UKIDSS detections of cool brown dwarfs - proper motions of 14 known $>$T5 dwarfs and discovery of three new T5.5-T6 dwarfs

Abstract

AIMS: We contribute to improving the census of cool brown dwarfs (late-T and Y dwarfs) in the immediate solar neighbourhood. METHODS: By combining near-infrared (NIR) data of UKIDSS with mid-infrared WISE and other available NIR (2MASS) and red optical (SDSS $z$-band) multi-epoch data we detect high proper motion (HPM) objects with colours typical of late spectral types ($>$T5). We use NIR low-resolution spectroscopy for the classification of new candidates. RESULTS: We determined new proper motions for 14 known T5.5-Y0 dwarfs, many of them being significantly ($>$2-10 times) more accurate than previous ones. We detected three new candidates, ULAS J0954+0623, ULAS J1152+0359, and ULAS J1204-0150, by their HPMs and colours. Using previously published and new UKIDSS positions of the known nearby T8 dwarf WISE J0254+0223 we improved its trigonometric parallax to 165$\pm$20 mas. For the three new objects we obtained NIR spectroscopic follow-up with LBT/LUCIFER classifying them as T5.5 and T6 dwarfs. With their estimated spectroscopic distances of about 25-30 pc, their proper motions of about 430-650 mas/yr lead to tangential velocities of about 50-80 km/s typical of the Galactic thin disk population.

5 pages, 2 figures, plus 3 pages with 5 tables (online material), accepted for publication in Astronomy and Astrophysics

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• Seventy six T dwarfs from the UKIDSS LAS: benchmarks, kinematics and an updated space density
• A Volume-Limited Sample of Ultracool Dwarfs. I. Construction, Space Density, and a Gap in the L/T Transition
• The Hawaii Infrared Parallax Program. IV. A Comprehensive Parallax Survey of L0-T8 dwarfs with UKIRT
• A Proper Motion Survey Using the First Sky Pass of NEOWISE-Reactivation Data
• Trigonometric parallaxes of young field L dwarfs
• Photometric brown-dwarf classification. II. A homogeneous sample of 1361 L and T dwarfs brighter than J = 17.5 with accurate spectral types
• Characterization of High Proper Motion Objects from the Wide-field Infrared Survey Explorer
• Neighbours hiding in the Galactic plane - a new M/L dwarf candidate for the 8pc sample
• A Brown Dwarf Census from the SIMP Survey
• First T dwarfs in the VISTA Hemisphere Survey
• An overlooked brown dwarf neighbour (T7.5 at d~5pc) of the Sun and two additional T dwarfs at about 10pc
• The Collapse of the Wien Tail in the Coldest Brown Dwarf? Hubble Space Telescope Near-Infrared Photometry of WISE J085510.83-071442.5
• A low-mass triple system with a wide L/T transition brown dwarf component: NLTT 51469AB/SDSS 2131-0119