Astrochemistry at Millimetre and Submillimetre Wavelengths
Abstract: The focus of this thesis is a series of observational tests, aiming to clarify the chemical and physical origin of interstellar molecules. Spectral lines at millimetre and submillimetre wavelengths, caused by rotational transitions is CO, H2O, NH3, CH3OH, CH3SH, C2H3CN, and several of their isotopologues, have been observed towards regions of star-formation in the Galaxy. Maps of extended H2O and CO emission from the Orion nebula demonstrate that the water probably is localised to the photon-dominated region at the surface of the molecular cloud, at higher abundances than previously thought. Water is also observed in absorption from its ground-state towards the massive star-forming region Sgr B2. Curiously enough, a water abundance similar to the one reported for Orion is found in the low-excitation gas in one of the Galactic spiral arms. Ammonia absorption was also observed from diffuse spiral arm clouds along the same line-of-sight, but at about an order of magnitude lower abundance. The observed water and water absorptions caused by the Sgr B2 cloud itself are successfully modelled without invoking a morphological component of hot gas. Two independent methods of analysis are applied to observations of methanol (CH3OH) and its 13C isotopologue in the cold envelopes of young stellar objects. Both methods indicate that methanol is mainly formed by hydrogenation of CO on cold dust grains. A study comparing the interstellar abundances of CH3SH (methyl mercaptan) and CH3OH unveil a possible trend of lower relative CH3SH abundances in more evolved objects. However, the significance of this trend, in relation to the chemical origin of these molecules, needs to be further investigated. In addition, searches for two pre-biotic molecules, namely vinyl acetylene (C2H3CCH) and amino acetonitrile (H2NCH2CN), resulting in improved upper abundance limits are presented. A comprehensive conclusion of this thesis is that in order to exploit the full capacity of high-quality observations there is a serious need for additional theoretical and laboratory investigations of processes like proton-exchange reactions, collision rates, freeze-out and desorption, all taking into account different isotopologues and spin-types.
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