Global ETD Search
Search theses and dissertations gathered from participating repositories worldwide. Every result links back to the library that holds it. No account is needed.
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Showing 1 to 20 of 21 for “"Accretion Discs"”.
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On the Dynamics of Noncircular Accretion Discs
The classical picture of an accretion disc is of a geometrically thin, nearly axisymmetric, fluid flow undergoing supersonic circular motion and slowly accreting due to angular momentum transport by the disc turbulence. There are, however, strong theoretical and observational grounds for …
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Exploring the Variability of Accretion Discs with Stochastic Models
… create corresponding fluctuations in the accretion rate which then propagate through the disc, is often used to explain these characteristic features in the variability. While this theory has been extensively explored analytically, there has thus far been little numerical work …
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Variability of black-hole accretion discs: a theoretical study
Accretion discs are fluid-dynamical entities which surround many black holes. Observations reveal that these systems exhibit variability on a range of time scales. This thesis investigates phenomena occurring in black-hole accretion discs which are likely to induce high-frequency quasi-periodic …
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Shearing waves and the MRI dynamo in stratified accretion discs
Accretion discs efficiently transport angular momentum by a wide variety of as yet imperfectly understood mechanisms, with profound implications for the disc lifetime and planet formation. We discuss two different methods of angular momentum transport: first, generation of acoustic waves by mixing …
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Zonal flows in accretion discs and their role in gravito-turbulence
… the evolution of zonal flows in self-gravitating accretion discs and their resulting effect on disc stability; it also studies the process of disc gravito-turbulence, with particular emphasis given to the way the turbulent state is able to extract energy from the background flow and sustain itself …
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Accretion processes in cataclysmic variable stars: insights from optical transient surveys
… stars are binary stars mostly characterised by accretion from a main sequence donor star to a white dwarf star. Multiple CV subclasses exist with varia tions in the nature of the CV, many of which have accretion discs surrounding the accretor. This study focuses on two sub-classes of CVs, namely …
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Hydromagnetic Oscillations and Instabilities in Astrophysical Discs
… magnetized and differentially rotating, accretion flows provide an environment for the propagation of waves and the growth of instabilities unlike those normally encountered on Earth. In this dissertation, I investigate the properties of oscillations and instabilities in accretion discs, …
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Neutron Star Mergers at the Dawn of Multimessenger Astrophysics: massive binaries, accretion disks and phase transitions
… of April, 2019, on the characterization of the accretion discs formed from the merger of a binary system composed by two neutron stars and on the effects of a hadron to quark phase transition that can occurs during such mergers.
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Modelling the balmer line profiles in the spectra of symbiotic stars
… their origin is ascribed to emission from an accretion disc. The role which accretion discs play within the class of symbiotic stars however, is much more uncertain and modelling the lines on this basis could, if successful, help to strengthen the evidence for their presence in symbiotic …
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Shear instabilities in stellar objects: linear stability and non-linear evolution
… in various astrophysical objects, including accretion discs and stellar interiors. Due to observational limitations the complex dynamics in stellar interiors that result in turbulent motions, mixing processes, and magnetic field generation, are not entirely understood. It is therefore …
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Applications of Gaussian Processes at Extreme Lengthscales: From Molecules to Black Holes
… the latent emission signature from a black hole accretion disc. Furthermore, GPs are currently the workhorse model for Bayesian optimisation, a methodology foreseen to be a vehicle for guiding laboratory experiments in scientific discovery campaigns. The first contribution of this thesis is to …
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Protoplanetary disc evolution and dispersal
In this thesis I have studied how discs around young stars evolve and disperse. In particular, I build models which combine viscous evolution with photoevaporation, as previous work suggests that photoevaporation can reproduce the observed disc evolution and dispersal time-scales. The main question …
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Nonlinear hydrodynamic instability and turbulence in eccentric astrophysical discs
… of inertial waves inherent to eccentric discs is studied by way of a new local numerical model. Mode coupling of tidal deformation with the disc eccentricity is known to produce exponentially growing eccentricities at mean-motion resonances, most prominently via the 3:1 eccentric Lindblad …
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Hydrodynamic and Magnetohydrodynamic Convection in Accretion Disks
… of convection perpendicular to the plane of accretion disks have been discussed for several decades. Recent simulations combining convection and the magnetorotational instability have given fresh impetus to the debate, as the interplay of the two processes can enhance angular momentum …
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Astronomical Interferometry at Submillimetre Wavelengths
… for the evolved star VY CMa and observations of discs around young stars. Spectra of the 321 GHz H20 line from VY CMa confirm that maser amplification is responsible for the emission. Continuum emission, spatially offset from the masers, was also detected with the interferometer, as was the CO …
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The Dynamics of Magnetised Warped Discs
The study of warped discs and the study of magnetised discs have generally been performed in relative isolation, and comparatively little attention has been given to the complex interplay between these two aspects of accretion disc physics. This thesis aims to provide a synthesis of these disparate …
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Understanding X-ray reflection as a probe of accreting black holes
… surrounding an accreting black hole from the accretion disc is investigated in the context of probing the structure of the central regions as well as the physical processes that power some of the brightest objects seen in the Universe. A method is devised to measure the emissivity profile of …
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Black Hole Feedback in New Regimes: Modelling Dwarf Galaxies with Active Galactic Nuclei
… To examine the interplay between SNe and AGN accretion in more detail, I run a series of cosmological zoom-in simulations. I find that AGN feedback in tandem with more realistic SN feedback can be a successful alternative quenching mechanism to strong SN feedback in dwarfs, provided that the …
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The complex interplay between AGN jet feedback and galaxy evolution
… Chapter 2, I introduce my model for black hole accretion through a (warped) α-disc and feedback in the form of a Blandford-Znajek jet and describe its numerical implementation. In the remainder of Chapter 2, I perform benchmarking of the model and explore the effects of AGN jet feedback on the …
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ASSESSING THE ROLE OF EXTERNAL PHOTOEVAPORATION OF PROTOPLANETARY DISCS IN NEARBY STAR-FORMING REGIONS
… formation and diversity. Most stars (and their discs) in the Galaxy form in massive clusters where the intense UV radiation emitted by OBA-type stars heats the outermost layers of protoplanetary discs, producing thermal winds that are launched from the disc surface. This causes outside-in disc …
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