Research Note

UKIDSS detections of cool brown dwarfs

Proper motions of 14 known  >T5 dwarfs and discovery of three new T5.5-T6 dwarfs^⋆,⋆⋆

Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
e-mail: rdscholz@aip.de; gbihain@aip.de; oschnurr@aip.de; jstorm@aip.de

Received: 2 February 2012
Accepted: 8 April 2012

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 detected high proper motion (HPM) objects with colours typical of late spectral types (>T5). We used 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 which are 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 ± 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^-1, typical of the Galactic thin-disk population.