Bibliography: pages 167-169. / Photographic (B,V) light curves of 213 LMC and 180 SMC cepheids (and W Virginis stars) have been obtained and the relevant light-curve parameters derived. Comparison of this work with stars in common with photoelectric data indicate that the <V> and (<B> - <V>) magnitudes differ from their photoelectric counterparts by ±0ᵐ.09 and ±0ᵐ.12 respectively for the fainter cepheids (log P <0.8) and by ±0ᵐ.06 and ±0ᵐ.07 respectively for the brighter cepheids (log P >0.8).

The velocity function of MIGHTEE-H I Early Science data is presented. This is the first velocity function that is based on a blind radio interferometric survey. As a precursor, understanding the systematics that affect the Early Science velocity function will optimise the full survey's analysis. PYMULTINEST and the Busy Function are employed to estimate the linewidths of the low spectral resolution data. The performance of PYMULTINEST in estimating known linewidths of simulated H I profiles with varying spectral resolution is assessed. The simulation study shows that the estimated linewidths of the Early Science data, using this novel method, are robust and are recovered within the uncertainty. The effects of cosmic variance, instrumental linewidth broadening and Doppler linewidth broadening on the velocity function are quantified within the context of the limitations of the Early Science data. The MIGHTEE-H I Early Science velocity function is compared with the velocity functions from previous large-scale H I surveys, namely the Arecibo Legacy Fast ALFA (ALFALFA) survey and the H I Parkes All-Sky Survey (HIPASS). There is general agreement with the ALFALFA and HIPASS results, when taking linewidth broadening into account, given that the MIGHTEE-H I Early Science data is strongly affected by cosmic variance. In particular, cosmic variance introduces an average uncertainty of ∼ 24% in the measured Early Science volume densities. The larger effective area of the full survey will reduce the impact of cosmic variance. The full survey velocity function can be further optimised by estimating the rotational velocities using kinematic modelling, and correcting the measured linewidths for instrumental broadening, Doppler broadening, turbulent motion and inclination effects.

This thesis presents radio frequency studies of three novae, namely V445 Puppis, V3890 Sagitarii and V339 Delphini. The new data, in this thesis, represent some of the most detailed and comprehensive radio light curves to date. In these systems a thermonuclear eruption occurs on the surface a white dwarf following extensive periods of accretion of material from a companion star. The result of the thermonuclear eruption is the explosive ejection of the outer layers of the accumulated material. Since the ejected material emits at radio wavelengths at some point in its evolution, radio data obtained for the three objects are utilised to test the hypothesis of radio emission models. First, radio data of V445 Pup are presented. V445 Pup is the only helium nova observed to date; its eruption in late 2000 showed high velocities up to 8500 km s°1, and a remarkable bipolar morphology cinched by an equatorial dust disc. Here we present multi-frequency radio observations of V445 Pup obtained with the Very Large Array (VLA) spanning 1.5°43.3 GHz, to 2001 January and 2008 March (ª 89 ° 2700 days after eruption). The radio light curve is dominated by synchrotron emission over these seven years, and shows four distinct radio flares. Resolved radio images obtained in the VLA's A configuration show that the synchrotron emission hugs the equatorial disc, and comparisons to near-IR images of the nova clearly demonstrate that it is the densest ejecta—not the fastest ejecta—that are the sites of the synchrotron emission in V445 Pup. The data are consistent with a model where the synchrotron emission is produced by a wind from the white dwarf impacting the dense equatorial disc, resulting in shocks and particle acceleration. The individual synchrotron flares may be associated with density enhancements in the equatorial disc and/or velocity variations in the wind from the white dwarf. This overall scenario is similar to a common picture of shock production in hydrogen-rich classical novae, but V445 Pup is remarkable in that these shocks persist for almost a decade, much longer than the weeks or months for which shocks are typically observed in classical novae. Second, the radio observations following the 2019 August eruption of V3890 Sgr obtained with the MeerKAT radio telescope at 1.28 GHz are presented. The radio light curve spans from day 1 to 300 after discovery and is dominated by synchrotron emission produced as the expanding ejected nova envelope interacts with the dense wind from an evolved companion in the binary system. The radio emission is detected early on day 6 and increases rapidly with the radio flux peaking after day 15. The radio luminosity increases due to a decrease in the opacity of the circumstellar material in front of the shocked material and fades as the density of the surrounding medium decreases and the velocity of the shock losses energy and decelerates. Modelling of the light curve provides an estimated mass-loss rate of M ˙ wind º 10°8 MØ yr°1 for the red giant wind. V3890 Sgr hosts a massive white dwarf, shows presence of structured circumbinary material consistent with the surrounding environment present in some supernovae type Ia (SNe Ia) and therefore it is possible to be a SNe Ia progenitor. Finally, radio observations of V339 Del obtained using the Karl G. Jansky Very Large Array and the Arcminute Microkelvin Imager Large Array (AMI) following the 2013 eruption are presented. The AMI data were obtained every two days resulting in the most detailed radio light curve of a classical nova known to date. The radio data are used to test the hypothesis that the observations were due to a bipolar shaped ejecta as suggested from emission line modelling at optical wavelength. Here, the morphology is utilised in predicting the ejected mass of V339 Del during the eruption. The radio light curve is modelled in the morpho-kinematical program SHAPE using a linearly expanding ionized ejecta, assuming that the nova emission is dominated by thermal free-free emission. Considering a bipolar geometry for the nova ejecta, the radio light curve fluxes can be replicated using an ejected mass of Mej º 0.1°8£10°5 MØ. The mass estimate of the ejecta compares well with estimates of emission line modelling in optical spectroscopy and also ∞-ray emission modelling. High resolution radio images of V339 Del obtained with the VLA's A configuration initially show a spherical structure when the ejecta is optically thick and later a ring structure when the ejecta is optically thin. The change in optical depth and apparent morphology is a further indication of a non-spherical geometry. V339 Del shows evidence of shock powered emission in the first 100 days after the nova eruption. This thesis emphasizes the role of radio data towards a better understanding of the effects of thermonuclear eruptions in novae. This work highlights the importance of obtaining well sampled radio light curves of novae which give more insight into the physical processes that follow nova explosions, which may have been overlooked before due to sparsely sampled radio light curves. Resolved radio images of novae are also crucial in determining the morphology of the ejecta hence provide clues on the approach to modelling radio emission from novae. This is particularly relevant given the new generation of sensitive radio telescope arrays such as MeerKAT, the next generation Very Large Array, and ultimately the Square Kilometre Array.

