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Cataclysmic Variables (CVs) are close binary stars in which a white dwarf accretes gas via Roche lobe overflow from a low mass near-main sequence companion. The high gravitational potential of the white dwarf causes very high infall velocities, and the thermalisation of the kinetic energy of the accreting gas close to the white dwarf makes CVs bright X-ray sources.
In some cases, the white dwarf is strongly magnetic. In the AM Her type systems (or polars), the magnetic field is so strong that the stars are rotationally locked and the gas flows onto the white dwarf in a narrow steam controlled by its magnetic field as shown below (by Darren Baskill).
At lower magnetic fields (and wider separations) the accreting gas probably forms a disk of some sort, although close in the gas must follow the dipole-like field lines onto the white dwarf. These systems are called intermediate polars (or DQ Her systems). The drawing to the right shows one of the major components of an intermediate polar, showing the accretion flow (by Dr Simon Rosen).
At lower fields still, the disk probably extends down to the white
dwarf
surface, where we expect a hot interaction region to be formed between
the rapidly rotating disk material and the more slowly rotating white
dwarf.
The painting to the top left shows a non-magnetic CV.
Most CVs are non-magnetic and show dwarf-novae outbursts due to an increased accretion rate onto the white dwarf. Study of these systems allows us to probe accretion physics at various gas densities. The magnetic CVs show a huge wealth of periodic phenomena due the the orbital period of the system and the spin period of the white dwarf (and sometimes due to the interaction of these periods). Study of these systems allows us to get geometrical information about the system and multiple views of the magnetically confined accretion region at various orientations, both of which can give fundamental knowledge of the stars, the magnetic field and the physical processes close to the white dwarf.
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Page maintained by Keith Sohl (kbs@star.le.ac.uk) |  |