Sochi, T; (2012) Atomic and Molecular Aspects of Astronomical Spectra. Doctoral thesis , UCL (University College London).

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Abstract

In the first section of this thesis, we present the atomic part of our investigation. A C2+ atomic target was prepared and used to generate theoretical data required in the investigation of recombination lines that arise from collisions between electrons and ions in thin plasma found in planetary nebulae and other astrophysical objects. The R-matrix method of electron scattering theory was used to describe the C2+ plus electron system. Theoretical data concerning bound and autoionizing states were generated in the intermediate-coupling approximation by R-matrix and Autostructure codes and compared to experimental data. The comparison revealed very good agreement. These theoretical data were then used to generate dielectronic recombination data for C+ which include transition lines, oscillator strengths, radiative transition probabilities, as well as emissivity and dielectronic recombination coefficients. The data were cast in the form of a line list, called SS1, containing 6187 optically-allowed transitions which include many C II lines observed in astronomical spectra. The data were validated by comparison to C+ recombination data obtained from a number of sources in the literature. This line list was used to analyze the spectra from a number of astronomical objects, mainly planetary nebulae, and identify their electron temperature where the observational data were obtained from the literature. The electron temperature investigation was also extended to include free electron energy analysis which uses observational and theoretical data of FF and FB transitions to investigate the long-standing problem of discrepancy between the results of recombination and forbidden lines analysis and its possible connection to the electron distribution (Maxwellian or non-Maxwellian). In the course of this investigation two elaborate methods, one for finding and analyzing resonances (K-matrix method) and the other for analyzing and identifying electron temperature from astronomical spectra (least squares minimization), were employed. A computer program for atomic transition analysis was also developed and used as the main tool for obtaining the line list and analyzing the observational spectra. In the second section of the thesis we present the results of our molecular investigation; the generation of a comprehensive, calculated line list of frequencies and transition probabilities for the singly deuterated isotopologue of H3+, H2D+. The line list, which is the most comprehensive one of its kind in existence, contains over 22 million rotational-vibrational transitions occurring between more than 33 thousand energy levels and covers frequencies up to 18500 cm-1. All energy levels with rotational quantum number, J, up to 20 are considered, making the line list useful for temperatures up to at least 3000 K. About 15% of these levels are fully assigned with approximate rotational and vibrational quantum numbers. The list is calculated using a previously proposed, highly accurate, ab initio model implemented in a high-accuracy computer code based on a two-stage discrete variable representation (DVR) approach. Various consistency checks were carried out to test and validate the results. All these checks confirmed the accuracy of the list. A temperature-dependent partition function, valid over a more extended temperature range than those previously published, and cooling function are presented. Temperature-dependent synthetic spectra for the temperatures T=100, 500, 1000 and 2000 K in the frequency range 0-10000 cm-1 were also generated and presented graphically.

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