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
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
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 Hα 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 Ks 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.
Key words: stars: individual: CFBDS J111807-064016 / stars: individual: 2MASS J111806.99-064007.8 / stars: low-mass / brown dwarfs / binaries: general
Based on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/DAPNIA, at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council (NRC) of Canada, the Institut National des Sciences de l’Univers of the Centre National de la Recherche Scientifique (CNRS) of France, and the University of Hawaii. This work is based in part on data products produced at TERAPIX and the Canadian Astronomy Data Centre as part of the Canada-France-Hawaii Telescope Legacy Survey, a collaborative project of NRC and CNRS. Based on observations made with the ESO New Technology Telescope at the La Silla Observatory under programme ID 082.C-0506(A) and the ESO Very Large Telescope at Paranal Observatory under programme ID 385.C-0242(A). Based on observations (director discretionary time) made with the Canada-France-Hawaii Telescope at Mauna Kea Observatory.
© ESO, 2014