Microlensing by a wide-separation planet: detectability and boundness

TL;DR

This paper investigates the detectability of wide-separation planets via microlensing, establishing conditions under which their boundness can be inferred based on source size, separation, and observational thresholds.

Contribution

It introduces an analytic condition for detecting the boundness of wide-separation planets in microlensing events considering source size and separation.

Findings

Detection range for Earth-mass planets is ~10 AU.

Detection range for Jupiter-mass planets is ~30 AU.

Planetary caustic analysis extends detection of boundness farther.

Abstract

We explore the detection condition of a wide-separation planet through the perturbation induced by the planetary caustic for various microlensing parameters, especially for the size of the source stars. By constructing the fractional deviation maps at various positions in the space of microlensing parameters, we find that the pattern of the fractional deviation depends on the ratio of the source radius to the caustic size, and the ratio satisfying the observational threshold varies with the star-planet separation. We have also obtained the upper limits of the source size that allows the detection of the signature of the host star as a function of the separation for given observational threshold. It is shown that this relation further leads one to a simple analytic condition for the star-planet separation to detect the boundness of wide-separation planets as a function of the mass ratio and the source radius. For example, when 5% of the detection threshold is assumed, for a source star with the radius of ~1 R_sun, an Earth-mass planet and a Jupiter-mass planet can be recognized of its boundness when it is within the separation range of ~10 AU and ~30 AU, respectively. We also compare the separation ranges of detection by the planetary caustic with those by the central caustic. It is found that when the microlensing light curve caused by the planetary caustic happens to be analyzed, one may afford to support the boundness of the wide-separation planet farther than when that caused by the central caustic is analyzed. Finally, we conclude by briefly discussing the implication of our findings on the next-generation microlensing experiments.