Hubble Space Telescope ultraviolet imaging and high-resolution spectroscopy of water photodissociation products in comet Hyakutake (C/1996 B2)

Comet Hyakutake (C/1996 B2) provided a target of opportunity for performing a systematic study of water photodissociation products in which we obtained data from three instruments on the Hubble Space Telescope (HST). The HST Goddard High Resolution Spectrograph (GHRS) was used to measure the line profile of hydrogen Ly alpha (H Ly alpha) at six locations around the coma of the comet, ranging from the nucleus to a displacement of 100,000 km, and covering different directions compared with the comet-sun line. GHRS yielded line profiles with a spectral resolution (FWHM similar to 4 km s(-1)) that was a factor of 2-3 better than any previous H Ly alpha or H alpha ground-based measurements. The Wide Field Planetary Camera 2 (WFPC2) and the Woods filter were used to obtain H Ly alpha images of the inner coma. Th faint object spectrograph (FOS) was used to determine the OH production rate and monitor its variation throughout the HST observing sequence. The GHRS H Ly alpha line profiles show the behavior of a line profile that is optically thick in the core for positions near the nucleus (<5000 km) and gradually becoming more optically thin at larger displacements and lower column abundances. A composite H Ly alpha image constructed from four separate WFPC3 exposures is consistent with the relative fluxes seen in GHRS observations and clearly shows the dayside enhancement of a solar illuminated optically thick coma. These data were analyzed self-consistently to test our understanding of the detailed physics and chemistry of the expanding coma and our ability to obtain accurate water production rates from remote observations of gaseous hydrogen (H) and hydroxyl (OH), the major water dissociation products. Our hybrid kinetic/hydrodynamic model of the coma combined with a spherical radiative transfer calculation is able to account for (1) the velocity distribution of H atoms, (2) the spatial distribution of the H Ly alpha emission in the inner coma, and (3) the absolute intensities of H and OH emissions, giving a water production rate of (2.6 +/- 0.4) x 10(29) s(-1) on 1996 April 4.