A brown dwarf orbiting an M-dwarf: MOA 2009–BLG–411L^⋆

¹ Probing Lensing Anomalies Network, http://planet.iap.fr  
² Microlensing Follow Up Network, http://www.astronomy.ohio-state.edu/~microfun  
³ The Optical Gravitational Lensing Experiment, http://ogle.astrouw.edu.pl  
⁴ Microlensing Observations in Astrophysics, http://www.phys.canterbury.ac.nz/moa  
⁵ Robotic Telescope Network, http://robonet.lcogt.net  
⁶ Microlensing Network for the Detection of Small Terrestrial Exoplanets, http://www.mindstep-science.org  
⁷ Department of Astronomy, Ohio State University, 140 West 18th Avenue, Columbus, OH 43210, USA
⁸ University of Canterbury, Department of Physics & Astronomy, Private Bag 4800, Christchurch 8020, New Zealand
⁹ European Southern Observatory (ESO), Casilla 19001, Vitacura 19, Santiago, Chile
¹⁰ European Southern Observatory, Karl-Schwarzschild-Stra β 2, 85748 Garching bei München, Germany
¹¹ Institut d’Astrophysique de Paris, CNRS, Université Pierre & Marie Curie, 98bis Bd Arago, 75014 Paris, France
¹² Astronomisches Rechen-Institut (ARI), Zentrum für Astronomie der Universität Heidelberg (ZAH), Mönchhofstrasse 12ŋ-14, 69120 Heidelberg, Germany
¹³ Scottish Universities Physics Alliance, School of Physics & Astronomy, University of St Andrews, North Haugh, St Andrews, KY16 9SS, UK
¹⁴ University of Notre Dame, Department of Physics, 225 Nieuwland Science Hall, Notre Dame, IN 46556, USA
¹⁵ University of Texas, McDonald Observatory, 16120 St Hwy Spur 78, Fort Davis TX 79734, USA
¹⁶ Institute of Geophysics and Planetary Physics (IGPP), L-413, Lawrence Livermore National Laboratory, PO Box 808, Livermore, CA 94551, USA
¹⁷ Physics Department, Faculty of Arts and Sciences, University of Rijeka, Omladinska 14, 51000 Rijeka, Croatia
¹⁸ Technical University of Vienna, Dept. of Computing, Wiedner Hauptstrasse 10, Vienna, Austria
¹⁹ School of Mathematics and Physics, University of Tasmania, Private Bag 37, Hobart, 7001 Tasmania, Australia
²⁰ NASA Exoplanet Science Institute, Caltech, MS 100-22, 770 South Wilson Avenue, Pasadena, CA 91125, USA
²¹ Perth Observatory, Walnut Road, Bickley, Perth 6076, Australia
²² South African Astronomical Observatory, PO Box 9, 7925 Observatory, South Africa
²³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
²⁴ Department of Physics, Institute for Basic Science Research, Chungbuk National University, 361-763 Chongju, Korea
²⁵ Korea Astronomy and Space Science Institute, 61-1, Whaam-Dong, Youseong-Gu, 305-348 Daejeon, Korea
²⁶ Bronberg Observatory, Pretoria, South Africa
²⁷ Warsaw University Observatory. Al. Ujazdowskie 4, 00-478 Warszawa, Poland
²⁸ Universidad de Concepción, Departamento de Física, Astronomy Group, Casilla 160-C, Concepción, Chile
²⁹ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
³⁰ Institute of Information and Mathematical Sciences, Massey University, Private Bag 102-904, North Shore Mail Centre, Auckland, New Zealand
³¹ Department of Earth and Space Science, Osaka University, 560-0043 Osaka, Japan
³² Jodrell Bank Centre for Astrophysics, University of Manchester, Manchester M13 9PL, UK
³³ Department of Physics, Konan University, Nishiokamoto 8-9-1, 658-8501 Kobe, Japan
³⁴ Nagano National College of Technology, 381-8550 Nagano, Japan
³⁵ Department of Physics, University of Auckland, Private Bag 92019, 1142 Auckland, New Zealand
³⁶ Tokyo Metropolitan College of Industrial Technology, 116-0003 Tokyo, Japan
³⁷ Department of Physics and Astrophysics, Faculty of Science, Nagoya University, 464-8602 Nagoya, Japan
³⁸ Mount John Observatory, PO Box 56, 8770 Lake Tekapo, New Zealand
³⁹ Las Cumbres Observatory, 6740B Cortona Dr, suite 102, Goleta, CA 93117, USA
⁴⁰ Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead CH41 1LD, UK
⁴¹ School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, UK
