2009ApJ...691..452H

A microlensing zone refers to the range of planet-star separations where the probability of detecting a planetary signal is high. Its conventional definition as the range between ∼0.6 and 1.6 Einstein radii of the primary lens is based on the criterion that a major caustic induced by a planet should be located within the Einstein ring of the primary lens. However, current planetary lensing searches focus on high-magnification events to detect perturbations induced by another caustic always located within the Einstein ring very close to the primary lens (stellar caustic) and thus a new definition of a lensing zone is needed. In this paper, we determine this lensing zone. By applying a criterion that detectable planets should produce signals ≥5%, we find that the new lensing zone varies depending on the planet/star mass ratio unlike the fixed range of the classical zone regardless of the mass ratio. The lensing zone is equivalent to the classical zone for a planet with a planet/star mass ratio q ∼ 3x10^–4 and becomes wider for heavier planets. For a Jupiter-mass planet, the lensing zone ranges from 0.25 to 3.9 Einstein radii, corresponding to a physical range between ∼0.5 and 7.4 AU for a typical Galactic event. The wider lensing zone of central perturbations for giant planets implies that the microlensing method provides an important tool to detect planetary systems composed of multiple ice-giant planets.