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

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.

A Volume-Limited Sample of Ultracool Dwarfs. I. Construction, Space Density, and a Gap in the L/T Transition

“…Figure 1 shows the spectral type distribution of our sample, featuring a clear deficit at early-T spectral types. This deficit was predicted by evolutionary models (e.g., Saumon & Marley 2008, hereinafter SM08) and seen in previous photometrically-selected samples (Burgasser 2007a;Metchev et al 2008;Reid et al 2008b;Marocco et al 2015;Best et al 2018), and is now confirmed in our volume-limited sample, indicating that brown dwarfs cool through these spectral types (T eff ≈ 1400 − 1100; Luhman et al 2007;Cushing et al 2008;Stephens et al 2009;King et al 2010;Deacon et al 2012a,b;Reylé et al 2014;Filippazzo et al 2015;Deacon et al 2017a; in a relatively short time. We also note an uneven distribution of L0-L5 dwarfs whose origin is unclear given that our sample is 90% complete at these types (Section 2.3).…”

COol Companions ON Ultrawide orbiTS (COCONUTS). I. A High-gravity T4 Benchmark around an Old White Dwarf and a Re-examination of the Surface-gravity Dependence of the L/T Transition

“…The only object in our sample whose L bol was not derived from these three approaches is CFBDS J111807 − 064016, for whichReylé et al (2014) interpolated theBaraffe et al (2003) evolutionary models using the age of the primary star and the object's T eff derived from atmospheric models(Allard et al 2012(Allard et al , 2013 to compute the L bol .…”