New brown dwarfs in Upper Sco using UKIDSS Galactic Cluster Survey science verification data

We present first results from a deep (J = 18.7), wide-field (6.5 deg(2)) infrared (ZY JHK) survey in the Upper Sco association conducted within the science verification phase of the United Kingdom Infrared Telescope Infrared Deep Sky Survey Galactic Cluster Survey (GCS). Cluster members define a sequence well separated from field stars in the (Z - J, Z) colour-magnitude diagram. We have selected a total of 164 candidates with J = 10.5-18.7 mag from the (Z - J, Z) and (Y - J, Y) diagrams. We further investigated the location of those candidates in the other colour-magnitude and colour-colour diagrams to weed out contaminants. The cross-correlation of the GCS catalogue with the Two-Micron All-Sky Survey data base confirms the membership of 116 photometric candidates down to 20 Jupiter masses as they lie within a 2 sigma circle centred on the association mean motion. The final list of cluster members contains 129 sources with masses between 0.3 and 0.007 M-circle dot. We extracted a dozen new low-mass brown dwarfs below 20 M-Jup, the limit of previous surveys in the region. Finally, we have derived the mass function in Upper Sco over the 0.3-0.01 M-circle dot mass range, best fit by a single segment with a slope of index alpha = 0.6 +/- 0.1, in agreement with previous determination in open clusters.