published_or_final_version / Physics / Master / Master of Philosophy

published_or_final_version / abstract / Physics / Master / Master of Philosophy

published_or_final_version / Physics / Master / Master of Philosophy

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No description available.

No description available.

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Magnetohydrodynamic dynamo in disc-like astrophysical bodies has been considered for some time. Important astrophysical objects like accretion discs, protostellar and protoplanetary nebulae and galaxies are thought to regenerate a magnetic field through a dynamo mechanism. Although there is a well developed theory for describing the regeneration of magnetic field in these objects, there are not any specific methods how to calculate such magnetic fields in general case. In this work, after a description of the dynamo theory, the specific method for solving the nonspherical dynamo is introduced. The unique feature of this method is accommodation of variable magnetic diffusivity in order to model the shape of a disc-like body. The detailed construction of the method is presented, as well as description of mathematical and numerical methods used for obtaining the solution. The method of checking the model with respect to well established spherical models is also presented. Finally, some examples are calculated and discussion is given on the behaviour of calculated magnetic field and possible astrophysical implications.

A two fluid hydrodynamical model describing the modification of a stellar wind flow due to its interaction with galactic cosmic-rays is investigated. The two fluids consist of the thermal stellar wind gas and the galactic cosmic-rays. A polytropic one fluid model is used to describe the stellar wind gas, and the cosmic-rays modify the wind via their pressure gradient. The cosmic-rays are considered to be a hot low density gas of negligible mass flux, but with a significant pressure and energy flux compared to the thermal gas. The equations used are essentially those employed in two fluid hydrodynamical models of cosmic-ray shock acceleration by the first order Fermi mechanism, but suitably modified to apply in a spherical geometry and including the effects of gravity on the flow. The stellar wind consists of a transonic flow with a termination shock, and subsonic flow outside the shock. The model shows the deceleration of the wind upstream of the shock by the positive galactic cosmic-ray pressure gradient. The dissertation first discusses the fluid polytropic stellar winds and how to insert shocks in the flow. The hydrodynamical equations governing cosmic-ray modified winds are then introduced followed by a discussion of the physics of the interaction between the thermal stellar wind and the cosmic-rays. A description of the singularities of the equations is also presented. The system of equations is first solved by a finite difference method in the test particle approximation in which the cosmic-rays do not modify the flow, with appropriate boundary conditions applied at infinity, at the wind termination shock, and at the star. A perturbation scheme to determine the modification of the wind by the cosmic-rays is then developed. This scheme applies when the modification of the wind by the cosmic-rays is sufficiently small. Finally a numerical iteration is employed to exactly solve the equations. This latter method has the advantage that it can be applied when there is a considerable modification of the wind by the cosmic-rays.

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