Toward the Detection of Exoplanet Transits With Polarimetry

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 • scattering angle and low optical depth experienced by photons at the limb. In addition to the ratio R 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-tototal 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.

Polarimetric Detection of Exoplanets Transiting T and L Brown Dwarfs

“…The net polarization of the L and T dwarfs during the transit is calculated by multiplying the intensity and scattering polarization at each radial point along the disk of the object with the fractional circumference occulted by the projection of the planet over the surface of the primary dwarf. The disk-averaged polarization during the transit phase or the transit polarization is given by Sengupta & Marley (2016); Wiktorowicz & Laughlin (2014); Carciofi & Magalhaes (2005).…”

Spatially resolved spectroscopy across stellar surfaces

“…Besides limb darkening, continuum radiation also carries some small degree of polarization; this depends on the wavelength and could be detected in exoplanet transit polarimetry (Kostogryz et al 2015(Kostogryz et al , 2016Wiktorowicz & Laughlin 2014).…”

Polarization in Exoplanetary Systems Caused by Transits, Grazing Transits, and Starspots

“…Unfortunately, the transiting system HAT-P-54, which has the largest polarization, is very faint, m V = 13.5 and can be observed with a large telescope only. In their recently submitted paper, Wiktorowicz & Laughlin (2014) reported on failed attempt of detecting transit polarization of HD 80606 because of the low sensitivity and systematics of their polarimetry. This system is not included in our sample, as it is not meeting our selection criteria.…”