Detection of CN Gas in Interstellar Object 2I/Borisov

The detection of interstellar objects passing through the solar system offers the promise of constraining the physical and chemical processes involved in planetary formation in other extrasolar systems. While the effect of outgassing by 1I/2017 U1 ('Oumuamua) was dynamically observed, no direct detection of the ejected material was made. The discovery of the active interstellar comet 2I/Borisov means spectroscopic investigations of the sublimated ices is possible for this object. We report the first detection of gas emitted by an interstellar comet via the near-UV emission of CN from 2I/Borisov at a heliocentric distance of r = 2.7 au on 2019 September 20. The production rate was found to be Q(CN) = (3.7 ± 0.4) × 1024 s−1, using a simple Haser model with an outflow velocity of 0.5 km s−1. No other emission was detected, with an upper limit to the production rate of C2 of 4 × 1024 s−1. The spectral reflectance slope of the dust coma over 3900 Å < λ < 6000 Å is steeper than at longer wavelengths, as found for other comets. Broadband Rc photometry on 2019 September 19 gave a dust production rate of Afρ = 143 ± 10 cm. Modeling of the observed gas and dust production rates constrains the nuclear radius to 0.7–3.3 km assuming reasonable nuclear properties. Overall, we find the gas, dust, and nuclear properties for the first active interstellar object are similar to normal solar system comets.