Astrometric Method for Breaking the Photometric Degeneracy between Binary‐Source and Planetary Microlensing Perturbations

Abstract

An extrasolar planet can be detected by microlensing because the planet can perturb the smooth lensing light curve created by the primary lens. However, it was shown by Gaudi that a subset of binary-source events can produce light curves that closely resemble those produced by a significant fraction of planet/star lens systems, causing serious contamination of samples of suspected planetary systems detected via microlensing. In this paper, we show that if a lensing event is observed astrometrically, one can unambiguously break the photometric degeneracy between the binary-source and the planetary lensing perturbations. This is possible because while the planet-induced perturbation in the trajectory of the lensed source image centroid shifts points away from the opening of the unperturbed elliptical trajectory, the perturbation induced by the binary-source companion always points toward the opening. Therefore, astrometric microlensing observations using future high-precision interferometers will be important for solid confirmation of microlensing planet detections.