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

TL;DR

This study enhances the census of cool brown dwarfs near the Sun by combining multi-epoch data to identify high proper motion objects, accurately measure their motions, classify new candidates spectroscopically, and refine the distance and velocity estimates of known objects.

Contribution

It provides improved proper motion measurements for known T5.5-Y0 dwarfs, discovers three new T5.5-T6 brown dwarfs, and refines the parallax of a well-known T8 dwarf, advancing the understanding of nearby cool brown dwarfs.

Findings

Proper motions for 14 known dwarfs were significantly improved.

Three new T5.5-T6 brown dwarfs were discovered and classified.

The new objects are estimated to be 25-30 pc away with velocities typical of the Galactic thin disk.

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.