Microphysics of dust in a distant comet C/2017 K2 (PanSTARRS) retrieved by means of polarimetry

Abstract

We conducted the earliest polarimetric survey of Comet C/2017 K2 (PanSTARRS) over 10 epochs during February–March 2021, while the comet's heliocentric distance was decreasing from 7.135 au to 6.801 au. Within an aperture radius ρ = 10,000 km, we found slightly varying negative polarization with median value PQ ≈ –2.1% at phase angle α ≈ 8° This appears in quantitative agreement with measurements of other distant comets. The negative polarization reveals submicron and micron-sized dust particles within the coma constrain the imaginary part of the refractive index in at least one type of particles to Im(m) ≤ 0.04. Models earlier developed in Kochergin et al. [1] for the negative polarization in distant Comet 29P/Schwassmann–Wachmann are similar to the negative polarization of Comet C/2017 K2 (PanSTARRS), suggesting its coma could be populated by Mg-rich silicate particles and/or water-ice particles, which are mixed with organic particles and/or amorphous-carbon particles. Modeling dust motion suggests that the inner coma is populated by particles whose age is up to 15 days since their emanation from the nucleus.