Blue not brown: UKIDSS T dwarfs with suppressed K-band flux

Abstract: We have used blue near-infrared colours to select a group of 12 spectroscopically-confirmed 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 radial velocity = 0 km/s. From this, ULAS J0926+0835 is found to have U=62 km/s and V=-140km/s and ULAS J1319+1209 is found to have U=192 km/s and V=-92 km/s. 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 and is located at a distance of 19 pc. U, V, W space velocity components are calculated for Hip 73786A and B, finding that U=-48 +/- 7 km/s, V=-75 +/- 4 km/s and W=-44 +/- 8 km/s. 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.