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

Abstract

On 31 August 2019, an interstellar comet was discovered as it passed through the Solar System (2I/Borisov). On the basis of initial imaging observations, 2I/Borisov seemed to be similar to ordinary Solar System comets1,2—an unexpected characteristic given the multiple peculiarities of the only known previous interstellar visitor, 1I/‘Oumuamua3,4,5,6. Spectroscopic investigations of 2I/Borisov identified the familiar cometary emissions from CN (refs. 7,8,9), C2 (ref. 10), O i (ref. 11), NH2 (ref. 12), OH (ref. 13), HCN (ref. 14) and CO (refs. 14,15), revealing a composition similar to that of carbon monoxide-rich Solar System comets. At temperatures greater than 700 kelvin, comets also show metallic vapours that are produced by the sublimation of metal-rich dust grains16. Observation of gaseous metals had until very recently17 been limited to bright sunskirting and sungrazing comets18,19,20 and giant star-plunging exocomets21. Here we report spectroscopic observations of atomic nickel vapour in the cold coma of 2I/Borisov at a heliocentric distance of 2.322 astronomical units—equivalent to an equilibrium temperature of 180 kelvin. Nickel in 2I/Borisov seems to originate from a short-lived nickel-containing molecule with a lifetime of \({340}_{-200}^{+260}\) seconds at 1 astronomical unit and is produced at a rate of 0.9 ± 0.3 × 1022 atoms per second, or 0.002 per cent relative to OH and 0.3 per cent relative to CN. The detection of gas-phase nickel in the coma of 2I/Borisov is in line with the recent identification of this atom—as well as iron—in the cold comae of Solar System comets17.

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Fig. 1: Emission lines from gaseous atomic nickel in the near-UV spectrum of 2I/Borisov.
Fig. 2: Observed and modelled spatial profiles of nickel emission.
Fig. 3: Haser scalelengths of the observed nickel emission.

Data availability

The X-shooter raw data are available in the ESO archive at https://archive.eso.orgSource data are provided with this paper.

Code availability

The EsoReflex pipeline is available from the ESO website at https://www.eso.org/sci/software/esoreflex/. All custom codes are direct implementations of standard methods and techniques, described in detail in Methods.

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Acknowledgements

We thank K. Rusek for help with proposal writing, M. Ratajczak and M. Gromadzki for introducing us to X-shooter data reduction, and P. Kozyra for discussion on nickel-containing molecules. This work is based on observations collected at the ESO under ESO programme 0104.C-0933(B). We thank the ESO staff for support. We are also grateful for support from the National Science Centre of Poland through ETIUDA scholarship no. 2020/36/T/ST9/00596 to P.G. and SONATA BIS grant no. 2016/22/E/ST9/00109 to M.D., and we acknowledge support from the Polish Ministry of Science and Higher Education through grant no. DIR/WK/2018/12.

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P.G. and M.D. wrote the telescope time proposal, searched for the origin of the detected spectral lines and wrote the paper. P.G. prepared the observations, reduced and calibrated the data, identified the emitting species and measured the spectral lines. M.D. created the fluorescence model, retrieved the scalelengths and calculated the production rate.

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Correspondence to Piotr Guzik or Michał Drahus.

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Peer review information Nature thanks Ryan Fortenberry and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Fig. 1 Complete spectrum of comet 2I/Borisov from X-shooter UVB arm.

a, Flux-calibrated spectrum with fitted dust continuum (see Methods). b, Same as a but with the dust-continuum component removed. Major emission features are labelled. c, Modelled spectrum of nickel fluorescence emission (see Methods) scaled to best match the two brightest lines.

Extended Data Fig. 2 Distribution of Monte Carlo-simulated production rates.

The distribution was constructed from the production rates corresponding to the results of the Monte Carlo simulation in Fig. 3b (see Methods). Results are presented in three groups according to the assumed PSF equal to 0.65 (blue), 1.0 (red) and 1.5 (green) arcsec.

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Guzik, P., Drahus, M. Gaseous atomic nickel in the coma of interstellar comet 2I/Borisov. Nature 593, 375–378 (2021). https://doi.org/10.1038/s41586-021-03485-4

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