This thesis presents several observational studies based on radio variability and interstellar scintillation of extragalactic flat-spectrum radio sources. Such sources are commonly called blazars, a term used to describe the phenomenon observed when the jet of a radio-loud Active Galactic Nucleus (AGN) is directed towards the observer. These sources provide unique laboratories for studying the physics of relativistic jets. Observations of selected samples of blazars, made with the Australia Telescope Compact Array (ATCA) and the Australia Telescope Long Baseline Array are presented here. Statistics for long-term (months--years) and short-term (intraday) variability in both total and linearly polarized flux density at several frequencies are presented. The sensitivity and flux density measurement accuracy of the ATCA make it particularly useful for observations of intraday variability (IDV). Resolving the question of what is the mechanism for radio IDV was of great importance at the time this thesis was being undertaken, since if intrinsic, IDV implies extremely high brightness temperatures, far in excess of the Inverse Compton limit for incoherent synchrotron radiation. Most source models are fundamentally based on the assumption that the radiation from radio to optical, and sometimes soft X-ray, energies is produced by the incoherent synchrotron mechanism, so any result which challenges this has serious implications. There is now strong evidence that interstellar scintillation (ISS) is the principal cause of radio IDV, which substantially lowers the implied source brightness temperatures from those calculated assuming intrinsic variability. Some of the results presented in this thesis have made an important contribution to the paradigm shift from IDV to ISS, by showing unequivocally that the rapid IDV observed in PKS 1257-326 is a result of scintillation due to a nearby scattering screen in the ionised interstellar medium (ISM) of our Galaxy. This unusual source, serendipitously discovered during the course of my PhD, has also proved extremely valuable in showing that ISS can be used as a probe of microarcsecond-scale source structure and also of the local Galactic ISM. Such high angular resolution is not currently achievable even with space interferometer baselines. / Thesis (Ph.D.)--Physics, 2003.

This thesis presents several observational studies based on radio variability and interstellar scintillation of extragalactic flat-spectrum radio sources. Such sources are commonly called blazars, a term used to describe the phenomenon observed when the jet of a radio-loud Active Galactic Nucleus (AGN) is directed towards the observer. These sources provide unique laboratories for studying the physics of relativistic jets. Observations of selected samples of blazars, made with the Australia Telescope Compact Array (ATCA) and the Australia Telescope Long Baseline Array are presented here. Statistics for long-term (months--years) and short-term (intraday) variability in both total and linearly polarized flux density at several frequencies are presented. The sensitivity and flux density measurement accuracy of the ATCA make it particularly useful for observations of intraday variability (IDV). Resolving the question of what is the mechanism for radio IDV was of great importance at the time this thesis was being undertaken, since if intrinsic, IDV implies extremely high brightness temperatures, far in excess of the Inverse Compton limit for incoherent synchrotron radiation. Most source models are fundamentally based on the assumption that the radiation from radio to optical, and sometimes soft X-ray, energies is produced by the incoherent synchrotron mechanism, so any result which challenges this has serious implications. There is now strong evidence that interstellar scintillation (ISS) is the principal cause of radio IDV, which substantially lowers the implied source brightness temperatures from those calculated assuming intrinsic variability. Some of the results presented in this thesis have made an important contribution to the paradigm shift from IDV to ISS, by showing unequivocally that the rapid IDV observed in PKS 1257-326 is a result of scintillation due to a nearby scattering screen in the ionised interstellar medium (ISM) of our Galaxy. This unusual source, serendipitously discovered during the course of my PhD, has also proved extremely valuable in showing that ISS can be used as a probe of microarcsecond-scale source structure and also of the local Galactic ISM. Such high angular resolution is not currently achievable even with space interferometer baselines. / Thesis (Ph.D.)--Physics, 2003.

The brightness and proximity of many classical Be stars makes them perfect laboratories for studying the physics of astrophysical disks. They are also among the most popular targets for optical/IR interferometers, which are able to fully resolve their circumstellar disks, to which much of the recent progress in our understanding of these enigmatic objects is owed. The current consensus is that classical Be stars eject material from the stellar surface into Keplerian orbits, thus forming a disk, whose subsequent evolution is governed by turbulent viscosity, which is the basis of the so-called viscous decretion disk (VDD) model. Among the main results of the present work is arguably the best-constrained model of a particular Be star β CMi. The VDD predictions were confronted also with radio observations, which allowed for the first determination of the physical extent of a Be disk. This result subsequently led to the detection of a binary companion, which is truncating the disk by tidal forces. Extending the sample to include five more targets led to revealing a similar outer disk structure in all of them. The range of explanations includes the most plausible scenario, in which the truncation of Be disks by (unseen) companions is much more common than previously thought.