The composition of cometary ices as inferred from measured production rates of volatiles
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The present work describes how it is possible to characterize the chemical and physical properties of comets, using high-dispersion spectroscopy in the mid-infrared (between about 2 to 5 µm); in particular, inorganic and organic parent molecules were detected and analyzed in two comets, 8P/Tuttle and C/2007 W1 (Boattini), using CRIRES at ESO Very Large Telescope. A model that describes the v1 vibrational transition of the HCN molecule in cometary comae has been developed, and a review of the mathematical steps needed to retrieve the chemical composition from measured emission line fluxes in comets is given. 8P/Tuttle was observed in January 2008; it is an Halley type comet, with an orbital period of 13.6 years, and an orbital inclination of about 55 degrees. The comparison between the chemical composition of 8P/Tuttle and other comets reveals that this comet is depleted in CO, CH4, C2H6, and HCN, but at the same time it is enriched in CH3OH. Different explanations for this unusual behaviour are given: it could be that 8P/Tuttle is constituted of two quite different cometesimals, or more simply, that it belongs to a new taxonomical class, not observed before. For this comet HCN and CN production rates (the latter retrieved from optical observations) were compared: CN production rate was found to be higher than the HCN ones, suggesting that the latter cannot be considered the only parent molecule for the CN radical. Comet C/2007 W1 (Boattini) is a dynamically new comet that was observed for the first time in May 2008 and in the beginning of July 2008. The comparison of its composition with the one of already observed comets reveals that C/2007 W1 is an organic enriched comet, charachterized by a high amounts of volatile material; in this comet, HCN and CN production rates were found to be consistent one another. Finally, possible correlations between dynamical and chemical classes were investigated for different comets, but no obvious relationships were found.