A Comprehensive Study of the H Lyman-alpha Line Profile and Water Photochemistry in Comet Hyakutake (1996 B2)

Details A Comprehensive Study of the H Lyman-alpha Line Profile and Water Photochemistry in Comet Hyakutake (1996 B2) A Comprehensive Study of the H Lyman-alpha Line Profile and Water Photochemistry in Comet Hyakutake (1996 B2)

Sep 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996dps....28.0933c&link_type=abstract

American Astronomical Society, DPS meeting #28, #09.33; Bulletin of the American Astronomical Society, Vol. 28, p.1094

Astronomy and Astrophysics

Astronomy

1

Scientific paper

Comet Hyakutake (1996 B2) provided a target-of-opportunity for performing a systematic study of water photodissociation products (H, OH and O) in which we combined data from three instruments on the Hubble Space Telescope (HST) with ground-based long-slit echelle spectroscopy. The HST Goddard High Resolution Spectrograph (GHRS) was used to measure the line profile of H Lyman-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 ~ 7\ km/s) that was a factor of more than 2 better than any previous Lyman-alpha measurements and 50% better than any ground-based measurements of H Balmer-alpha . Images were made using the Wide Field Planetary Camera 2 (WFPC2) and the Woods filter, which basically provides a Lyman-alpha image 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. Spatial profiles of the forbidden O((1) D) 6300 Angstroms emission and combined spatial and resolved line profiles of H Balmer-alpha to be compared with the Lyman-alpha profiles were obtained with the Hamilton echelle spectrograph on 24-inch coude auxiliary telescope at Lick observatory. The GHRS Lyman-alpha line profiles clearly show the behavior of a line profile that is optically thick in the core for positions near the nucleus ( < 5000 km) and becoming more optically thin at larger displacements (30,000 to 100,000 km) and correspondingly lower column abundances. A composite Lyman-alpha image constructed from four separate exposures is consistent with the relative fluxes seen in GHRS observations, and shows clear evidence of the dayside enhancement of a solar illuminated optically thick coma. Radiative transfer calculations are in reasonable agreement with the scale of the day-night asymmetry, and with the overall shape and brightness of the coma observed. A comparison with the ground-based observations, and preliminary combined analyses are presented.

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