Spectrophotometry and the Development of Emissions for C/1996 B2 (Comet Hyakutake)

Abstract

An analysis of the spectrophotometry of C/1996 B2 (Hyakutake) from 0.55 μm to 1.05 μm recorded between February 17 and April 17, 1996, is presented. We derive Afρ values and production rates of H2O, C2, NH2, and CN. In general we find the Haser model to be substantiated with no inconsistencies for different aperture sizes and different heliocentric and geocentric distances. Comet Hyakutake is the dustiest comet in our database of 39 comets (U. Fink and M. Hicks 1996.Astrophys. J.459,729–743) and both the dust and the H2O production rates follow a heliocentric dependence of ∼r−1.5, lower than the ∼r−2.5dependence found for P/Halley by U. Fink (1994.Astrophys. J.423,461–472). The Afρ values and the H2O production rates track the visual lightcurve quite well. Strong evidence for quenching of OI emissions close to the nucleus was observed in the March data due to the comet's small geocentric distance. While the CN production rate also has a dependence of ∼r−1.5with a CN/H2O ratio typical of most comets, the C2production rate has a much steeper slope, ∼r−2.5, and the C2/H2O ratio evolves from a typical cometary ratio to one that is exceedingly rich in C2. We feel that this is evidence for a significant CHON contribution to the overall C2production. The NH2production is considerably flatter and follows roughly a ∼r−0.85law. In February and March, Comet Hyakutake exhibited the highest relative NH2abundance of any comet in our database, but reverts to more a normal value in April. All together, we feel that the behavior of the comet's Afρ and production rates throughout its apparition argue for a more primordial comet than may be suggested by the orbital elements alone.