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Accredition disc phenomena in cataclysmic variables

Friday, 3 October, 2008 - 15:00
Campus: Brussels Humanities, Sciences & Engineering campus
Faculty: Science and Bio-engineering Sciences
Christina Papadaki
phd defence

This thesis, devoted to accretion disc phenomena in cataclysmic variables (CVs), is based on the photometric and spectroscopic study of several non-magnetic CVs in which a fully grown disc has developed. Accretion discs are among the most common structures that exist: stars form through a disc of material whose residues will become planetary systems. A large fraction of binary systems have accretion discs in one or other stage of their evolution. Accretion discs are also important in X-ray binaries where matter is transferred towards a neutron star or a black hole. Finally, even galaxies have generally discs. However, one of the great unknowns is the source of viscosity, responsible for the angular momentum transfer in the disc and thus for the accretion.

In non-magnetic CVs the radiation from the system is dominated by the disc. The study of such systems reveals thus mostly the structure of the accretion disc and therefore the underlying viscosity sources. As these binary systems have orbital periods of a few hours and orbital separations of the order of one solar radius, their study is made possible on small time scales. They are thus the ideal systems to probe the structure of accretion discs and to try to understand the source of anomalous viscosity.

As far as photometry is concerned, the majority of data, approximately 200 nights of allocated time, was collected at several observatories including, the Uccle Observatory in Belgium, the South African Astronomical Observatory, Kryoneri and Skinakas in Greece, Hoher List Observatory in Germany, and the Cerro Tololo International Observatory in Chile. The selected systems were followed over a long period of time and with regularity, with a very high temporal sampling, so as to detect long-term trends, modulations on timescales of the orbital period, of the precession of the disc and very short timescales associated with flickering and possible quasi-periodic oscillations.

On the spectroscopic side, this thesis makes use of data, on two systems, collected at the 3.6m NTT (La Silla, Chile). These high-resolution time-resolved spectra give us the opportunity to probe the accretion disc structure in detail via the simultaneous study of several emission lines and the application of the indirect imaging technique of Doppler tomography.