1 Department of Physics and Astronomy, Science and Technology, Aarhus University2 Probing Lensing Anomalies Network3 Microlensing Follow Up Network4 Optical Gravitational Lensing Experiment5 Microlensing Observations in Astrophysics6 Robotic Telescope Network7 Microlensing Network for the Detection of Small Terrestrial Exoplanets8 Ohio State University9 Department of Physics and Astronomy10 European Southern Observatory (ESO)11 European Southern Observatory12 Université Pierre and Marie Curie13 Zentrum für Astronomie der Universität Heidelberg (ZAH)14 University of St Andrews15 McDonald Observatory16 Lawrence Livermore National Laboratory17 University of Rijeka18 Dept. of Computing19 University of Tasmania20 Caltech, MS 100-2221 Perth Observatory22 Space Telescope Science Institute23 Korea Astronomy and Space Science Institute24 Bronberg Observatory25 University of Cambridge26 North Shore Mail Centre27 Osaka University28 University of Manchester29 Konan University30 Nagano National College of Technology31 University of Auckland32 Tokyo Metropolitan College of Industrial Technology33 Mount John Observatory34 Las Cumbres Observatory35 Twelve Quays House36 University of Exeter37 University of Warwick38 University of Copenhagen39 Max Planck Institute for Astronomy40 Charles University41 INFN42 Sharif University of Technology43 Georg-August Universität44 Institut d'Astrophysique et de Géophysique45 Keele University46 Wendelstein Calar Alto Pixellensing Project47 Tel-Aviv University48 University of Tokyo49 Graduate University for Advanced Studies (Sokendai)50 Université de Toulouse51 unknown52 Molehill Astronomical Observatory53 Universidad de Valencia54 Institute for Advanced Study55 National Astronomical Observatory of Japan56 Department of Physics and Astronomy, Science and Technology, Aarhus University
Context. Caustic crossing is the clearest signature of binary lenses in microlensing. In the present context, this signature is diluted by the large source star but a detailed analysis has allowed the companion signal to be extracted. Aims. MOA 2009-BLG-411 was detected on August 5, 2009 by the MOA-Collaboration. Alerted as a high-magnification event, it was sensitive to planets. Suspected anomalies in the light curve were not confirmed by a real-time model, but further analysis revealed small deviations from a single lens extended source fit. Methods. Thanks to observations by all the collaborations, this event was well monitored. We first decided to characterize the source star properties by using a more refined method than the classical one: we measure the interstellar absorption along the line of sight in five different passbands (VIJHK). Secondly, we model the lightcurve by using the standard technique: make (s,q,α) grids to look for local minima and refine the results by using a downhill method (Markov chain Monte Carlo). Finally, we use a Galactic model to estimate the physical properties of the lens components. Results. We find that the source star is a giant G star with radius 9 R⊙. The grid search gives two local minima, which correspond to the theoretical degeneracy s ≡ s-1. We find that the lens is composed of a brown dwarf secondary of mass MS = 0.05 M⊙ orbiting a primary M-star of mass MP = 0.18 M⊙. We also reveal a new mass-ratio degeneracy for the central caustics of close binaries. Conclusions. As far as we are aware, this is the first detection using the microlensing technique of a binary system in our Galaxy composed of an M-star and a brown dwarf.
Astronomy and Astrophysics, 2012, Vol 547, Issue November