Protoplanetary disks are the essential link between molecular clouds and planetary systems. Protoplanetary disk evolution determines the resulting planetary systems. In this dissertation, I focus on tracing evolution in populations of disks by analyzing large samples of disks. This dissertation is made possible by new observations and physically-motivated models. The main processes that I use to study disk evolution are the dust growth and evolution that occur in the outer disk and the accretion of gas from the inner disk onto the star. Lynds 1641 (L1641) is a star-forming region in the Orion Molecular Cloud A, and it has great potential as a laboratory for protoplanetary disk evolution. I present observations of disks in L1641 from the Herschel Space Observatory and the Atacama Large Millimeter Array (ALMA). The far-infrared Herschel data are sensitive to micron-sized dust grains in the outer disk atmosphere, and the radio ALMA data trace the millimeter-sized dust grains in the disk midplane. I use accretion disk models to show that the far-infrared data are consistent with disks that show signs of dust evolution, even in this young (~1.5 Myr) region. I compare the L1641 millimeter data to other surveys and show that L1641 is at a stage of evolution between young protostellar systems and more evolved disks where planet formation is already well-underway. Accretion of material onto a star is an important mechanism in protoplanetary disk dispersal and heating. The classical magnetospheric accretion paradigm is well-understood and established for low-mass stars. However, higher-mass stars may not have the magnetic field strength for magnetospheric accretion to occur. I present a large survey of intermediate-mass systems with near-infrared spectra. I use the accretion-tracing Brγ line to find trends with system properties and find a break in the accretion rate—stellar mass relationship, which may indicate a break in the accretion mechanism. Additionally, I use magnetospheric accretion models to reproduce the observations to determine the accretion properties in a subset of objects, finding that these models can reproduce fast-moving emission.

Includes abstract. / Includes bibliographical references. / This dissertation presents a new algorithm that was developed to perform autonomous fitting of gravitational microlensing lightcurves. The algorithm combines features of extant evolutionary algorithms with some novel ones, and fares well on the problem of fitting binary lens microlensing lightcurves, as well as on a number of other difficult optimisation problems. Furthermore, the new algorithm is conceptually simple, versatile and robust, and parallelises trivially.

