Evidence for Collisional Effects in the Radial Distributions of OH and C in the Coma of C/1995 O1 (Hale-Bopp)

At perihelion C/1995O1 (Hale-Bopp) had the largest water production rate of any modern comet. This circumstance had significant implications for conditions in the coma and for the validity of many of the assumptions and approximations used to interpret them. In particular, we consider the effect of the Hale-Bopp's large 'collision sphere', and its significance for the energy budget, photochemical scale lengths, and the velocity distribution in and beyond the inner coma. With a production rate Q_H2O of ~ 10^31 s^-1 , Hale-Bopp's collisionally thick coma was substantially more than 10x larger than in the typical comet, and was comparable to or greater than the scale lengths of several parent species. Analysis of the polarimetric and wide field properties of OH and C radial profiles in observations taken on 8 April, 1997 show clear evidence of formation from parents within the collision sphere and subsequent thermalization of the daughters with the background gas. Dissociation inside the collision sphere will pump energy (particularly from water to OH + H) into the coma and disrupt the partition of excess energy to daughter species. This significantly affects conditions throughout the entire coma by introducing a radial dependence in the temperature and bulk outflow velocity in the collision sphere and by redistributing the velocities for the individual daughter products relative to their formation in the ballistic regime. Since determination of the production rate and photochemical scale lengths of different coma species is tightly coupled to a knowledge of the velocity structure, the large collision sphere of Hale-Bopp therefore has an impact on the accuracy of production rate estimates.