WISE J061213.85-303612.5: a new T-dwarf binary candidate

Abstract

Context. T and Y dwarfs are among the coolest and least luminous objects detected, and they can help to understand the properties of giant planets. Up to now, there are more than 350 T dwarfs that have been identified thanks to large imaging surveys in the infrared, and their multiplicity properties can shed light on the formation process. Aims. The aim of this work is to look for companions around a sample of seven ultracoool objects. Most of them have been discovered by the WISE observatory and have not been studied before for multiplicity. Methods. We observed a sample six T dwarfs and one L9 dwarf with the Laser Guide Star (LGS) and NAOS-CONICA, the adaptive optics (AO) facility, and the near infrared camera at the ESO Very Large Telescope. We observed all the objects in one or more near-IR filters (JHK s ). Results. From the seven observed objects, we have identified a subarcsecond binary system, WISE J0612-3036, composed of two similar components with spectral types of T6. We measure a separation of ρ = 350 ± 5 mas and a position angle of PA = 235 ± 1 • . Using the mean absolute magnitudes of T6 dwarfs in the 2MASS JHK s bands, we estimate a distance of d = 31 ± 6 pc and derive a projected separation of ρ ∼ 11 ± 2 au. Another target, WISE J2255-3118, shows a very faint object at 1. 3 in the K s image. The object is marginally detected in H, and we derive a near infrared color of H − K s > 0.1 mag. HST/WFC3 public archival data reveals that the companion candidate is an extended source. Together with the derived color, this suggests that the source is most probably a background galaxy. The five other sources are apparently single, with 3-σ sensitivity limits between H = 19-21 for companions at separations ≥0. 5. Conclusions. WISE 0612-3036 is probably a new T-dwarf binary composed of two T6 dwarfs. As in the case of other late T-dwarf binaries, it shows a mass ratio close to 1, although its projected separation, ∼11 au, is larger than the average (∼5 au). Additional observations are needed to confirm that the system is bound.

Constraining the multiplicity statistics of the coolest brown dwarfs: binary fraction continues to decrease with spectral type

“…Burgasser et al (2003Burgasser et al ( , 2006 probed T-dwarfs with spectral types spanning from T0−T8 and measured binary rates of ∼10%, with all identified systems having separations <5 AU and mass ratios >0.8. These results confirm the idea of a decreasing binary fraction within the brown dwarf mass regime and suggest a more compact and symmetric substellar binary population at later spectral types (Huélamo et al 2015).…”

“…This fraction decreases to 50−60% for Solar-type stars (Duquennoy & Mayor 1991;Raghavan et al 2010) and around 30−40% of M-stars are found in multiple systems (Fischer & Marcy 1992;Delfosse et al 2004;Janson et al 2012). Surveys probing old (1−10 Gyr) brown dwarfs from the field (Close et al 2003;Burgasser et al 2006;Gelino et al 2011;Huélamo et al 2015) observed a substantially lower binary rate (∼10−20%) than in the stellar population, extending the trend of a decreasing binary fraction with later spectral type seen in the stellar regime. As stellar binary frequency decreases with decreasing primary mass, the semi-major axis distribution peaks at closer separations and mass ratios shift towards unity.…”

Properties of the Nearby Brown Dwarf Wisep J180026.60+013453.1

“…The space motion is consistent with the young thin disk, and W1800+0134 has a 100% chance of being a field object and a 0% chance of belonging to any known moving group according to the model of Gagné et al (2014). As discussed in the discovery paper, there is no spectroscopic evidence of an unresolved companion; in Huélamo et al (2015)'s survey, W1800+0134 did not show any companion down to 0.3 arcseconds in the H-band. The apparent magnitudes and parallax are consistent with a single L7.5 dwarf.…”

“…We wish to recognize C. Luginbuhl, J. Munn, and T. Tilleman for their many contributions to the USNO infrared astrometry program. We thank Radostin Kurtev for helpful discussions of the Huélamo et al (2015) imaging survey. This work is based in part on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Science and Technology Facilities Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), CNPq (Brazil) and CONICET (Argentina).…”

Spectral characterization of newly detected young substellar binaries with SINFONI

“…As such, several substellar binaries in the Galactic field have been identified; many being old, cool, and of very late spectral type (e.g. Gelino et al 2011;Dupuy et al 2015;Huélamo et al 2015). Efforts have also been put into constraining young stellar associations and Young Moving Groups (YMGs) (e.g.…”