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D. G. Schleicher, T. L. Farnham (Lowell Obs.), W. R. Williams (Vanderbilt U.), B. R. Smith (N. Ariz. U.), C. C. Cheung (Brandeis U.)
Studies of the morphological features present in a cometary coma can lead to an understanding of several basic physical properties of the cometary nucleus, such as rotation rate, pole orientation, and location and composition of active regions on the surface. The recent apparition of Comet Hale-Bopp (1995 O1) has provided an excellent opportunity to study the morphology of dust and gas jets as a function of nucleus rotation. Narrowband images obtained with the Hall 42-inch (1.1-m) telescope at Lowell Observatory within a week of perihelion have been flux calibrated and, for each gas species -- OH, NH, CN, C2, and C3 -- continuum subtracted. Each species' images were then phased using our best-determined rotation period of 0.471 day to follow a full rotation cycle. Full rotational spirals are readily visible in the gas images, indicating that the rotational axis of the nucleus at perihelion is pointing only 10-15 degrees from the Earth. Modeling reveals that two high-latitude source regions can reproduce the majority of the features visible in the dust images. Gas jets arising from the same regions reproduce many of the gas features, but an additional strong gas source (apparently at a low latitude) is also required. While jets from the various gas species are cospatial, dust jets are not, due to significantly larger outflow velocities of the gas: v(dust)=1.0 km/s; v(gas)>2 km/s (far from the nucleus). Spatial profiles are also consistent with these unusually large outflow velocities, and have been used to successfully extrapolate to model-independent total abundances of each species. Jet intensities as a function of position angle imply that 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. This research was supported by NASA and NSF.