  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.