⁴² Department of Physics, University of Warwick, Coventry, CV4 7AL, UK
⁴³ Department of Physics, Broida Hall, University of California, Santa Barbara CA 93106-9530, USA
⁴⁴ IRAP, Université de Toulouse, CNRS, 14 avenue Edouard Belin, 31400 Toulouse, France
⁴⁵ Niels Bohr Institute and Centre for Star and Planet Formation, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
⁴⁶ University Observatory Munich, Scheinerstrasse 1, 81679 München, Germany
⁴⁷ Max Planck Institute for Astronomy, Königstuhl 17, 69117- Heidelberg, Germany
⁴⁸ Institute of Theoretical Physics, Charles University, V Holešovičkách 2, 18000 Prague, Czech Republic
⁴⁹ Bellatrix Observatory, via Madonna de Loco 47, 03023 Ceccano, Italy
⁵⁰ Dipartimento di Fisica, Università di Salerno, Fisciano, INFN, sez. di Napoli, IIASS, Vietri sul Mare, Italy
⁵¹ Armagh Observatory, College Hill, Armagh, BT61 9DG, UK
⁵² Department of Physics, Sharif University of Technology, PO Box 11155-9161, Tehran, Iran
⁵³ Institut für Astrophysik, Georg-August Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
⁵⁴ Institut d’Astrophysique et de Géophysique, Allée du 6 Août, Sart Tilman, Bât. B5c, 4000 Liège, Belgium
⁵⁵ Astrophysics Group, Keele University, Newcastle-under-Lyme, ST5 5BG, UK
⁵⁶ INAF, Osservatorio Astronomico di Brera, 23846 Merate ( LC), Italy
⁵⁷ Deutsches SOFIA Institut, Universität Stuttgart, Pfaffenwaldring 31, 70569 Stuttgart, Germany
⁵⁸ SOFIA Science Center, NASA Ames Research Center, Mail Stop N211-3, Moffett Field CA 94035, USA
⁵⁹ Department of Astronomy, Kyoto University, 606-8502 Kyoto, Japan
⁶⁰ School of Mathematical Sciences, Queen Mary University of London, Mile End Road, London E1 4NS, UK
⁶¹ The Wendelstein Calar Alto Pixellensing Project, http://www.usm.uni-muenchen.de/people/arri/wecapp.html  
⁶² Possum Observatory, Patutahi, Gisbourne, New Zealand
e-mail: john_drummond@xtra.co.nz
⁶³ School of Physics and Astronomy and Wise Observatory, Tel-Aviv University, Tel-Aviv 69978, Israel
e-mail: shai@wise.tau.ac.il; dani@wise.tau.ac.il; shporer@wise.tau.ac.il; david@wise.tau.ac.il
⁶⁴ Auckland Observatory, Auckland, New Zealand
e-mail: gwchristie@christie.org.nz
⁶⁵ Farm Cove Observatory, Centre for Backyard Astrophysics, Pakuranga, Auckland, New Zealand
e-mail: farmcoveobs@xtra.co.nz
⁶⁶ Kiso Observatory, Institute of Astronomy, The University of Tokyo, 10762-30, Mitake, Kiso, 397-0101 Nagano, Japan
⁶⁷ Teide Observatory  
⁶⁸ The Graduate University for Advanced Studies (Sokendai), 2-21-1 Oosawa, Mitaka, 181-8588 Tokyo, Japan
⁶⁹ CALMIP, DTSI Université Paul Sabatier, Université de Toulouse, 31062 Toulouse, France
⁷⁰ Perimeter Institute for Theoretical Physics, 31 Caroline Street North, Waterloo, ON N2L 2Y5, Canada
⁷¹ Korea Astronomy and Space Science Institute, 776 Daedukdae-ro, Yuseong-gu, 305-348 Daejeon, Republic of Korea (South Korea)
⁷² International Institute for Advanced Scientific Studies, via G. Pellegrino 19, 84019  – Vietri sul Mare ( SA), Italy
⁷³ Department of Physics, University of Salerno, via Ponte Don Melillo, 84084  – Fisciano ( SA), Italy
⁷⁴ Department of Physics & Astronomy, Aarhus Universitet, Ny Munkegade, 8000 Aarhus C, Denmark
⁷⁵ Molehill Astronomical Observatory, Auckland, New Zealand
⁷⁶ Departamento de Astronomía y Astrofísica, Universidad de Valencia, 46100 Burjassot, Valencia, Spain
⁷⁷ Dept. of Physics, Texas A&M University, College Station, TX, USA
⁷⁸ Institute for Advanced Study, Einstein Drive, Princeton, NJ 08540, USA
⁷⁹ AUT University, Auckland, New Zealand
⁸⁰ Okayama Astrophysical Observatory, National Astronomical Observatory of Japan, Asaguchi, 719-0232 Okayama, Japan

Received: 6 June 2012
Accepted: 27 August 2012

Abstract

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 M_S = 0.05   M_⊙ orbiting a primary M-star of mass M_P = 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.