High polarization degree of the continuum of comet 2P/Encke based on spectropolarimetric signals during its 2017 apparition

¹ Department of Physics and Astronomy, Seoul National University, 1 Gwanak, Seoul 08826, Republic of Korea
e-mail: ynkwon@astro.snu.ac.kr; ishiguro@astro.snu.ac.kr
² Laboratory of Infrared High-Resolution Spectroscopy, Koyama Astronomical Observatory, Kyoto Sangyo University, Motoyama Kamigamo, Kita-ku, Kyoto 603-8555, Japan
³ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁴ Hiroshima Astrophysical Science Center, Hiroshima University, Kagamiyama 1-3-1, Higashi-Hiroshima 739-8526, Japan
⁵ Okayama Observatory, Kyoto University, 3037-5 Honjo, Kamogata, Asakuchi, Okayama 719-0232, Japan
⁶ Ishigakijima Astronomical Observatory, National Astronomical Observatory of Japan, 1024-1 Arakawa, Ishigaki, Okinawa 907-0024, Japan
⁷ Nishi-Harima Astronomical Observatory, Center for Astronomy, University of Hyogo, 407-2, Nishigaichi, Sayo, Hyogo 679-5313, Japan
⁸ Institute of Astronomy, Graduate School of Science, The University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan
⁹ Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8551, Japan
¹⁰ Faculty of Science, Department of Earth and Planetary Sciences, Hokkaido University, Kita 10, Nishi 8, Kita-ku, Sapporo 060-0810, Japan
¹¹ Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan
¹² Graduate School of Science and Engineering, Kagoshima University, 1-21-35 Korimoto, Kagoshima 890-0065, Japan
¹³ Graduate School of Science and Engineering, Saitama University, Simo-okubo 135, Sakura-ku, Saitama 338-8570, Japan

Received: 27 July 2018
Accepted: 3 October 2018

Abstract

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 150 cm 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 0.82 μm, 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 (0.51–0.56 μm), the NH₂ α bands (0.62–0.69 μm), and the CN-red system (0.78–0.94 μm) 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 0.50–0.97 μm.

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.