Water production rates and activity of interstellar comet 2I/Borisov

Extrasolar comets offer a glimpse into the building blocks, formation, and evolution of other planetary systems. Impacts by exocomets may significantly alter the atmospheres of exoplanets [1], or furnish the exchange of volatiles and complex molecules between different planetary systems. However, the identification of interstellar objects based on chemical signatures has been hampered by the lack of understanding of what drives the chemical taxonomy of comets in the first place [2, 3]. With an eccentricity of 3.357, there is no question regarding the extrasolar origins of 2I/Borisov (Minor Planet Center; MPEC 2019-W50). At the time of its discovery, at 3 au from the Sun, the appearance of Borisov was that of a 'regular' comet, featuring a prominent tail and coma. This implies that it contains sublimating volatiles, which was first confirmed by the detection of the emission of gaseous CN [4].

To determine 2I/Borisov's production rate of water, the dominant molecules of gas in most comets, we used the UltraViolet Optical Telescope (UVOT) of the Neil Gehrels-Swift observatory at six epochs spaced before and after perihelion on Dec. 8.55, 2019 UTC (-2.56 AU to 2.54 AU) [5]. Through OH, we unambiguously detected water around 2I/Borisov for the first time, with production rates increasing steadily before perihelion, and the rate of increase is quicker than that of most dynamically new comets and at the lower end of the wide range of Jupiter-family comets. After perihelion, the water production rate decreased much more rapidly than that of all previously observed comets. Our sublimation model constrains the minimum radius of the nucleus, which indicates that a significant fraction of the surface of Borisov is active. Dust content surrounding the nucleus was also detected by Swift, and A(0)fr calculations show a variation with a slight trend peaking before the perihelion, lower than previous and concurrently published values. These results confirm that 2I/Borisov is carbon-chain depleted and enriched in NH₂ relative to water.

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