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Session 62 - Comet Hyakutake Encounter.
Display session, Wednesday, June 12
Great Hall,
Ultraviolet spectra covering the wavelength range from 129.5 to 327.7 nm were obtained during HST observations of comet Hyakutake (C/1996 B2) on 1 April 1996. The region below 220.0 nm is dominated by emission from CO. At least 25 bands of the CO Fourth Positive Group (4PG) were detected, including optically thin bands in the v'=14 progression pumped via the solar H Ly-\alpha line. Seven or more bands of the forbidden CO Cameron band system were detected. These latter bands are produced primarily by prompt emission following the photodissociation of CO_2 and provide a unique tracer of CO_2 in the cometary nucleus. Preliminary calculations indicate that the production rates for both CO and CO_2 were \mbox\sim 1-2 \times 10^28 s^-1, which corresponds to an abundance relative to H_2O of \sim5%. The S_2 molecule was clearly detected via fluorescence in multiple bands between 285.0 and 312.0 nm. A rough estimate of the S_2 relative abundance is \sim0.001%. This is the first time that S_2 has been definitely detected in a comet since its initial discovery during IUE observations of the Earth-grazing comet IRAS-Araki-Alcock (C/1983 H1) in 1983. The Hyakutake spectra have several as yet unidentified emissions, which we are currently investigating. WFPC2 images of Hyakutake were taken through B, V, and R filters on 26 March and 1 April at a plate scale of \sim8 km pixel^-1. These images provide a detailed view of the multiple jets in the inner coma and should also provide direct information on the size of the nucleus.
