Hubbl E Space T El Escope 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 T elescope (HST ). The HST Goddard High Resolution Spectrograph (GHRS) was used to measure the line proÐle of hydrogen Lya (H Lya) at six locations around the coma of the comet, ranging from the nucleus to a displacement of 100,000 km, and covering di†erent directions compared with the cometsun line. GHRS yielded line proÐles with a spectral resolution (FWHM D4 km s~1) that was a factor of 2È3 better than any previous H Lya or Ha ground-based measurements. The Wide Field Planetary Camera 2 (WFPC2) and the Woods Ðlter were used to obtain H Lya images of the inner coma. The faint object spectrograph (FOS) was used to determine the OH production rate and monitor its variation throughout the HST observing sequence. The GHRS H Lya line proÐles show the behavior of a line proÐle 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 Lya image constructed from four separate WFPC2 exposures is consistent with the relative Ñuxes 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 Lya 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)] 1029 s~1 on 1996 April 4. Subject headings : comets : individual (Hyakutake) È molecular processes È ultraviolet : solar system