Protoplanetary discs set the initial conditions for planet formation. By combining observations with detailed modelling, it is possible to constrain the physics and chemistry in such discs. I have used the detailed thermo-chemical disc model ProDiMo to explore the characteristics of the gas in protoplanetary discs, particularly in Herbig Ae objects. I have assessed the ability of various observational data to trace the disc properties. This has involved a number of different approaches. Firstly I compute a series of disc models with increasing mass, in order to test the diagnostic powers of various emission lines, in particular as gas mass tracers. This approach is then expanded to a large multiparameter grid of ~ 10 5 disc models. I have helped to develop a tool for analysing and plotting the huge quantity of data presented by such a model grid. Following this approach I move on to a detailed study of the Herbig Ae star HD 163296, attempting to fit the large wealth of available observations simultaneously. These include new Herschel observations of the far-infrared emission lines, as well as interferometric CO observations and a large number of continuum data. This study addresses the topical issues of the disc gas/dust ratio, and the treatment of the disc outer edge. It explores the effects of dust settling, UV variability and stellar X-ray emission on the disc chemistry and line emission. There is possible evidence for gas-depletion in the disc of HD 163296, with the line emission enhanced by dust settling, which would indicate a later evolutionary stage for this disc than suggested by other studies. Finally, I work to improve the treatment of the gas heating/cooling balance in ProDiMo, by introducing a non-LTE treatment of the atomic hydrogen line transitions and bound-free continuum transitions. I explore the effects of this on the disc chemical and thermal structure, and assess its impact in terms of the observable quantities.

Stars are likely to form with non-zero initial angular momentum, and will consequently possess a substantial gaseous protostellar disc in the early phases of their evolution. At this early stage, the disc mass is expected to be comparable to the mass of the protostar. The disc’s self-gravity therefore plays an important role in the subsequent evolution of the system, regulating the accretion of matter onto the protostar, as well as being potentially capable of forming low mass stars and massive planets by disc fragmentation. The protostellar disc may later evolve into a protoplanetary disc, providing the feedstock for planet formation. Therefore, if the current stellar populations and exoplanetary systems are to be understood, an understanding of the evolution of protostellar discs is crucial, especially their earliest self-gravitating phases. I have used various methods of numerical simulation to probe the physics of self-gravitating protostellar discs and their constituents. When constructing a model for self-gravitating protostellar discs, including detailed thermodynamics and radiative transfer is essential. I have developed two distinct numerical techniques for incorporating radiative transfer into Smoothed Particle Hydrodynamics (SPH) simulations. The first allows the modelling of frequency-averaged radiative transfer during the SPH simulation, in effect approximating radiative SPH (RSPH) with only a marginal increase in runtime (around 6%). The second takes the output from SPH simulations, and creates synthetic, wavelength-dependent telescope images and spectra of SPH systems. This allows the direct construction of observables from SPH simulations, providing, for the first time, a direct connection between the output of SPH simulations and observations. I have used these numerical methods to analyse, in detail, the local angular momentum transport induced by self-gravity in protostellar discs, testing the robustness of the “pseudo-viscous” analytical approximation for local disc stresses. I confirm that semi-analytical disc modellers are justified in using the pseudo-viscous approximation in some cases, but I also outline the limits in which non-local transport effects causes the approximation to fail. Also, I have investigated the evolution of protostellar discs when perturbed by a secondary companion, in particular identifying whether such events will in general trigger a) a disc fragmentation event, or b) a stellar outburst event. For case a), I found no significant evidence that perturbation by a companion improves the possibility of disc fragmentation in compact discs - in case b), I found that stellar outburst events do indeed occur, but they are unlikely to be seen by observers due to their rare occurrence, as well as due to self-obscuration effects.

