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978-3-8439-2280-7, Reihe Physik
3D Extension and Applications of the KOSMA-tau PDR Model
193 Seiten, Dissertation Universität Köln (2015), Softcover, A5
The components of the interstellar medium (ISM) are continuously heated due to energy input from different sources, one being the radiation of young and massive stars. In photon-dominated regions (PDRs) the interstellar far-ultraviolet (FUV; 6-13.6 eV) radiation field determines the energy balance and the chemistry of the ISM. Consequently, PDRs form in different astrophysical scenarios, for instance at the edges of irradiated molecular clouds. Depending on the cloud’s characteristics, stars may form from dense, embedded cores. To understand the interaction between FUV radiation and star formation it is necessary to gain detailed knowledge of the physical conditions and the intrinsic structure of the ISM in molecular clouds and PDRs. Cooling of the gas in PDRs is dominated by fine structure line emission by atoms and ions, as well as rotational and vibrational line emission by molecules. The KOSMA-tau PDR model simulates the chemical and physical structure and the line emission of spherical clouds (“clumps”) in the ISM.
In this thesis the KOSMA-tau PDR code is used to simulate transitions from atomic to molecular hydrogen. Numerically derived properties of the clumps, like their HI column densities or H_2 mass fractions, are compared to analytic formulae provided by Sternberg et al. (2014) as well as to results from McKee & Krumholz (2010). Observations suggest that the ISM can be modelled using fractal structures. Furthermore, it has been shown that a superposition of spherical clumps, having a specific mass-spectrum and a specific mass-size relation, can be used to mimic the structure of the ISM. In this thesis, I introduce an extension of the KOSMA-tau code, denoted KOSMA-τ 3D, which can be used to model star forming regions with arbitrary three-dimensional (3D) geometry. The Orion Bar PDR, a well-known and luminous star forming region with an interesting edge-on geometry, is used as a test-case for the new 3D code. New HIFI/Herschel data from the HEXOS guaranteed-time key program and complementary data from the Caltech Submillimeter Observatory (CSO) are fitted. Furthermore, I present simulation results, based on the clumpy edge-on cavity wall suggested by Hogerheijde et al. (1995) and on a cylindrical filament geometry. Simulations and observations are compared in terms of the layered positions of the emission peaks, the “chemical stratification”, and the line integrated intensities at the peak positions.