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

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