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Astron. Astrophys. 321, 424-433 (1997)


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Lensing of unresolved stars towards the Galactic Bulge

C. Alard 1, 2

1 Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge. CB3 0HA, United Kingdom
2 Centre d'Analyse des Images de l'INSU, Bâtiment Perrault, Observatoire de Paris, 61 Avenue de l'Observatoire, F-75014, Paris, France

Received 5 September 1996 / Accepted 18 September 1996

Abstract

This paper presents an analysis of lensing of unresolved background stars. Previous theoritical calculations of the lensing rates and optical depths considered only resolved stars. However, if a faint unresolved star is lensed close enough to a resolved star, the event will be seen by the microlensing experiment and attributed to the brighter star. The blending biases the duration, making the contribution of the unresolved stars very significant for short events. This contribution might be confused with lensing by brown dwarfs. The exact rates of these blended events are extremely sensitive to the limiting magnitude achieved in the microlensing search. Calculations of the optical depth and rates are provided here, and illustrated in the case of the DUO and OGLE experiments. The additional contribution of unresolved stars is very significant. It probably explains the high optical depth and rates observed towards the Galactic Bulge for turn-off stars. The blended unresolved event can be identified using either the color shift or the light curve shape. However, neither of these two methods is apropriate to identify a large number of blended events towards the Bulge. In some case with good photometry and small impact parameter an indentification is possible. This is illustrated by the case of the OGLE 5 event, which clearly appears as a case of lensing of an unresolved star. The recent results obtained by the PLANET collaboration indicate that a high resolution and dense sampling of the light curve is possible, and will make possible to correct the blending bias, as demonstrated for OGLE 5. This possiblity, is certainly better than a statistical estimation of the lensing rates, which are always prone to some uncertainty. But, at this time, except in the case of the MACHO measurement of the optical depth for the clump giants, the analysis of Microlensing events found in the various microlensing experiments requires the uses of modelisations of the contribution of unresolved stars.

Key words: methods: data analysis – gravitational lensing – Galaxy: stellar content; center

Send offprint requests to: C. Alard

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998

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