Constraints on the Spin-Pole Orientation, Jet Morphology and Rotation of Interstellar Comet 2I/Borisov with Deep HST Imaging

Abstract

We present high resolution, deep imaging of interstellar comet 2I/Borisov taken with the Hubble Space Telescope/Wide Field Camera 3 (HST/WFC3) on 2019 December 8 UTC and 2020 January 27 UTC (HST GO 16040, PI Bolin) before and after its perihelion passage in combination with HST/WFC3 images taken on 2019 October 12 UTC and 2019 November 16 UTC (HST GO/DD 16009, PI Jewitt) before its outburst and fragmentation of March 2020, thus observing the comet in a relatively undisrupted state. We locate 1-2\arcsec~long (2,000 - 3,000 km projected length) jet-like structures near the optocenter of 2I that appear to change position angles from epoch to epoch. With the assumption that the jet is located near the rotational pole supported by its stationary appearance on $\sim$10-100 h time frames in HST images, we determine that 2I's pole points near $α$ = 322$\pm$10$^\circ$, $δ$ = 37$\pm$10$^\circ$ ($λ$ = 341$^\circ$, $β$ = 48$^\circ$) and may be in a simple rotation state. Additionally, we find evidence for possible periodicity in the HST time-series lightcurve on the time scale of $\sim$5.3 h with a small amplitude of $\sim$0.05 mag implying a lower limit on its $b/a$ ratio of $\sim$1.5 unlike the large $\sim$2 mag lightcurve observed for 1I/`Oumuamua. However, these small lightcurve variations may not be the result of the rotation of 2I's nucleus due to its dust-dominated light-scattering cross-section. Therefore, uniquely constraining the pre-Solar System encounter, pre-outburst rotation state of 2I may not be possible even with the resolution and sensitivity provided by HST observations.