A Ground-Based Albedo Upper Limit for HD 189733b from Polarimetry
Wiktorowicz et al
We present 50 nights of polarimetric observations of HD 189733 in B band using the POLISH2 aperture-integrated polarimeter at the Lick Observatory Shane 3-m telescope. This instrument, commissioned in 2011, is designed to search for Rayleigh scattering from short-period exoplanets due to the polarized nature of scattered light. Since these planets are spatially unresolvable from their host stars, the relative contribution of the planet-to-total system polarization is expected to vary with an amplitude of order 10 parts per million (ppm) over the course of the orbit. Non-zero and also variable at the 10 ppm level, the inherent polarization of the Lick 3-m telescope limits the accuracy of our measurements and currently inhibits conclusive detection of scattered light from this exoplanet. However, the amplitude of observed variability conservatively sets a 3σ upper limit to the planet-induced polarization of the system of 58 ppm in B band, which is consistent with a previous upper limit from the POLISH instrument at the Palomar Observatory 5-m telescope (Wiktorowicz 2009). A physically-motivated Rayleigh scattering model, which includes the depolarizing effects of multiple scattering, is used to conservatively set a 3σ upper limit to the geometric albedo of HD 189733b of Ag less than 0.37. This value is consistent with the value Ag=0.226±0.091 derived from occultation observations with HST STIS (Evans et al. 2013), but it is inconsistent with the large Ag=0.61±0.12 albedo reported by (Berdyugina et al. 2011).