2015A&A...581A..73L

The aim of the project is to characterise the two components of the brown dwarf system nearest to the Sun, WISE J104915.57-531906.1 (also called Luhman 16AB) at optical and near-infrared wavelengths. We obtained high signal-to-noise intermediate-resolution (R∼6000-11000) optical (600-1000nm) and near-infrared (1000-2480nm) spectra of each component of Luhman 16AB with the X-Shooter instrument on the Very Large Telescope. We classify the primary and secondary of the Luhman16 system as L6-L7.5 and T0±1, respectively, in agreement with previous measurements published in the literature. We present measurements of the lithium pseudo-equivalent widths, which appear of similar strength in both components (8.2±1.0Å for the L and 8.4±1.5Å for the T component). The presence of lithium (⁷Li) in both components implies masses lower than 0.06M_☉, while the comparison with models suggests lower limits of 0.04M_☉. The detection of lithium in the T component is the first of its kind. Similarly, we assess the strength of other alkali lines (e.g. pseudo-equivalent widths of 6-7Å for RbI and 4-7Å for CsI) present in the optical and near-infrared regions and compare with estimates for L and T dwarfs. We also derive effective temperatures and luminosities of each component of the binary: -4.66±0.08dex and 1305⁺¹⁸⁰_–135K for the L dwarf and -4.68±0.13dex and 1320⁺¹⁸⁵_–135K for the T dwarf. According to our radial velocity determinations, the binary does not appear to belong to any of the well-known moving group. Our preliminary theoretical analysis of the optical and J-band spectra indicates that the L- and T-type spectra can be reproduced with a single temperature and gravity but different relative chemical abundances, which strongly affects the spectral energy distribution of L/T transition objects.