Microlens Parallaxes with SIRTF

The Space Infrared Telescope Facility (SIRTF) will drift away from the Earthat about 0.1 AU/yr. Microlensing events will therefore have differentcharacteristics as seen from the satellite and the Earth. From the difference,it is possible in principle to measure v-tilde, the transverse velocity of thelens projected onto the observer plane. Since v-tilde has very different valuesfor different populations (disk, halo, Large Magellanic Cloud), suchmeasurements could help identify the location, and hence the nature, of thelenses. I show that the method previously developed by Gould for measuring suchsatellite parallaxes fails completely in the case of SIRTF: it is overwhelmedby degeneracies which arise from fact that the Earth and satellite observationsare in different band passes. I develop a new method which allows forobservations in different band passes and yet removes all degeneracies. Themethod combines a purely ground-based measurement of the "parallax asymmetry"with a measurement of the delay between the time the event peaks at the Earthand satellite. In effect, the parallax asymmetry determines the component ofv-tilde in the Earth-Sun direction, while the delay time measures the componentof v-tilde in the direction of the Earth's orbit.Comment: 21 pages plus 3 figure