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[12.10] Modeling the Fluorescence of the Carbon Monoxide Fourth Positive Bands in Cometary Comae

P. D. Feldman (JHU), H. A. Weaver (JHU/APL)

Observations of the carbon monoxide Fourth Positive Band system (A\,¹\Pi~-~X\,¹\Sigma⁺) in vacuum ultraviolet spectra of comets made by various space observatories over the past 26 years have demonstrated a range of over an order of magnitude in the relative abundance of CO to that of water. Recent radio, infrared, and ultraviolet observations have provided constraints on the physical state of CO in the inner coma while recent laboratory work on the molecular parameters, particularly for high vibrational levels of the upper A\,¹\Pi state, have made possible accurate modeling of the observed emissions. We have constructed a fluorescence model that includes saturation of solar flux absorption along the line-of-sight to the Sun, redistribution of photons along the line-of-sight to the Earth, and the Swings effect. The model is compared with moderate resolution (~ Å) spectra of comets C/2002 C1 (Ikeya-Zhang) and C/1996 B2 (Hyakutake) obtained with the Hubble Space Telescope. The latter is particularly interesting in showing fluorescence from v' = 9 and 14, excited by solar \ion{O}{1} \lambda1302 and \ion{H}{1} Lyman-\alpha, respectively. The goal of these models is to provide a quantitative footing for the determination of the relative CO/H₂O abundance in a diverse sample of comets.

Support for this work was provided by NASA through grant GO-09496.04-A from the Space Telescope Science Institute.

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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.