* Astronomy

Title: WISE J061213.85-303612.5: a new T-dwarf binary candidate
Author: N. Huélamo, V.D. Ivanov, R. Kurtev, J.H. Girard, J. Borissova, D. Mawet, K. Muzic, C. Cáceres, C.H.F. Melo, M. F. Sterzik, D. Minniti

T and Y-dwarfs are among the coolest and least luminous objects detected, and they can help to understand the properties of giant planets. Their multiplicity properties can shed light on the formation process. 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. 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 rho = 350±5 mas and a position angle of PA=235±1°. Using the mean absolute magnitudes of T6 dwarfs in the 2MASS JHKs bands, we estimate a distance of d=31±6 pc and derive a projected separation of rho = 11±2 au. Another target, WISE J2255-3118, shows a very faint object at 1.3 arcsec in the K_s image. The object is marginally detected in H, and we derive a near infrared colour 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 colour, this suggests that the source is most probably a background galaxy. The five other sources are apparently single, with 3-sigmasensitivity limits between H=19-21 for companions at separations \geq 0.5 arcsec. 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.

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