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Session 62 - Comet Hyakutake Encounter.
Display session, Wednesday, June 12
Great Hall,
We present analysis of both narrowband photometry and CCD imaging of Comet Hyakutake (1996 B2) obtained at Lowell Observatory using the IHW comet filters. Photometry results include production rates measured for OH, NH, CN, C_2, C_3, and a measure of the dust production, Af\rho. Several hundred CCD images were obtained in filters isolating CN, C_2, C_3, CO^+, and H_2O^+, and continuum at 4845 Å\ and 6840 ÅModerate time resolution (of order 10-30 minutes) in both the photometry and the imaging over three nights near closest approach to Earth enables us to evaluate temporal variations in the various emission species and dust, related to the comet's rotational state and active area distribution. Well--defined periodic brightness variations were measured and are consistent with single--peaked lightcurves with periods of about either 6.25 or 8.55 hr, or double--peaked lightcurves with periods of 9.80, 12.45, 13.0, or 17.2 hr. Preliminary analysis of the imaging data indicates the morphology repeats with a period of between 6.0 and 6.4 hr within an individual night, but not for other periods less than 10 hr. The combined results strongly imply that the rotation period of the comet is approximately 6.25 hr and that a single active region controls the photometric variations. Photometry from the night of closest approach (25 March) yield mean production rates (log Q(X) molecules/sec) of: 28.93 for OH; 26.60 for CN; and 26.78 for C_2, while the log of the dust production was 3.75. Production rates the prior two nights were significantly higher. Additional photometric data obtained prior to and after closest approach indicated that the dependence of the gas species initially was flat or very shallow with heliocentric distance but then sharply steepened, while that of the dust has maintained a constant slope of -2.5 [cf. IAUC 6344]. The ratio of the trace species to water (as measured by OH) classify this comet as ``typical'' in our current taxonomy [cf. A'Hearn et al., 1995, Icarus 118, 223-270].
