MICROLENSING EVENTS - THIN DISK, THICK DISK, OR HALO

The three recently announced candidate microlensing events toward the Large Magellanic Cloud (LMC) could arise from compact objects in a thin disk, a thick disk, or a halo. We discuss five observations that could discriminate among these possibilities: (1) The optical depth to the LMC, which can be measured from the rate and duration of events. The ratios of upper limits to the optical depths are 1:2.5:15 for the thin disk, thick disk, and halo models, so a large optical depth can rule out the thin disk and thick disk models. (2) The ratios of the optical depths to the SMC and LMC for the three models: 0.6, 0.6, and 1-1.5. (3) Deviations from the normal shape of the microlensing light curve, caused by parallax effects. The ratios of the fraction of events with detectable parallax effects are 70:15:1. If events are identified in real time and are observed with high-precision relative photometry (approximately 0.02 mag), then parallax effects can be measured for most thin-disk events and approximately 15% of thick-disk events. (4) The characteristic masses inferred from the mean duration of events. These are in the ratio of 1.2:1:1.5, so this can be at most a marginal test of the three models. (5) The ratios of optical depths to the bulge (in excess of that produced by known stars) and to the LMC. These are 45, 5.5, and 1 for a thin disk, a thick disk, and a halo. At present the observations favor a thick-disk or halo origin of the lenses, or similar contributions from more than one structure, but do not exclude the thin disk as the main contributor.