Physics
Scientific paper
Oct 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000phdt.........2a&link_type=abstract
PhD Thesis
Physics
Accretion Disc, Magnetic Field, Outflow, Stellar Magnetosphere, Young Stars
Scientific paper
In this thesis we address issues relating to the evolution of magnetised accretion discs including the effects of outflows. We construct vertically averaged, radially global, time-dependent models including magnetic torques, viscosity, ν, and magnetic diffusivity, η. We follow the evolution of an initially uniform external magnetic field permeating a disc. We find that when the radial inflow in the disc is calculated self-consistently field lines can be inclined sufficiently to the vertical for centrifugal acceleration of an outflow to occur, provided that η/ν <~0.5. We also follow the evolution of a disc with a magnetic field generated through a non-linear α-Ω dynamo. Using a form of α which is appropriate for magnetohydrodynamical turbulence in discs we find that magnetic fields of dipolar symmetry are produced. The magnetic field strength and field line inclinations are in general sufficiently large to launch a centrifugally accelerated wind, even for η/ν > 1. The evolution of an externally magnetised accretion disc with an outflow is also simulated. We find that for constant η/ν steady states with unrealistically large magnetic fields are produced. Self-consistency arguments are used to propose a variable diffusivity which is a function of the magnetic field. When this diffusivity is implemented the subsequent disc evolution is in general cyclic. During outbursts both the mass accretion and the mass loss rate increase on average by two orders of magnitude. The applicability of this model in observed outbursts in Fuors and X-ray binaries is also explored. Finally we study two-dimensional, force-free equilibria of a star-disc magnetosphere. The poloidal magnetic field is found to be significantly inflated when it is sufficiently wound up. The magnetic torque resulting in this interaction is found to be much smaller than the torque associated with an untwisted poloidal magnetic field. For more information: http://www-star.qmw.ac.uk/~va
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