Toward the Detection of Exoplanet Transits with Polarimetry

TL;DR

This paper explores the potential of using polarimetry to detect exoplanet transits, highlighting the unique signals during ingress and egress, and reports on initial observational challenges and future prospects.

Contribution

It introduces the concept of polarimetric transits, identifies promising systems like HD 80606b, and discusses the technical challenges and necessary improvements for successful detection.

Findings

Polarimetric signals are strongest during ingress and egress.

Test observations faced systematic noise issues.

Further reduction in systematic noise is needed for detection.

Abstract

In contrast to photometric transits, whose peak signal occurs at mid-transit due to occultation of the brightest region of the disk, polarimetric transits provide a signal upon ingress and egress due to occultation of the polarized stellar limb. Limb polarization, the bright corollary to limb darkening, arises from the $90^\circ$ scattering angle and low optical depth experienced by photons at the limb. In addition to the ratio $R_{\rm p} / R_*$, the amplitude of a polarimetric transit is expected to be controlled by the strength and width of the stellar limb polarization profile, which depend on the scattering-to-total opacity ratio at the stellar limb. We present a short list of the systems providing the highest expected signal-to-noise ratio for detection of this effect, and we draw particular attention to HD 80606b. This planet is spin/orbit misaligned, has a three-hour ingress, and has a bright parent star, which make it an attractive target. We report on test observations of an HD 80606b ingress with the POLISH2 polarimeter at the Lick Observatory Shane 3-m telescope. We conclude that unmodeled telescope systematic effects prevented polarimetric detection of this event. We outline a roadmap for further refinements of exoplanet polarimetry, whose eventual success will require a further factor of ten reduction in systematic noise.