Microlens Parallax Asymmetries toward the Large Magellanic Cloud

If the microlensing events now being detected toward the Large MagellanicCloud (LMC) are due to lenses in the Milky Way halo, then the events shouldtypically have asymmetries of order 1% due to parallax from the reflex motionof the Earth. By contrast, if the lenses are in the LMC, the parallax effectsshould be negligible. A ground-based search for such parallax asymmetries wouldtherefore clarify the location of the lenses. A modest effort (2 hours pernight on a 1 m telescope) could measure 15 parallax asymmetries over 5 yearsand so marginally discriminate between the halo and the LMC as the source ofthe lenses. A dedicated 1 m telescope would approximately double the number ofmeasurements and would therefore clearly distinguish between the alternatives.However, compared to satellite parallaxes, the information extracted fromground-based parallaxes is substantially less useful for understanding thenature of the halo lenses (if that is what they are). The backgrounds ofasymmetries due to binary-source and binary-lens events are estimated to beapproximately 7% and 12% respectively. These complicate the interpretation ofdetected parallax asymmetries, but not critically.Comment: Submitted to ApJ, 17 pages, including 2 embedded figure