Unusually high CO abundance of the first active interstellar comet

Abstract

Comets spend most of their lives at large distances from any star, during which time their interior compositions remain relatively unaltered. Cometary observations can therefore provide direct insight into the chemistry that occurred during their birth at the time of planet formation1. To date, there have been no confirmed observations of parent volatiles (gases released directly from the nucleus) of a comet from any planetary system other than our own. Here, we present high-resolution interferometric observations of 2I/Borisov, the first confirmed interstellar comet, obtained using the Atacama Large Millimeter/submillimeter Array (ALMA) on 15–16 December 2019. Our observations reveal emission from hydrogen cyanide (HCN) and carbon monoxide (CO) coincident with the expected position of 2I/Borisov’s nucleus, with production rates Q(HCN) = (7.0 ± 1.1) × 1023 s−1 and Q(CO) = (4.4 ± 0.7) × 1026 s−1. While the HCN abundance relative to water (0.06–0.16%) appears similar to that of typical, previously observed comets in our Solar System, the abundance of CO (35–105%) is among the highest observed in any comet within 2 au of the Sun. This shows that 2I/Borisov must have formed in a relatively CO-rich environment—probably beyond the CO ice-line in the very cold, outer regions of a distant protoplanetary accretion disk, as part of a population of small icy bodies analogous to our Solar System’s own proto-Kuiper belt.

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Fig. 1: ALMA molecular flux maps.
Fig. 2: ALMA spectra of 2I/Borisov.
Fig. 3: ALMA CO autocorrelation spectrum.
Fig. 4: Cometary CO/HCN mixing ratios.

Data availability

This work makes use of ALMA dataset ADS/JAO.ALMA#2019.01008.T, which is available for download from the ALMA Science Archive (http://almascience.nrao.edu/aq/) following a 1-year proprietary period. All data that support the findings of this study are available on resonable request from the corresponding author.

Code availability

The radiative transfer model required to reproduce the results of this study is available on reasonable request from the corresponding author.

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Acknowledgements

We thank R. Simon for setting up the ALMA scheduling blocks, and D. Cruikshank for discussions on the composition of Kuiper belt objects. This work was supported by the National Science Foundation (under grant no. AST-1614471), and by the Planetary Science Division Internal Scientist Funding Program through the Fundamental Laboratory Research (FLaRe) work package, as well as the NASA Astrobiology Institute through the Goddard Center for Astrobiology (proposal 13-13NAI7-0032). Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. ALMA is a partnership of ESO, NSF (USA), NINS (Japan), NRC (Canada), NSC and ASIAA (Taiwan) and KASI (Republic of Korea), in cooperation with the Republic of Chile. The JAO is operated by ESO, AUI/NRAO and NAOJ. The NRAO is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. TRAPPIST is a project funded by the Belgian Fonds (National) de la Recherche Scientifique (Fonds de la Recherche Scientifique–FNRS) under grant FRFC 2.5.594.09.F. E.J. is a FNRS Senior Research Associate. N.X.R. was supported by the NASA Postdoctoral Program, administered by the Universities Space Research Association.

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M.A.C. performed the data reduction and radiative transfer modelling, and generated most of the text and figures. S.N.M. obtained the ALMA observations and helped write the manuscript. N.B. performed independent radiative transfer calculations and statistical comparisons. D.B.-M. made Fig. 4. E.A.B. wrote part of the interpretation. N.X.R. worked on Supplementary Table 1, and generated upper limits. A.J.R. helped obtain the observations and identify spectral lines. E.J. provided ancillary optical data from TRAPPIST. S.B.C., J.C., D.C.L., L.P., Y.-J.K., J.B. and M.J.M. contributed to the interpretation and helped write the manuscript.

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Correspondence to M. A. Cordiner.

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Cordiner, M.A., Milam, S.N., Biver, N. et al. Unusually high CO abundance of the first active interstellar comet. Nat Astron 4, 861–866 (2020). https://doi.org/10.1038/s41550-020-1087-2

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