1 Instituto de Astrofísica de Canarias (IAC), Calle Vía Láctea s/n, 38200 La Laguna, Tenerife, Spain
e-mail: firstname.lastname@example.org; email@example.com; firstname.lastname@example.org
2 Departamento de Astrofísica, Universidad de La Laguna (ULL), 38205 La Laguna, Tenerife, Spain
3 Centro de Astrobiología (CSIC-INTA), Ctra. Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
4 Consejo Superior de Investigaciones Científicas, CSIC, Spain
5 Main Astronomical Observatory of the National Academy of Sciences of Ukraine, Ukraine
6 Center for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield, Hertfordshire AL10 9AB, UK
7 Universidad Politécnica de Cartagena, Campus Muralla del Mar, Cartagena, 30202 Murcia, Spain
Received: 5 September 2014
Accepted: 30 June 2015
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 (7Li) 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+180-135 K for the L dwarf and −4.68 ± 0.13 dex and 1320+185-135 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.
Key words: techniques: spectroscopic / brown dwarfs / binaries: general / stars: fundamental parameters
Based on observations collected at the European Southern Observatory, Chile, under DDT programme 290.C-5200(B) (PI Lodieu).
FITS files of the reduced spectra are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (188.8.131.52) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/581/A73
© ESO, 2015