Detection of CN gas in Interstellar Object 2I/Borisov

The detection of Interstellar Objects passing through the Solar System offersthe promise of constraining the physical and chemical processes involved inplanetary formation in other extrasolar systems. While the effect of outgassingby 1I/2017 U1 ('Oumuamua) was dynamically observed, no direct detection of theejected material was made. The discovery of the active interstellar comet2I/Borisov means spectroscopic investigations of the sublimated ices ispossible for this object. We report the first detection of gas emitted by aninterstellar comet via the near-UV emission of CN from 2I/Borisov at aheliocentric distance of r = 2.7 au on 2019 September 20. The production ratewas found to be Q(CN) = (3.7±0.4)×1024 s−1, using a simpleHaser model with an outflow velocity of 0.5 km s−1. No other emission wasdetected, with an upper limit to the production rate of C2 of4×1024 s−1. The spectral reflectance slope of the dust coma over3900 Å <λ<6000 Å\ is steeper than at longer wavelengths, asfound for other comets. Broad band Rc photometry on 2019 September 19 gave adust production rate of Afρ=143±10 cm. Modelling of the observed gas anddust production rates constrains the nuclear radius to 0.7−3.3 km assumingreasonable nuclear properties. Overall, we find the gas, dust and nuclearproperties for the first active Interstellar Object are similar to normal SolarSystem comets.