No description available.

This thesis investigates the response of a circumstellar accretion disc to the fly-by of a perturbing mass on a parabolic orbit, both with and without the presence of dissipation. Observed protostellar number densities and velocity dispersions imply that collisions between protostellar systems are likely during the pre-main sequence phase of their evolution. Observations of the Orion Nebula have revealed 'truncated' discs and with this in mind the post-encounter material distributions are examined and are found to be promising candidates for the observed systems. Finally, the isolated evolution of the disc remnants are studied with a view to studying the motions of 'precessing' or 'wandering' jets.

In this thesis we investigate a number of roles that scalar fields can play in cosmology. In particular it is generally believed that the early Universe underwent a period of very rapid expansion. We call this epoch <i>inflation</i>. Initially we investigate the evolution of two slow-rolling scalar fields with potentials of the form <i>V = V_oφ</i>^-^a exp(-<i>bφ^m</i>). By considering different values of the parameters a, b and <i>m</i>, we drive several new inflationary solutions in which one field just evolves in the background and is not important for the inflationary dynamics. In addition, we find new solutions where both fields are dynamically important during inflation. Moreover, we discuss the evolution of perturbations in both the scalar fields and the spacetime metric, concentrating on the production of entropy perturbations. We find that for a large region in parameter space and initial conditions, tensor modes are negligible, and that adiabatic and isocurvature perturbations are essentially uncorrelated. We move on to consider perturbations in the Randall-Sundrum braneworld scenario. At energies higher than the brane tension, the dynamics of a scalar field rolling down a potential are modified compared with the predictions of General Relativity. These modifications imply, among other things, that steeper potentials can be used to drive an epoch of slow-roll inflation. The evolution of entropy and adiabatic modes during inflation driven by two scalar fields confined to the brane is studied. We show that the amount of entropy perturbation produced during inflation is suppressed compared to the predictions made by General Relativity. As a consequence, the initial conditions do not matter in multiple field inflation in braneworlds if inflation is driven at energies much higher than the brane tension. Following this, we study the evolution of slow-roll inflation in a five-dimensional braneworld model with two boundary branes and a bulk scalar field. Assuming that the inflationary scale is below the brane tension, we can employ the moduli space approximation to study the dynamics of the system. Detuning the brane tension results in a potential for the moduli fields which we show will not support a period of slow-roll inflation. We then study an inflation field confined to the positive tension brane, to which the moduli fields are non-minimally coupled. We discuss in detail the two cases of <i>V (χ) </i>= ½m² χ²</i> and <i>V (χ) </i>= l<i> χ⁴</i> and demonstrate that increasing the coupling results in spectra which are further away from scale-invariance. Furthermore, there is an increase in the tensor mode production, while entropy perturbations are subdominant. Finally, we point out that the five-dimensional spacetime is unstable during inflation because the negative tension brane collapses.

This thesis aims to study both the stellar and the gaseous components of a sample of GEHRs and analyse the possible mechanisms that govern these forming regions. As there are no giant HII regions in our Galaxy, GEHRs from our Local Group of galaxies provide us with the data needed to understand these regions. Much has been done in this direction, and I review the latest efforts and our current knowledge in Chapter 1. Chapter 2 is focused on the analysis of the stellar population of the object that can better help us to understand GEHRs: 30 Doradus, our closest and largest neighbouring starburst. I have performed spectral classification over 175 stars in the region which allowed me to study their spatial distribution and combine with new photometry to estimate the initial mass function (IMF) and the star formation history of the stellar cluster. The results indicate a normal IMF and the existence of three distinct bursts of star formation. I also discuss the evidence found towards and against mass segregation of stars in the cluster. I have used a subset of the mentioned spectroscopic data to study the kinematics of the stars in 30 Doradus in Chapter 3. From the data I found evidence of partial dynamical mass segregation supporting the findings of the previous chapter. Assuming the cluster is virialised, I have also estimated its dynamical mass, and found it to be several times larger than the one estimated photometrically. However, more and higher resolution spectra are needed to better constrain the final value. In Chapter 4 I investigate the effect that an underlying population of binary stars could introduce in the kinematical analysis mentioned above. I have used Monte Carlo techniques to construct an artificial population of binary stars, and simulated observations of such a population to measure its kinematical parameters.

