Simulation of the water and carbon monoxide production rates of comet Hale–Bopp using a quasi 3-D nucleus model

Abstract

The activity of comet C\\1995 O1 (Hale–Bopp) has been simulated using a multidimensional comet model (Enzian et al., 1997). In this model the comet nucleus is considered to be formed by a fluffy ice-dust matrix. Heat and gas diffusion inside the rotating nucleus is taken into account in radial and meridional direction. A quasi tri-dimensional solution is obtained through the dependency of the boundary conditions on the hour angle. The ice phase is considered to be amorphous water ice including solid carbon monoxide, both trapped in amorphous water ice and as an independent phase. Comet Hale–Bopp is of particular interest due to its strong activity and the significant CO production at large heliocentric distance. The model results provide a satisfying fit to the observed CO production rates (until perihelion) in the case of an amorphous ice composition and a CO source located below the surface, and not at the surface. During the ACM96 meeting in June 1996 we predicted that the increase in carbon monoxide production levels off at about 3 AU and that the production rates increase again at about 1.5 AU (inbound). The gas production rates will then reach their maximum slightly after the comets perihelion : 6×1030 molecules s−1 for CO and 2×1031 molecules s−1 for H2O (upper limit for a nucleus with a radius of 20 km). An attempt is made to take an extended H2O source in the coma into account. These predictions were compatible with recent observations of comet Hale–Bopp.