OGLE 2008–BLG–290: an accurate measurement of the limb darkening of a galactic bulge K Giant spatially resolved by microlensing

¹ Probing Lensing Anomalies Network,
² LATT, Université de Toulouse, CNRS, 14 avenue Edouard Belin, 31400 Toulouse, France
³ Institute of Theoretical Physics, Charles University, V Holešovičkách 2, 18000 Prague, Czech Republic
⁴ Microlensing Follow Up Network,
⁵ Department of Astronomy, Ohio State University, 140 West 18th Avenue, Columbus, OH 43210, USA
⁶ The Optical Gravitational Lensing Experiment,
⁷ Warsaw University Observatory. Al. Ujazdowskie 4, 00-478 Warszawa, Poland
⁸ University of Canterbury, Department of Physics & Astronomy, Private Bag 4800, Christchurch 8020, New Zealand
⁹ Institut d'Astrophysique de Paris, CNRS, Université Pierre & Marie Curie, 98bis Bd Arago, 75014 Paris, France
¹⁰ Microlensing Observations in Astrophysics,
¹¹ University of Texas, McDonald Observatory, 16120 St Hwy Spur 78, Fort Davis TX 79734, USA
¹² Institute of Information and Mathematical Sciences, Massey University, Private Bag 102-904, North Shore Mail Centre, Auckland, New Zealand
¹³ Robotic Telescope Network,
¹⁴ European Southern Observatory (ESO), Karl-Schwarzschild-Straße 2, 85748 Garching bei München, Germany
¹⁵ Dipartimento di Fisica, Universita' di Salerno and INFN, sez. di Napoli, Italy
¹⁶ Astronomisches Rechen-Institut (ARI), Zentrum für Astronomie der Universität Heidelberg (ZAH), Mönchhofstrasse 12-14, 69120 Heidelberg, Germany
¹⁷ Microlensing Network for the Detection of Small Terrestrial Exoplanets,
¹⁸ Scottish Universities Physics Alliance, School of Physics & Astronomy, University of St Andrews, North Haugh, St Andrews, KY16 9SS, UK
¹⁹ Physics Department, Faculty of Arts and Sciences, University of Rijeka, Omladinska 14, 51000 Rijeka, Croatia
²⁰ School of Mathematics and Physics, University of Tasmania, Private Bag 37, Hobart, Tasmania 7001, Australia
²¹ Dark Cosmology Centre, Københavns Universitet, Juliane Maries Vej 30, 2100 København, Denmark
²² European Southern Observatory (ESO), Casilla 19001, Vitacura 19, Santiago, Chile
²³ University Observatory Munich, Scheinerstrasse 1, 81679 München, Germany
²⁴ The Wendelstein Calar Alto Pixellensing Project,
²⁵ South African Astronomical Observatory, PO Box 9 Observatory 7925, South Africa
²⁶ Bronberg Observatory, Pretoria, South Africa
²⁷ Department of Astronomy, Kyoto University, Kyoto 606-8502, Japan
²⁸ Solar-Terrestrial Environment Laboratory, Nagoya University, Nagoya, 464-8601, Japan
²⁹ Las Cumbres Observatory, 6740B Cortona Dr, suite 102, Goleta, CA 93117, USA
³⁰ Department of Physics, Broida Hall, University of California, Santa Barbara CA 93106-9530, USA
³¹ Perth Observatory, Walnut Road, Bickley, Perth 6076, Australia
³² Institute of Geophysics and Planetary Physics (IGPP), L-413, Lawrence Livermore National Laboratory, PO Box 808, Livermore, CA 94551, USA
³³ Technical University of Vienna, Dept. of Computing, Wiedner Hauptstrasse 10, Vienna, Austria
³⁴ NASA Exoplanet Science Institute, Caltech, MS 100-22, 770 South Wilson Avenue, Pasadena, CA 91125, USA
³⁵ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
³⁶ School of Mathematical Sciences, Queen Mary University of London, Mile End Road, London E1 4NS, UK
³⁷ Department of Physics, Institute for Basic Science Research, Chungbuk National University, Chongju 361-763, Korea
³⁸ Korea Astronomy and Space Science Institute, 61-1, Whaam-Dong, Youseong-Gu, Daejeon 305-348, Korea
³⁹ Universidad de Concepción, Departamento de Física, Astronomy Group, Casilla 160-C, Concepción, Chile
⁴⁰ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁴¹ Mount John Observatory, PO Box 56, Lake Tekapo 8770, New Zealand
⁴² Department of Physics, Konan University, Nishiokamoto 8-9-1, Kobe 658-8501, Japan
⁴³ Nagano National College of Technology, Nagano 381-8550, Japan
⁴⁴ Department of Physics, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand
⁴⁵ Tokyo Metropolitan College of Industrial Technology, Tokyo 116-0003, Japan
⁴⁶ Department of Physics and Astrophysics, Faculty of Science, Nagoya University, Nagoya 464-8602, Japan
⁴⁷ School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, UK
⁴⁸ Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead CH41 1LD, UK
⁴⁹ Deutsches SOFIA Institut, Universität Stuttgart, Pfaffenwaldring 31, 70569 Stuttgart, Germany
⁵⁰ SOFIA Science Center, NASA Ames Research Center, Mail Stop N211-3, Moffett Field CA 94035, USA
⁵¹ Jodrell Bank Centre for Astrophysics, University of Manchester, Manchester M13 9PL, UK
⁵² Department of Physics, University of Warwick, Coventry, CV4 7AL, UK
⁵³ Armagh Observatory, College Hill, Armagh, BT61 9DG, Ireland
⁵⁴ Institut für Astrophysik, Georg-August Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
⁵⁵ Bellatrix Observatory, via Madonna de Loco 47, 03023 Ceccano, Italy
⁵⁶ Department of Physics, Sharif University of Technology, PO Box 11155-9161, Tehran, Iran
⁵⁷ Institut d'Astrophysique et de Géophysique, Allée du 6 Août, Sart Tilman, Bât. B5c, 4000 Liège, Belgium
⁵⁸ Astrophysics Group, Keele University, Newcastle-under-Lyme, ST5 5BG, UK
⁵⁹ INAF, Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy
⁶⁰ Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse, 85748 Garching, Germany

Received: 13 January 2010
Accepted: 5 May 2010

Abstract

Context. Not only is gravitational microlensing a successful tool for discovering distant exoplanets, but it also enables characterization of the lens and source stars involved in the lensing event.

Aims. In high-magnification events, the lens caustic may cross over the source disk, which allows determination of the angular size of the source and measurement of its limb darkening.

Methods. When such extended-source effects appear close to maximum magnification, the resulting light curve differs from the characteristic Paczyński point-source curve. The exact shape of the light curve close to the peak depends on the limb darkening of the source. Dense photometric coverage permits measurement of the respective limb-darkening coefficients.

Results. In the case of the microlensing event OGLE 2008-BLG-290, the K giant source star reached a peak magnification at about 100. Thirteen different telescopes have covered this event in eight different photometric bands. Subsequent light-curve analysis yielded measurements of linear limb-darkening coefficients of the source in six photometric bands. The best-measured coefficients lead to an estimate of the source effective temperature of about 4700 K. However, the photometric estimate from colour-magnitude diagrams favours a cooler temperature of 4200 ± 100 K.

Conclusions. Because the limb-darkening measurements, at least in the CTIO/SMARTS2 - and -bands, are among the most accurate obtained, the above disagreement needs to be understood. A solution is proposed, which may apply to previous events where such a discrepancy also appeared.