Origin and production of C(^1D) atoms in cometary comae

Astronomy and Astrophysics – Astrophysics

Scientific paper

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Comets, Ultraviolet: Solar System, Molecular Processes

Scientific paper

The abundance of carbon atoms in the metastable (1) D state near the cometary nucleus provides an important diagnostic of one of the principal sources of carbon in the cometary coma. This quantity may be determined in two independent ways: measurement of (1) D -(1) P fluorescence at 1931 Angstroms and by prompt emission of the (1) D -(3) P doublet at 9823/9850 Angstroms . The latter is analogous to the [Oi ] lambda lambda 6300/6364 emission that is often used to determine the cometary water production rate, but has not been extensively exploited to date. We have re-examined the Ci lambda 1931 emission observed in some bright comets by the International Ultraviolet Explorer, and have compared these data to both the brighter resonance transitions, Ci lambda lambda 1657 and 1561, and the CO Fourth Positive band system when the latter are observed with sufficient signal-to-noise ratio. We find a strong correlation between the derived C((1) D ) and CO production rates that suggests that photodissociation of CO is the primary source of the observed C((1) D ) atoms in the coma. The photodissociation rate required by these data is significantly higher than the rates currently in the literature. Dissociative recombination of CO(+) is found to be only a minor source of C((1) D ). In the future, ground-based observations of the 9823/9850 Angstroms doublet at sufficiently high spectral resolution should provide a means for routinely determining the CO abundance relative to that of water in comets and how this ratio varies from comet to comet, with important implications to the physical aging of comets.

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