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It is currently believed that angular momentum transport in accretion disks is mediated by magnetohydrodynamic (MHD) turbulence driven by the magnetorotational instability (MRI). More than 15 years after its discovery, an accretion disk model that incorporates the MRI as the mechanism driving the MHD turbulence is still lacking. This dissertation constitutes the first in a series of steps towards establishing the formalism and methodology needed to move beyond the standard accretion disk model and incorporating the MRI as the mechanism enabling the accretion process. I begin by presenting a local linear stability analysis of a compressible, differentially rotating flow and addressing the evolution of the MRI beyond the weak-field limit when magnetic tension forces due to strong toroidal fields are considered. Then, I derive the first formal analytical proof showing that, during the exponential growth of the instability, the mean total stress produced by correlated MHD fluctuations is positive and leads to a net outward flux of angular momentum. I also show that some characteristics of the MHD stresses that are determined during this initial phase are roughly preserved in the turbulent saturated state observed in local numerical simulations. Motivated by these results, I present the first mean-field MHD model for angular momentum transport driven by the MRI that is able to account for a number of correlations among stresses found in local numerical simulations. I point out the relevance of a new type of correlation that couples the dynamical evolution of the Reynolds and Maxwell stresses and plays a key role in developing and sustaining the MHD turbulence. Finally, I address how the turbulent transport of angular momentum depends on the magnitude of the local shear. I show that turbulent MHD stresses in accretion disks cannot be described in terms of shear-viscosity.

The initial study herein assessed mineral digestibility in situations when reducedamounts of inorganic and organic (proteinates) trace minerals (TM) were fed in finishingpigs, and their long-term effects on body mineral status. The second study was aslaughter-investigation that tested the impact of lean growth potential on phosphorusbody accretion from 30 to 110 kg.Organic TM exhibited neither improvement in digestibility nor in total retention;fecal excretion responded quantitatively to mineral intake independently of the source.Contents of copper in kidney and zinc in liver were higher for pigs fed the organic form.Phosphorus content was linearly related to live weight, empty body weight, andnitrogen content. Phosphorus accretion was very similar for both genetic backgrounds,with gilts retaining more mineral (Pandlt;0.05) than barrows. A N/P deposition ratio of 5.14was determined for pigs of both genders and genetic backgrounds to further predictphosphorus requirements based on protein accretion.Reduction of TM waste from growing-finishing pigs may be best addressed bylimiting their dietary inclusion rather than by using organic forms. Data from the secondstudy may be useful for an accurate estimation of phosphorus requirements, whichaccounts for variations in lean accretion rate.

There is no abstract available for this thesis. / Department of Physics and Astronomy

Cataclysmic variable stars (CVs) are interacting binary systems. One of the stars (referred to as the primary) is a white dwarf, the other (referred to as the secondary) is usually a late main sequence star such as a red dwarf. Due to the closeness of the two stars, the white dwarf accretes gasses from the secondary. If the white dwarf does not possess a strong magnetic field, these gasses go into orbit, forming an accretion disk around the primary. If the white dwarf does possess a strong magnetic field, the gasses cannot form an accretion disk because they are entrained by the magnetic field lines. Cataclysmic variable stars in which the magnetic field is strong enough to prevent the formation of the accretion disk are called AM Herculis-type systems, after their prototype. In this study, the time-resolved spectroscopy of two AM Herculis-type binary systems, QQ Vul and EF Eri, are analyzed. In addition, Doppler Tomography, an analysis technique previously applied primarily to cataclysmic variable stars with accretion disks, is applied to these systems. / Department of Physics and Astronomy

There is no abstract available for this thesis. / Department of Physics and Astronomy