Constraints on the spin-pole orientation, jet morphology, and rotation of interstellar comet 2I/Borisov with deep HST imaging

Creators: Bolin, Bryce T.; Lisse, Carey M.

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 2020 March, thus observing the comet in a relatively undisrupted state. We locate 1–2 arcsec long (2000–3000 km projected length) jet-like structures near the optocentre 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 ∼10–100 h time frames in HST images, we determine that 2I's pole points near α = 322 ± 10° and δ = 37 ± 10° (λ = 341° and β = 48°) and may be in a simple rotation state. Additionally, we find evidence for possible periodicity in the HST time-series light curve on the time-scale of ∼5.3 h with a small amplitude of ∼0.05 mag implying a lower limit on its b/a ratio of ∼1.5 unlike the large ∼2 mag light curve observed for 1I/'Oumuamua. However, these small light-curve 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.

Additional Information

© 2020 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model). Accepted 2020 July 21. Received 2020 July 21; in original form 2020 June 3. Published: 27 July 2020. Based on observations with the NASA/ESA HST obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. Support for program number (GO 16040) was provided through a grant from the STScI under NASA contract NAS5-26555. We would like to acknowledge the anonymous reviewer whose helpful comments significantly improved the quality of this manuscript. While this manuscript was in review, two other studies by Manzini et al. (2020) and Kim et al. (2020) focusing on the observations of 2I by HST and its pole solution were accepted and or submitted on arxiv respectively. The results in both papers complement our work and enhance the scientific discussion in this manuscript. We would like to thank R. Jedicke and G. Helou for helpful discussion on the interpretation of low-amplitude light-curve data. We would also like to thank Y. R. Fernandez for helpful discussion on the interpretation of the comet coma data and for useful comments on constraining the pole. We thank D. Bodewits for advice in planning the HST observations and data reduction. Data Availability: The data underlying this article will be shared on reasonable request to the corresponding author. The data are also available at the Mikulski Archive for Space Telescopes (MAST).

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