The Fourth Positive System of Carbon Monoxide in the Hubble Space Telescope Spectra of Comets

The rich structure of the AΠ−XΣ system of carbon monoxide accounts for many of the spectral features seen in long slit HST -STIS observations of comets 153P/Ikeya-Zhang, C/2001 Q4 (NEAT), and C/2000 WM1 (LINEAR), as well as in the HST -GHRS spectrum of comet C/1996 B2 (Hyakutake). A detailed CO fluorescence model is developed to derive the CO abundances in these comets by simultaneously fitting all of the observed A−X bands. The model includes the latest values for the oscillator strengths and state parameters, and accounts for optical depth effects due to line overlap and self-absorption. A complete fitting of the CO spectral features using this model leads to the first identification of a molecular hydrogen line pumped by solar H I Lyman-β longward of 1200 Å in the spectrum of comet 153P/Ikeya-Zhang. Pumping by strong solar lines also plays an important role in CO fluorescence, as shown by the detection in the spectrum of comet C/1996 B2 (Hyakutake) of bands from the (14-v) and (9-v) progressions pumped by solar H I Lyman-α and O I λ1302, respectively. Using spectra extracted at increasing distances from the comet nucleus or averaging over increasing effective apertures, the model fits yield radial profiles of CO column density that are consistent with a predominantly native source for all the comets observed by STIS. The derived CO production rates are 1.54 ± 0.09 × 10 molecules s for 153P/Ikeya-Zhang, 1.76 ± 0.16 × 10 molecules s for C/2001 Q4 (NEAT), and 3.56 ± 0.20 × 10 molecules s for C/2000 WM1 (LINEAR). In the absence of spatial information for comet C/1996 B2 (Hyakutake), we estimate a CO production rate of 4.97± 0.07× 10 molecules s, assuming an entirely native source. The CO abundances relative to water in these comets span a wide range, from 0.44±0.03% for C/2000 WM1 (LINEAR), 7.2±0.4% for 153P/Ikeya-Zhang, 8.8± 0.8% for C/2001 Q4 (NEAT) to 20.9± 0.3% for C/1996 B2 (Hyakutake). For comets C/2000 WM1 and C/2001 Q4 we can compare these results with those derived from nearly simultaneous observations by the Far Ultraviolet Spectroscopic Explorer. Subject headings: comets:individual (153P/Ikeya-Zhang, C/2001 Q4 (NEAT), C/1996 B2 (Hyakutake), C/2000 WM1 (LINEAR)) — carbon monoxide — ultraviolet: solar system