Thesis (Ph.D.) - University of St Andrews, March 2007.

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The total luminosity of the Rho Ophiuchi molecular cloud is derived from IRAS data and is found to match the luminosity of known embedded sources very closely. High resolution 60 and 100 micron band IRAS images have been reduced to yield equilibrium color temperature maps and 60 micron band dust optical depth maps for the region. These data along with optically thin C18O column density data are used to evaluate dust grain sizes and compositions via competing grain models. Radiative modeling shows that a standard power law distribution of graphite and silicate grains is responsible for IRAS 60 and 100 micron band emissions. These grains are heated to about one tenth of the cloud's depth in the core region. Their optical depths closely follow molecular column density structure, but these grains are considerably colder than the molecular gas. We also find that a 10 nm minimum particle radius cutoff is appropriate for the 60 and 100 micron band emissions while very small grains or PAH molecules dominate the cloud's 12 and 25 micron band emissions.

The Oort Cloud (OC) is a roughly spherical cloud of comets surrounding the solar system, stretching from well beyond the orbit of Neptune, half way to the nearest star. This body of comets is interesting because it contains a record of the gravitational perturbations suffered by the solar system over its lifetime. Here, we investigate the effects of a particular class of perturbing objects - enormous complexes of molecular gas called giant molecular clouds (GMCs). Recent evidence has shown that the classical picture of Oort Cloud formation is inadequate to describe certain properties of the OC. To re-investigate the dynamical evolution of the Oort Cloud, we simulate the Sun's emergence from its natal molecular cloud, and its subsequent encounters with GMCs. While the role of giant molecular clouds in OC formation has been explored before, they have been implemented in a general way, not explicitly taking into account the 3D structure of the cloud. In this research, we draw on an extensive body of evidence which suggests that GMCs are not uniform, diffuse objects, but are instead organized into high density clumps, connected by a very diffuse inter-clump medium. Recent research has shown that GMCs are likely to be fractal in nature, and so we have modeled them as fractal distributions with dimension 1.6. We then perform N-body simulations of the passage of the Sun and its Oort Cloud through such a molecular cloud. We find that the fractal structure of the GMC is, in fact, an important parameter in the magnitude of the cometary energy change. The significant energy changes occur as a result of interactions with the GMC substructure, not simply as a result of its overall density distribution. We find that interactions with GMCs can be quite destructive to the OC, but can also serve to move comets from tightly bound orbits to less tightly bound orbits, thus partially replacing those lost to stripping. Simulations of the Sun's relatively slow exit from its birth GMC paint a picture of a potentially very destructive era, in which a large portion of the OC's evolution may have occured. / Thesis (Master, Physics, Engineering Physics and Astronomy) -- Queen's University, 2009-09-21 13:05:17.527

Thesis (Ph. D.)--McMaster University, 1999. / Includes bibliographical references (leaves 247-252). Also available via World Wide Web.

Thesis (Ph.D.) -- McMaster University, 1997. / Includes bibliographical references (p. 218-233). Also available via World Wide Web.

Thesis (M.S.)--Ohio State University, 2005. / Title from first page of PDF file. Document formatted into pages; contains xiii, 107 p.; also includes graphics Includes bibliographical references (p. 103-107). Available online via OhioLINK's ETD Center

Molecular clouds are imperative to astronomy as the sites of all known star formation. The problem of how molecular clouds are formed in spiral galaxies is approached numerically, by modelling the response of a gas disk to a spiral potential. The importance of spiral shocks is highlighted as a dominant formation mechanism for molecular clouds in grand design galaxies, where a strong density wave is present. The spiral shock both increases the density of the interstellar gas significantly, and produces structure in the spiral arms. The gas evolves into discrete clumps, which are shown to contain substantial densities of molecular hydrogen, and are therefore identified as molecular clouds. The formation of these clouds requires that the interstellar medium (ISM) is cold and inhomogeneous. The passage of an inhomogeneous gas distribution through a spiral potential further shows that supersonic velocities are induced as the gas shocks. This can explain the velocity dispersion relation observed in molecular clouds. Finally, the shearing of clumps of gas in the spiral arms leads to the formation of inter-arm structures, which are commonly observed in spiral galaxies.

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