Note on the HST Image of Comet 2I/Borisov Taken on 2019 October 12

TL;DR

This paper analyzes a Hubble Space Telescope image of comet 2I/Borisov, revealing a fan-like dust feature likely composed of large grains released at large distances from the Sun, with implications for comet composition.

Contribution

It proposes that the observed feature is a halo of large dust grains released at large heliocentric distances, a novel interpretation for interstellar comet activity.

Findings

Identification of a fan-like dust feature in the coma.

Large dust grains likely released at >10 AU from the Sun.

Potential absence of silicate features in infrared observations.

Abstract

The image of comet 2I/Borisov taken by the Hubble Space Telescope on October 12 shows a fan-like feature in the inner coma, which extends over a few arcseconds from the nuclear condensation, its axis pointing a little west of the north. It is proposed that this protrusion is a halo of centimeter-sized and larger dust grains or pebbles released at low velocities from the nucleus at large heliocentric distances (on the order of 10 AU or more) before perihelion and subjected to very low radiation pressure accelerations. Unlike potential microscopic dust from ongoing activity, this population of large grains should show no silicate feature near 10 microns in the infrared and its presence in 2I/Borisov can be tested observationally. The existence of material of a similar size range in Oort Cloud comets with perihelia near or beyond the snow line has long been documented by the characteristic orientation of their tails.