Activities and Origins of Interstellar Comet 2I/Borisov

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EPSC Abstracts Vol. 14, EPSC2020-993, 2020, updated on 28 Sep 2021 https://doi.org/10.5194/epsc2020-993 Europlanet Science Congress 2020 © Author(s) 2021. This work is distributed under the Creative Commons Attribution 4.0 License.

Activities and origins of interstellar comet 2I/Borisov

Ze-Xi Xing1,2, Dennis Bodewits2, John W. Noonan3, Paul D. Feldman4, Michele T. Bannister5, Davide Farnocchia6, Walt M. Harris7, Jian-Yang Li8, Kathleen E. Mandt9, and Joel Wm. Parker10 1The University of Hong Kong, Laboratory for Space Research, Department of Physics, Hong Kong, Hong Kong ([email protected]) 2Department of Physics, Leach Science Center, Auburn University, Auburn, AL, USA ([email protected]) 3Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA ([email protected]) 4Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD, USA ([email protected]) 5School of Physical and Chemical Sciences—Te Kura Matū, University of Canterbury, Christchurch, New Zealand ([email protected]) 6Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA (davide.farnocchia@jpl.nasa.gov) 7Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA ([email protected]) 8Planetary Science Institute, Tucson, AZ, USA ([email protected]) 9Space Exploration Sector, Johns Hopkins Applied Physics Laboratory, Laurel, MD, USA ([email protected]) 10Department of Space Studies, Southwest Research Institute, Boulder, CO, USA ([email protected])

We will present the results of coordinated observations of 2I/Borisov with the Neil Gehrels-Swift observatory (Swift) and Hubble Space Telescope (HST), which allowed us to provide the first glimpse into the ice content and chemical composition of the protoplanetary disk of another star.Comets are condensed samples of the gas, ice and dust that were in a star’s protoplanetary disk during the formation of its planets, and inform our understanding on how chemical compositions and abundances vary with distance from the central star. Their orbital migration distributes volatiles [1], organic material and prebiotic chemicals around their host system [2]. In our Solar System, hundreds of comets have been observed remotely, and a few have been studied up close by space missions [3]. Similarly, interstellar comets offer a glimpse into the building blocks, formation, and evolution of other planetary systems. However, knowledge of extrasolar comets has been limited to what could be gleaned from distant, unresolved observations of cometary regions around other stars. 2I/Borisov, discovered in Aug. 2019, is the first notably active interstellar comet discovered in our Solar System [4].

We used the UltraViolet Optical Telescope (UVOT) of Swift to determine 2I/Borisov’s water production rates and dust content surrounding the nucleus at six epochs spaced before and after perihelion on Dec. 8.55, 2019 UTC (-2.56AU to 2.54AU) [5]. Water production rates increased steadily before perihelion at a rate of increase quicker than that of most dynamically new comets but slower than most Jupiter-family comets. After perihelion, the water production rate decreased much more rapidly than that of all previously observed comets. We used a sublimation model to constrain the active area and minimum radius of the nucleus, and found that a significant fraction of the surface of Borisov is active.

We also used Cosmic Origins Spectrograph (COS) on the HST during four epochs around the perihelion and clearly detected the emissions of several bands of the CO Fourth Positive system, which we used to derive CO production rates [6]. Comparing these with the water production rates determined by Swift, we found that after perihelion, the coma of 2I/Borisov contains substantially more CO than H2O gas. Our abundances were more than three times higher than previously measured for any comet in the inner (<2.5 au) Solar System [3]. The derived high abundance ratio of CO/H2O and high elemental abundance of carbon relative to oxygen firmly sets 2I/Borisov apart from solar system comets, and suggest that the physical and chemical environment were Borisov was formed are substantially different from those in our solar system [6, 7] .

Fig. 1 Volatile production rates as a function of time relative to perihelion [6]. The production rates of CO measured with HST/COS (this work) and the water production rate measured by Swift (based on OH, open circles; 5) and the Very Large Telescope/UVES (based on OH, 8), and at the Apache Point Observatory (based on [OI], 9). Arrows indicate 3-σ upper limits, and error bars indicate 1-σ stochastic uncertainties. The grey line indicates the temporal trend of water production rates used to derive the elemental composition.

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