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In the first three chapters we consider a Direc plane wave impinging upon a Schwarzschild black hole. The interaction is characterised by three quantities: the absorption cross section, the scattering cross section, and the partial polarization. If the wavelength λ of the incident wave is much smaller than the event horizon <i>r­_s</i>, these quantities may be approximated classically. If λ ≫ <i>r_s</i> then the black hole may be treated parturbatively, and these quantities may be approximated through a Born series expansion. In the intermediate regime, λ ~ <i>r_s, </i>a partial wave expansion is necessary. We consider all three regimes and present significant new results for absorption, scattering and Mott polarization. In the next chapter we show the existence of a spectrum of normalisable fermionic bound states on the Schwarzschild background. These states are the gravitational analogues of the hydrogenic electron orbitals. We compute their energy levels, decay times and wavefunctions, and discuss their potential significance. Two further chapters treat wave interaction with the rotating (Kerr) black hole. As a preliminary step, we review the theory of spin-weighted spheroidal harmonics and methods for their calculation. These functions are used to separate the Dirac field on the Kerr background into angular and radial parts. The radial equations are then solved numerically to find phase shifts and transmission coefficients. We extend the partial wave analysis to the rotating case, and present accurate scattering and absorption cross sections.  The effect of spin-rotation coupling on the cross sections is discussed. The final chapter is a study of fermionic emission from higher-dimensional rotating black holes. If theories with large extra dimensions are well-founded then black holes may arise at TeV energies. Black holes created through high-energy particle collisions would decay via the emission of standard model fields on the brane, and gravitons in the bulk. Their emission spectra would provide a clear signature of the underlying higher-dimensional spacetime. Motivated by this intriguing possibility, we apply numerical methods to compute the spectra and angular profiles of fermionic emission on the brane. We show that the net emission is greatly enhanced by the addition of extra dimensions; that the emission is concentrated in the plane of rotation; and that neutrinos are emitted preferentially into the southern hemisphere. Finally, the effect of fermionic emission on the evolution of a 5D black hole is considered in detail.

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The Cambridge Optical Aperture Synthesis Telescope, or COAST, was conceived by Baldwin and co-workers at the Cavendish Laboratory in 1987 as the simplest possible separated element array for optical closure phase imaging. At the outset if was not clear that the technical difficulties associated with co-phasing the multiple elements simultaneously could be overcome. In this dissertation I describe work which spans the transition between the "engineering and construction" phase and the "astronomical" phase of the COAST project. The contents of each Chapter are as follows: • In Chapter 2 we begin by considering the analysis of visibility-amplitude and closure-phase measurements, concentrating in particular upon the problem of model comparison. We develop a Bayesian framework for the problem, which we then use consistently throughout the remainder of the dissertation. • Chapter 3 is primarily concerned with the design and construction of the array and is therefore mainly a review. Also considered are the range of array configurations presenting available and the throughput of the optical beam-combination system. • Chapter 4 describes the practicalities of data acquisition at COAST and also discusses the estimation of visibility amplitudes and closure-phases from the raw data. The first angular diameter measurements from the array illustrate the quality of visibility amplitude data. • Chapter 5 is by far the most important in this work: the first true images from <I>any </I>separated-element optical array are presented. These represent the first proof that closure-phase imaging using a long-baseline optical interferometer is possible. • In Chapter 6 we consider observations of Betelgeuse which are important both technically and astrophysically. • Chapter 7 is concerned with observations of the Mira-type variable star R Leo. These revealed a pulsation-phase coherent variation in angular diameter at four wavelengths across two periods during 1996 and 1997.

