Optical Imaging Polarimetry of Comet 21P/Giacobini–Zinner during Its 2018 Apparition

Abstract We conducted polarimetric observations of comet 21P/Giacobini–Zinner in the optical wavelength region using imager PICO equipped with 50 cm telescope for Public Outreach at Mitaka Campus National Astronomical Observatory Japan on UT 2018 September 16, a solar phase angle 77° comet. used Gunn’s i ′-band filter to cover where sunlight reflected by dust grains dominates cometary spectra. The intensity map showed an elongated coma structure along east–west direction. linear polarization degree was almost uniform and no evidence jet or arc structures inner coma. 22% coma, which is consistent other this previous apparitions reported literature. measured angles (179°) were nearly perpendicular position scattering plane 271°. There significant changes properties (composition, porosity, size distribution) from its 1985 apparitions. spectral gradient flat range ∼510–830 nm ∼77° during apparition.