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

Abstract

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°, 46 days before perihelion. Observations were made through the medium-band SED500 (λ5019/246 Å) and broadband r-sdss (λ6200/1200 Å) 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 Fem/Fcont ratios were 1.5 and 2.9 in the SED500 and the r-sdss filters near the nucleus but dropped sharply to ~0.2 and ~1 at the distance ~2.500 km, respectively. Then these ratios began to increase at distances ~12,000 km from the nucleus, the ratio was ~0.3 (SED500) and ~ 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, ~8% in the r-sdss filter, was observed in the dust shell which was shifted by ~1.000 km towards the Sun. Polarization sharply dropped to ~4% at the distance ~3.000 km and then gradually increased with wave-like fluctuations with the distance from the nucleus, reaching ~8% at the distance ~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 (~1.000 km), and reached 11–12% at the distance ~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 ~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.