Time-dependent, three-dimensional fluid model of the outer coma, with application to the comet Hale-Bopp gas spirals

We describe here the first tridimensional, time-dependent model of a gas coma, and its first application to observed rotationally induced gas coma structures. The present version of the model uses inviscid flow equations and, because of it, is applicable rigorously only to highly productive comets, such as comet C1995/O1 Hale-Bopp in the inner Solar System. Using parameters suitable for this comet, and an arbitrary aspherical nucleus shape and rotation mode, the model is capable to generate large-scale gas spirals of the kind observed in this comet, thus demonstrating that the existence of such spirals does not require surface heterogeneity (active/inactive areas). This result does not in itself exclude heterogeneity. The question whether, in the absence of direct nucleus observations, any reliable interpretation of such structures is possible is raised.