CFBDS J111807-064016: A new L/T transition brown dwarf in a binary system^⋆

¹ Institut UTINAM, CNRS UMR 6213, Observatoire des Sciences de l’Univers THETA Franche-Comté-Bourgogne, Université de Franche Comté, Observatoire de Besançon, BP 1615, 25010 Besançon Cedex, France
e-mail: celine@obs-besancon.fr
² UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, 38041 Grenoble, France
³ Département de physique and Observatoire du Mont Mégantic, Université de Montréal, CP 6128, Succursale Centre-Ville, Montréal, QC H3C 3J7, Canada
⁴ Centre de Recherche Astrophysique de Lyon, CNRS UMR 5574, Université de Lyon, École Normale Supérieure de Lyon, 46 allée d’Italie, 69364 Lyon Cedex 7, France

Received: 19 June 2013
Accepted: 4 September 2013

Abstract

Binary systems with a substellar companion are quite rare and provide interesting benchmarks. They constrain the complex physics of substellar atmospheres, because several physical parameters of the substellar secondary can be fixed from the much better characterized main-sequence primary. We report the discovery of CFBDS J111807-064016, a T2 brown-dwarf companion to 2MASS J111806.99-064007.8, a low-mass M4.5-M5 star. The brown dwarf was identified from the Canada France Brown Dwarf Survey. At a distance of 50–120 pc, the 7.7′′ angular separation corresponds to projected separations of 390-900 AU. The primary displays no H_α emission, placing a lower limit on the age of the system of about 6 Gyr. The kinematics is also consistent with membership in the old thin disc. We obtained near-infrared spectra, which together with recent atmosphere models allow us to determine the effective temperature and gravity of the two components. We derived a system metallicity of [Fe/H] = −0.1 ± 0.1 using metallicity-sensitive absorption features in our medium-resolution K_s spectrum of the primary. From these parameters and the age constraint, evolutionary models estimate masses of 0.10 to 0.15 M_⊙ for the M dwarf and 0.06 to 0.07 M_⊙ for the T dwarf. This system is a particularly valuable benchmark because the brown dwarf belongs to the early-T class: the cloud-clearing that occurs at the L/T transition is very sensitive to gravity, metallicity, and detailed dust properties, and produces a large scatter in the colours. This T2 dwarf, with its metallicity measured from the primary and its mass and gravity much better constrained than those of younger early-Ts, will anchor our understanding of the colours of L/T transition brown dwarfs. It is also one of the most massive T dwarfs, just below the hydrogen-burning limit, and all this makes it a prime probe for brown-dwarf atmosphere and evolution models.