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A.J. Lovell (Agnes Scott College), N. Kallivayalil (Mt. Holyoke College), F.P. Schloerb (FCRAO), M.R. Combi, K.C. Hansen, T.I. Gombosi (Univ. of Michigan)
We present the results of an investigation of the role of electrons in the collisional excitation of rotational transitions of HCN in comets Hyakutake and Hale-Bopp. For the physical conditions in these comets, cross-sections for collisions between electrons and HCN are computed to be as high as 1.22 \times 10-12 cm2. These large cross-sections result in an enlarged collision region in the inner coma, beyond what would be expected as solely as a result of collisions with water molecules. For comet Hyakutake, the collision region is estimated to extend 50,000 km from the nucleus, corresponding to more than 10 arcminutes at perigee. For comet Hale-Bopp, the collision region extends 160,000 km from the nucleus, or approximately 170 arcseconds at its perihelion. These computations can be used to interpret HCN spectral line maps, which appear to require an enlarged collision-dominated portion of the coma.