Detection of CN gas in Interstellar Object 2I/Borisov

TL;DR

This paper reports the first detection of CN gas emission from interstellar comet 2I/Borisov, providing insights into its composition and properties, which are similar to those of typical Solar System comets.

Contribution

It presents the first spectroscopic detection of gas from an interstellar comet, specifically CN emission, and analyzes its dust and nuclear characteristics.

Findings

Detected CN gas emission at 2.7 au heliocentric distance.

Estimated CN production rate of approximately 3.7×10^{24} s^{-1}.

Nuclear radius constrained to 0.7-3.3 km, similar to Solar System comets.

Abstract

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) = $(3.7\pm0.4)\times10^{24}$ 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 C$_2$ of $4\times10^{24}$ s$^{-1}$. The spectral reflectance slope of the dust coma over $3900$ \AA\ $< \lambda< 6000$ \AA \ is steeper than at longer wavelengths, as found for other comets. Broad band $R_c$ photometry on 2019 September 19 gave a dust production rate of $Af\rho=143\pm10$ cm. Modelling 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.