Astron. Astrophys. 330, 753-763 (1998)

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

G.P. Tozzi ¹, P.D. Feldman ² and M.C. Festou <sup>3

1</sup> Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50125 Firenze, Italy (tozzi@arcetri.astro.it)
² Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD 21218, USA
³ Observatoire Midi-Pyrénées, 14, avenue E. Belin, F-31400 Toulouse, France

Received 5 June 1997 / Accepted 15 September 1997

Abstract

The abundance of carbon atoms in the metastable ¹D 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 ¹D -¹P fluorescence at 1931 Å and by prompt emission of the ¹D -³P doublet at 9823/9850 Å . The latter is analogous to the [O I ] 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 C I 1931 emission observed in some bright comets by the International Ultraviolet Explorer, and have compared these data to both the brighter resonance transitions, C I 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(¹D) and CO production rates that suggests that photodissociation of CO is the primary source of the observed C(¹D) 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(¹D). In the future, ground-based observations of the 9823/9850 Å 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.

Key words: comets ultraviolet: solar system molecular processes

Send offprint requests to: G.P. Tozzi

© European Southern Observatory (ESO) 1998

Online publication: January 16, 1998
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