The presence of a giant planet in a low-viscosity disc can create a gap edge in the disc's radial density profile sharp enough to excite the Rossby wave instability. This instability may evolve into dust-trapping vortices that might explain the `banana-shaped' features in recently observed asymmetric transition discs with inner cavities. Previous hydrodynamical simulations of planet-induced vortices have neglected the time-scale of hundreds to thousands of orbits to grow a massive planet to Jupiter size. In this work, we study the effect of a giant planet's runaway growth time-scale on the lifetime and characteristics of the resulting vortex. For two different planet masses (1 and 5 Jupiter masses) and two different disc viscosities (alpha= 3 x 10-4 and 3 x 10-5), we compare the vortices induced by planets with several different growth time-scales between 10 and 4000 planet orbits. In general, we find that slowly-growing planets create significantly weaker vortices with lifetimes and surface densities reduced by more than 50 per cent. For the higher disc viscosity, the longest growth time-scales in our study inhibit vortex formation altogether. Additionally, slowly-growing planets produce vortices that are up to twice as elongated, with azimuthal extents well above 180. in some cases. These unique, elongated vortices likely create a distinct signature in the dust observations that differentiates them from the more concentrated vortices that correspond to planets with faster growth time-scales. Lastly, we find that the low viscosities necessary for vortex formation likely prevent planets from growing quickly enough to trigger the instability in self-consistent models.

MMed, Ophthalmology,Faculty of Health Sciences, University of the Witwatersrand, 2009 / Objectives To determine in a cohort of 69 African patients with large optic discs and large optic cups, that proportion of patients with physiologic cupping (normal eyes) misdiagnosed as glaucomatous. To evaluate the possible relationship between optic disc size and central corneal thickness. Design and method A case series of 69 patients with large discs (vertical disc height measuring 1.8mm) and large cups (vertical cup to disc ratio 0.6) was evaluated to determine what proportion had glaucoma and what proportion was normal. Patients categorized as normal were further evaluated to determine what proportion were previously misdiagnosed and treated for glaucoma. Patients with a suspected diagnosis of glaucoma, normal tension glaucoma or primary open angle glaucoma were recruited from the glaucoma clinic at St John Eye Hospital. Outcome measures included corrected vertical disc height (VDH), vertical cup to disc ratio (CDR), central corneal thickness (CCT), the relationship between VDH and vertical cup height, the relationship between VDH and CCT, adjusted intraocular pressure (A-IOP), retinal nerve fiber layer analysis and visual fields. vii Results Sixty-nine African patients (138 eyes) with large discs and large cups were evaluated. 41 (59%) were females and 28 (41%) were males. The mean age was 56 years. Of the 69 patients, 51 (74%) had physiologic cupping (normal eyes) and 18 (26%) patients were glaucomatous. Of the group of 51 patients with physiologic cupping, there were 9 patients who were previously misdiagnosed with glaucoma and who had received treatment. VDH ranged between 1.9 and 3.2mm (mean ±SD, 2.3±0.26mm). The distribution analysis of VDH measurements noted the largest cluster around 2.3mm. CCT ranged between 454μm and 618μm (mean±SD, 516±37μm). 107 (77.5%) of the 138 eyes had CCT < 544μm. Conclusion Large cup to disc ratio in relation to large disc size can be normal. It can be misdiagnosed as glaucomatous if objective retinal nerve fiber layer analysis is not carried out. In this study, 9 (18%) patients from a group of 51 patients with physiologic cupping were misdiagnosed as glaucomatous. There was no linear correlation between CCT and VDH in this study. Pearson’s correlation coefficient was 0.13. The majority (77.5%) of eyes had thin corneas (CCT < 544μm).

No description available.

