Other
Scientific paper
Apr 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001phdt........11h&link_type=abstract
Thesis (PhD). BOSTON UNIVERSITY, Source DAI-B 61/10, p. 5369, Apr 2001, 259 pages.
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4
Scientific paper
The photo dissociation region (PDR) is an area in the interstellar medium where far-ultraviolet radiation from OB stars dominates the physical and chemical processes and conditions. I study properties of the PDRs near the ionized hydrogen (H II) regions, which surround massive, hot stars. Carbon is one of the most abundant species after hydrogen and helium. I concentrate my study on the properties of carbon in its various forms to further our understanding of PDR physics. PDRs have been studied in detail in only a small number of classic ``test bed'' sources with a limited range of physical and geometrical conditions. I study a much larger sample of H II regions with a large range of radiative excitation conditions, selected from H II region radio-recombination line surveys. I observe these sources in emissions from ionized (C II), neutral (C I), and molecular carbon (CO; the carbon monoxide molecule) to obtain carbon properties in a broad range of PDR conditions. To observe my sources I use the ISO satellite and the AST/RO telescope which I helped to build. In addition I use published data from the IRAS satellite and other sources. I compare these observations with theoretical predictions of existing PDR models. My major conclusions include: the C I intensity is due mainly to local heating; the correlation between C I and CO observations and H II radio observations strongly supports a spherically-structured PDR model; PDR linewidths and H II region excitation parameters are positively correlated. Using the combined data I derive the density and the incident far-ultraviolet emission flux in my sample of PDRs. I have identified a number of problems in applying the current homogeneous slab PDR models to the observational results. When these problems are addressed the PDR models can explain the infrared observational results within error limits, however the models do not explain the neutral and molecular observations. I find that spherical and clumpy PDR models can help to interpret these results. Based on my study I suggest that current PDR models should include a range of gas turbulent velocities and address the geometry and clumpiness of the PDRs.
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