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Wednesday 04 Jul 2012: What's Wrong with the Accretion Disks in Quiescent Dwarf Novae?
Dr Koji Mukai - NASA/GSFC
Physics, 4th floor 14:00-14:45
Accretion disks play a crucial role in a wide range of objects, from active galactic nuclei to star formation. While some are hot, ionized, actively accreting and therefore luminous, others are cold, mostly neutral, accreting at a low rate and therefore faint. Although it is important to understand both states, luminous disks are easier to study and therefore are arguably much better understood. To improve the situation, one may wish to study dwarf novae, which are a large subset of cataclysmic variables (close binaries in which a white dwarf is accreting from a Roche-lobe filling, late-type mass donor) whose disks switch between faint and luminous states. In quiescence, the disk is cold and faint, and the matter transferred from the secondary cannot all accrete onto the white dwarf, or so the standard disk instability model goes. Yet quiescent dwarf novae are moderately bright X-ray sources, indicating active accretion onto the white dwarf, at a level much too high compared to theoretical expectations. I will elaborate on this and other observational clues pointing to the shortcomings of our current understanding of the accretion disks in quiescent dwarf novae.
