Microlensing Characterization of Wide-Separation Planets

With their excellent photometric precision and dramatic increase in monitoring frequency, future microlensing survey experiments are expected to be sensitive to very short timescale, isolated events caused by free-floating and wide-separation planets with masses as low as a few lunar masses. The scientific value of these detections would be greatly enhanced if their nature (bound or unbound) could be accurately characterized and if the planet masses could be measured. Here we present a comprehensive discussion of the ability of microlensing to detect and characterize wide-separation planets. We estimate the probability of measuring the planetary Einstein radius theta(E,p) for bound and free-floating planets; this is one of the two additional observables required to measure the planet mass. We carry out detailed simulations of the planetary events expected in next-generation surveys and estimate the resulting uncertainty in theta(E,p) for these events. We show that, for main-sequence sources and Jupiter-mass planets, the caustic structure of wide-separation planets with projected separations of less than or similar to20 AU substantially increases the probability of measuring the dimensionless source size and thus determining theta(E,p) compared to the case of unbound planets. In this limit where the source is much smaller than the caustic, the effective cross section to measure theta(E,p) to 10% is similar to25% larger than the full width of the caustic. Measurement of the lens parallax is possible for low-mass planetary events through combined observations from the ground and a satellite located in an L2 orbit; this would complete the mass measurements for such wide-separation planets. Finally, short-duration events caused by bound planets can be routinely distinguished from those caused by free-floating planets for planet-star separations less than or similar to20 AU from either the deviations due to the planetary caustic or (more often) the low-amplitude bump from the magnification due to the parent star.