The ultracool-field dwarf luminosity-function and space density from the Canada-France Brown Dwarf Survey^⋆,⋆⋆

¹ Observatoire de Besançon, Université de Franche-Comté, Institut Utinam, UMR CNRS 6213 , BP 1615, 25010 Besançon Cedex, France
e-mail: celine@obs-besancon.fr
² School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews KY16 9SS, UK
³ Herzberg Institute of Astrophysics, National Research Council, 5071 West Saanich Rd, Victoria, BC V9E 2E7, Canada
⁴ Canada-France-Hawaii Telescope Corporation, 65-1238 Mamalahoa Highway, Kamuela, HI96743, USA
⁵ Laboratoire d’Astrophysique de Grenoble, Université J. Fourier, CNRS, UMR5571, Grenoble, France
⁶ Département de physique and Observatoire du Mont Mégantic, Université de Montréal, C.P. 6128, Succursale Centre-Ville, Montréal, QC H3C 3J7, Canada
⁷ McDonald Observatory, University of Texas at Austin, 1 University Station C1402, Austin, TX 78712-0259, USA

Received: 3 September 2009
Accepted: 7 August 2010

Abstract

Context. Thanks to recent and ongoing large scale surveys, hundreds of brown dwarfs have been discovered in the last decade. The Canada-France Brown Dwarf Survey is a wide-field survey for cool brown dwarfs conducted with the MegaCam camera on the Canada-France-Hawaii Telescope.

Aims. Our objectives are to find ultracool brown dwarfs and to constrain the field brown-dwarf luminosity function and the mass function from a large and homogeneous sample of L and T dwarfs.

Methods. We identify candidates in CFHT/MegaCam i′ and z′ images and follow them up with pointed near infrared (NIR) imaging on several telescopes. Halfway through our survey we found  ~ 50 T dwarfs and  ~ 170 L or ultra cool M dwarfs drawn from a larger sample of 1400 candidates with typical ultracool dwarfs i′ − z′ colours, found in 780 square degrees.

Results. We have currently completed the NIR follow-up on a large part of the survey for all candidates from mid-L dwarfs down to the latest T dwarfs known with utracool dwarfs’ colours. This allows us to draw on a complete and well defined sample of 102 ultracool dwarfs to investigate the luminosity function and space density of field dwarfs.

Conclusions. We found the density of late L5 to T0 dwarfs to be objects pc^-3, the density of T0.5 to T5.5 dwarfs to be objects pc^-3, and the density of T6 to T8 dwarfs to be objects pc^-3. We found that these results agree better with a flat substellar mass function. Three latest dwarfs at the boundary between T and Y dwarfs give the high density  objects pc^-3. Although the uncertainties are very large this suggests that many brown dwarfs should be found in this late spectral type range, as expected from the cooling of brown dwarfs, whatever their mass, down to very low temperature.