We develop and present models within a semi-analytic merger tree based framework for the evolution of the brightest quasar population from redshifts z ~ 6 to the present, based on Eddington limited accretion on to supermassive black holes (SMBHs) and driven by galaxy mergers. We first demonstrate that even the simplest possible model can uncover constraints on the nature of quasar and SMBH evolution due to the rapid growth required to produce the bright high redshift quasars. This model is then gradually built up by sequentially adding in simplistic but plausible recipes for gas cooling, star formation and various modes of feedback with the aim not of producing a single highly-tuned fit to the observed evolution but rather of isolating and understanding the effects of these processes upon such evolution. Our final models are able to successfully reproduce the basic observed evolution of the brightest quasars, from the initial rise to the subsequent turnover in quasar numbers and then the sharp drop in numbers at the lowest redshifts, while simultaneously matching the universal cold gas density and star formation rate. The areas where our simplistic recipes fail are highlighted and we suggest several ways in which these problems might be rectified. The overall picture which emerges from our work is one where early rapid growth of SMBHs in galaxies is eventually slowed by self-regulation where feedback from the central source drives out further fuel and sets up the M - s relation. As galaxies then join groups and clusters at still later redshifts, however, further significant accretion is instead prevented by a suppression of gas cooling which is driven by some other mode of feedback. We lastly present preliminary results that suggest direct radiative heating by the central source is not an effective mechanism for this later mode of feedback and offer some alternatives.

No description available.

The thermal and mechanical structure of the lithosphere is the primary control on a planet’s geology, and is most commonly investigated using global or regional topography and gravity data. In this dissertation, the relationship between the two is used to investigate the lithosphere on the Moon and under the Earth’s ocean basins. Analysis of the relationship between gravity and topography on the nearside of the Moon, using line-of-sight accelerations from Doppler tracking of the Lunar Prospector spacecraft, indicates that <i>T_e</i> increased from less than 7 km to greater than 40 km between the time the oldest terrain formed and the most recent giant impacts. The increase is most simply interpreted in terms of the cooling and thickening of the lunar lithosphere, although the variation of <i>T_e</i>is not well-described using simple thermal history models. Modelling of sometimes sparse topographic measurements above large seamounts using grids of satellite-derived sea-surface gravity indicates that, in the Earth’s oceans, <i>T_e</i> coincides roughly with the depth to the 300 ± 100°C isotherm at the time of loading. However, there is considerable scatter, and the interpretation is complicated by viscous relaxation, breaks in the plate, and uncertainty about the density of the load. The cooling thermal plate model has been used for the last 30 years to describe the subsidence of the ocean floor. However, an analysis of the average seafloor depth as a function of age shows that, in the absence of thickened crust and dynamic topography generated by mantle convection, the plate model does not provide a good description of the average topography of the ocean floor at ages greater than approximately 85 million years. There is evidence for a slight, temporary swallowing between the ages of 85-130 million years in the Pacific and Atlantic oceans, which is consistent with the outcome of early numerical experiments on the instability of a cooling thermal boundary layer. Nevertheless, the thermal plate model, with a plate thickness of approximately 90 km, does appear to provide a good description of the average subsidence of, and heat flux through, the oldest sea floor. An analysis of the relationship between gravity and topography over dynamic swells and depressions in the Pacific Ocean, Atlantic Ocean, and Central Africa indicates that gravity and topography are highly coherent at intermediate and long wavelengths, and that the admittance does not vary significantly with wavelength. This is in contrast to the predictions of early theoretical calculations. The long-wavelength admittance over marine dynamic topography is 30 ± 5 mGal km^-1, and the admittance over dynamic topography in central Africa is approximately 40 mGal km^-1. An analysis of the results of asthenospheric seismic tomography under the Pacific Ocean, using a recently-published parameterisation of SV-velocity in terms of temperature and depth, indicates that the intermediate and long-wavelength gravity field does not result primarily from temperature anomalies within the lower part of the asthenosphere.

