Includes abstract. / Includes bibliographical references. / Velocity dispersions are used to determine the stability of galactic disks against gravitational collapse to form stars, in some star formation recipes, in studies of gas dynamics, and to determine how much turbulence there is in the interstellar medium. Atomic hydrogen (HI) dispersions have been used in the studies of star formation and large-scale turbulence, despite stars forming in molecular clouds and the inner regions of galaxies being dominated by molecular gas. Carbon monoxide (CO) has been used as a tracer for molecular gas. In this work HI and CO dispersions were determined for a sample of nearby galaxies and they were compared to determine what the relationship between HI and molecular gas dispersions is.

Black hole X-ray binaries (BHXBs) are stellar binary systems consisting of a black hole (BH) and a companion star. They are known to produce X-ray emission through the accretion of mass from the companion star onto the black hole via an accretion disc, as well as radio emission originating from their jets. My thesis splits into two projects. On one hand, I focus on the connection between the X-ray emitting accretion disc and the radio jets of BHXBs in general, by studying the quasi-simultaneous evolution of the radio fluxes and the X-ray fluxes from 17 BHXBs. This connection, also known as the radio/X-ray correlation has been studied and updated over the past years. New observations of new and known sources have shown that another population of X-ray binaries exists (referred to as outliers), lying below the standard radio/X-ray correlation. I investigate whether the mass of the black hole component of BHXBs can explain the existence of these outliers. In my second project, I focus on an exotic source, known as SS433. It has a supercritical accretion disc and displays precessing relativistic jets. I investigate whether these jets are made up of proton-electron plasma or electron-positron plasma. Circular polarization (CP) is a good diagnostics for understanding the particle composition of radio jets. Therefore we have observed the circular polarized flux densities of SS433 using the Australia Telescope Compact Array (ATCA) for a broad range of frequencies between 1:4 - 10 GHz. From those observations, a CP spectrum can be constructed and the spectral index can be estimated. There are 4 ways of producing CP emission and the spectral index helps us to constrain the CP production mechanism. In addition, the kinematics of propelling a proton-electron plasma in a jet is different from that of electronpositron plasma. I simulate various plausible models for the energy content of the jets and thereby aim to constrain the particle composition of the jets. Black hole X-ray binaries (BHXBs) are stellar binary systems consisting of a black hole (BH) and a companion star. They are known to produce X-ray emission through the accretion of mass from the companion star onto the black hole via an accretion disc, as well as radio emission originating from their jets. My thesis splits into two projects. On one hand, I focus on the connection between the X-ray emitting accretion disc and the radio jets of BHXBs in general, by studying the quasi-simultaneous evolution of the radio fluxes and the X-ray fluxes from 17 BHXBs. This connection, also known as the radio/X-ray correlation has been studied and updated over the past years. New observations of new and known sources have shown that another population of X-ray binaries exists (referred to as outliers), lying below the standard radio/X-ray correlation. I investigate whether the mass of the black hole component of BHXBs can explain the existence of these outliers. In my second project, I focus on an exotic source, known as SS433. It has a supercritical accretion disc and displays precessing relativistic jets. I investigate whether these jets are made up of proton-electron plasma or electron-positron plasma. Circular polarization (CP) is a good diagnostics for understanding the particle composition of radio jets. Therefore we have observed the circular polarized flux densities of SS433 using the Australia Telescope Compact Array (ATCA) for a broad range of frequencies between 1:4 - 10 GHz. From those observations, a CP spectrum can be constructed and the spectral index can be estimated. There are 4 ways of producing CP emission and the spectral index helps us to constrain the CP production mechanism. In addition, the kinematics of propelling a proton-electron plasma in a jet is different from that of electron-positron plasma. I simulate various plausible models for the energy content of the jets and thereby aim to constrain the particle composition of the jets.

Includes abstract. Includes bibliographical references (leaves 82-88).

The general equations describing the time-independent structure of gaseous circumstellar disks in vertical hydrostatic equilibrium are derived. The possibility of turbulent instablilty and the resulting viscosity laws are discussed.

Aims: In this study, we quantify the star formation activity in bars as a function of the host stellar mass and the morphological type. We use a sample of 70 barred spiral galaxies in the nearby universe taken from the Spitzer Survey of Stellar Structure in Galaxies (S⁴G). The data are combined with Hα data from various optical telescopes. (KPNO, JKT, OHP and CTIO) Methods: We overlay continuum-subtracted Hα emission on 3.6μm images from Spitzer (S⁴G) to quantify the distribution of star formation in each bar. We discern the star formation distribution into three classes: morph-(1): Minimal to no star formation in the bar but intense star formation activity at the bar ends and in the nuclear region; morph-(2): Star formation along the bar, in the nuclear region to the bar ends; morph-(3): Star formation only at the bar ends. To understand the location of the bar and its properties, we used ellipse fitting to identify the bar properties, including the bar size and the bar strength. We also use the host stellar mass of each galaxy measured from the 3.6μm integrated magnitudes from S⁴G to investigate how the three star formation distributions are related to the stellar mass. The host galaxies are divided into Early-Type (SB0-SBbc) and Late-Type Barred Spirals (SBc-SBm) - these morphological types correlate with a variety of galaxy properties that have historically been used to better understand galaxy formation and evolution. For the next stage of this work, we will compare the star formation activity in one galaxy from the sample, NGC 1097 with molecular gas distribution mapped using ALMA. This will be an investigation of the relationship between gas kinematics and star formation using numerical simulations of gas flow. [Please note: this thesis fulltext has been deferred until 9 December 2016]