Includes abstract. / Includes bibliographical references. / Multifibre spectrographic observations of the bipolar helium nova V445 Puppis were made over fivenights (two nights in January 2006 and three nights in January 2007) using the IMACS Integral Field Lnit spectrograph on the Magellan I Telescope. Of these. spectra from the two best nights. 4 January 2006 and 8 January 2007, were reduced. extracted. calibrated and analysed in this thesis. The spectra extracted from sewn target frames in 2006 and six target frames in 2007 were combined for -100 individual fibres. The two-dimensional spectra were then recombined into a data cube to create a three-dimensional picture of the emission from the nova.

Includes bibliographical references (leaves 67-69). / This work gives an account of the study of the metallicity [Fe/H] distribution (gradient) in the oldest population in the Large Magellanic Cloud (LMC), by making use of the available RR Lyrae data from the Optical Gravitational Lensing Experiment III (OGLE III). RR Lyrae stars are amongst the oldest objects in the universe and they have a range in element (metal) abundances. Measuring the distribution of metallicities of RR Lyrae stars in a galaxy gives one clues to the origin of galaxies. It is known that the pulsation periods of RR Lyraes is broadly correlated with their metallicity. This fact has been used for investigating the metallicity distribution of RR Lyrae stars in the LMC. I have found an indication that the proportion of metal poor RR Lyrae stars increases with distance from the centre of the LMC. In addition, an attempt was made to improve the metallicity-period relation by introducing the Fourier parameters, but this was unsuccessful. Lastly, a comparison is made with estimates of metallicity gradients of other LMC populations.

Includes bibliographical references (p. 111-113). / The large (10m) aperture of the Southern African Large Telescope (SALT) coupled with the unique capabilities of the Robert Stobie Spectrograph (RSS), promises unparalleled prospects for polarimetric observations on an 8 - 10 m class telescope. RSS is a complex and highly versatile first-generation instrument of the SALT. RSS-VIS, the visible arm spanning 320-900 nm, employs a high UV-transmitting optical design to support UV spectroscopy down to the atmospheric cutoff at 320nm (rare on large telescopes).

In this MSc thesis, I investigated the kinematics of Hα emission from the hot ionized and NaD absorption from cool neutral gas in a sample of 40 nearby Luminous Infrared Galaxies (LIRGs: LIR ≥1011L) from Supernovae and starbursts in the infrared, SUNBIRD survey imaged with the Southern African Large Telescope (SALT). This project can be seen in two major areas. The first is analyzing the emission and absorption spectra, in this case from the hot ionized gas and cool gas respectively. Two routines were used for the profile fitting, to get the centroid wavelength of both the emission and absorption lines; Gaussian and Gaussian-Hermite functions were used. The fitting result provides an indication of the origin of an outflowing gas; nuclear or disk driven?. Secondly, I plotted and compared the velocities of hot ionized and cool neutral gas. Three different methods were used to estimate the systemic velocity Vsys for the accurate estimation of outflowing velocity. For instance, an outflowing NaD upto 380 kms−1 and inflowing gas up to -100 kms−1 relative to Hα is seen at the optical nucleus of the NGC6240[PA=45] and NGC1204[PA=253] galaxies respectively. I tested if there was a relation between star formation rate (SFR) and an outflowing gas and also whether the inflowing gas triggering new nuclear SF. I find the gas of two of our targets escaping the potential well of the host galaxy. There was also an expectation that the out flowing velocity would correlate with SFR, unfortunately I do not see any correlation, though it needs further investigation of all offset velocities of the gas with respect to stellar absorption lines and it also requires detail analysis of the of projection or orientation effect on the galaxy.

Includes abstract. / Includes bibliographical references (leaves 96-99). / In this thesis, a deep neutral hydrogen (HI) galaxy search for galaxies hidden by the Galactic Bulge is presented. The HI Parkes deep HI Zone of Avoidance (ZOA) survey has been extended towards the higher latitudes in the Galactic Bulge region. The surveyed region covers 332° ≤ l ≤ 36° for the region 5° ≤ │b│ ≤ 10° and 352° ≤ l ≤ 24° for the region 10° ≤ │b│ ≤ 15°.