VLT X-Shooter spectroscopy of the nearest brown dwarf binary

Abstract

Aims. 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. Methods. We obtained high signal-to-noise intermediate-resolution (R ~ 6000-11 000) optical (600-1000 nm) and near-infrared (1000-2480 nm) spectra of each component of Luhman 16AB with the X-Shooter instrument on the Very Large Telescope. Results. We classify the primary and secondary of the Luhman 16 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 ± 1.0 Å for the L and 8.4 ± 1.5 Å for the T component). The presence of lithium ( 7 Li) in both components implies masses lower than 0.06 M ⊙, while the comparison with models suggests lower limits of 0.04 M ⊙. 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 6-7 Å for RbI and 4-7 Å 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 ± 0.08 dex and 1305 K for the L dwarf and -4.68 ± 0.13 dex and 1320 K 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.