Long- and Short-Term Photometric Behavior of Comet Hyakutake (1996 B2)

Narrowband filter photometry observations of Comet Hyakutake (1996 B2) were used to investigate this cometʹs short-term variability as well as its behavior for the apparition as a whole. Utilizing measurements obtained on a total of 13 nights between February 9, 1996, and April 14, 1996, we find that the heliocentric distance (rH) dependence of the production rates of OH and NH were much shallower than those for either the carbon-bearing species or the visible dust. Based on the OH measurements, the derived water rH-dependence was also significantly less steep than expected from a basic water vaporization model and required an effective active surface area of about 29 km2 at rH=1.8 AU, 16 km2 at rH=1 AU, and only 13 km2 at rH=0.6 AU. This decrease in active area may be due to seasonally induced variations of a heterogeneous surface, or due to a decreasing contribution of gas from icy grains in the innermost coma. The relative abundances of the minor gas species place Hyakutake into the “typical” category of comets in the AʹHearn et al. (1995, Icarus118, 223–270) taxonomic classification system. The spectrum is generally redder at shorter wavelengths throughout the apparition; however, the dust color progressively changes from being significantly reddened (37%/1000 Å) at large rH to near-solar at small rH. This change of color with distance implies a significant change in grain sizes or a changing proportion between two or more grain populations. r outburst was initiated near March 19.9, just prior to the cometʹs close approach to Earth. The characteristic recovery from the outburst differed among the observed species, with OH recovering most rapidly, essentially returning to its baseline values by March 25. The spatial radial fall-off of OH throughout this interval was consistent with the expected nominal spatial distribution, while CN and C2 displayed fall-offs consistent with a distributed source, and the dust fall-off was significantly less steep than 1/ρ, possibly due to fragmenting grains. Rotational lightcurve amplitudes were largest for the OH, NH, and dust, again consistent with the carbon-bearing species primarily originating from a distributed source. Significant variations were observed in the lightcurve amplitude and phase shifts as functions of aperture size. Finally, a refined value for the rotation period of 0.2614±0.0003 day was determined.