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T.L. Farnham, D.G. Schleicher (Lowell Obs.), C.C. Cheung (Brandeis U.)
Images of Comet Hale--Bopp (1995 O1) were obtained on a total of 39 nights during the first half of 1997 using the Hall 42-inch telescope at Lowell Observatory. In order to investigate the time-variable coma morphology, the new Hale-Bopp narrowband filters were used with a 20482 CCD to isolate emission from OH, NH, CN, C2, and C3, and to isolate the continuum at 3448, 4450, 5260, and 7128 Å. Photometric conditions on many of the nights allow us to apply absolute flux calibration to the images and, for the gas species, perform accurate continuum subtraction. Several findings have been made based on a preliminary analysis of images obtained within a week of perihelion. For instance, while the dust grains in the coma overall are near-solar in color, the dust concentrated in the well-defined dust arcs is much redder than the sun -- by about 0.2 mag per 1000 Å\ in the near-UV -- possibly implying significant variations in particle size with location. These images also reveal gas jets with much smaller sunward-tailward brightness asymmetries than the dust jets, i.e. complete spirals are easily detected in the gas images while the primary dust jet is only barely evident in the anti-solar direction. The spatial locations of the dust jets also differ significantly from those of the gas species, possibly due to different outflow velocities. The images, combined with our previously determined apparent rotation period of 0.4712±0.0002 days, are being used to obtain a rotational sequence for each species, and these sequences will also be presented. This research was supported by NSF and NASA.