Comet 2P/Encke in apparition of 2017: II. Polarization and color

We present results of imaging polarimetry of comet 2P/Encke performed on January 23, 2017 at the heliocentric (1.052 au) and geocentric (1.336 au) distances and phase angle 46.8 degrees, 46 days before perihelion. Observations were made through the medium-band SED500 (lambda 5019/246 angstrom) and broadband r-sdss (7x6200/1200 A) filters with the multimode focal reducer SCORPIO-2 at the 6-m BTA telescope of the Special Astrophysical Observatory (Russia). Dust in comet 2P/Encke was mainly concentrated in the near-nucleus region of the coma: the maximum dust/gas to leave F-em/F-cont ratios were 1.5 and 2.9 in the SED500 and the r-sdss filters near the nucleus but dropped sharply to similar to 0.2 and similar to 1 at the distance similar to 2.500 km, respectively. Then these ratios began to increase at distances similar to 12,000 km from the nucleus, the ratio was similar to 0.3 (SED500) and similar to 1.3 (r-sdds). There were significant variations of polarization over the coma, which correlated with the variations in the dust color and dust/gas ratio. The maximum degree of polarization, similar to 8% in the r-sdss filter, was observed in the dust shell which was shifted by similar to 1.000 km towards the Sun. Polarization sharply dropped to similar to 4% at the distance similar to 3.000 km and then gradually increased with wave-like fluctuations with the distance from the nucleus, reaching similar to 8% at the distance similar to 12,000 km. A similar change in polarization was observed in the SED500 filter. After correction for gas contamination, using the dust/gas ratios from spectroscopy made on the same night, the values of polarization appeared to be 4% in the near-nucleus region (similar to 1.000 km), and reached 11-12% at the distance similar to 12,000 km in both filters. We also found an effect of nucleus polarization on the polarization of the dust coma in comet Encke in the r-sdss filter. The maximum value of the nucleus contamination was similar to 0.7%. Changes in polarization and color across the 2P/Encke coma indicate changes in physical properties of the dust particles with the distance from the nucleus. Our Sh-matrix computer simulations of light scattering by Gaussian particles allow us to suggest that the observed trends in color and polarization are mainly result from changing particle size.