High-Resolution Spectroscopy of the A-X and B-X System of CH in Comet Austin (1990 V).

Abstract

We analyzed the A-X(0-0) band of CH, which appears in high-resolution spectra of comet Austin (1990 V), in order to understand fluorescence and collisional processes that influence the rotational structure of the A-X(00) band. Some of the weak lines of the A-X (0-0) band are clearly resolved, which have not been previously resolved with relatively low-resolution spectroscopy. We unambiguously confirmed the B-X (0-0) band lines around 3890 Å, which had been suspected previously, but it had not been clearly identified because of strong adjacent CN and C3 bands. In order to analyze the cometary spectra we have conducted two different fluorescence calculations: a single-cycle fluorescence and fluorescent equilibrium. The fluorescent equilibrium model includes infrared and ultraviolet fluorescence processes as well as electron and neutral collisional effects, and therefore the model is a function of cometocentic distance. We found that single-cycle fluorescence models with a Boltzmann distribution in the X state fit the observed spectra better than the fluorescent equilibrium models. However, single-cycle fluorescence models with two different temperatures (130 K for F1 state and 250 K for F2 state) in the X state fit the Austin spectra significantly better than the single-cycle fluorescence model with the same temperature (150 K) for F1 and F2 states. This suggests that we are observing two different Boltzmann distributions of nascent, short-life CH radicals right after they were produced by photodissociations of parent molecules. We presented g-factors of the A-X (0-0) and B-X (0-0) bands as a function of heliocentric velocity based on single-cycle fluorescence models with a 150 K distribution in the X state. We have calculated the expected intensity of the fundamental band (v' = 1 → 0) of CH and discussed the detectability of this band near 2730 cm-1. We also discussed possible parent molecules of CH and long lifetimes of the parent molecules, which may explain extensive emissions of CH up to 105km from the nucleus despite its short lifetime.