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Astrophysics Colloquium

Characterizing the Nucleus, Morphology and Activity of Interstellar Comet 2I/Borisov
Presented by Bryce Bolin
Infrared Processing and Analysis Center, California Institute of Technology

Thursday, November 19, 2020
Virtual Event starts at 11:00 A.M.

We present visible and near-infrared photometric and spectroscopic observations of interstellar object 2I/Borisov taken from 2019 September 10 to 2020 January 27 using ground-based facilities from the ZTF, the Keck Telescope, the GROWTH network, the APO ARC 3.5m, and the NASA/IRTF 3.0 m combined high-resolution observations from HST. The photometry, taken in filters spanning the visible and NIR range shows 2I having a reddish object becoming neutral in the NIR. The lightcurve from recent and pre-discovery data reveals a brightness trend suggesting the recent onset of significant H2O sublimation with the comet being active with super volatiles such as CO at heliocentric distances >6 AU, consistent with its extended morphology. Using the advanced capability to significantly reduce the scattered light from the coma enabled by high-resolution NIR images from Keck adaptive optics taken on 2019 October 04, we estimate a diameter of 2I's nucleus of <1.4 km. We use the size estimates of 1I/'Oumuamua and 2I/Borisov to roughly estimate the slope of the ISO size-distribution resulting in a slope of ~3.4+/-1.2, similar to Solar System comets radii > 1 km.

In addition to the ground-based observations, we combine our 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 before and after its perihelion passage in combination with HST/WFC3 images taken on 2019 October 12 UTC and 2019 November 16 UTC before its outburst and fragmentation of March 2020, thus observing the comet in a relatively undisrupted state. We locate 1-2" long (2000 - 3000 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, we determine that 2I's pole points near RA = 322 deg, dec = 37 deg, ecliptic longitude = 341 deg, ecliptic latitude = 48 deg. From our pole solution, we find that 2I experienced large seasonal effects during its perihelion passage with the sub-Solar latitude changing by as much as ~70 degrees which may explain the large changes in volatile production seen by other observatories during this period. In addition, we find evidence for a periodicity in the time-series lightcurve of ~5.3 h and a small amplitude of ~0.05 implying a b/a axial ratio of ~1.5 when combined with our pole solution, unlike the b/a of 4-10 found for 1I/`Oumuamua.

In addition, we comment on the galactic dynamics of these objects and the prospects for where they came from in the Milky Way.

JPL Contact: Chengxing Zhai

About the Speaker
Bryce Bolin, Ph.D, 2018 in Sciences de la Planete et de l'Univers from l'Université Côte d'Azur, works on observations of near-Earth Objects by the Zwicky Transient Facility at Caltech/IPAC. His background includes ground and space-based observations of near-Earth objects, Main Belt asteroids, comets and the occasional interstellar object. Bolin did his Ph.D. work with Drs. Marco Delbo' and Alessandro Morbidelli on primordial asteroid families while studying in Nice, France. Before that, Bolin was a research analyst at the Institute for Astronomy working with Robert Jedicke and Richard Wainscoat on observations of near-Earth objects and comets with the PanSTARRS telescope and the construction of the Near-Earth Object model.

SVCP Astrophysics

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