Physics Theses and Dissertations
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This collection contains some of the theses and dissertations produced by students in the University of Oregon Physics Graduate Program. Paper copies of these and other dissertations and theses are available through the UO Libraries.
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Browsing Physics Theses and Dissertations by Subject "Astronomy"
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Item Open Access Host stellar population properties and the observational selection function of type Ia supernovae(University of Oregon, 2010-09) Johnson, Elsa M., 1971-Supernovae Ia are viable standard candles for measuring cosmological distances because of their enormous light output and similar intrinsic brightness. However, dispersion in intrinsic brightness casts doubt on the overall reliability of supernovae as cosmological distance indicators. Moreover, as shown in this thesis, the dependence of peak brightness on host galaxy properties significantly contributes to this dispersion. As a result, there is good reason to doubt that the nearby sample of supernovae Ia is identical to the distant samples, which occur in host galaxies that are billions of years younger. This study explores the validity of supernovae Ia as standard candles by examining regions of nearby galaxies that hosted supernovae and modeling their observational selection function. The approach is two-fold. First, photometry is performed on the stellar population environment of supernovae to characterize that region as a function of supernova type. Then, the observational selection function is simulated to determine the true supernovae production rate of the z < 0.1 redshift limit. We find that, on average, type Ia events occur in redder and older populations; underluminous supernovae Ia occur in regions that seem to be preferentially dusty, whereas normal Ia coming from the same galaxy type occur in a wide range of extinction environments. Furthermore, redder peak colors correspond to redder underlying population colors. This finding implies that dust extinction effects can cause systematic errors in the luminosity calibration of Ia events Finally, a single supernova rate does not adequately describe all supernovae Ia within z < 0.1. A rate of 0.25 SNu describes the population up to z < 0.03, and a much smaller rate, 0.1 SNu or less, describes supernovae past this distance. This finding indicates that observed supernova rates per galaxy remain biased by sample selection effects and that the intrinsic rate is likely uncertain by a factor of 2 to 3.Item Open Access Linear stability analysis of nonaxisymmetric instabilities in self-gravitating polytropic disks(University of Oregon, 2011-03) Hadley, Kathryn Z., 1955-An important problem in astrophysics involves understanding the formation of planetary systems. When a star-forming cloud collapses under gravity its rotation causes it to flatten into a disk. Only a small percentage of the matter near the rotation axis falls inward to create the central object, yet our Sun contains over 99% of the matter of our Solar System. We examine how global hydrodynamic instabilities transport angular momentum through the disk causing material to accrete onto the central star. We analyze the stability of polytropic disks in the linear regime. A power law angular velocity of power q is imposed, and the equilibrium disk structure is found through solution of the time-independent hydrodynamic equations via the Hachisu self-consistent field method. The disk is perturbed, and the time-dependent linearized hydrodynamic equations are used to evolve it. If the system is unstable, the characteristic growth rate and frequency of the perturbation are calculated. We consider modes with azimuthal e im[varphi] dependence, where m is an integer and [varphi] is the azimuthal angle. We map trends across a wide parameter space by varying m , q and the ratios of the star-to-disk mass M * /M d and inner-to-outer disk radius r - /r + . We find that low m modes dominate for small r - /r + , increasing to higher r - /r + as M * /M d increases, independent of q . Three main realms of behavior are identified, for M * << M d , M * [approximate] M d and M * >> M d , and analyzed with respect to the I, J and P mode types as discussed in the literature. Analysis shows that for M * << M d , small r - /r + disks are dominated by low m I modes, which give way to high m J modes at high r - /r + . Low m J modes dominate M * [approximate] M d disks for small r - /r + , while higher m I modes dominate for high r - /r + . Behavior diverges with q for M * >> M d systems with high q models approximating M * [approximate] M d characteristics, while low q models exhibit m = 2 I modes dominating where r - /r + < 0.60.