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Title: Molecular hydrogen line emission from photodissociation regions
Author: Chrysostomou, Antonio
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1993
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The work presented in this thesis is dedicated to the study of the physical properties of photodissociation regions (PDRs), the surface layers of molecular clouds which are irradiated by ultraviolet radiation. The structure of PDRs is investigated with the development of an analytical model which incorporates the essential heating and cooling mechanisms in a PDR. The main parameters in the model are the density and the incident ultraviolet radiation field, above the ambient value in the solar neighbourhood, impinging on the surface (G0) which dissociates the molecules in the PDR. It is demonstrated that when the ratio (n/G0) is high (> 100 cm-3) the attenuation of ultraviolet photons is dominated by H2 self shielding which brings the HI/H2 transition zone close to the surface of the cloud (Av < 1). When the ratio is of order unity then the attenuation of ultraviolet photons is dominated by dust grains in the PDR. In this case, the HI/H2 transition zone occurs at a depth of Av ~ 2 - 3. Images of the PDR in the northern bar of M17 show that there is a spatial coincidence, accurate to ~ 1 arcsec, of the H2 and 3.28 μm emission regions (the 3.28 μm emission being a tracer of the hot edge of the PDR delineated by the HII/HI transition) placing a lower limit to the density in the clumps of 105 cm-3. This coincidence is also observed in other PDR sources (eg. NGC 2023) and can be readily explained if the sources are clumpy. It is not clear in the northern bar of M17, where G0 ~ 10^4, whether shielding by dust or H_2 molecules is dominating the attenuation of ultraviolet photons. A uniform, high density PDR model is sufficient to reproduce the observed H_2 line intensity, however the images clearly reveal structures at the 2 arcsec level suggesting that a clumpy model is a realistic solution.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available