Rotation periods and astrometric motions of the Luhman 16AB brown dwarfs by high-resolution lucky-imaging monitoring^⋆,⋆⋆

¹ Max-Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
e-mail: mancini@mpia.de
² INAF–Osservatorio Astrofisico di Torino, via Osservatorio 20, 10025 Pino Torinese, Italy
³ Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK
⁴ Astrophysics Group, Keele University, Keele ST5 5BG, UK
⁵ Department of Physics, University of Salerno, Via Giovanni Paolo II 132, 84084 Fisciano (SA), Italy
⁶ Istituto Nazionale di Fisica Nucleare, Sezione di Napoli, 80126 Napoli, Italy
⁷ SUPA, University of St Andrews, School of Physics & Astronomy, North Haugh, St Andrews, Fife KY16 9SS, UK
⁸ Niels Bohr Institute & Centre for Star and Planet Formation, University of Copenhagen, Østervoldgade 5, 1350 Copenhagen K, Denmark
⁹ Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile
¹⁰ Department of Physics, Sharif University of Technology, PO Box 11155-9161, Tehran, Iran
¹¹ Astronomisches Rechen-Institut, Zentrum für Astronomie, Universität Heidelberg, Mönchhofstrasse 12-14, 69120 Heidelberg, Germany
¹² Centre for Electronic Imaging, Dept. of Physical Sciences, The Open University, Milton Keynes MK7 6AA, UK
¹³ Planetary and Space Sciences, Dept. of Physical Sciences, The Open University, Milton Keynes MK7 6AA, UK
¹⁴ Qatar Environment and Energy Research Institute, Qatar Foundation, Tornado Tower, Floor 19, PO Box 5825, Doha, Qatar
¹⁵ NASA Exoplanet Science Institute, MS 100-22, California Institute of Technology, Pasadena, CA 91125, USA
¹⁶ Istituto Internazionale per gli Alti Studi Scientifici (IIASS), 84019 Vietri Sul Mare (SA), Italy
¹⁷ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
¹⁸ Yunnan Observatories, Chinese Academy of Sciences, 650011 Kunming, PR China
¹⁹ Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, 650011 Kunming, PR China
²⁰ Korea Astronomy and Space Science Institute, 305-348 Daejeon, Republic of Korea
²¹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
²² Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
²³ Institut d’Astrophysique et de Géophysique, Université de Liège, 4000 Liège, Belgium

Received: 5 July 2015
Accepted: 25 October 2015

Abstract

Context. Photometric monitoring of the variability of brown dwarfs can provide useful information about the structure of clouds in their cold atmospheres.The brown-dwarf binary system Luhman 16AB is an interesting target for such a study, because its components stand at the L/T transition and show high levels of variability. Luhman 16AB is also the third closest system to the solar system, which allows precise astrometric investigations with ground-based facilities.

Aims. The aim of the work is to estimate the rotation period and study the astrometric motion of both components.

Methods. We have monitored Luhman 16AB over a period of two years with the lucky-imaging camera mounted on the Danish 1.54 m telescope at La Silla, through a special i + z long-pass filter, which allowed us to clearly resolve the two brown dwarfs into single objects. An intense monitoring of the target was also performed over 16 nights, in which we observed a peak-to-peak variability of 0.20 ± 0.02 mag and 0.34 ± 0.02 mag for Luhman 16A and 16B, respectively.

Results. We used the 16-night time-series data to estimate the rotation period of the two components. We found that Luhman 16B rotates with a period of 5.1 ± 0.1 h, in very good agreement with previous measurements. For Luhman 16A, we report that it rotates more slowly than its companion, and even though we were not able to get a robust determination, our data indicate a rotation period of roughly 8 h. This implies that the rotation axes of the two components are well aligned and suggests a scenario in which the two objects underwent the same accretion process. The 2-year complete data set was used to study the astrometric motion of Luhman 16AB. We predict a motion of the system that is not consistent with a previous estimate based on two months of monitoring, but cannot confirm or refute the presence of additional planetary-mass bodies in the system.