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