Hnc and Hcn in Comets

We use a multifluid hydrodynamical-chemical model of cometary comae to predict the HNC and HCN abundances in comets Hyakutake and Hale-Bopp for a variety of initial parameters. We find, in general, that the presence of methanol in the nuclear ices inhibits the synthesis of HNC from HCN through proton transfer reactions in the coma. For Hale-Bopp, we find that because of the large gas production rates, isomerization of HCN to HNC driven by the impact of fast hydrogen atoms can explain the HNC abundances, and we predict that the HNC/HCN ratio should increase to ∼18% as the comet approaches perihelion, as observed. For Hyakutake, we find that gas-phase reactions in the coma cannot produce the observed HNC/HCN ratio and so conclude that the observed HNC is either nuclear or is the decay product of an unknown parent. Subject headings: comets: general — comets: individual (Hale-Bopp 1995 O1, Hyakutake (C/1996 B2))