2011MNRAS.418.2604L

We present the results of a deep (J ∼ 21 at 5σ) infrared photometric survey of a 0.95-deg² area in the central region of the Upper Sco association. The photometric observations consist of a deep Y+J-band image obtained with the Wide Field Camera on the United Kingdom Infra-Red Telescope (UKIRT) with part coverage in Z complemented by the methane ON and OFF imaging survey conducted with the Wide-field InfraRed Camera on the Canada–France–Hawaii Telescope. We have selected five potential T-type objects belonging to the Upper Sco association on the basis of their blue methane colours and their J- CH_4 off_colours. We have also identified a sample of seven to eight Upper Sco member candidates bridging the gap between known cluster M types and our new T-type candidates. These candidates were selected based on their positions in various colour–magnitude diagrams, and they follow the sequence of known Upper Sco members identified in the UKIRT Infrared Deep Sky Survey Galactic Clusters Survey. We present additional membership constraints using proper motion estimates from the multiple epochs available with us. We also present optical and near-infrared spectra obtained with the X-Shooter spectrograph on the Very Large Telescope for five L-type candidates covering the 0.6–2.5 µm wavelength range, none of which is confirmed as a young brown dwarf. We discuss the lack of detection of new candidate members as well as the possible turn-down in the Upper Sco mass function as we are approaching the fragmentation limit.