Identifying nearby field T dwarfs in the UKIDSS Galactic Clusters Survey

Abstract

We present the discovery of two new late-T dwarfs identified in the UKIRT Infrared Deep Sky Survey (UKIDSS) Galactic Clusters Survey (GCS) Data Release 2 (DR2). These T dwarfs are nearby old T dwarfs along the line of sight to star-forming regions and open clusters targeted by the UKIDSS GCS. They are found towards the α Per cluster and Orion complex, respectively, from a search in 54 deg2 surveyed in five filters. Photometric candidates were picked up in two-colour diagrams, in a very similar manner to candidates extracted from the UKIDSS Large Area Survey (LAS) but taking advantage of the Z filter employed by the GCS. Both candidates exhibit near-infrared J-band spectra with strong methane and water absorption bands characteristic of late-T dwarfs. We derive spectral types of T6.5 ± 0.5 and T7 ± 1 and estimate photometric distances less than 50 pc for UGCS J030013.86+490142.5 and UGCS J053022.52−052447.4, respectively. The space density of T dwarfs found in the GCS seems consistent with discoveries in the larger areal coverage of the UKIDSS LAS, indicating one T dwarf in 6–11 deg2. The final area surveyed by the GCS, 1000 deg2 in five passbands, will allow expansion of the LAS search area by 25 per cent, increase the probability of finding ultracool brown dwarfs, and provide optimal estimates of contamination by old field brown dwarfs in deep surveys to identify such objects in open clusters and star-forming regions.