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

TL;DR

This study reports the first spectroscopic detection of gaseous atomic nickel in the cold coma of interstellar comet 2I/Borisov, revealing similarities with Solar System comets and providing insights into its composition and sublimation processes.

Contribution

It presents the novel detection of gaseous nickel vapor in the cold coma of an interstellar comet, expanding understanding of cometary compositions beyond the Solar System.

Findings

Gaseous nickel detected at 180 K in 2I/Borisov's coma.

Nickel production rate is approximately 0.9 x 10^{22} atoms/sec.

Nickel abundance is about 0.3% relative to CN.

Abstract

On 31 August 2019, an interstellar comet was discovered as it passed through the Solar System (2I/Borisov). Based on initial imaging observations, 2I/Borisov appeared to be completely similar to ordinary Solar System comets - an unexpected characteristic after the multiple peculiarities of the only previous known interstellar visitor 1I/'Oumuamua. Spectroscopic investigations of 2I/Borisov identified the familiar cometary emissions from CN, C2, O I, NH2, OH, HCN, and CO, revealing a composition similar to that of carbon monoxide-rich Solar System comets. At temperatures >700 K, comets additionally show metallic vapors produced by the sublimation of metal-rich dust grains. However, due to the high temperature needed, observation of gaseous metals has been limited to bright sunskirting and sungrazing comets and giant star-plunging exocomets. Here we report spectroscopic detection of atomic nickel vapor in the cold coma of 2I/Borisov observed at a heliocentric distance of 2.322 au - equivalent to an equilibrium temperature of 180 K. Nickel in 2I/Borisov seems to originate from a short-lived nickelbearing molecule with a lifetime of $340^{+260}_{-200}$ s at 1 au and is produced at a rate of $0.9 \pm 0.3 \times 10^{22}$ atoms s$^{-1}$, or 0.002% relative to OH and 0.3% relative to CN. The detection of gas-phase nickel in the coma of 2I/Borisov is in line with the concurrent identification of this atom (as well as iron) in the cold comae of Solar System comets.