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J. L. Fisker, D. S. Balsara (University of Notre Dame)
For a non-magnetized white dwarf the accretion disk extends all the way to the surface, where matter impacts and spreads as new matter continuously piles up behind it. Using the RIEMANN code we have simulated the boundary layer on an axisymmetric grid to see how its structure depends on magneto-rotational instabilities (MRI) and hydrodynamical turbulence.
We have investigated the MRI instabilities directly by solving the 3d MHD equations and we have also solved the 3d compressible Navier-Stokes equations where the MRI instabilities and turbulence have been parametrized by different \alpha-viscosities.
The high viscosity cases show a supersonically spreading boundary layer which sets off gravity waves in the surface matter which makes them susceptible to the rapid development of gravity wave and/or Kelvin-Helmholtz instabilities. This BL is optically thick and extends more than 30 degrees from the disk plane after a Keplerian rotation period. The low viscosity cases also show a spreading BL, but here the accretion flow does not set off gravity waves and it is optically thin.
This work is sponsored by the Joint Institute of Nuclear Astrophysics (http://www.JINAweb.org) under NSF-PFC grant PHY02-16783
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.