An extra-solar 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 a sample 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 binary-source and 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, while the perturbation
induced by the binary source companion points always towards the opening.
Therefore, astrometric microlensing observations by using future high-precision
interferometers will be important for solid confirmation of microlensing planet
detections.Comment: total 5 pages, including 1 figure and no table, ApJ, submitted,
better quality pdf file is avalilable at
http://astroph.chungbuk.ac.kr/~cheongho/publication.htm