Gravitational Microlensing with the Space Interferometry Mission

The Space Interferometry Mission (SIM), with its launch date planned for 2005, has as its goal astrometry with ∼ 1 μ arcsecond accuracy for stars as faint as 20th mag. If the SIM lives to expectations it can be used to measure astrometric displacements in the light centroid caused by the gravitational microlensing in the events detected photometrically from the ground. The effect is typically ∼ 0.1 mas, i.e. two orders of magnitude larger than planned SIM’s accuracy. Therefore, it will be possible to determine the mass, the distance, and the proper motion of almost any MACHO capable of inducing a photometric microlensing event towards the galactic bulge or the Magellanic Clouds, even though no light from the MACHO has to be detected. For strong microlensing events in which the source is photometrically resolved, like the recent MACHO 95-30 event, SIM’s astrometry combined with accurate ground based photometry will allow the determination of the stellar effective temperature. The masses of nearby high proper motion stars and brown dwarfs can be measured accurately with the SIM taking advantage of the fact that their effective cross sections for astrometric effects due to gravitational lensing are as large as ∼ 1 arcminute. Subject headings: astrometry – galaxy: structure – galaxy: halo – gravitational lensing – stars: brown dwarfs – stars: fundamental parameters –