Transit polarimetry of exoplanetary system HD189733

TL;DR

This paper investigates how a transiting exoplanet like HD189733 causes measurable linear polarization in the star's light by breaking symmetry, with detailed modeling of limb polarization and star spots to understand the polarization signals.

Contribution

It provides the first detailed radiative transfer calculations of limb polarization for HD189733, including effects of star spots, to predict polarization variations during transits.

Findings

Maximum polarization depends on limb polarization variation.

Transit causes measurable flux and polarization changes.

Star spots influence polarization signals.

Abstract

We present and discuss a polarimetric effect caused by a planet transiting the stellar disk thus breaking the symmetry of the light distribution and resulting in linear polarization of the partially eclipsed star. Estimates of this effect for transiting planets have been made only recently. In particular, we demonstrate that the maximum polarization during transits depends strongly on the centre-to-limb variation of the linear polarization of the host star. However, observational and theoretical studies of the limb polarization have largely concentrated on the Sun. Here we solve the radiative transfer problem for polarized light and calculate the centre-to-limb polarization for one of the brightest transiting planet host HD189733 taking into account various opacities. Using that we simulate the transit effect and estimate the variations of the flux and the linear polarization for HD189733 during the event. As the spots on the stellar disk also break the limb polarization symmetry we simulate the flux and polarization variation due to the spots on the stellar disk.