Microlensing events from measurements of the deflection

Microlensing events are now regularly being detected by monitoring the flux of a large number of potential sources and measuring the combined magnification of the images. This phenomenon could also be detected directly from the gravitational deflection, by means of high-precision interferometric astrometry. Relative astrometry at the level of 10 mu as may become possible in the near future. Contrary to the photometric method, astrometry allows the detection of microlensing events for impact parameters much larger than the Einstein radius, increasing by a large factor the number of detectable events. With ground-based interferometers, the gravitational deflection can be measured by astrometric monitoring of a bright star having a background star within a small angular separation. This type of monitoring program will be carried out for the independent reasons of discovering planets from the angular motion they induce on the nearby star around which they are orbiting and of measuring parallaxes, proper motions, and orbits of binary stars. We discuss three applications of the measurement of gravitational deflections by astrometric monitoring: measuring the mass of the bright stars that are monitored, measuring the mass of brown dwarfs or giant planets around the bright stars, and detecting microlensing events by unrelated objects near the line of sight to the two stars. We discuss the number of stars whose mass could be measured by this procedure. We also give expressions for the number of expected microlensing events by unrelated objects, which could be stars, brown dwarfs, or other compact objects accounting for dark matter in the halo or in the disk; these could be detected with space interferometers.