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The CO2/CO Abundance Ratio in 1P/Halley and Several Other Comets Observed by IUE and HST

We report the detection of CO Cameron band emission in the ultraviolet spectra of four moderately active comets (including 1P/Halley) obtained by the International Ultraviolet Explorer (IUE) satellite. The motivation was the observation of Cameron band emission, a tracer of the CO2 production rate, in the spectra of two comets observed by the Hubble Space Telescope (HST). We examined IUE spectra of those comets for which CO Fourth Positive emission was detected so that the CO production rate could be derived simultaneously. Also, we used the (1, 0) band at 1993 Å, at the extreme end of the spectral range of the short-wavelength prime (SWP) spectrograph rather than the (0, 0) or (0, 1) bands at 2063 and 2155 Å, respectively, which fall at the low-sensitivity end of the long-wavelength camera. For 1P/Halley, the CO2 production rate that we derive, based on a (1, 0) Cameron band emission observation made by IUE at the time of the Giotto encounter, is in good agreement with the in situ result. With the exception of comet Levy, the production rate of CO2 relative to water in all of these comets is found to have a value close to that found for 1P/Halley. However, the two comets observed by HST appear to be deficient in CO (but not in CO2) relative to those observed by IUE. Since these two comets had lower water production rates than any of those observed by IUE, this suggests, albeit from a very small sample, that the overall level of activity of a comet may be related to its relative CO abundance.