The dynamical evolution of substructure within dark matter halos is of central importance in determining many aspects of galaxy formation and galaxy evolution in cold dark matter cosmologies. The overall sequence in which the different stellar components of galaxies are assembled, the survival of galactic disks, the number of dwarf satellites orbiting giant galaxies, and the nature of stellar material in galactic halos all depend on the dynamics of halo substructure. In this thesis, I develop an analytic description of the evolution of substructure within a dark matter halo, and use it to construct a semi-analytic model of the formation and evolution of disk galaxies. Substructure within an individual halo is modelled as a set of distinct subhalos, orbiting in a smooth background. These subhalos evolve through three main processes: dynamical friction, tidal mass loss, and tidal heating. By including analytic descriptions of these three processes explicitly in a simple orbital integration scheme, it is possible to reproduce the results of high-resolution numerical simulations at a fraction of the computational expense. The properties of a subhalo can be estimated with an accuracy of 20%, until it has lost most of its mass or been disrupted. Using this description of satellite dynamics, I construct a semi-analytic model for the evolution of a galaxy or cluster halo. I show that this model reproduces the basic features of numerical simulations, and use it to investigate two major problems in current galaxy formation scenarios: the prediction of excessive substructure in galaxy halos, and the survival of galactic disks in halos filled with substructure. I show that the small number of dwarf galaxies observed in the Local Group can be explained by considering the effects of reionisation on star formation in small halos. The stellar luminosities predicted in this case match the observed luminosities of local satellites. The predicted spatial distribution, sizes and characteristic velocities of dwarf galaxies are also consistent with those observed locally. Many of these satellite galaxies are disrupted by tidal stripping or encounters. I investigate the properties of their debris, and show that its total mass and spatial distribution are similar to those of the stellar halo of the Milky Way. Furthermore, the stars in this debris are mainly old, satisfying another observational constraint on models of galaxy formation. Some satellites have been disrupted fairly recently, however, suggesting that coherent tidal streams may still be visible at the present day. Finally, I investigate the effects of encounters on the central disk within the main halo. I find that the rate of disruptive encounters drops off sharply after the galaxy is assembled, such that the typical disk has remained undisturbed for the past 8–10 billion years. Less disruptive encounters are more common, and disks are often heated as they re-form after their last disruption, producing components like the thick disk of the Milky Way. These results may resolve the long-standing uncertainty about disk ages in hierarchical, cold dark matter cosmologies. It is less clear whether the bulge-to-disk mass ratios predicted by the model, for the currently favoured LCDM cosmology, are consistent with observations. The relative mass of the bulge in typical disk galaxies may place an upper limit on the age of their stellar contents. / Graduate

Class I protostars are thought to represent an early stage in the lifetime of protoplanetary disks, when they are still embedded in their natal envelope. Here we measure the disk masses of 10 Class I protostars in the Taurus Molecular Cloud to constrain the initial mass budget for forming planets in disks. We use radiative transfer modeling to produce synthetic protostar observations and fit the models to a multi-wavelength data set using a Markov Chain Monte Carlo fitting procedure. We fit these models simultaneously to our new Combined Array for Research in Millimeter-wave Astronomy 1.3 mm observations that are sensitive to the wide range of spatial scales that are expected from protostellar disks and envelopes so as to be able to distinguish each component, as well as broadband spectral energy distributions compiled from the literature. We find a median disk mass of 0.018 M-circle dot on average, more massive than the Taurus Class II disks, which have median disk mass of similar to 0.0025 M-circle dot. This decrease in disk mass can be explained if dust grains have grown by a factor of 75 in grain size, indicating that by the Class II stage, at a few Myr, a significant amount of dust grain processing has occurred. However, there is evidence that significant dust processing has occurred even during the Class I stage, so it is likely that the initial mass budget is higher than the value quoted here.

M.Ing. (Electrical and Electronic Engineering) / During the past decade the optical disc has become increasingly popular. Write-once optical recording systems will mainly be used in data storage systems in which archival aspects or mass storage requirements prevail. In write-once optical data storage one is faced with an asymmetry between marks and non-marks due to a practical lower limit of the mark size. In some optical fibre communications there is also an asymmetry present in injection lasers and it may be feasible to use asymmetrical codes. In this study information theoretical methods are used to find values of channel capacity for sequences complying with binary asymmetrical runlength constraints. Different coding methods are used to construct encoders and decoders for generating and decoding these sequences with high values of efficiency. The power spectra of maxentropic binary asymmetrical runlength limited sequences complying with different runlength constraints are also investigated.

