A Ground-based Albedo Upper Limit for HD 189733b from Polarimetry
We present 50 nights of polarimetric observations of HD 189733 in the 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 the order of 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 99.7% confidence upper limit to the planet-induced polarization of the system of 60 ppm in the B band, which is consistent with a previous upper limit from the POLISH instrument at the Palomar Observatory 5-m telescope. A physically motivated Rayleigh scattering model, which includes the depolarizing effects of multiple scattering, is used to conservatively set a 99.7% confidence upper limit to the geometric albedo of HD 189733b of A_g < 0.40. This value is consistent with the value A_g = 0.226 ±0.091 derived from occultation observations with Hubble Space Telescope STIS, but it is inconsistent with the large A_g = 0.61 ± .012 albedo reported by Berdyugina et al.
© 2015. The American Astronomical Society. Received 2015 July 13; accepted 2015 September 29; published 2015 October 27. We would like to acknowledge the tireless efforts of the Lick Observatory staff. This work was performed (in part) under contract with the California Institute of Technology (Caltech) funded by NASA through the Sagan Fellowship Program executed by the NASA Exoplanet Science Institute. S.J.W. and L.A.N. acknowledge support from the NASA Origins of Solar Systems program through grant NNX13AF63G. P.K. and Y.L.Y. acknowledge support from an NAI Virtual Planetary Laboratory grant from the University of Washington to the Jet Propulsion Laboratory and California Institute of Technology. Research at Lick Observatory is partially supported by a generous gift from Google. Facility: Shane (POLISH2) - .
Accepted Version - 1507.03588.pdf
Published - Wiktorowicz_2015.pdf