As the number of known exoplanetary systems has grown, it has become increasing apparent that our current understanding of planet formation is insufficient to explain the broad but distinct distributions of planets and planetary systems we observe. In particular, constructing a coherent model of planetary formation and migration within a circumstellar disc which is capable of producing both hot Jupiters or Solar System-like planetary system is high challenging. Resolved observations of where planets form and how they influence their parent discs provides essential information for tackling this important question. A promising technique for detecting close-in companions is Sparse Aperture Masking (SAM). The technique uses a mask to transform a single aperture telescope into a compact interferometric array capable of reliably detecting point sources at the diffraction limit or closer to a bright star with superior contrasts than extreme AO systems at the cost of smaller fields of view. Applying image reconstruction techniques to the interferometric information allows an observer to recover detailed structure in the circumstellar material. In this thesis I present work on the interpretation of SAM interferometry data on protoplanetary discs through the simulation of a number of scenarios expected to be commonly seen, and the application of this technique to a number of objects. Analysing data taken as part of a SAM survey of transitional and pre-transitional discs using the Keck-II/NIRC2 instrument, I detected three companion candidates within the discs of DM\,Tau, LkH\alpha\,330, and TW\,Hya, and resolved a gap in the disc around FP\,Tau as indicated by flux from the disc rim. The location of all three of the companions detected as part of the survey are positioned in interesting regions of their parent discs. The candidate, LkH\alpha\,330\,b is a potentially cavity opening companion due to its close radial proximity to the inner rim of the outer disc. DM\,Tau\,b is located immediately outside of a ring of dusty material largely responsible for the NIR comment of the disc SED, similar to TW\,Hya\,b located in a shallow gap in the dust disc outside another ring of over-dense dusty material which bounds a deep but narrow gap. Both of these companion candidates maybe migrating cores which are feeding from the enriched ring of material. I conducted a more extensive study of the pre-transitional disc, V1247\,Ori, covering three epochs and the H-, K- and L-wavebands. Complementary observations with VLT/SPHERE in H\alpha and continuum plus SMA observations in CO (2-1) and continuum were performed. The orientation and geometry of the outer disc was recovered with the SMA data and determine the direction of rotation. We image the inner rim of the outer disc in L-band SAM data, recovering the rim in all three epochs. Combining all three data sets together we form a detailed image of the rim. In H- and K-band SAM data we observe the motion of a close-in companion candidate. This motion was found to be too large to be adequately explained through a near-circular Keplerian orbit within the plane of the disc around the central star. Hence an alternate hypothesis had to be developed. I postulated that the fitted position of the companion maybe influenced by the emission from the disc rim seen in the L-band SAM data. I constructed a suite of model SAM data sets of a companion and a disc rim and found that under the right conditions the fitted separation of a companion will be larger than the true separation. Under these conditions we find the motion of the companion candidate to be consistent with a near-circular Keplerian orbit within the plane of the disc at a semi-major axis of \sim6\,au. The H\alpha data lack the necessary resolution to confirm the companion as an accreting body, but through the high contrast sensitivities enabled by the state of the art SPHERE instrument I was able to rule out any other accreting body within the gap, unless deeply embedded by the sparse population of MIR emitting dust grains previously inferred to reside within the gap. Through the combination of SAM and SMA data we constrain the 3-D orientation of the disc, and through multi-wavelength SAM observation identify a close-in companion potentially responsible for the gap clearing and asymmetric arm structures seen in previous observations of this target. During my PhD I have contributed to the field of planet formation through the identification of four new candidate protoplanets observed in the discs of pre-main sequence stars. To do so I have quantified the confidence levels of companion fits to SAM data sets and formed synthetic data from models of asymmetric structures seen in these discs. I have described for the first time the effects of extended sources of emission on the fitted results of companion searches within interferometric data sets. I have combined SAM data sets from two separate telescopes with different apertures and masks to produce reconstructed image of an illuminated disc rim with superior uv-coverage. I have used the expertise I have developed in this field to contribute to a number of other studies, including the study of the young star TYC\,8241\,2652\,1, resulting in the rejection of a sub-stellar companion as the cause of the rapid dispersal of the star`s disc. The companion candidates I have identified here should be followed up to confirm their presence and nature as accreting protoplanets. Objects such as these will provide the opportunity for more detailed study of the process of planet formation in the near future with the next generation of instruments in the JWST and E-ELT.

