2MASS J03105986+1648155 AB - a new binary at the L/T transition

Context. The transition from the L to the T spectral type of brown dwarfs is marked by a very rapid transition phase, remarkable brightening in the J-band, and higher binary frequency. Despite being an active area of inquiry, this transition region still remains one of the most poorly understood phases of brown dwarf evolution. Aims. We resolved the L dwarf 2MASS J03105986+1648155 for the first time into two almost equally bright components straddling the L/T transition. Since such a coeval system with common age and composition provides crucial information on this special transition phase, we monitored the system over similar to 3 years to derive first orbital parameters and dynamical mass estimates, as well as a spectral type determination. Methods. We obtained resolved high angular resolution, near-IR images with both HST and the adaptive optics instrument NACO at the VLT, including the laser guide star system PARSEC. Results. Based on two epochs of astrometric data, we derive a minimum semi-major axis of 5.2 +/- 0.8 AU. The assumption of a face-on circular orbit yields an orbital period of 72 +/- 4 years and a total system mass of similar to 30-60 M(Jup). This places the masses of the individual components of the system at the lower end of the mass regime of brown dwarfs. The achieved photometry allowed a first spectral type determination of L9 +/- 1 for each component. In addition, this seems to be only the fifth resolved L/T transition binary with a flux reversal. Conclusions. While ultimate explanations for this effect are still lacking, the 2MASS J03105986+ 1648155 system adds an important benchmark object for improving our understanding of this remarkable evolutionary phase of brown dwarfs. Additionally, the observational results of 2MASS J03105986+1648155 AB derived with the new PARSEC AO system at the VLT show the importance of this technical capability. The updated AO system allows us to significantly extend the sample of brown dwarfs observable with high resolution from the ground, hence to reveal more of their physical properties.