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D. McCarthy (U. Arizona), S. Stolovy (CalTech), S. Kern, H. Campins, S. Larson, T. Ferro (U. Arizona)
Diffraction-limited images of comet Hale-Bopp (C/1995 O1) have been obtained at 1.87-2.2 microns by NICMOS/HST and provide information on the coma's composition and dynamics and on the nuclear size and activity. These images record the dynamical evolution of one or more significant outbursts at a heliocentric distance of 2.49 AU, beginning <7 hours prior to the NICMOS images. The inner coma (radius <2500 km) is dominated by a spiral feature which expands and becomes more diffuse over the 1.7 hour observation period. Linear and circular morphologies are also apparent as well as several static linear features extending to the edge of the field-of-view. Radial brightness profiles are highly asymmetric and only approach a 1/r decline at distances beyond >11,000 km. Images in a narrow-band filter at 2.04 microns exhibit a ~4% absorption feature relative to nearly simultaneous images at wavelengths of 2.2, 1.90, and 1.87 microns. This absorption is attributed to water ice in the coma grains. Although both the depth of this feature and the comet's integrated brightness remain constant during the observation, the spatial extent of the 2.04 micron absorption increases. The nucleus is revealed by a classic point-spread function (<165 km) with a flux density consistent with a 40 km diameter and 4% albedo.