Determination of stellar shape in microlensing event MOA 2002-BLG-33

¹ Jodrell Bank Observatory, Macclesfield, Cheshire SK11 9DL, UK e-mail: njr@jb.man.ac.uk
² Solar Terrestrial Environment Laboratory, Nagoya University, Nagoya 464-8601, Japan
³ Department of Physics, Notre Dame University, Notre Dame, IN 46556, USA
⁴ Institute of Information and Mathematical Sciences, Massey University, Auckland, New Zealand
⁵ Department of Physics, University of the Free State, Bloemfontein, South Africa
⁶ Instituto de Astrofísica de Andalucía, CSIC, Apartado 3004, 18080 Granada, Spain
⁷ Institute for Geophysics and Planetary Physics, Lawrence Livermore National Laboratory, CA 94563, USA
⁸ Hubble Fellow, Caltech Astronomy, MS 105-24, California Institute of Technology, 1200 E. California Bl., Pasadena, California
⁹ CEA, DSM, DAPNIA, Centre d'Études de Saclay, 91191 Gif-sur-Yvette Cedex, France
¹⁰ Department of Physics and Astronomy, University of Canterbury, Private Bag 4800, Christchurch, New Zealand
¹¹ Universität Hamburg, Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
¹² School of Physics and Astronomy, Tel-Aviv University, Tel-Aviv 69978, Israel
¹³ Laboratoire de l'Accélérateur Lineaire, IN2P3 CNRS, Université de Paris-Sud, 91405 Orsay Cedex, France
¹⁴ Faculty of Science, University of Auckland, Private Bag 92019, Auckland, New Zealand
¹⁵ School of Chemical and Physical Sciences, Victoria University, PO Box 600, Wellington, New Zealand
¹⁶ Department of Astrophysical Sciences, Princeton University, Princeton NJ 08544, USA
¹⁷ Department of Astronomy, Ohio State University, Columbus OH 43210, USA

Received: 10 February 2005
Accepted: 2 May 2005

Abstract

We report a measurement of the shape of the source star in microlensing event MOA 2002-BLG-33. The lens for this event was a close binary whose centre-of-mass passed almost directly in front of the source star. At this time, the source star was closely bounded on all sides by a caustic of the lens. This allowed the oblateness of the source star to be constrained. We found that where a and b are its semi-major and semi-minor axes respectively. The angular resolution of this measurement is approximately 0.04 . We also report HST images of the event that confirm a previous identification of the source star as an F8-G2 turn-off main-sequence star.