A new "circuit" model is developed to study non-linear effects in holography. The model links classical signal detection theory to holography, for it clearly shows that the reconstructed images in a thin hologram can be computed from mathematical formulas obtained for the output of non-linear detectors. In preparation for hologram analysis, the results for the (time-) autocorrelation of electrical ʋth-law devices in response to signal plus noise are extended to memoryless non-linear detectors with arbitrary characteristics. Mathematical parallels are next established between holography and the non-linear detection of signals, and these are incorporated in the model. The ready-made formulas for electrical detectors apply directly to give formulas for the (space-)autocorrelation of holograms of diffuse objects. The autocorrelation function predicts distorted multi-pie images, their relative positions, orientations, widths, and strengths. The multiplicity of images is due to the generation of harmonics; the background halo-like noise components added to the faithful images are mainly due to the multiplicity of the first harmonic itself. The analysis is further expanded to include a still more general case in which the reflected light from the object is depolarized. A decrease in signal-to-noise ratio (decrease in fringe visibility); a loss of information, and an augmentation of non-linear distortion could be expected. A simple way of reducing these effects is suggested. Finally, holography with more than one reference beam is studied. An exact formulation is given for double-reference-beam holograms. For multiple-reference-beam holograms, only approximate closed forms are presented. It is shown that the addition of extra reference beams during the recording step of the hologram may amplify the faithful images without increasing their background noise significantly. Experiments confirm the theoretical expectations. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

We present new Atacama Large Millimeter/sub-millimeter Array (ALMA) 1.3 mm continuum observations of the SR 24S transition disk with an angular resolution less than or similar to 0'.18 (12 au radius). We perform a multi-wavelength investigation by combining new data with previous ALMA data at 0.45 mm. The visibilities and images of the continuum emission at the two wavelengths are well characterized by a ring-like emission. Visibility modeling finds that the ring-like emission is narrower at longer wavelengths, in good agreement with models of dust-trapping in pressure bumps, although there are complex residuals that suggest potentially asymmetric structures. The 0.45 mm emission has a shallower profile inside the central cavity than the 1.3 mm emission. In addition, we find that the (CO)-C-13 and (CO)-O-18 (J = 2-1) emission peaks at the center of the continuum cavity. We do not detect either continuum or gas emission from the northern companion to this system (SR 24N), which is itself a binary system. The upper limit for the dust disk mass of SR 24N is less than or similar to 0.12 M-circle plus, which gives a disk mass ratio in dust between the two components of M-dust,M-SR 24S/M-dust,M-SR 24N greater than or similar to 840. The current ALMA observations may imply that either planets have already formed in the SR 24N disk or that dust growth to millimeter sizes is inhibited there and that only warm gas, as seen by rovibrational CO emission inside the truncation radii of the binary, is present.

Using Keck/HIRES spectra (Delta v similar to 7 km s(-1)) we analyze forbidden lines of [O I] 6300 angstrom, [O I] 5577 angstrom. and [S II] 6731 angstrom. from 33 T Tauri stars covering a range of disk evolutionary stages. After removing a high-velocity component (HVC) associated with microjets, we study the properties of the low-velocity component (LVC). The LVC can be attributed to slow disk winds that could be magnetically (magnetohydrodynamic) or thermally (photoevaporative) driven. Both of these winds play an important role in the evolution and dispersal of protoplanetary material. LVC emission is seen in all 30 stars with detected [O. I] but only in two out of eight with detected [S. II], so our analysis is largely based on the properties of the [O. I] LVC. The LVC itself is resolved into broad (BC) and narrow (NC) kinematic components. Both components are found over a wide range of accretion rates and their luminosity is correlated with the accretion luminosity, but the NC is proportionately stronger than the BC in transition disks. The full width at half maximum of both the BC and NC correlates with disk inclination, consistent with Keplerian broadening from radii of 0.05 to 0.5 au and 0.5 to 5 au, respectively. The velocity centroids of the BC suggest formation in an MHD disk wind, with the largest blueshifts found in sources with closer to face-on orientations. The velocity centroids of the NC, however, show no dependence on disk inclination. The origin of this component is less clear and the evidence for photoevaporation is not conclusive.

We develop a simple model to predict the radial distribution of planetesimal formation. The model is based on the observed growth of dust to millimeter-sized particles, which drift radially, pile-up, and form planetesimals where the stopping time and dust-to-gas ratio intersect the allowed region for streaming instability-induced gravitational collapse. Using an approximate analytic treatment, we first show that drifting particles define a track in metallicity-stopping time space whose only substantial dependence is on the disk's angular momentum transport efficiency. Prompt planetesimal formation is feasible for high particle accretion rates (relative to the gas, (M) over dot(p)/(M) over dot greater than or similar to 3 x 10(-2) for alpha = 10(-2)), which could only be sustained for a limited period of time. If it is possible, it would lead to the deposition of a broad and massive belt of planetesimals with a sharp outer edge. Numerically including turbulent diffusion and vapor condensation processes, we find that a modest enhancement of solids near the snow line occurs for centimeter-sized particles, but that this is largely immaterial for planetesimal formation. We note that radial drift couples planetesimal formation across radii in the disk, and suggest that considerations of planetesimal formation favor a model in which the initial deposition of material for giant planet cores occurs well beyond the snow line.