Understanding the evolution of massive stars and their deaths in core-collapse supernovae is of fundamental importance not only to stellar astrophysics, but has implications for other broader areas of astronomy. Massive stars are thought to be the main drivers of the chemical and dynamical evolution of galaxies through their strong stellar winds and their explosive deaths. Supernovae are principally separated into two categories, those without hydrogen (Type I) and those with (Type II). The plateau subclass of Type II supernovae (SNe II-P) are thought to arise from the explosions of red supergiants (RSGs), which have initial masses greater than 8-10 M° and have retained their hydrogen envelopes before core collapse. Until the discovery of the 8 M° red supergiant (RSG) that exploded as SN 2003gd, there had been no direct confirmation that SNe II-P did indeed arise from the explosion of RSGs. Before this detection there had been only two other unambiguous detections of Type II progenitors, neither of which fitted the evolutionary scenario that is commonly accepted. These were the progenitors of the peculiar Type II SN 1987A (Sk —69°202), which was a blue supergiant (BSG), and the Type IIb SN 1993J that arose in a massive interacting binary system. There is at the moment only a handful of well studied and documented SNe II-P. The best observed supernova of this class to date is SN 1999em, which was followed for over 600 days at many different wavelength ranges. Because of the questions that still surround the progenitors of supernovae, it is of vital importance to understand the nature of the supernovae themselves as well as to compare them with the observed progenitor masses. This thesis tells the tales of two SNe II-P, 2003gd and 2004A, which have unambiguous detections of their progenitors with masses of 8 and 9M°, respectively. SN 2003gd is of particular importance because of the question of its nature. It was found to have a lower tail luminosity than is normal for SNe II-P, indicative of a lower ejected nickel mass. However, the supernova does not belong to the proposed group of low-luminosity supernovae, which could either be the result of the low-energy explosion of massive or low-mass stars. In the high-mass model the collapsing core forms a black hole and a significant amount of fallback of material occurs, giving the low-luminosity and low-nickel mass. Instead, SN 2003gd appears to be a member of a continuous heterogeneous group of SNe II-P. This thesis also includes a study of Sher 25, a curious BSG in NGC 3603, which has an impressive emission line nebula in the form of an apparent circumstellar ring and bipolar nebulae. The appearance of the nebula of Sher 25 bears a striking similarity to the rings around the remnant of SN 1987A, which were almost certainly there before the supernova exploded.

No description available.

The astrophysical content of this thesis is based on radio observations of ten extended radio sources. Each source was observed at several different frequencies. Since this type of study had not been widely attempted in the past, the first chapters give the preliminary instrumental and technical developments needed. After an introductory chapter, the improvements made to the geometry of the One-Mile telescope are given. The best method of making polarization maps with the same telescope is described in Chapter 3. Since much of the project involved comparing maps at different resolutions, methods were derived to convolve maps to a common resolution (Chapter 4). Chapters 5-11 form the astrophysical section of the thesis. Maps of the radio galaxy 3C 452 at four frequencies are given in Chapter 5. The spectral distribution across the source was found to be consistent with that expected from a simple model of synchrotron losses in the radiating electrons. Extensive mapping of the large radio galaxy 3C 31 is presented in Chapter 6, together with the spectral and polarization properties of the source. Remarkable jets were found and the directions of the magnetic fields within them were determined. Several models of jets could be ruled out by these results. Chapters 7-11 contain results from a systematic study of six powerful radio galaxies and one quasar (3C 47, 79, 219, 234, 300, 382, 430). Total intensity and polarization maps were made of each source at two frequencies and the compact components were mapped with a much higher resolution. The magnetic field structure in the extended regions of the sources was modelled using a Monte Carlo procedure and the existence of large scale components to the magnetic fields was inferred. The thermal electron densities within the sources were also derived and maps of the magnetic field direction are given for five of the sources. The spectral distributions across each source were also obtained and evidence for re-acceleration of relativistic electrons in the extended source components was found. The implications of the results from the seven sources for the various source models are considered.