Chemical processing in the coma as the source of cometary HNC

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.

References

  1. 1

    Irvine, W. M.et al. Spectroscopic evidence for interstellar ices in comet Hyakutake. Nature 383, 418–420 (1996).

    ADS  CAS  Article  Google Scholar 

  2. 2

    Lovas, F. J. Recommended rest frequencies for observed interstellar molecular microwave transitions — 1991 revision. J. Phys. Chem. Ref. Data 21, 181–271 (1992).

    ADS  CAS  Article  Google Scholar 

  3. 3

    Bockele'e-Morvan, D., Padman, R., Davies, J. K. & Crovisier, J. Observations of submillimeter lines of CH3OH, HCN, and H2CO in comet P/Swift-Tuttle with the James Clerk Maxwell Telescope. Planet. Space Sci. 42, 655–662 (1994).

    ADS  CAS  Article  Google Scholar 

  4. 4

    Lovell, A. J. et al. HCO+ imaging of comet C/1995 O1 Hale-Bopp. Astrophys. J. Lett. 497, L117–L121 (1998).

    ADS  CAS  Article  Google Scholar 

  5. 5

    Wink, J.et al. Evidence for extended sources and temporal modulations in molecular observations of C/1995 O1 (Hale-Bopp) at the IRAM interferometer. Earth Moon PlanetsAbstr. (in the press).

  6. 6

    Biver, N.et al. Evolution of the outgassing of Comet Hale-Bopp (C/1995 O1) from radio observations. Science 275, 1915–1918 (1997).

    ADS  CAS  Article  Google Scholar 

  7. 7

    Biver, N.et al. Long-term evolution of the outgassing of comet Hale-Bopp from radio observations. Earth Moon Planets(in the press).

  8. 8

    Irvine, W. M.et al. in The Far Infrared and Submillimetre Universe (ed. Wilson, A.) 277–280 (SP-401, ESA, Noordwijk Netherlands, (1997).

    Google Scholar 

  9. 9

    Notesco, G. & Bar-Nun, A. Trapping of methanol, hydrogen cyanide, and n-hexane in water ice, above its transformation temperature to the crystalline form. Icarus 126, 336–341 (1997).

    ADS  CAS  Article  Google Scholar 

  10. 10

    Tacconi-Garman, L. E., Schloerb, F. P. & Claussen, M. J. High spectral resolution observations and kinematic modeling of the 1667 MHz hyperfine transition of OH in comets Halley (1982i), Giacobini-Zinner (1984e), Hartley-Good (1985l), Thiele (1985m), and Wilson (1986l). Astrophys. J. 364, 672–686 (1990).

    ADS  Article  Google Scholar 

  11. 11

    Lovell, A. J.et al. HCO+ ion-molecule chemistry in comet C/1995 O1 Hale-Bopp. Earth Moon Planets(in the press).

  12. 12

    Bergin, E. A. & Langer, W. D. Chemical evolution in pre-protostellar and protostellar cores. Astrophys. J. 486, 316–328 (1997).

    ADS  CAS  Article  Google Scholar 

  13. 13

    Irvine, W. H.et al. Chemistry in cometary comae. Faraday Discuss. 109(in the press).

  14. 14

    Herbst, E. What are the products of polyatomic ion-electron dissociative recombination reactions? Astrophys. J. 222, 508–516 (1978).

    ADS  CAS  Article  Google Scholar 

  15. 15

    Shiba, Y., Hirano, T., Nagashima, U. & Ishii, K. Potential energy surfaces and branching ratio of the dissociative recombination reaction HCNH+ + e: An ab initio molecular orbital study. J. Chem. Phys. 108, 698–705 (1998).

    ADS  CAS  Article  Google Scholar 

  16. 16

    Hirota, T., Yamamoto, S., Mikami, H. & Ohishi, M. Abundances of HCN and HNC in dark cloud cores. Astrophys. J.(in the press).

  17. 17

    Mumma, M. J., Weissman, P. R. & Stern, S. A. in Protostars and Planets III (eds Levy, E. H. & Lunine, J. I.) 1177–1252 (Univ. Arizona Press, Tucson, (1993).

    Google Scholar 

  18. 18

    Fomenkova, M. in From Stardust to Planetesimals (eds Pendleton, Y. J. & Tielens, A. G. G. M.) 415–421 (ASP Conf. Ser.122, Astron. Soc. Pacific, San Francisco, (1997).

    Google Scholar 

  19. 19

    Rodgers, S. D. & Chamley, S. B. HNC and HCN in comets. Astrophys. J. Lett.(in the press).

  20. 20

    Crovisier, J. Rotational and vibrational synthetic spectra of linear parent molecules in comets. Astron. Astrophys. Suppl. 68, 223–258 (1987).

    ADS  CAS  Google Scholar 

  21. 21

    Festou, M. C., Rickman, H. & West, R. M. Comets II. Models, evolution, origin and outlook. Astron. Astrophys. Rev. 5, 37–163 (1993).

    ADS  Article  Google Scholar 

  22. 22

    Goldsmith, P. F., Langer, W. D., Ellder, J., Irvine, W. M. & Kollberg, E. Determination of the HNC to HCN abundance ratio in giant molecular clouds. Astrophys. J. 249, 524–531 (1981).

    ADS  CAS  Article  Google Scholar 

  23. 23

    Goldsmith, P. F., Irvine, W. M., Hjalmarson, & & Å & Ellder, J. Variations in the HCN/HNC abundance ratio in the Orion moelcular cloud. Astrophys. J. 310, 383–391 (1986).

    ADS  CAS  Article  Google Scholar 

  24. 24

    Olofsson, H., Johansson, L. E. B., Hjalmarson, & & Å. & Nguyen-Quang-Rieu High sensitivity molecular line observations of IRC +10216. Astron. Astrophys. 107, & 128–144 (1982).

    ADS  CAS  Google Scholar 

  25. 25

    Schmidt, H. U., Wegman, R., Huebner, W. F. & Boice, D. C. Cometary gas and plasma flow with detailed chemistry. Comput. Phys. Commun. 49, 17–59 (1988).

    ADS  CAS  Article  Google Scholar 

  26. 26

    Proc. 1st Int. Conf. on Comet Hale-Bopp (to be published as a special issue of Earth, Moon, Planets).

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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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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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