Debris disks are a fundamental component of exoplanetary systems. Understanding their relationship with host stars and neighboring planets can help contextualize the evolution of exoplanetary systems. In order to further that goal, this thesis addresses some extreme outlier examples of debris disk systems. First, the highly asymmetric debris disk around HD 111520 is resolved and analyzed at multiple wavelengths to create a self-consistent model of the disk thermal emission and scattered light. The best-fit model is proposed to be an asymmetric disk from a recent collision of large, icy bodies on one side of the disk. In contrast, most debris disks are thought to be in a steady collisional cascade and this disk model could represent a relatively rare event in the creation of debris disks. Secondly, an optical spectroscopic survey of stars is conducted on stars where far-infrared observations exist to detect the presence of debris disks. Specifically, AF-type stars are targeted in order to provide context regarding the Lambda Boo phenomenon, where stars are found to be specifically refractory metal-poor. One mechanism for this was hypothesized to be from planetary scattering of debris disks, causing the accretion of volatiles from comets. The findings were that over the entire unbiased sample, stars which were refractory metal poor tended to be the stars with brightest debris disks. This supports a planet-disk hypothesis underlying the accretion of volatile gases, since debris disks undergoing active planetary stirring are brighter. This would mean about 13\% of stars with debris disk are undergoing strong planetary scattering based on the occurrence rate of Lambda Boo stars relative to debris disk stars. These two tacks in our observational understanding of these extreme examples of debris disks provide constraints on the volatility at work. / Graduate

Context. Whether or not a self-gravitating accretion disk fragments is still an open issue. There are many different physical and numerical explanations for fragmentation, but simulations often show a non-convergent behavior for ever better resolution. Aims. We aim to investigate the influence of different numerical limiters in Godunov type schemes on the fragmentation boundary in self-gravitating disks. Methods. We have compared the linear and non-linear outcomes in two-dimensional shearingsheet simulations using the VANLE ER and the SUPERBEE limiter. Results. We show that choosing inappropriate limiting functions to handle shock-capturing in Godunov type schemes can lead to an overestimation of the surface density in regions with shallow density gradients. The effect amplifies itself on timescales comparable to the dynamical timescale even at high resolutions. This is exactly the environment in which clumps are expected to form. The effect is present without, but scaled up by, self-gravity and also does not depend on cooling. Moreover it can be backtracked to a well known effect called oversteepening. If the effect is also observed in the linear case, the fragmentation limit is shifted to larger values of the critical cooling timescale.

An important part of automobile safety is the braking system. Disc brakes have been widely used in automobiles for sped retardation for over 30 years. During that time, they have developed from a simple disc to a complex disc with channels, vanes, holes and grooves. The stopping capability of disc brakes is affected by the rate at which heat is dissipated by forced convection and the thermal capacity of the rotor. Catastrophic failure of brake rotors can occur during rapid increases or decreases in rotor temperature where regions of high temperature gradients result in high thermal strains. There is little information in the public domain regarding the relative merits of different disc brake rotor geometries, particularly in terms of airflow patterns, heat transfer rates, and internal thermal gradients. The aim of this research project was to investigate how geometrical variations affect the thermal performance of bi-directional disc brake rotors, particularly for high performance applications. Dynamometer testing showed that respectable increases in braking performance are achievable with relatively simple machining modifications. Tuft and smoke visualization techniques provided a preliminary understanding of the airflow in the passages of three distinct bi-directional rotor designs. Particle Image Velocimetry was used for detailed flow measurements which supported the numerical simulations. Computational Fluid Dynamics was used to predict the airflow and heat transfer associated with eight bi-directional brake rotor designs. The results show that 'pillared' passage designs can achieve higher heat transfer rates than traditional straight radial vane designs and that the heat loss from pillared rotors is generally more uniform than from vaned rotors. Subsequent conjugate heat transfer simulations found that temperature gradients inside pillared rotors are typically lower than inside vaned rotors. Thus failure rates due to excessive thermal strain are expected to be lower for pillared rotors. It was shown that rotor selection based solely on heat transfer rates is inappropriate and different passage designs are suited to different applications. The findings of this research will directly benefit local disc brake manufacturers, who do not have resources to conduct thorough studies comparing the thermal characteristics of different brake rotor designs.