Prompt Emission by Oh in Comet Hyakutake

Laboratory studies predict that the photo-dissociation of H2O by solar Lya photons in the comae of comets should lead to a small percentage of OH in high rotational states of the A (2)Sigma(+) electronic state. These states should promptly emit a near-ultraviolet (near-UV) photon in a transition to the X (2)Pi state. From there, the radicals decay to the lowest rotational states by direct rotational transitions and via ro-vibrational cascade in the 1-0 vibrational band, all within the X (2)Pi state. Normally in Earth-based observations the lines are extremely weak compared to the fluorescence of OH in sunlight. Since the prompt emission rate is directly proportional to the column density of water, whereas the fluorescent emission of OH is proportional to the column density of OH, the lines due to prompt emission are relatively strongest very close to the nucleus, a region not often accessible from Earth. We report here the first spectrally resolved detection of near-UV prompt emission by cometary OH, as seen in the bright comet Hyakutake (C/1996 B2), on the day of its closest approach to Earth. All the expected doublets are seen, in both the P and Q branches, up to rotational quantum number N' = 22 with relative strengths in good agreement with the laboratory results. The fluxes of the lines are in agreement with what one expects from the various measurements of the water production rate of the comet, including that deduced from the OH fluorescent lines in these spectra.