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T.L. Farnham (Univ. Texas), D.G. Schleicher (Lowell Observatory), W.R. Williams (Vanderbilt U.), B.R. Smith (N. Ariz. U.)
We obtained an extensive set of images of comet Hale-Bopp (1995 O1) on 39 nights from January through May 1997 at the Lowell Observatory 42-inch Hall telescope. In order to investigate the time-variable coma morphology, the new Hale-Bopp narrowband filters were utilized with a 20482 CCD to isolate gas emission and reflected sunlight from the dust. The data used in this study were flux calibrated and the underlying continuum was removed from images of the gas species. The images in the months around perihelion clearly exhibit a spiral-shaped arc morphology, produced by a jet on a rotating nucleus. A full spiral is clearly observed in the CN images when a bulk profile is removed, and, though not as obvious, the full spiral can also be distinguished in the dust images. By measuring the distances of the dust arcs at various position angles as a function of time, we derived an apparent rotation period of 0.4712±.0002 days (11.31 hrs) over the interval from mid-February to early May. More recently, we have applied a two-jet Monte Carlo model to both the dust and CN images to determine the orientation of the rotation pole, and to find the locations of the active regions. The results of this analysis show that the dust morphology is well-matched with two primary jets, at latitudes of +51\circ and +61{\circ}. These regions are active throughout the full rotation cycle, though only at a very low level on the night side. Evidence for a third jet is consistent with a near-equatorial source, but this position is not well constrained. At perihelion, the rotation axis is oriented within about 10 degrees of the Earth, which allows the complete spirals to be observed (the Earth lies within the cones swept out by the rotating active regions). Models of the images at other dates indicate that there is a systematic movement of the pole, apparently resulting from precession or complex rotation. This movement keeps the pole oriented such that complete spirals are visible from the Earth for much longer than would be possible if the nucleus were in a simple rotation state. The final results from this analysis and their implications will be presented. This research was supported by NASA and NSF.
The author(s) of this abstract have provided an email address for comments about the abstract: farnham@lowell.edu