The Coma Morphology Due to an Extended Active Region and the Implications for the Spin State of Comet Hale-Bopp

We show that the circular character of continuum structures observed in the coma of comet Hale-Bopp around the perihelion passage is most likely due to a dust jet from a large extended active region on the surface. The coma morphology due to a wide jet is different from that due to a narrow jet. The latter shows foreshortening effects due to observing geometry; a wider jet produces more circular features. This circularization effect provides a self-consistent explanation for the evolution of near-perihelion coma morphology. No changes in the direction of the rotational angular momentum vector are required during this period, in contrast to the models of Schleicher et al. This circularization effect also enables us to produce near-circular coma features in the southeast quadrant in late February of 1997, and therefore it puts into question the basic premise on which Sekanina bases his morphological arguments for a gravitationally bound satellite nucleus.