Blue not brown: UKIRT Infrared Deep Sky Survey T dwarfs with suppressed K-band flux

Abstract

We have used blue near-infrared colours to select a group of 12 spectroscopically confirmed United Kingdom InfraRed Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS) T dwarfs later than T4. From amongst these, we identify the first two kinematic halo T-dwarf candidates. Blue near-infrared colours have been attributed to collisionally induced hydrogen absorption, which is enhanced by either high surface gravity or low metallicity. Proper motions are measured and distances estimated, allowing the determination of tangential velocities. U and V components are estimated for our objects by assuming Vrad= 0. From this, ULAS J0926+0835 is found to have U= 62 km s-1 and V=-140 km s-1, and ULAS J1319+1209 is found to have U= 192 km s-1 and V=-92 km s-1. These values are consistent with potential halo membership. However, these are not the bluest objects in our selection. The bluest is ULAS J1233+1219, with J-K=-1.16 ± 0.07, and surprisingly this object is found to have young disc-like U and V. Our sample also contains Hip 73786B, companion to the metal-poor K5 dwarf Hip 73786. Hip 73786 is a metal-poor star, with [Fe/H] =-0.3 ± 0.1 and is located at a distance of 19 ± 0.7 pc. U, V, W space velocity components are calculated for Hip 73786A and B, finding that U=-48 ± 7 km s-1, V=-75 ± 4 km s-1 and W=-44 ± 8 km s-1. From the properties of the primary, Hip 73786B is found to be at least 1.6-Gyr old. As a metal-poor object, Hip 73786B represents an important addition to the sample of known T dwarf benchmarks.