Gaseous atomic nickel in the coma of interstellar comet 2I/Borisov

The detection of 2I/Borisov in 2019 created a unique opportunity for an up-close study of a cometary body that formed in another planetary system. Initial imaging observations revealed striking similarities between 2I/Borisov and ordinary Solar System comets, further confirmed by multiple spectroscopic investigations. This fact is surprising given that the only previous known interstellar visitor 1I/'Oumuamua had numerous puzzling properties that have not been fully explained to date. We observed 2I/Borisov with the VLT/X-Shooter spectrograph on three nights in late January 2020 UT at a heliocentric distance of 2.322 a.u. The spectrum covers the wavelength range between 300 nm and 2.5 μm with a resolution of 4100, and was obtained with a total on-source integration time of ~3 hr. Apart from typical cometary spectral emission lines, we found several previously unidentified lines in the near UV, which we attributed to gaseous atomic nickel (Guzik & Drahus 2021, Nature 593, 375-378). The spatial profile of the detected lines is consistent with nickel emission from a short-lived parent molecule having a lifetime of 340⁺²⁶⁰_-200 s at 1 a.u. The estimated nickel production rate is equal to 0.9 ± 0.3 × 10²² atoms per second, which is roughly 0.3% of the production rate of CN. Thus, gaseous nickel is a very minor constituent of the coma of 2I/Borisov. The detection of this species in a distant interstellar comet is unexpected, given that unbound nickel atoms and other heavy elements have previously been observed only in very hot cosmic environments, including the atmospheres of ultra-hot exoplanets and evaporating sungrazing comets, such as C/1965 S1 (Ikeya-Seki).