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D. G. Schleicher (Lowell Observatory), D. J. Osip (MIT)
We present results from narrowband photometry of Comet Hyakutake (1996 B2) obtained at Lowell Observatory from 1996 February 9 to April 14. Previous analysis of these data and corresponding CCD images of the dust coma were used to determine Hyakutake's rotation period as being 6.23±0.03 hr (Schleicher, Millis, Osip, and Lederer, 1998, Icarus131, 233). New analyses have investigated the trends of dust and gas production as a function of heliocentric distance, the characteristics of the major outburst which occured a few days prior to perigee, and the nature of the trends in derived production rates as a function of aperture size and rotational phase. We have also determined a refined rotation period of 6.266±0.008 hr, after accounting for these effects and combining our data with the CCD lightcurve measurements of Lisse et al. (1999, Icarus 140, 189). The combined lightcurves permits us to constrain the onset time of the outburst to Mar 20.0±0.3. We unexpectedly measured a change in dust color as a function of heliocentric distance, with the UV continuum (3650 Å) having an r-dependence 0.31 steeper that the continuum at 4845 Å. The r-dependences of the carbon--bearing species, CN, C2, and C3, were all similar to the dust, while OH and NH were much more shallow -- significantly less than expected with a standard water vaporization model, possibly due to a strong seasonal effect. Our derived water production implies a peak active area on the nucleus of about 30 km2, and a minimum effective radius of 1.5 km. The ratio of the trace species to water classifies this comet as ``typical'' in our current compositional taxonomy [cf. A'Hearn et al.,1995, Icarus 118, 223]. This research was supported by NASA.