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This thesis is concerned with the role of stellar encounters and collisions in dense clusters. In Chapter 1 I discuss basic collision cross sections and rates, and briefly discuss the cluster cores in which they occur. A short discussion of the history of stellar-collision calculations is then given, followed by an overview of the stellar exotica that may be produced in collisions between stars. In Chapter 2 stellar collisions involving luminous red-giant stars are investigated. Recent observations of the central parsec of the Galaxy reveal that highly-luminous giants are missing from the innermost 0.1 <I>pc</I>. The Galactic core contains a very dense stellar cluster, which suggests that the giants may have been destroyed in stellar collisions. Collision rates are calculated for such giants, and simulations between large red giants and other single stars are performed. In Chapter 3 the role of the stellar collisions in the formation of stars of masses > 10M⊙ is examined. Accretion of ambient gas by stars in young cluster cores give rise to multiple stellar collisions to occur. Mergers between the stars is expected to build up stars of masses > 10M⊙. This may provide a solution to the problem of massive-star formation, in which the radiation pressure of stellar cores of masses < 10M⊙ is sufficiently high to halt the infall of gas onto the core, thereby limiting the stellar mass. Simulations of distant encounters and physical collisions between young stars are performed in order to compute tidal-capture radii and the nature of merged objects. The findings are then used to recompute massive-star formation time-scales. In Chapter 4 collision rates involving binary stars are calculated. Stellar evolution models are analysed to examine which stellar species are most likely to undergo collisions. Simulations of encounter and collisions are then performed in order to compute tidal-capture radii, mass loss and merger radii between stars of various masses and for stars at various stages of evolution. The data produced is intended for use in simulations of stellar cluster evolution by N-body codes in which stellar evolution and close stellar encounters are included.

Firstly, we present reviews of the background atomic, magnetohydrodynamic, and solar, physics involved in the analysis of the problem. We proceed to produce a time dependent code to calculate the Ionisation state distribution of Iron and of Argon, using some of the recent data for Ionisation coefficients by Arnaud & Raymond (1992), and Fournier et al. (1997, 1998). We also look at some data for the excited ionised states using the Chianti database amongst others. Taking the engine developed, we continue by applying it to a three stage nanoflare simulation for the heating, spanning a 2000km range. This model is drawn from various previous models developed in the literature. We look for the indications of the non Local Thermodynamic Equilibrium (LTE) processes via the concept of "signature ions". By this we mean the presence of highly charged ions which would not be expected if the temperature were simply constant and everywhere was in LTE. We make a calculation of the actual experimental line of sight results that we would obtain, if we were to take Emission Measure calculations to ascertain the Ionisation distribution within the Solar Atmosphere. We also take a look at the problem of Diagnostics based on Emission Measure Analysis, and investigate the fundamental nature of the Inversion Problem. Simply stated, it is difficult to obtain the differential emission measure (DEM) of the Transition Region because the only information that we have available to us is the intensity of spectral lines emitted from the Sun. This consists of the integral of the DEM convolved with a function representing the physics of the transition. The problems associated with the inversion of the integral, form much of the discussion together with the philosophical implications of using phenomenology as an alternative.

This thesis is concerned with interannual variability in the stratosphere, in particular the tropical oscillation called the Quasi-Biennial Oscillation (QBO) and its interaction with the extratropical circulation. A three dimensional mechanistic model is used to investigate interannual variability, with a tropical QBO being forced in the model by either a relaxation scheme or a simple gravity wave parameterisation scheme. The tropospheric forcing of large-scale planetary waves is represented by imposing a geopotential perturbation at the artificial lower boundary of the model. The influence of the QBO on the extratropics is examined to see if there is a preferred mode of variability in the extratropics, and how the extratropical response to the QBO is affected by the strength of extratropical wave forcing in the model. The influence of other details of the QBO, specifically its phase relationships with the annual cycle, its height structure and its time structure are investigated. The correlation between the QBO and the extratropical circulation is seen to be consistent with the Holton-Tan mechanism for the extratropical QBO; this correlation is modulated by the amplitude of extratropical geopotential wave forcing. The extratropical dynamics are seen to affect phase preference relative to the annual cycle, irregularity in period and asymmetry between different phases of the QBO simulated in the three dimensional model. It is hypothesised that this effect comes about through the extratropical wave driving of a seasonally varying equatorial upwelling. This is investigated further in a one dimensional (equatorial) model and a simple zero dimensional model, by imposing the equatorial upwelling as a function of time. The one dimensional model exhibits phase preference, irregularity and asymmetry, consistent with the hypothesis, though cannot explain all equatorial features of the three dimensional model model. The occurrence of "stalled" descents in the parameterised QBO in the three dimensional model is noted and examined.