This thesis describes the radio and optical follow-up of 176 sources taken from the 15-GHz 9^th Cambridge Survey. The initial motivation for this was to establish the radio source population at high frequency in the expectation that a significant fraction of the sources would prove to be young objects with spectra peaking at high (~ 5 – 20 GHz) frequencies. Simultaneous radio observations in the GHz to tens of GHz range have provided a continuum radio spectrum for each object and allowed them to be classified on the basis of their simultaneous spectral index. Between one fifth and one quarter of the sources in the flux-limited samples have spectra rising between 1.4 and 4.8 GHz – this is double the fraction of such sources found in surveys conducted at lower frequency. Observations at optical wavelengths are described and the results are analysed. Further radio follow-up at sub-arcsecond resolution has sought to measure the angular size of a sample of all objects, but this has largely failed to resolve the rising-spectrum sources. A 2-3 year study of the variability of the 9C sources at 15GHz has been carried out and the rising spectrum class of objects is seen to vary more than the flat- and steep-spectrum classes. In addition to the observational work, theoretical models predicting the evolution of infant radio sources are presented and used to make predictions about the observed fractions of rising-spectrum objects in surveys high frequency. The modelling results are found to be consistent with the observational data, all suggest that the rising-spectrum class is highly contaminated by objects which are not necessarily young, but instead owe their rising spectra to the effects of relativistic beaming.

No description available.

Star clusters are fundamental building blocks of galaxies, and yet very little is known about young star clusters as they evolve unseen in the embedded phase. Using four band photometry of star clusters in NGC 3256 we are able to determine the chance of survival for star clusters over the course of the embedded phase, and combined with other data sets show a relationship between survivability and the star formation rate density. We also perform a detailed study of the XUV-disc phenomenon and their cluster population. Firstly we measure the broad properties of several XUV-disc galaxies in comparison to normal galaxies using surface photometry and individual object photometry. We reveal that these galaxies show a range of properties, some showing sharp edges in their Hα profiles whilst others do not, although all show a smoothly declining UV profile which extends beyond the optical edge of the disc. The HII regions in these outer discs appear to have luminosities consistent with single ionising O-type stars and have implied mass which suggest that the formation of the most massive stars is governed by stochastic processes. We also examine the spectra of HII regions in two XUV-disc galaxies, NGC 4625 and NGC 3621. Both galaxies show a shallower abundance gradient in the outer disc compared to the inner disc, yet NGC 4625 also shows a discontinuity close to the optical edge. This is of particular interest as NGC 3621 shows no sharp Hα and no discontinuity whilst NGC 4625 displays both of these features. The HII regions of both of these galaxies show properties different to bright inner disc HII regions, making abundance calculations less reliable.

No description available.

The goal of this thesis is to address several issues in gravitational physics, in particular black hole physics, within a quantum mechanical framework. Of course this requires the existence of such a quantum mechanical theory of gravity. Superstring theory is a consistent perturbative theory of gravity. However, many aspects of the theory turn out to be non-perturbative and in this sense the theory is still under construction. The theory of strings contains a large variety of <I>p-</I>dimensional spatially extended objects, now called <I>p</I>-branes. These <I>p</I>-branes are dynamical objects embedded in space-time: they interact between each other, intersect each other and bound themselves together. In this thesis I shall study some of these <I>p</I>-brane configurations. I shall start by using supergravity which is a large distance (low energy) approximation to string theory, to construct many new <I>p</I>-brane configurations. The emphasis will be on the technics to construct the supergravity solutions describing these configurations and on the symmetries of the underlying theory that render their existence. I shall not attempt to give a complete classification of the <I>p</I>-brane configurations folklore nor shall I address issues related to the global structure of the corresponding gravitational geometry. In turn, the focus will be on the microscopic and quantum mechanical description of some of these <I>p</I>-brane configurations, namely the D-brane bound states. In particular, I shall study in detail a D-5 brane configuration with a constant magnetic field on its worldvolume. The gravitational counterpart is a five-dimensional black hole. The quantum mechanical description of this system will allow the calculation of the Bekenstein-Hawking entropy from counting microscopic states of the D-brane system. The scattering of a scalar particle off the black hole (and therefore Hawking radiation) are also studied. I end by making a proposal for an effective string description of black hole dynamics. In the process I shall use results in perturbative string theory, gauge theory and gravitational theory. In this sense, this thesis is rather interdisciplinary which is a remarkable characteristic acquired by superstring theory.

No description available.