OGLE 2004–BLG–254: a K3 III Galactic bulge giant spatially resolved by a single microlens

¹ PLANET/Robonet Collaboration
² Astronomisches Rechen-Institut (ARI), Zentrum für Astronomie der Universität Heidelberg (ZAH), MönchhofStr. 12-14, 69120 Heidelberg, Germany e-mail: cassan@ari.uni-heidelberg.de
³ Institut d'Astrophysique de Paris, UMR 7095 CNRS – Université Pierre & Marie Curie, 98bis Bd Arago, 75014 Paris, France
⁴ Observatoire Midi-Pyrénées, Laboratoire d'Astrophysique, UMR 5572, Université Paul Sabatier – Toulouse 3, 14 avenue Edouard Belin, 31400 Toulouse, France
⁵ European Southern Observatory (ESO), Casilla 19001, Vitacura 19, Santiago, Chile
⁶ University of St Andrews, School of Physics & Astronomy, North Haugh, St Andrews, KY16 9SS, UK
⁷ University of Tasmania, Physics Department, GPO 252C, Hobart, Tasmania 7001, Australia
⁸ Institute of Theoretical Physics, Charles University, V Holešovičkách 2, 180 00 Prague, Czech Republic
⁹ Niels Bohr Institute, Astronomical Observatory, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
¹⁰ University of Canterbury, Department of Physics & Astronomy, Private Bag 4800, Christchurch, New Zealand
¹¹ University of Notre Dame, Physics Department, 225 Nieuwland Science Hall, Notre Dame, IN 46530, USA
¹² Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹³ University of Texas, McDonald Observatory, Fort Davis TX 79734, USA
¹⁴ Dept Physics / Boyden Observatory, University of the Free State, Bloemfontein 9300, South Africa
¹⁵ Institute of Geophysics and Planetary Physics, L-413, Lawrence Livermore National Laboratory, PO Box 808, Livermore, CA 94550, USA
¹⁶ DSM/DAPNIA, CEA Saclay, 91191 Gif-sur-Yvette cedex, France
¹⁷ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
¹⁸ Technical University of Vienna, Dept. of Computing, Wiedner Hauptstrasse 10, Vienna, Austria
¹⁹ Department of Astronomy, University of Florida, 211 Bryant Space Science Center, Gainesville, FL 32611-2055, USA
²⁰ Perth Observatory, Walnut Road, Bickley, Perth 6076, Australia
²¹ South African Astronomical Observatory, PO Box 9 Observatory 7935, South Africa
²² OGLE Collaboration
²³ Warsaw University Observatory. Al. Ujazdowskie 4, 00-478 Warszawa, Poland
²⁴ Universidad de Concepción, Departamento de Física, Casilla 160-C, Concepción, Chile
²⁵ Jodrell Bank Observatory, The University of Manchester, Macclesfield, Cheshire SK11 9DL, UK

Received: 25 October 2005
Accepted: 4 September 2006

Abstract

Aims. We present an analysis of OGLE 2004–BLG–254, a high-magnification () and relatively short duration ( days) microlensing event in which the source star, a Bulge K-giant, has been spatially resolved by a point-like lens. We seek to determine the lens and source distance, and provide a measurement of the linear limb-darkening coefficients of the source star in the I and R bands. We discuss the derived values of the latter and compare them to the classical theoretical laws, and furthermore examine the cases of already published microlensed GK-giants limb-darkening measurements.

Methods. We have obtained dense photometric coverage of the event light curve with OGLE and PLANET telescopes, as well as a high signal-to-noise ratio spectrum taken while the source was still magnified by , using the UVES/VLT spectrograph. We have performed a modelling of the light curve, including finite source and parallax effects, and have combined spectroscopic and photometric analysis to infer the source distance. A Galactic model for the mass and velocity distribution of the stars has been used to estimate the lens distance.

Results. From the spectrum analysis and calibrated color-magnitude of the event target, we found that the source was a K3 III Bulge giant, situated at the far end of the Bulge. From modelling the light curve, we have derived an angular size of the Einstein ring as, and a relative lens-source proper motion mas/yr. We could also measure the angular size of the source, as, whereas given the short duration of the event, no significant constraint could be obtained from parallax effects. A Galactic model based on the modelling of the light curve then provides us with an estimate of the lens distance, mass and velocity as kpc, and (at the lens distance) respectively. Our dense coverage of this event allows us to measure limb darkening of the source star in the I and R bands. We also compare previous measurements of linear limb-darkening coefficients involving GK-giant stars with predictions from ATLAS atmosphere models. We discuss the case of K-giants and find a disagreement between limb-darkening measurements and model predictions, which may be caused by the inadequacy of the linear limb-darkening law.