Astrometric and photometric initial mass functions from the UKIDSS Galactic Clusters Survey: IV Upper Sco

Abstract: We present the results of a proper motion wide-field near-infrared survey of the entire Upper Sco (USco) association (~160 square degrees) released as part of the UKIRT Infrared Deep Sky (UKIDSS) Galactic Clusters Survey (GCS) Data Release 10 (DR10). We have identified a sample of ~400 astrometric and photometric member candidates combining proper motions and photometry in five near-infrared passbands and another 286 with HK photometry and 2MASS/GCS proper motions. We also provide revised membership for all previously published USco low-mass stars and substellar members based on our selection and identify new candidates, including in regions affected by extinction. We find negligible variability between the two $K$-band epochs, below the 0.06 mag rms level. We estimate an upper limit of 2.2% for wide common proper motions with projected physical separations less than ~15000 au. We derive a disk frequency for USco low-mass stars and brown dwarfs between 26 and 37%, in agreement with estimates in IC348 and sigma Ori. We derive the mass function of the association and find it consistent with the (system) mass function of the solar neighbourhood and other clusters surveyed by the GCS in the 0.2-0.03 Msun mass range. We confirm the possible excess of brown dwarfs in USco.