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Chemical processing in the coma as the source of cometary HNC

Nature volume 393, pages 547–550 (1998)Cite this article

Abstract

The discovery of hydrogen isocyanide (HNC) in comet Hyakutake with an abundance (relative to hydrogen cyanide, HCN) similar to that seen in dense interstellar clouds raised the possibility that these molecules might be surviving interstellar material1. The preservation of material from the Sun's parent molecular cloud would provide important constraints on the processes that took place in the protostellar nebula. But another possibility is that HNC is produced by photochemical processes in the coma, which means that its abundance could not be used as a direct constraint on conditions in the early Solar System. Here we show that the HNC/HCN ratio determined for comet Hale–Bopp varied with heliocentric distance in a way that matches the predictions of models of gas-phase chemical production of HNC in the coma, but cannot be explained if the HNC molecules were coming from the comet's nucleus. We conclude that HNC forms mainly by chemical reactions in the coma, and that such reactions need to be considered when attempting to deduce the composition of the nucleus from observations of the coma.

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Figure 1: The J(4–3) lines of HNC (dashed lines) and HCN (solid lines, spectra multiplied by 0.25) for comet Hale–Bopp.
Figure 2: Comparison of an observed J = 4–3 HCN line (solid line; for indicated date) with a radiative transfer model (dashed line).
Figure 3: Dependence of the HNC/HCN ratio on heliocentric distance r.

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Acknowledgements

We thank B. Marsden and D. Tholen for rapidly providing us with the best available cometary elements, and the staff at the JCMT for their assistance with the observations. This work was partly supported by the NSF and NASA.

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Authors and Affiliations

  1. Five College Radio Astronomy Observatory, 619 LGRC, University of Massachusetts, Amherst, 01003, Massachusetts, USA

    William M. Irvine, James E. Dickens, Amy J. Lovell, F. Peter Schloerb & Matthew Senay

  2. Smithsonian Astrophysical Observatory, MS-66, 60 Garden Street, Cambridge, 02138-1596, Massachusetts, USA

    Edwin A. Bergin

  3. Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, 96822, Hawaii, USA

    David Jewitt

  4. Herzberg Institute of Astrophysics, National Research Council of Canada, Ottawa, K1A 0R6, Ontario, Canada

    Henry E. Matthews

  5. Joint Astronomy Centre, 660 N. A'ohoku Place, University Park, Hilo, 96720, Hawaii, USA

    Henry E. Matthews

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  1. William M. Irvine
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  2. Edwin A. Bergin
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Correspondence to William M. Irvine.

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Irvine, W., Bergin, E., Dickens, J. et al. Chemical processing in the coma as the source of cometary HNC. Nature 393, 547–550 (1998). https://doi.org/10.1038/31171

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