PROBING THE SPATIAL DISTRIBUTION OF EXTRASOLAR PLANETS WITH GRAVITATIONAL MICROLENSING

摘要

To search for extrasolar planets, current microlensing follow-up experiments are monitoring events caused by stellar-mass lenses, hoping to detect the planet's signature of the short-duration perturbation in the smooth lensing light curve of the primary. According to this strategy, however, it is possible to detect only planets located within a narrow region of separations from central stars. As a result, even if a large sample of planets are detected and the separations from their central stars are determined, it will be difficult to draw meaningful result about the spatial distribution of planets. An additional channel of microlensing planet detection is provided if the monitoring frequency of survey experiments is dramatically increased. From high-frequency monitoring experiments, such as the recently proposed Galactic Exoplanet Survey Telescope mission, one can detect two additional populations of planets, which are free-floating planets and bound planets with wide orbits around central stars. In this paper, we investigate the lensing properties of events caused by wide-orbit planets and find that the light curves of a significant fraction of these events will exhibit signatures of central stars, enabling one to distinguish them from events caused by free-floating planets. Because of the large primary/planet mass ratio, the effect of the central star endures to considerable separations. We find that for a Jupiter-mass planet the signatures of the central star can be detected with fractional deviations of ≥ 5% from the best-fitting single-lens light curves for approx> 80% of events caused by bound planets with separations approx < 10 AU, and the probability is still substantial for planets with separations up to ～ 20 AU. Therefore, detecting a large sample of these events will provide useful information about the distribution of extrasolar planets around their central stars. Proper estimation of the probability of distinguishing wide-orbit and free-floating planets will also be important for the correct determination of the frequency of free-floating planets, whose microlensing sample will be contaminated by wide-orbit planets.