Hubble Space Telescope Pre-perihelion ACS/WFC Imaging Polarimetry of Comet Ison (c/2012 s1) at 3.81 AU

We present polarization images of Comet ISON (C/2012 S1) taken with the Hubble Space Telescope (HST) on UTC 2013 May 8 (r(h) = 3.81 AU, Delta = 4.34 AU), when the phase angle was alpha approximate to 12.degrees 16. This phase angle is approximately centered in the negative polarization branch for cometary dust. The region beyond 1000 km (similar to 0.32 arcsec approximate to 6 pixels) from the nucleus shows a negative polarization amplitude of p% similar to -1.6%. Within 1000 km of the nucleus, the polarization position angle rotates to be approximately perpendicular to the scattering plane, with an amplitude p% similar to +2.5%. Such positive polarization has been observed previously as a characteristic feature of cometary jets, and we show that Comet ISON does indeed harbor a jet-like feature. These HST observations of Comet ISON represent the first visible light, imaging polarimetry with subarcsecond spatial resolution of a Nearly Isotropic Comet beyond 3.8 AU from the Sun at a small phase angle. The observations provide an early glimpse of the properties of the cometary dust preserved in this Oort-Cloud comet.