular separation of the components is 1.″31 ± 0.″14, corresponding to a projected physical separation of 37 ± 8 au.

     
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  3. Abstract

    We present the discovery of CWISE J050626.96+073842.4 (CWISE J0506+0738), an L/T transition dwarf with extremely red near-infrared colors discovered through the Backyard Worlds: Planet 9 citizen science project. Photometry from UKIRT and CatWISE give a (JK)MKOcolor of 2.97 ± 0.03 mag and aJMKO− W2 color of 4.93 ± 0.02 mag, making CWISE J0506+0738 the reddest known free-floating L/T dwarf in both colors. We confirm the extremely red nature of CWISE J0506+0738 using Keck/NIRES near-infrared spectroscopy and establish that it is a low-gravity, late-type L/T transition dwarf. The spectrum of CWISE J0506+0738 shows possible signatures of CH4absorption in its atmosphere, suggesting a colder effective temperature than other known, young, red L dwarfs. We assign a preliminary spectral type for this source of L8γ–T0γ. We tentatively find that CWISE J0506+0738 is variable at 3–5μm based on multiepoch WISE photometry. Proper motions derived from follow-up UKIRT observations combined with a radial velocity from our Keck/NIRES spectrum and a photometric distance estimate indicate a strong membership probability in theβPic moving group. A future parallax measurement will help to establish a more definitive moving group membership for this unusual object.

     
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  4. Abstract We present medium-resolution ( λ /Δ λ  = 2700), near-infrared spectral standards for field L0–L2, L4, and L7–Y0 dwarfs obtained with the Near-Infrared Echellette Spectrometer on the Keck II 10 m telescope. These standards allow for detailed spectral comparative analysis of cold brown dwarfs discovered through ongoing ground-based projects such as Backyard Worlds: Planet 9, and forthcoming space-based spectral surveys such as the James Webb Space Telescope, SPHEREx, Euclid, and the Nancy Grace Roman Space Telescope. 
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  5. Abstract We have used data from the UKIRT Hemisphere Survey to search for substellar members of the Hyades cluster. Our search recovered several known substellar Hyades members, and two known brown dwarfs that we suggest may be members based on a new kinematic analysis. We uncovered thirteen new substellar Hyades candidates, and obtained near-infrared follow-up spectroscopy of each with IRTF/SpeX. Six candidates with spectral types between M7 and L0 are ruled out as potential members based on their photometric distances (≳100 pc). The remaining seven candidates, with spectral types between L5 and T4, are all potential Hyades members, with five showing strong membership probabilities based on BANYAN Σ and a convergent point analysis. Distances and radial velocities are still needed to confirm Hyades membership. If confirmed, these would be some of the lowest mass free-floating members of the Hyades yet known, with masses as low as ∼30 M Jup . An analysis of all known substellar Hyades candidates shows evidence that the full extent of the Hyades has yet to be probed for low-mass members, and more would likely be recovered with deeper photometric and astrometric investigations. 
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  6. Abstract

    We present the discovery of VVV J165507.19−421755.5, a mid-T dwarf found through ongoing unWISE-based proper motion searches. A near-infrared spectrum of this object obtained with the NIRES instrument on the Keck II telescope indicates a spectral classification of T5. Using data from the VISTA Variables in the Via Lactea (VVV) catalog with a 9 year baseline, we measure a proper motion of (μαcos(δ),μδ) = (−631.0 ± 1.3, −315.0 ± 1.4) mas yr−1and a trigonometric parallax ofπabs = 66.0 ± 4.8 mas, corresponding to a distance of 15.2 ± 1.1 pc. The trigonometric parallax agrees well with our photometric distance estimate (16.13.9+5.1pc) assuming that VVV J165507.19−421755.5 is a single T5 dwarf. VVV J165507.19−421755.5 is a new member of the 20 parsec census.

     
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  7. Abstract While stars are often found in binary systems, brown dwarf binaries are much rarer. Brown dwarf–brown dwarf pairs are typically difficult to resolve because they often have very small separations. Using brown dwarfs discovered with data from the Wide-field Infrared Survey Explorer (WISE) via the Backyard Worlds: Planet 9 citizen science project, we inspected other, higher-resolution, sky surveys for overlooked cold companions. During this process, we discovered the brown dwarf binary system CWISE J0146−0508AB, which we find has a very small chance alignment probability based on the similar proper motions of the components of the system. Using follow-up near-infrared spectroscopy with Keck/NIRES, we determined component spectral types of L4 and L8 (blue), making CWISE J0146−0508AB one of only a few benchmark systems with a blue L dwarf. At an estimated distance of ∼40 pc, CWISE J0146−0508AB has a projected separation of ∼129 au, making it the widest-separation brown dwarf pair found to date. We find that such a wide separation for a brown dwarf binary may imply formation in a low-density star-forming region. 
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  8. null (Ed.)