2018A&A...620A.161K

Context. Spectropolarimetry is a powerful technique for investigating the physical properties of gas and solid materials in cometary comae without mutual contamination, but only a few spectropolarimetric studies have been conducted to extract each component.
Aims. We attempt to derive the continuum (i.e., scattered light from dust coma) polarization degree of comet 2P/Encke, free of the influence of molecular emissions. The target is unique in that its orbit is dynamically decoupled from Jupiter, like the main-belt asteroids, but it ejects gas and dust like ordinary comets.
Methods. We observed the comet using the Hiroshima Optical and Near-Infrared Camera attached to the Cassegrain focus of the 150cm Kanata telescope on UT 2017 February 21 when the comet was at the solar phase angle of α=75.7°.
Results. We find that the continuum polarization degree with respect to the scattering plane is P_cont,r=33.8±2.7% at the effective wavelength of 0.82µm, which is significantly higher than those of cometary dust in a high-P_max group at similar phase angles. Assuming that an ensemble polarimetric response of the dust of 2P/Encke as a function of phase angle is morphologically similar with those of other comets, its maximum polarization degree is estimated to P_max≥40% at α_max~=100°. In addition, we obtain the polarization degrees of the C₂ swan bands (0.51-0.56µm), the NH₂ α bands (0.62-0.69µm), and the CN-red system (0.78-0.94µm) in a range of 3-19%, which depend on the molecular species and rotational quantum numbers of each branch. The polarization vector is aligned nearly perpendicularly to the scattering plane with an average of 0.4° over a wavelength range of 0.50-0.97µm.
Conclusions. From the observational evidence, we conjecture that the high polarization degree of 2P/Encke might be attributable to a dominance of large dust particles around the nucleus, which have remained after frequent perihelion passages near the Sun.