First T dwarfs in the VISTA Hemisphere Survey^⋆,⋆⋆

¹ Instituto de Astrofísica de Canarias (IAC), Calle Vía Láctea s/n, 38200 La Laguna, Tenerife Spain
e-mail: nlodieu@iac.es
² Departamento de Astrofísica, Universidad de La Laguna (ULL), 38205, La Laguna, Tenerife, Spain
³ Centre for Astrophysics Research, Science and Technology Research Institute, University of Hertfordshire, Hatfield AL10 9AB, UK
⁴ Departamento de Astronomia, Universidad de Chile, Casilla 36-D, Santiago, Chile
⁵ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁶ Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK
⁷ Sternberg Astronomical Institute, Lomonosov Moscow State University, Universitetsky pr. 13, 119992 Moscow, Russia
⁸ Centro Astronómico Hispano Alemán, Calle Jesús Durbán Remón 2-2, 04004 Almería, Spain
⁹ Departamento de Astrofisica, Centro de Astrobiologia (INTA-CSIC), ESAC Campus, PO Box 78, 28691 Villanueva de la Cañada, Spain
¹⁰ Universidad Politécnica de Cartagena, Campus Muralla del Mar, Cartagena, 30202 Murcia, Spain
¹¹ School of Physics & Astronomy, Queen Mary University of London, Mile End Road, London E1 4NS, UK

Received: 7 August 2012
Accepted: 8 October 2012

Abstract

Aims. The aim of the project is to improve our current knowledge of the density of T dwarfs and the shape of the substellar initial mass function by identifying a magnitude-limited sample of T 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 at infrared 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 preliminary data release to extract candidates with red mid-infrared colours and near- to mid-infrared colours characteristics of cool brown dwarfs.

Results. The VHS DR1 vs. WISE search returned tens of T dwarf candidates, 13 of which are presented here, including two previously published in the literature and five new ones confirmed spectroscopically with spectral types between T4.5 and T8. We estimate that 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 in our sample is the faintest of its spectral class with J = 19.28 mag. The other six T dwarf candidates remain without spectroscopic follow-up. We also improve our knowledge on the proper motion accuracy for three bright T dwarfs by combining multi-epoch data from public databases (DENIS, 2MASS, VHS, WISE, Spitzer).