Photometric Microlens Parallaxes with SIM

Abstract

Astrometric measurements of microlensing events can in principle determine both the "parallax" \tilde r_E and the "proper motion" μof an individual event which (combined with the Einstein time scale t_E) in turn yield the mass, distance, and transverse velocity of the lens. We show, however, that the parallax measurements are generically several orders of magnitude less precise than the proper-motion measurements. Fortunately, astrometric measurements by the Space Interferometry Mission (SIM) are simultaneously **photometric** measurements, and since SIM will be in solar orbit, these allow SIM to be used as a classical (photometric) parallax satellite. We show that SIM photometric parallaxes are of comparable precision to its astrometric proper-motion measurements. For I=15 bulge stars, complete solutions with about 5% accuracy in mass, distance, and transverse velocity can be obtained from about 5 hours of observation, 100 to 10,000 times shorter than would be required for a purely astrometric solution of similar precision. Thus, it should be possible to directly measure the mass functions of both the bulge and the inner disk (including both dark and luminous objects) with only a few hundred hours of SIM observations.