ORIGIN AND PRODUCTION OF C(D-1) ATOMS IN COMETARY COMAE

The abundance of carbon atoms in the metastable D-1 state near the com etary nucleus provides an important diagnostic of one of the principal sources of carbon in the cometary coma. This quantity may be determin ed in two independent ways: measurement of D-1-P-1 fluorescence at 193 1 Angstrom and by prompt emission of the D-1-P-3 doublet at 9823/9850 Angstrom. The latter is analogous to the [O I] lambda lambda 6300/6364 emission that is often used to determine the cometary water productio n rate, but has not been extensively exploited to date. We have re-exa mined the C I lambda 1931 emission observed in some bright comets by t he International Ultraviolet Explorer, and have compared these data to both the brighter resonance transitions, CI lambda lambda 1657 and 15 61, and the CO Fourth Positive band system when the latter are observe d with sufficient signal-to-noise ratio. We find a strong correlation between the derived C(D-1) and CO production rates that suggests that photodissociation of CO is the primary source of the observed C(D-1) a toms in the coma. The photodissociation rate required by these data is significantly higher than the rates currently in the literature. Diss ociative recombination of CO+ is found to be only a minor source of C( D-1). In the future, ground-based observations of the 9823/9850 Angstr om doublet at sufficiently high spectral resolution should provide a m eans for routinely determining the CO abundance relative to that of wa ter in comets and how this ratio varies from comet to comet, with impo rtant implications to the physical aging of comets.