First T dwarfs in the VISTA Hemisphere Survey

School of Physics & Astronomy, Queen Mary University of London, Mile End Road, London E1 4NS, United KingdomOctober 19, 2012; October 19, 2012ABSTRACTAims.The aim of the project is to improve our current knowledge of the density of T dwarfs and the shape of the substellar initialmass function by identifying a magnitude-limited sample ofT dwarfs in the full southern sky.Methods. We present the results of a photometric search aimed at discovering cool brown dwarfs in the Southern sky imaged atinfrared wavelengths by the Visible and Infrared Survey Telescope for Astronomy (VISTA) and the Wide Infrared Survey Explorer(WISE) satellite mission. We combined the first data release (DR1) of the VISTA Hemisphere Survey (VHS) and the WISE prelimi-nary data release to extract candidates with red mid-infrared colours and near- to mid-infrared colours characteristics of cool browndwarfs.Results.The VHS DR1 vs. WISE search returned tens of T dwarf candidates, 13 of which are presented here, including two previ-ously published in the literature and five new ones confirmed s pectroscopically with spectral types between T4.5 and T8. We estimatethat the two T6 dwarfs lie within 16 pc and the T4.5 within 25 pc. The remaining three are 30–50 pc distant. The only T7 dwarf inour sample is the faintest of its spectral class with J =19.28 mag. The other six T dwarf candidates remain without spectroscopicfollow-up. We also improve our knowledge on the proper motion accuracy for three bright T dwarfs by combining multi-epoch datafrom public databases (DENIS, 2MASS, VHS, WISE, Spitzer).Key words. Stars: low-mass stars and brown dwarfs — techniques: photom etric — techniques: spectroscopic — Infrared: Stars —surveys