Doctor of Philosophy / We investigate the linear growth and vertical structure of the magnetorotational instability (MRI) in weakly ionised, stratified accretion discs. The magnetic field is initially vertical and perturbations have vertical wavevectors only. Solutions are obtained at representative radial locations from the central protostar for different choices of the initial magnetic field strength, sources of ionisation, disc structure and configuration of the conductivity tensor. The MRI is active over a wide range of magnetic field strengths and fluid conditions in low conductivity discs. For the minimum-mass solar nebula model, incorporating cosmic ray and x-ray ionisation and assuming that charges are carried by ions and electrons only, perturbations grow at 1 AU for B < 8G. For a significant subset of these strengths (200mG < B < 5 G), the growth rate is of order the ideal MHD rate (0.75 Omega). Hall conductivity modifies the structure and growth rate of global unstable modes at 1 AU for all magnetic field strengths that support MRI. As a result, at this radius, modes obtained with a full conductivity tensor grow faster and are active over a more extended cross-section of the disc, than perturbations in the ambipolar diffusion limit. For relatively strong fields (e.g. B > 200 mG), ambipolar diffusion alters the envelope shapes of the unstable modes, which peak at an intermediate height, instead of being mostly flat as modes in the Hall limit are in this region of parameter space. Similarly, when cosmic rays are assumed to be excluded from the disc by the winds emitted by the magnetically active protostar, unstable modes grow at this radius for B < 2 G. For strong fields, perturbations exhibit a kink at the height where x-ray ionisation becomes active. Finally, for R = 5 AU (10 AU), unstable modes exist for B < 800 mG (B < 250 mG) and the maximum growth rate is close to the ideal-MHD rate for 20 mG < B < 500 mG (2 mG < B < 50 mG). Similarly, perturbations incorporating Hall conductivity have a higher wavenumber and grow faster than solutions in the ambipolar diffusion limit for B < 100 mG (B < 10 mG). Unstable modes grow even at the midplane for B > 100 mG (B ~ 1 mG), but for weaker fields, a small dead region exists. When a population of 0.1 um grains is assumed to be present, perturbations grow at 10 AU for B < 10 mG. We estimate that the figure for R = 1 AU would be of order 400 mG. We conclude that, despite the low magnetic coupling, the magnetic field is dynamically important for a large range of fluid conditions and field strengths in protostellar discs. An example of such magnetic activity is the generation of MRI unstable modes, which are supported at 1 AU for field strengths up to a few gauss. Hall diffusion largely determines the structure and growth rate of these perturbations for all studied radii. At radii of order 1 AU, in particular, it is crucial to incorporate the full conductivity tensor in the analysis of this instability, and more generally, in studies of the dynamics of astrophysical discs.

<p> </p><p>Gas turbine discs are heavily loaded due to centrifugal and thermal loads and are therefore designed for a service lifetime specified in hours and cycles. New probabilistic design criteria have been worked out at Siemens Industrial Turbomachinery AB and this report is intended to evaluate if existing turbine discs meet the new design criteria. The evaluation is composed of two tasks, estimation of failure risk and investigation of which parameters that have large effect on the results.</p><p> </p><p>The outcome from the evaluations show that the failure risks are smaller than the maximum failure risks allowed in the design criteria. Further, creep strain rate, temperature and creep rupture strain are identified to have large effect on the results in the first case. In the second case blade load and other mechanical loads as well as yield stress show large effect on the results.</p><p> </p>

Thesis (Ph. D.)--Ohio State University, 2006. / Title from first page of PDF file. Includes bibliographical references (p. 178-197).

Gas turbine discs are heavily loaded due to centrifugal and thermal loads and are therefore designed for a service lifetime specified in hours and cycles. New probabilistic design criteria have been worked out at Siemens Industrial Turbomachinery AB and this report is intended to evaluate if existing turbine discs meet the new design criteria. The evaluation is composed of two tasks, estimation of failure risk and investigation of which parameters that have large effect on the results. The outcome from the evaluations show that the failure risks are smaller than the maximum failure risks allowed in the design criteria. Further, creep strain rate, temperature and creep rupture strain are identified to have large effect on the results in the first case. In the second case blade load and other mechanical loads as well as yield stress show large effect on the results.