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Probing chemical evolution in molecular clouds with millimeter-wave observations
We present results for a study of gas-phase and grain chemical processes in the dense interstellar medium (ISM) using millimeter-wavelength observations of the pure rotational transitions of molecules. We mapped emission from a large number of molecules over a 8.5' x 10' region in the quiescent dark cloud core L134N. Statistical equilibrium calculations were performed to determine the kinetic temperature from observations of NH 3, the molecular hydrogen density from measurements of HC3N, N2H+, and CS, and the chemical abundances. We determined that the cloud can be described by a single kinetic temperature of [special characters omitted]10 K, and that the average density toward all lines of sight is [special characters omitted]. The determinations of these physical conditions were then used to derive abundances relative to HCO+ and CO for the remaining molecular species. Significant abundance variations were observed and are discussed in relation to current gas-phase chemistry models. We investigated grain processes using the ortho/para ratio of formaldehyde (H2CO). We find modification of this ratio from that expected for gas-phase formation (i.e., ortho/para [special characters omitted] 3) for dark clouds in the earliest stages of star formation. This indicates formation on and subsequent removal of formaldehyde from grain mantles, related to the increased energetics of the forming star. Due to activation barriers in the gas-phase formation of molecules by the addition of hydrogen atoms, highly saturated molecules are not expected to be abundant in molecular clouds unless molecules are released from grain surfaces, where the hydrogen atom has sufficient time to tunnel through the barrier. We determined that the abundance of methylenimine (CH2NH), the hydrogenated cousin to CN and HCN, is well matched by gas-phase chemistry models in quiescent cores but greatly enhanced in massive star forming regions, suggesting direct evidence for grain formation of CH2NH in star forming cores. We also report the first interstellar detection of the cyclic molecule ethylene oxide (c-C2H4O), a precursor to the formation of the sugar phosphates which comprise the backbone of our molecular genetic structure. Its formation pathways are thought to be from molecules whose formation is related to grains, and its large abundance is indicative of such processing.
Dickens, James E., "Probing chemical evolution in molecular clouds with millimeter-wave observations" (1999). Doctoral Dissertations Available from Proquest. AAI9920595.