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

We have used blue near-infrared colours to select a group of 12 spectroscopicallyconfirmed 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 kms 1 and V = 140 kms 1 and ULAS J1319+1209 is found to have U = 192 kms 1 and V = 92 kms 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;Wspace velocity components are calculated for Hip 73786A and B, finding that U = 48 ± 7 kms 1 , V = 75 ± 4 kms 1 and W = 44 ± 8 kms 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.