MOA-2007-BLG-197: Exploring the brown dwarf desert^⋆

¹ Sorbonne Universités, UPMC Univ Paris 6 et CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98 bis bd Arago, 75014 Paris, France
² University of Canterbury, Dept. of Physics and Astronomy, Private Bag 4800, 8020 Christchurch, New Zealand
³ Institute of Natural and Mathematical Sciences, Massey University, Private Bag 102-904, North Shore Mail Centre, Auckland, New Zealand
⁴ Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA
⁵ SUPA, School of Physics & Astronomy, North Haugh, University of St Andrews, KY16 9SS, Scotland, UK
⁶ Kavli Institute for Astronomy and Astrophysics, Peking University, Yi He Yuan Road 5, Hai Dian District, 100871 Beijing, PR China
⁷ IRAP, CNRS – Université de Toulouse, 14 av. E. Belin, 31400 Toulouse, France
⁸ CFHT Corporation, 65-1238 Mamalahoa Hwy, Kamuela, Hawaii 96743, USA
⁹ Department of Astronomy, Ohio State University, 140 W. 18th Ave., Columbus, OH 43210, USA
¹⁰ School of Math and Physics, University of Tasmania, Private Bag 37, GPO Hobart, 7001 Tasmania, Australia
¹¹ Niels Bohr Institutet, Københavns Universitet, Juliane Maries Vej 30, 2100 København Ø, Denmark
¹² Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹³ South African Astronomical Observatory, PO Box 9, Observatory 7935, South Africa
¹⁴ Department of Earth and Space Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
¹⁵ Qatar Environment and Energy Research Institute, Qatar Foundation, PO Box 5825, Doha, Qatar
¹⁶ Department of Physics, University of Rijeka, Radmile Matej vcić 2, 51000 Rijeka, Croatia
¹⁷ Technical University of Vienna, Department of Computing, Wiedner Hauptstrasse 10, 1040 Wien, Austria
¹⁸ Department of Physics, Chungbuk National University, 371-763 Cheongju, Korea
¹⁹ Department of Physics and Astronomy, San Francisco State University, 1600 Holloway Avenue, San Francisco, CA 94132, USA
²⁰ Korea Astronomy and Space Science Institute, 776 Daedukdae-ro, Daejeon, Korea
²¹ Las Cumbres Observatory Global Telescope Network, 6740 Cortona Drive, suite 102, Goleta, CA 93117, USA
²² Astronomisches Rechen-Institut, Zentrum für Astronomie derUniversität Heidelberg (ZAH), Mönchhofstraße 12-14, 69120 Heidelberg, Germany
²³ Perth Observatory, Walnut Road, Bickley, 6076 Perth, Australia
²⁴ International Centre for Radio Astronomy Research, Curtin University, Bentley, WA 6102, Australia
²⁵ Solar-Terrestrial Environment Laboratory, Nagoya University, 464-8601 Nagoya, Japan
²⁶ Okayama Astrophysical Observatory, National Astronomical Observatory of Japan, 3037-5 Honjo, Kamogata, Asakuchi, 719-0232 Okayama, Japan
²⁷ Nagano National College of Technology, 381-8550 Nagano, Japan
²⁸ Department of Physics, University of Auckland, 92019 Private Bag, 1010 Auckland, New Zealand
²⁹ Tokyo Metropolitan College of Aeronautics, 116-8523 Tokyo, Japan
³⁰ School of Chemical and Physical Sciences, Victoria University, 6147 Wellington, New Zealand
³¹ Institute of Information and Mathematical Sciences, Massey University at Albany, Private Bag 102904, 0745 North Shore, Auckland, New Zealand
³² Mt. John University Observatory, PO Box 56, 8770 Lake Tekapo, New Zealand
³³ Department of Physics, University of Auckland, Private Bag 92019, 1010 Auckland, New Zealand
³⁴ Department of Physics, Faculty of Science, Kyoto Sangyo University, 603-8555 Kyoto, Japan

Received: 2 February 2015
Accepted: 15 May 2015

Abstract

We present the analysis of MOA-2007-BLG-197Lb, the first brown dwarf companion to a Sun-like star detected through gravitational microlensing. The event was alerted and followed-up photometrically by a network of telescopes from the PLANET, MOA, and μFUN collaborations, and observed at high angular resolution using the NaCo instrument at the VLT. From the modelling of the microlensing light curve, we derived basic parameters such as, the binary lens separation in Einstein radius units (s ≃ 1.13), the mass ratio q = (4.732 ± 0.020) × 10^-2 and the Einstein radius crossing time (t_E ≃ 82 d). Because of this long time scale, we took annual parallax and orbital motion of the lens in the models into account, as well as finite source effects that were clearly detected during the source caustic exit. To recover the lens system’s physical parameters, we combined the resulting light curve best-fit parameters with (J,H,K_s) magnitudes obtained with VLT NaCo and calibrated using IRSF and 2MASS data. From this analysis, we derived a lens total mass of 0.86 ± 0.04 M_⊙ and a lens distance of D_L = 4.2 ± 0.3 kpc. We find that the companion of MOA-2007-BLG-197L is a brown dwarf of 41 ± 2 M_J observed at a projected separation of a_⊥ = 4.3 ± 0.1 AU, and orbits a 0.82 ± 0.04 M_⊙ G-K dwarf star. We then placed the companion of MOA-2007-BLG-197L in a mass-period diagram consisting of all brown dwarf companions detected so far through different techniques, including microlensing, transit, radial velocity, and direct imaging (most of these objects orbit solar-type stars). To study the statistical properties of this population, we performed a two-dimensional, non-parametric probability density distribution fit to the data, which draws a structured brown dwarf landscape. We confirm the existence of a region that is strongly depleted in objects at short periods and intermediate masses (P ≲ 30 d, M ~ 30−60 M_J), but also find an accumulation of objects around P ~ 500 d and M ~ 20 M_J, as well as another depletion region at long orbital periods (P ≳ 500 d) and high masses (M ≳ 50 M_J). While these data provide important clues on the different physical mechanisms of formation (or destruction) that shape the brown dwarf desert, more data are needed to establish their relative importance, in particular as a function of host star mass. Future microlensing surveys should soon provide more detections, in particular for red dwarf hosts, thus uniquely complementing the solar-type host sample.