* Astronomy

Title: VLT X-shooter spectroscopy of the nearest brown dwarf binary
Author: N. Lodieu (1,2), M. R. Zapatero Osorio (3), R. Rebolo (1,2,4), V. J. S. Bejar (1,2), Y. Pavlenko (5,6), A. Perez-Garrido (7) ((1) IAC, Tenerife, Spain, (2) ULL, Tenerife, Spain, (3) CAB CSIC-INTA Madrid, Spain, (4) CSIC, Spain, (5) MAO Kiev, Ukraine, (6) University of Hertfordshire, UK, (7) Universidad Politecnica de Cartagena, Spain)

The aim of the project is to characterise both components of the nearest brown dwarf sytem to the Sun, WISE J104915.57-531906.1 (=Luhman16AB) at optical and near-infrared wavelengths. We obtained high signal-to-noise intermediate-resolution (R~6000-11000) optical (600-1000 nm) and near-infrared (1000-2480nm) spectra of each component of Luhman16AB, the closest brown dwarf binary to the Sun, 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 appears of similar strength on both components (8.2±1.0 Angstroms and 8.4±1.5 Angstroms for the L and T components, respectively). The presence of lithium (Lithium 7) in both components imply masses below 0.06 Msun while comparison with models suggests lower limits of 0.04 Msun. 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 Angstroms for RbI and 4-7 Angstroms 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) for the L dwarf and -4.68±0.13 dex and 1320(+185)(-135) for the T dwarf, respectively. Using 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 impact strongly the spectral energy distribution of L/T transition objects.

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Title: Fast-evolving weather for the coolest of our two new substellar neighbours
Authors: M. Gillon (1), A. H. M. J. Triaud (2), E. Jehin (1), L. Delrez (1), C. Opitom (1), P. Magain (1), M. Lendl (3), D. Queloz (3) ((1) University of Liege, Belgium, (2) MIT, USA, (3) Geneva Observatory, Switzerland)

We present the results of an intense photometric monitoring in the near-infrared (~0.9 microns) with the TRAPPIST robotic telescope of the newly discovered binary brown dwarf WISE J104915.57-531906.1, the third closest system to the Sun at a distance of only 2 pc. Our twelve nights of photometric time-series reveal a quasi-periodic (P = 4.87±0.01 h) variability with a maximal peak-peak amplitude of ~11% and strong night-to-night evolution. We attribute this variability to the rotational modulation of fast-evolving weather patterns in the atmosphere of the coolest component (~T1-type) of the binary, in agreement with the cloud fragmentation mechanism proposed to drive the spectroscopic morphologies of brown dwarfs at the L/T transition. No periodic signal is detected for the hottest component (~L8-type). For both brown dwarfs, our data allow us to firmly discard any unique transit during our observations for planets >= 2 Earth radii. For orbital periods smaller than ~9.5 h, transiting planets are excluded down to an Earth-size.

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Title: Characterisation of the Nearby L/T Binary Brown Dwarf WISE J104915.57-531906.1 at 2 Parsecs from the Sun
Authors: A. Y. Kniazev, P. Vaisanen, K. Muzic, A. Mehner, H. M. J. Boffin, R. Kurtev, C. Melo, V. D. Ivanov, J. Girard, D. Mawet, L. Schmidtobreick, N. Huelamo, J. Borissova, D. Minniti, K. Ishibashi, S. B. Potter, Y. Beletsky, D. A. H. Buckley, S. Crawford, A. A. S. Gulbis, P. Kotze, B. Miszalski, T. E. Pickering, E. Romero Colmenero, T. B. Williams

WISE J104915.57-531906.1 is a candidate L/T brown dwarf binary located 2pc from the Sun. The pair contains the closest known brown dwarfs and is the third closest known system, stellar or sub-stellar. Here we report the first comprehensive follow-up observations of this newly uncovered system. We have determined the spectral types of both components (L8±1, T1±2) and their radial velocities (V_rad~23.1, 19.5 km/s) using the Southern African Large Telescope (SALT) and other facilities located at the South African Astronomical Observatory (SAAO). The relative radial velocity of the two components is smaller than the range of orbital velocities for theoretically predicted masses, implying that they form a gravitationally bound system. We report resolved near-infrared JHK_S photometry from the IRSF telescope at the SAAO which yields colours consistent with the spectroscopically derived spectral types. Our apparent magnitudes predict a distance of ~2.25pc, similar to the previous measurement. The available kinematic and photometric information excludes the possibility that the object belongs to any of the known nearby young moving groups or associations. Simultaneous optical polarimetry observations taken at the SAAO 1.9-m give a non-detection with an upper limit of 0.07%. For the given spectral types and absolute magnitudes, 1Gyr theoretical models predict masses of 0.04--0.05 solar masses for the primary, and 0.03--0.05 solar masses for the secondary. The objects remain in the sub-stellar regime even if they are 10Gyr old.

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Tiny Star System Found On Galactic Doorstep

Spanning the gap between large planets and small stars lies an astronomical gray area. This is the domain of brown dwarfs, dim stellar cinders glowing faintly in the darkness. And a binary pair of these stars has just been discovered only 6.5 light-years away, making it the third closest star system to our own.
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