Bringing Closure to Microlensing Mass Measurement

Interferometers offer multiple methods for studying microlensing events anddetermining the properties of the lenses. We investigate the study ofmicrolensing events with optical interferometers, focusing on narrow-angleastrometry, visibility, and closure phase. After introducing the basics ofmicrolensing and interferometry, we derive expressions for the signals in eachof these three channels. For various forecasts of the instrumental performance,we discuss which method provides the best means of measuring the lens angularEinstein radius theta_E, a prerequisite for determining the lens mass. If theupcoming generation of large-aperture, AO-corrected long baselineinterferometers (e.g. VLTI, Keck, OHANA) perform as well as expected, theta_Emay be determined with signal-to-noise greater than 10 for all bright events.We estimate that roughly a dozen events per year will be sufficiciently brightand have long enough durations to allow the measurement of the lens mass anddistance from the ground. We also consider the prospects for a VLTI survey ofall bright lensing events using a Fisher matrix analysis, and find that evenwithout individual masses, interesting constraints may be placed on the bulgemass function, although large numbers of events would be required.Comment: 23 pages, aastex, submitted to Ap