Polarimetry Microlensing of Close-in Planetary Systems

Abstract

A close-in giant planetary (CGP) system has a net polarization signal whose value varies depending on the orbital phase of the planet. This polarization signal is either caused by the stellar occultation or by reflected starlight from the surface of the orbiting planet. When the CGP system is located in the Galactic bulge, its polarization signal becomes too weak to be measured directly. One method for detecting and characterizing these weak polarization signatures due to distant CGP systems is gravitational microlensing. In this work, we focus on potential polarimetric observations of highlymagnified microlensing events of CGP systems. When the lens is passing directly in front of the source star with its planetary companion, the polarimetric signature caused by the transiting planet is magnified. As a result some distinct features in the polarimetry and light curves are produced. In the same way microlensing amplifies the reflection-induced polarization signal. While the planetinduced perturbations are magnified, whenever these polarimetric or photometric deviations vanish for a moment the corresponding magnification factor or the polarization component(s) is equal to the related one due to the planet itself. Finding these exact times in the planet-induced perturbations helps us to characterize the planet. In order to evaluate the observability of such systems through polarimetric or photometric observations of high-magnification microlensing events, we simulate these events by considering confirmed CGP systems as their source stars and conclude that the efficiency for detecting the planet-induced signal with the state-of-the-art polarimetric instrument (FORS2/VLT) is less than 0.1%. Consequently, these planet-induced polarimetry perturbations can likely be detected under favorable conditions by high-resolution and short-cadence polarimeters of the next generation.

Variation of the stellar color in high-magnification and caustic-crossing microlensing events

“…These objects perturb their host star brightness but also their color. These faint and small color distortions are optimally observable at long wavelengths, that is, the infrared, in HM or CC microlensing events through photometric or polarimetric observations (Graff & Gaudi 2000;Sajadian & Rahvar 2010;Sajadian & Hundertmark 2017). This channel of exoplanet detection is parallel to the main channel of planet detection in microlensing events where the gravitational effect of a planet in the lensing system affects the light path of an image of the source star (see, e.g., Mao & Paczynski 1991;Beaulieu et al 2006;Gaudi 2012).…”

“…To estimate the planet radiation, we consider two components, thermal and reflected ones. All details about calculating these radiations can be found in Sajadian & Hundertmark (2017).…”

Polarization in caustic-crossing binary microlensing events

“…In microlensing events, the intrinsic polarization signals of the source stars which are mostly unmeasurable can be magnified and generate some perturbations in polarization curves. The intrinsic polarization signals are made by second-order perturbations, for instance the stellar magnetic fields and spots, circumstellar disks, close-in giant planets, etc., ( e.g., Sajadian 2015;Sajadian & Rahvar 2015;Sajadian & Hundertmark 2017).…”