Optical Imaging Polarimetry of Comet 21P/Giacobini-Zinner with PICO in the 2018 Apparition

Comet 21P/Giacobini-Zinner (hereafter, 21P/G-Z) is one of Jupiter-family comets, with a Tisserand parameter with respect to Jupiter of 2.46 and an orbital period of ~6.6 years. This comet is characterized by (1) depletion in carbon-chain molecules like C₂and C₃as well as NH₂, (2) depleted in highly volatile parent species (e.g., C₂H₆, CH₃OH, and CO), and (3) negative linear polarization gradient in wavelength for reflected sunlight by cometary grains, indicative of existence of organic matter.

We performed optical imaging polarimetric observations of comet 21P/G-Z on UT 2018 September 16 using the Polarimetric Imager for COmets (PICO; Ikeda et al. 2007, PASJ, 59, 1017) mounted on the 50 cm Telescope for Public Outreach at the Mitaka Campus of the National Astronomical Observatory of Japan. We used the Gunn’s i'-band filter (its effective wavelength and FWHM are 761.5 nm and 151.0 nm, respectively). The scattered sunlight by cometary grains dominates in this wavelength region. The heliocentric and geocentric distances of the comet were 1.017 au and 0.398 au, respectively. During the observations, the solar phase angle of the comet 21P/GZ was 77.0 degrees. We derive the linear polarization degree of 22.0±0.1% for the coma (in average). We discuss the properties of dust grains in the coma of 21P/G-Z based on our and previous observations. This research was supported by JSPS grants (No. 20K14541).