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

TL;DR

This study presents imaging polarimetry of comet 2P/Encke in 2017, revealing how dust properties and polarization vary across the coma, indicating changes in particle size and composition with distance from the nucleus.

Contribution

It provides detailed polarimetric and color measurements of 2P/Encke's coma, linking polarization variations to dust particle size changes using light scattering simulations.

Findings

Dust/gas ratios decrease with distance from nucleus.

Polarization correlates with dust color and dust/gas ratio.

Particle size variations explain observed polarization and color trends.

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 deg, 46 days before perihelion. Observations were made through the medium-band SED500 ({\lambda}5019/246 {\AA}) and broadband r-sdss ({\lambda}6200/1200 {\AA}) 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 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 ~2500 km, respectively. Then these ratios began to increase at distances ~12000 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. 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.