36th DPS Meeting, 8-12 November 2004
Session 25 Comets Coma II
Oral, Thursday, November 11, 2004, 8:30-10:00am, Clark

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[25.09] A Self-Consistent Model for the Jet Morphology Observed in Comet Hale-Bopp (1995 O1) From 1995 to 1998

D. G. Schleicher (Lowell Obs.), T. L. Farnham (U. Maryland), W. Hawley (Wellesley College)

Studies of the morphological features in a comet's coma can be used to infer basic physical properties of the nucleus, including the rotation rate and pole orientation, and the location and composition of the active regions. The apparition of Comet Hale-Bopp (1995 O1) provided an excellent opportunity for this type of study as a function of nucleus rotation and changing viewing geometry. Hale-Bopp exhibited an increasing number of near-radial features through 1996, which evolved into a series of spiral arcs near perihelion and then returned to a radial morphology in late 1997. Numerous investigators have attempted to model Hale-Bopp's morphology for differing intervals, but most such models are unable to accurately reproduce the evolution and relative brightness of the morphological features and the bulk coma asymmetries throughout the apparition.

Here we report on the results of our analyses of both our own extensive imaging set and images from several other sources. In particular, full rotational sequences near perihelion, produced from phasing images over several nights, provide strong constraints on the pole orientation and the location of the strongest source regions, as do the position angle and widths of radial features at certain times. We emphasize that a comprehensive model can only be produced with relatively broad jets (15-30\circ), rather than with the narrow jets that have often been previously used. These broad sources not only allow the morphology to be modeled without the need for precession, but also naturally reproduce many of the complex features and overlapping arcs. Our preferred solution has the North pole located within a few degrees of RA=18h23m and Dec=-54\circ (obliquity of about 83\circ). A near-polar source (latitude of about -75\circ) dominates the morphology early in the apparition, while another high latitude source (+65\circ) becomes active at the beginning of 1997 and dominates through perihelion. At least two additional low-to-mid-latitude sources are required to produce the remaining morphological features. Thermal lags of several hours are required to reproduce the large east-west brightness asymmetries observed during the last half of 1996. Jet intensities as a function of position angle near perihelion imply that significant gas is released even at low sun angles, but that the dust grains important at visible wavelengths require significantly higher sun angles, implying a threshold for lifting dust grains off of the surface. These and other results will be presented, along with a comparison of model solutions with observations. This research was supported by NASA.

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