Planet Parameters in Microlensing Events

A planetary microlensing event occurs when a planet perturbs one of the two images created in a point-mass microlensing event, causing a deviation from the standard Paczyński curve. Determination of the two physical parameters that can be extracted from a planetary microlensing event, the planet/star mass ratio q, and the planet/star separation in units of the stellar Einstein ring, yp, is hampered by several types of degeneracies. There are two distinct and qualitatively different classes of planetary events: major and minor image perturbations. For major image perturbations, there is a potentially crippling continuous degeneracy in q which is of order δ d , where δd is the maximum fractional deviation of the planetary perturbation. Since the threshold of detection is expected to be δd ∼ 5%, this degeneracy in q can be a factor of ∼ 20. For minor image perturbations, the continuous degeneracy in q is considerably less severe, and is typically less than a factor 4. We show that these degeneracies can be resolved by observations from dedicated telescopes on several continents together with optical/infrared photometry from one of these sites. There also exists a class of discrete degeneracies. These are typically easy to resolve given good temporal coverage of the planetary 1 Alfred P. Sloan Foundation Fellow