Coma morphology and dust emission pattern of comet C/2020 F3 (NEOWISE)

Abstract

The recent close approach of comet C/2020 F3 (NEOWISE) allowed us to study the morphology of its inner coma. From the measurement of the dust ejection velocity on spiral structures expanding around the nucleus, we estimated a mean deprojected expansion velocity V d = 1.11±0.08 km s −1 . Assuming that a new shell formed after every rotation of the comet, a rotation period of 7.8±0.2 hours was derived. The spin axis orientation was estimated at RA 210 • ±10 • , Dec. +35 • ± 10 • . The coma morphology appears related to two strong, diametrically opposite emissions located at mid-latitudes on the nucleus. A qualitative modelling of the coma produced consistent results with a wide range of dust sizes (0.80 to 800 µm), with inversely correlated densities (0.003 to 3.0 g cm −3 ). Images taken with V j and r-Sloan filters showed a greater concentration of dust in the first two shells, and an increasing density of radicals emitting in the B and V band-passes from the third shell outwards. Striaelike structures in the tail suggest that dust particles have different sizes.