Includes bibliographical references (p. 107-111). / abstract

The objective of this project was to design and implement the telescope control software for the Alan Cousins Telescope (ACT), a robotic telescope at the Sutherland, South Africa, site of the South African Astronomical Observatory (SAAO). The approach adopted involved a high level of modularisation of the software and the use of software simulators in an effort to foster rapid software development and enable more rigorous software testing. Significant attention was also paid to new and emerging technologies and how these may be used within the specific context of robotic telescope control software. The software now acts as a demonstration platform for those technologies that were implemented.

A computer-controlled area scanner designed for use on close visual double stars is described. Techniques used in making observations and in subsequent reduction of the data are given. Problems encountered are discussed. Magnitude differences and magnitudes and colours of components are given for 153 bright southern close visual doubles. Separations are given for some of the stars. Absolute magnitudes are calculated for the primaries by several methods. Individual stars are discussed where appropriate. The accuracy of the results is discussed. No significant systematic errors are evident in the results, but systematic errors are present in the results of other authors. Suggestions are made for the future use of conventional photometers, scanners and other techniques in the field of visual double star photometry and astrometry.

The rapidly oscillating Ap (roAp) stars are a sub-group of the chemically peculiar stars of class 2 (CP2), which are characterised by peculiar spectra and anomalously strong lines of Sr, Cr, Eu and other rare earth elements. They have strong global dipole magnetic fields with effective strengths of up to a few thousand gausses. Stars showing these phenomena cover the spectral range B8p to F0 V-IV. About 20 years ago rapid non-radial pulsations were discovered in the coolest members of the CP2 group, namely the roAp stars. These pulsations are due to high over-tone, low degree p modes with periods between 5.6 and 15 minutes. Since then, studies of these rapid oscillations have revealed a lot of information about these stars. The eigenfrequency spectra of roAp stars can potentially reveal information such as their rotation periods, rotational inclinations, magnetic geometries, internal magnetic field strengths, radii, masses, luminosities and ages. Matthews et al. (1990, 1996) suggested a technique to empirically determine the T(Ƭ) relation for roAp stars. This technique involves comparing the pulsation amplitudes obtained from multi-colour photometry to the ones calculated from black-body pulsator models (assuming dipole mode pulsations). This comparison yielded limb-darkening coefficients which were used to determine T(Ƭ) in a way similar to what was done for the sun. Matthews et al. based their idea on the observed fact that pulsation amplitudes of roAp stars drop sharply with increasing wavelength. They thus explained this sharp decline of amplitude with wavelength in terms of the strongly wavelength dependent limb-darkening. The initial aim of this thesis was to investigate the technique proposed by Matthews et al., and to apply it to a number of roAp stars. However, when a linearised expression for the variation of the pulsation amplitude with wavelength, limb-darkening, inclination of the pulsation axis α, and ΔT/T₀ (where ΔT is the polar pulsational temperature semi-amplitude and T₀ is the atmospheric temperature) was derived, it was discovered that limb-darkening is too small an effect to account for the observed amplitudes. The result is based on the Wien approximation and uses the Planck function to represent intensity. Therefore, limb-darkening cannot be measured from the amplitude vs wavelength data. This analysis and the results thereof are reported in this thesis. Numerical models based on realistic treatment of the intensity spectra (obtained from model atmospheres) are used to confirm and refine the analytical results. The linearised expression mentioned above suggests that an important factor that explains the sharp decline of amplitude with wavelength is the variation of the ratio ΔT/T₀ with wavelength. Therefore, if the T(Ƭ) structure of a star is known a priori (from model atmospheres), the variation of ΔT with wavelength can be determined. This new technique, together with the variation of opacity with wavelength in the atmospheres of roAp stars, is applied to HD 134214:, HD 137949, HD 128898, HD 101065 and HR 3831 to determine ΔT cos α as a function of atmospheric depth. HR 3831 was observed at various rotation phases to investigate the effect of rotation on the derived ΔT cos α vs atmospheric depth relation. Preliminary results on this are included in this thesis. Bibliography: pages 107-113.

Includes abstract. Includes bibliographical references.