Chapter 1 describes the theory and previous observations of the S-Z effect, and its potential importance for determining the evolution of clusters and antecedents. In Chapter 2 I describe the detection with the RT of an S-Z decrement towards the quasar pair PC1643+4631A&B at <I>z</I> = 3.8, and also describe the optical and infrared followup to constrain the nature and distance of the cluster responsible; this work was shared with the authors of Jones et al. 1997 and Saunders et al. 1997, although additional work I have done is included. Deeper imaging proved necessary, and in Chapter 3 I detail the reduction I carried out of the broadband and narrowband imaging of the PC1643 field with the William Herschel Telescope. In Chapter 4, I examine the isophotal aperture catalogues produced with FOCAS and SExtractor to determine some of the limitations of the algorithms used, and assess the difficulties and systematic errors arising from the use of isophotal apertures. In Chapter 5, I present the results obtained using the catalogues made with FOCAS and SExtractor codes, including the identification of intermediate and high-redshift galaxy candidates selected using colour criteria. To further constrain the distributions of objects in the field of PC1643, I used the photometric redshift technique to produce estimated redshifts for objects in the field; both the methods used and results obtained are discussed in Chapter 6, which also covers the simulations I designed to examine the accuracy and problems involved in using photometric redshift estimates. I further determine that deep optical surveys are confusion-limited and discuss the effects this has on colour-selected samples. Chapter 7 reviews the findings of the thesis and suggests areas of further work.

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This thesis presents a new survey of the kinematics of clustered, low-mass star formation. The survey uses HARP, a new submillimetre heterodyne array at the James Clerk Maxwell Telescope (JCMT), to map the <i>J </i>= 3 → 2 rotational transitions of CO and its isotopologues ¹³CO and C¹⁸O across 630 sq. arcmin of the Perseus molecular cloud, in four regions: NGC1333, IC348, L1448 and L1455. An algorithm to account for variable calibration across the array is developed, which can be used to remove prominent artefacts in HARP data towards molecular clouds. A new catalogue of 850<i>μ</i>m SCUBA clumps is identified using two automated algorithms, c1find and GaussClumps. The clumps found are gravitationally bound with a mass distribution comparable to the stellar initial mass function. However, many previously reported properties heavily depend on the algorithm used to identify the clumps’ emission. The C¹⁸O <i>J = </i>3 → 2 linewidths of these clumps are supersonic, varying according to the clump age and environment and are smaller than predicted currently by gravoturbulent models. Localized velocity gradients are identified across many of the SCUBA clumps, which I interpret as signs of core rotation. The resultant angular monument distribution is consistent with the <i>smallest </i>momenta previously observed in similar studies. Clumps of C¹⁸O and ¹³CO gas are also identified using c1find, additional clumps with no continuum counterparts are not bound, but do obey Larson’s laws.

Computational models of Wolf-Rayet and other massive stars are presented at solar metallicity and at a range of non-solar metallicities from Z = 0.03 to Z = 0.0001. The effects of differing mass-loss rates, evolutionary paths and initial mass distributions are quantified and the resulting Wolf-Rayet star populations compared to observations in this and other galaxies. It is found that observations may be reproduced with lower (and more realistic) mass-loss rates than the ones currently used in the literature, with a number of exceptions. The enrichment from these stars is calculated and compared to that from other sources of CNO. It is found that, whilst nitrogen originates mainly from intermediate-mass stars and oxygen from type II supernovae as expected, the origin of carbon is not clear-out and depends on a variety of sources. The effect of input parameters such as the adopted AGB yields and the binary fraction are discussed. Binary star models are presented and compared to observation. It is found that the majority of Wolf-Rayet stars at low metallicities are those which originate from mass transfer in binary systems, but that Wolf-Rayet CNO enrichment at all metallicities is still primarily from the single stars. Finally, a further use for the set of massive star models evolved here is considered - their use as gamma-ray burst progenitor models. It is found that the expected circumstellar mass distribution around a Wolf-Rayet star at the end of its lifetime may help to explain some features of gamma-ray burst afterglow light curves.

This work describes several self-contained pieces of research which are all linked by their connection with optical interferometry. First is an instrumentational project involving the selection, installation and subsequent testing of second-generation optical detectors for the Cambridge Optical Aperture Synthesis Telescope (COAST). These new detectors extended the magnitude limit of COAST by over a magnitude in the I band, increasing its range of observable astronomical targets and making it the most sensitive separate-element interferometer in this band. Also described were two observational projects. The first was carried out using COAST and was made possible by the improved sensitivity of the instrument after installation of the new detectors. It involved observations of two Be stars, Gamma Cassiopeiae and Zeta Tauri, in a narrow Hα band. Non-zero closure phases were measured in both cases, indicating deviations from axisymmetry in the circumstellar discs of the two stars. The first true map of Gamma Cassiopeiae was presented. The second observational project involved measurements of the proto-planetary nebula IRC+10216 made with the UK InfraRed Telescope (UKIRT), using the interferometric technique of speckle masking to achieve close to diffraction-limited imaging. The results include the first ever K-L map and K-band polarisation map of this source. From these and other observations, we found evidence that the morphology of the complex inner dust cloud differs from the models suggested by most other authors, and the star is completely obscured in both the K and L bands.