Hubble Space Telescope Imaging Polarimetry of Comet 67P/Churyumov-Gerasimenko Obtained During the Rosetta Mission

We present pre- and post-perihelion, high-spatial resolution (0.05 arcsec/pixel) 0.6 micron imaging polarimetry of Comet 67P/Churyumov-Gerasimenko taken with the Advanced Camera for Surveys aboard the Hubble Space Telescope (HST). The pre-perihelion observations were obtained at two epochs chosen to bracket the times when the closest orbits of Rosetta were flown (down to 10 km for extended periods: 2014-Aug-19: rh = 3.52 au, Δ = 2.76 au, α ≈ 12.0˚) and the Philae landing took place (2014-Nov-17: rh = 2.96 au, Δ = 3.43 au, α ≈ 15.7˚). Our preliminary analyses of both pre-perihelion epochs shows that the polarization position angle lies in the scattering plane, thus is negative, with a degree of polarization p% ≈ -2%. The two post-perihelion epochs were matched to the first time after perihelion that the comet was observable with HST (2015-Oct-10: rh = 1.43 au, Δ = 1.80 au, α ≈ 33.5˚), and when the comet was again viewed at small phase angle (2016-Feb-19: rh = 2.40 au, Δ = 1.49 au, α ≈ 12.0˚). We discuss our polarimetry results in context with in situ measurements of dust particles obtained with the Rosetta spacecraft.