OH PROMPT EMISSION IN COMET IRAS-ARAKI-ALCOCK (1983-VII)

In a cometary atmosphere the photodissociation of water by solar ultraviolet radiation creates a small fraction of OH radicals in the A2Σ+ state in highly excited rotational levels which quickly decay to OH(X2Π) through spontaneous emission, producing a spectrum distinct from that of OH solar resonance fluorescence. J. L. Bertaux (1986, Astron. Astrophys. 160, L7-L10) has suggested that this photodissociative OH fluorescence, called prompt emission, can provide a direct measure of the water density distribution within ∼100 km of the cometary nucleus, but the observation requires very high spatial resolution. We examine the ultraviolet spectra of two comets obtained by the International Ultraviolet Explorer satellite, which differ significantly only with respect to the effective spatial resolution, to search for the presence of OH prompt emission. A comparison of the spectra of comets IRAS-Araki-Alcock (1983 VII) and Tuttle (1980 XIII), at comparable heliocentric distance and velocity, but at geocentric distances of 0.032-0.048 and 0.50 AU, respectively, in the wavelength region 2700-3200 Å shows evidence for OH prompt emission in Comet IRAS-Araki-Alcock. The water production rate obtained from the prompt emission reflects an instantaneous value, and can be compared to that derived from OH solar resonance fluorescence, which gives an average over the lifetimes of the parent and daughter molecules. Future observations of comets with the Hubble Space Telescope should make possible routine determinations of the spatial distribution of water near the nucleus. © 1991.