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C. Reylé (Observatoire de Besancon), D.C. Boice (SWRI, San Antonio)
S2 has been detected in only three comets: Comet IRAS-Araki-Alcock in 1983, Comet Hyakutake in 1996 and Comet Lee in 1999. It has a very short lifetime and is therefore concentrated in the innermost coma where dynamics, optical depth effects and chemistry are most important. Up to now, pure vibrational models have been used to analyse the data. To interpret recent high-resolution S2 observations of Comet Hyakutake, a model including the rotational structure of the molecule is needed.
We performed a time-dependent model where rotational levels are included and, furthermore, we combined the resulting spectra with a gas dynamics model of the coma with detailed chemistry. This model is suitable to interpret high spectral resolution observations and also to study the evolution of S2 fluorescence in different coma regions.
We show that the global population of the vibrational levels become constant very quickly, due to the numerous rotational levels that allow the electrons to reach higher levels at a faster rate. The use of a pure vibrational model to interpret the relative intensities, even for moderate resolution spectra such as those taken by IUE, can lead to errors in estimating the effective exposure time of S2 in the coma and therefore to false assumptions on the lifetime.
Support for this work was provided by NSF and the visiting researcher program of the University of Franche-Comté.