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Session 27 - Gravitational Lensing & Dark Matter.
Oral session, Monday, January 13
Frontenac Ballroom,
Microlensing events have been detected so far in photometric surveys, from the variation in the total magnification of two images of the source star as the lensing object passes by. These events may also be detected astrometrically, by measuring directly the deflection. The field of high precision interferometric astrometry is rapidly advancing: interferometers in the ground should now have accuracies of 50 \mu \arcsec , and various proposals for interferometric satellites call for accuracies better than 10 \mu \arcsec on large numbers of stars. This motivates this study of the applications of detecting microlensing events with astrometry. Contrary to the photometric method, microlensing events can be detected with astrometry for impact parameters much larger than the Einstein radius, increasing by a large factor the number of detectable events. Three different applications will be discussed. The first is to measure masses of bright, nearby stars, from the deflection on a background star. This can be done with ground-based interferometers, and the observations required are the same as those needed to discover planets around nearby stars. We show that there should be several bright stars over the sky with an adequate background star close to them that will undergo a microlensing event detectable over the next 10 years. Second, masses of brown dwarfs orbiting nearby bright stars could be measured with the same technique. Third, distant stars can be monitored to detect microlensing events by any lens near the line-of-sight. This requires an astrometric satellite able to monitor astrometrically large numbers of stars, doing 10 to 20 observations of each star over a period of several years. The proposed satellite GAIA satisfies these characteristics, and we show that it should detect thousands of microlensing events from known stars, as well as compact objects that might account for dark matter in the halo or in the disk.
