Caustic Structures and Detectability of Circumbinary Planets in Microlensing

TL;DR

This paper investigates the caustic structures and detectability of circumbinary planets in microlensing, providing new analytic expressions and estimates of detection fractions based on binary parameters.

Contribution

It introduces a semi-empirical analytic expression for planetary caustic locations in circumbinary microlensing and explores caustic dynamics across the parameter space.

Findings

Caustic structures vary with binary separation and orientation.

Planet orbital motion dominates caustic movement over binary motion.

Detection probability of circumbinary planets in microlensing is 5-50% for certain binary separations.

Abstract

Recent discoveries of circumbinary planets in Kepler data show that there is a viable channel of planet formation around binary main sequence stars. Motivated by these discoveries, we have investigated the caustic structures and detectability of circumbinary planets in microlensing events. We have produced a suite of animations of caustics as a function of the projected separation and angle of the binary host to efficiently explore caustic structures over the entire circumbinary parameter space. Aided by these animations, we have derived a semi-empirical analytic expression for the location of planetary caustics, which are displaced in circumbinary lenses relative to those of planets with a single host. We have used this expression to show that the dominant source of caustic motion will be due to the planet's orbital motion and not that of the binary star. Finally, we estimate the fraction of circumbinary microlensing events that are recognizable as such to be significant (5-50 percent) for binary projected separations in the range 0.1-0.5 in units of Einstein radii.