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W.M. Irvine, D. McGonagle (UMass), P. Bergman (Onsala Space Obs.), T.B. Lowe (JAC), H.E. Matthews (JAC & HIA), A. Nummelin (Chalmers), T.C. Owen (UHawaii)
In cold interstellar clouds HNC has an abundance which can approach or even exceed that of HCN, a gross dis-equilibrium which can be understood in terms of the kinetics of ion-molecule reactions at low temperatures and densities. The discovery of cometary HNC in comet C/1996 B2 (Hyakutake) thus seemed to provide evidence for the survival of interstellar ices in comets. Subsequent data for comet C/1997 O1 (Hale-Bopp) showed, however, a dependence of the HNC/HCN ratio on heliocentric distance that indicated that most HNC in that comet was produced in the cometary coma, rather than being a constituent of the comet's nucleus. Moreover, the initially apparently successful attempts to model HNC production in the coma have been shown to be too simplistic, and more realistic models have thus far failed to produce sufficient HNC to match the observations in comets less active than Hale-Bopp. We have therefore observed both HCN and HNC in 2 recent comets, bringing the number of comets with published measurements of the HNC/HCN abundance ratio to 6. The HNC/HCN ratio in comet C/2002 C1 (Ikeya-Zhang) appears to increase with decreasing heliocentric distance, indicating that the HNC is produced at least in part by processes in the coma. Both comets C/2000 WM1 (LINEAR) and C/2002 C1 (Ikeya-Zhang) exhibit values of the HNC/HCN ratio that appear to be too large (~0.10-0.20) to be matched by current models of coma chemistry. Cometary HNC may be a photodissociation product of organic grains or large organic polymers stored in the nucleus .
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.