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″ 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 ∼10-100 h time frames in HST images, we determine that 2I’s pole points near α = 322±10○, δ = 37±10○ (λ = 341○, β = 48○) 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 ∼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 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.

Compact pebbles and the evolution of volatiles in the interstellar comet 2I/Borisov

“…Rosetta observations of 67P revealed that the peak of water flux was confined to the subsolar latitude either on the northern hemisphere or the southern hemisphere, whereas, the hypervolatile CO 2 and CO gases were not specifically originating from the sunlit regions 34 . Two studies based on the HST observations of 2I/Borisov suggest that the sub-solar latitude of the nucleus evolved either from −35 • to 0 • or from −10 • to 70 • between August 2019 and January 2020 27,48 . The drastic drop in the water production rate of 2I/Borisov may have been due to the possibility that water sublimation was mainly coming from a confined region on the surface of the nucleus.…”

Constraining the orientation of the spin axes of extrasolar minor bodies 1I/2017 U1 (‘Oumuamua) and 2I/Borisov

“…On the other hand, the equatorial coordinates of the Sun when 2I/Borisov reached perihelion on December 8, 2019 were approximately (254 • .6, −22 • .7), so none of our predictions are close to the position favored by Sekanina (1981). This location is also inconsistent with the estimates provided by Ye et al (2020), Bolin & Lisse (2020), and Kim et al (2020); it is, however, somewhat consistent with the spin-axis direction computed by Manzini et al (2020). Most of the observed properties of interstellar comet 2I/Borisov have been found to be remarkably similar to those of known Solar System comets (see for example de León et al 2019León et al , 2020Fitzsimmons et al 2019;Opitom et al 2019;Cremonese et al 2020;Guzik et al 2020;Jewitt et al 2020b;Kareta et al 2020;Yang et al 2020).…”

“…Most of the observed properties of interstellar comet 2I/Borisov have been found to be remarkably similar to those of known Solar System comets (see for example de León et al 2019León et al , 2020Fitzsimmons et al 2019;Opitom et al 2019;Cremonese et al 2020;Guzik et al 2020;Jewitt et al 2020b;Kareta et al 2020;Yang et al 2020). Recent estimates, placed the rotation period of 2I/Borisov at about 5.3 h or perhaps 10.6 h (Bolin & Lisse 2020). Gladman et al (2019) had found a very slow rotation rate with a period of about 13 d. If confirmed, this would be the longest rotation period ever observed in a comet.…”

“…Together with our own estimates in the form of maps of statistical significance, the various estimates for the spin-axis orientation of 2I/Borisov are plotted in Figs. 21 and 26, bottom panels, as filled orange symbols with white error bars (when available): triangles (Ye et al 2020), square (Manzini et al 2020), star (Bolin & Lisse 2020), and circle (Kim et al 2020). The published solutions for the spin-axis orientation of 2I/Borisov are quite different and they may signal actual short-term changes in the values of the rotational properties of this object or problems with the interpretation of some of the observed features.…”

Time-series and Phase-curve Photometry of the Episodically Active Asteroid (6478) Gault in a Quiescent State Using APO, GROWTH, P200, and ZTF

“…It should also be noted that others (e.g., Moreno et al 2019) report no variation in photometry over a time span longer than these periods. This very small amplitude demonstrates the challenge of photometrically extracting a rotation period from an active body (see also Bolin & Lisse 2020, for an example).…”