Letter to the Editor

Probing the atmosphere of the bulge G5III star OGLE-2002-BUL-069 by analysis of microlensed Hα line^*

¹ PLANET collaboration member
² Institut d'Astrophysique de Paris, 98bis boulevard Arago, 75014 Paris, France
³ European Southern Observatory, Casilla 19001, Vitacura 19, Santiago, Chile
⁴ DSM/DAPNIA, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France
⁵ University of St. Andrews, School of Physics & Astronomy, North Haugh, St. Andrews KY16 9SS, UK
⁶ Technical University of Vienna, Dept. of Computing, Wiedner Hauptstrasse 10, Vienna, Austria
⁷ Niels Bohr Institute, Astronomical Observatory, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
⁸ Universität Potsdam, Astrophysik, Am Neuen Palais 10, 14469 Potsdam, Germany
⁹ University of Canterbury, Department of Physics & Astronomy, Private Bag 4800, Christchurch, New Zealand
¹⁰ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹¹ Observatoire Midi-Pyrénées, UMR 5572, 14 avenue Édouard Belin, 31400 Toulouse, France
¹² University of Tasmania, Physics Department, GPO 252C, Hobart, Tasmania 7001, Australia
¹³ Perth Observatory, Walnut Road, Bickley, Perth 6076, Australia
¹⁴ South African Astronomical Observatory, PO Box 9 Observatory 7935, South Africa
¹⁵ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
¹⁶ RSAA, Mount Stromlo and Siding Spring Observatories, ANU, Cotter Road, Weston Creek, Canberra ACT 2611, Australia

Corresponding author: J. P. Beaulieu, beaulieu@iap.fr

Received: 7 January 2004
Accepted: 11 March 2004

Abstract

We discuss high-resolution, time-resolved spectra of the caustic exit of the binary microlensing event OGLE 2002-BLG-069 obtained with UVES on the VLT. The source star is a G5III giant in the Galactic Bulge. During such events, the source star is highly magnified, and a strong differential magnification around the caustic resolves its surface. Using an appropriate model stellar atmosphere generated by the PHOENIX v2.6 code we obtain a model light curve for the caustic exit and compare it with a dense set of photometric observations obtained by the PLANET microlensing follow up network. We further compare predicted variations in the  equivalent width with those measured from our spectra. While the model and observations agree in the gross features, there are discrepancies suggesting shortcomings in the model, particularly for the  line core, where we have detected amplified emission from the stellar chromosphere after the source star's trailing limb exited the caustic. This achievement became possible by the provision of the very efficient OGLE-III Early Warning System, a network of small telescopes capable of nearly-continuous round-the-clock photometric monitoring, on-line data reduction, daily near-real-time modelling in order to predict caustic crossing parameters, and a fast and efficient response of a 8 m class telescope to a “Target-of-Opportunity” observation request.