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Accretion Disks
Prof. Donald Lynden-Bell, University of Cambridge
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Lecture 1
Following a brief history of the development of radio astronomy and the discovery of quasars that resulted, the theories developed to explain them are discussed. the theory of dissipative accretion discs around a central object is first developed for Newtonian gravity.
Lecture 2
The theory of accretion discs is further developed to cover their emission. Each ring of the disc achieves a different temperature.
The total emission from all such rings is summed to predict the total spectrum which for quasars maximises in the ultraviolet.
The theory is gneralised to account for the case of an accretion disc around either a static or a spinning black hole, and it is demonstrated how a static black hole evolves into a Kerr spinning hole with the maximum spin rate.
Lecture 3
After completing the theory desribed under lecture 2, we begin the subject of magnetohydrodynamics demonstrating Alfven's theorem that the magnetic lines of force are frozen into a highly conducting fluid. The conductivity needed to achieve this becomes much smaller on large scales.
After some brief remarks on the origin of cosmic magnetic fields via dynamo action, we consider the action of an accretion disc in stretching the field until it floats up out of the accretion disc into its surrounding corona.
We describe how further winding makes the magnetic field extend into a tall tower whose height grows with every turn.
Lecture 4
The theory of jets in radio galaxies, T Tauri stars, Herbig Haro objects, dying stars and possibly gamma-ray bursts is developed in terms of force-free towers of electromagnetic field that rapidly accelerate upwards as the surrounding pressure decreases. When the pressure decreases faster than 1/z^4, ram-pressure confinement takes over, but even that fails if the density falls faster than 1/z^6. Under such circumstances the jets become relativistic as in gamma-ray burst sources.
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