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Photon-dominated regions: development of a time-dependent model and application to astrophysical problems

Bell, TA; (2006) Photon-dominated regions: development of a time-dependent model and application to astrophysical problems. Doctoral thesis , UCL (University College London). Green open access

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Abstract

Photon-dominated regions occur in many regions of astrophysical interest and an understanding of their underlying chemical and physical processes can provide an insight into the conditions within them. This thesis describes the development and implementation of a time-dependent version of the UCL_PDR model, including comprehensive benchmarking as part of an international effort to understand the differences between individual models and improve their agreement in key areas. The code has been applied to calculate theoretical values of the CO-to-H2 conversion factor, XqOi to investigate its sensitivity to physical parameter variation. Xqo is found to vary significantly from its canonical value under certain conditions and by over an order of magnitude in the case of high density or low metallicity. By fitting observed line intensity ratios in a sample of nearby galaxies, PDR models have been constructed to represent the conditions found in a range of galaxy types. These are used to derive appropriate values of Xqo for such objects and to investigate the possibility of using higher transition lines of CO as more reliable mass tracers in these environments. A parameter space search has also been conducted using the model to look for conditions that produce significant column densities of H2 with low levels of emission that would be undetectable or overlooked by current surveys. A plausible region of parameter space is found to produce such molecular dark matter, capable of concealing significant masses of gas that may form reservoirs for future star formation. Additional applications of the ucl_pdr model are discussed, including a study of the chemistry within transient microstructure in the diffuse interstellar medium and models of the time-dependent expansion of a molecular shell around a massive star cluster, applied to observations of the central starburst in M 82.

Type: Thesis (Doctoral)
Title: Photon-dominated regions: development of a time-dependent model and application to astrophysical problems
Identifier: PQ ETD:593571
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Thesis digitised by ProQuest. Third party copyright material has been removed from the ethesis. Images identifying individuals have been redacted or partially redacted to protect their identity.
URI: https://discovery.ucl.ac.uk/id/eprint/1446236
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