The Fourth Positive System of Carbon Monoxide in theHubble Space TelescopeSpectra of Comets

Abstract

The rich A1Π - X1Σ+ system of carbon monoxide accounts for most 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. The derived 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, and 8.8% ± 0.8% for C/2001 Q4 (NEAT) to 20.9% ± 0.3% for C/1996 B2 (Hyakutake). The radial profiles of the CO column density are consistent with a predominantly native source for all the comets observed by STIS. A complete fitting of the CO spectral features using the fluorescence model leads to the first identification of a molecular hydrogen line pumped by solar H I Lyβ longward of 1200 A in the spectrum of comet 153P/Ikeya-Zhang. CO fluorescence due to pumping by solar H I Lyα and O I λ1302 is detected the spectrum of comet C/1996 B2 (Hyakutake).