The response of a turbulent accretion disc to an imposed epicyclic shearing motion |
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Authors: | Ulf Torkelsson Gordon I Ogilvie Axel Brandenburg James E Pringle Åke Nordlund Robert F Stein |
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Institution: | Institute of Astronomy, Madingley Road, Cambridge CB3 0HA;Chalmers University of Technology/Göteborg University, Department of Theoretical Physics, Astrophysics Group, S-412 96 Gothenburg, Sweden;Isaac Newton Institute for Mathematical Sciences, 20 Clarkson Road, Cambridge CB3 0EH;Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Straße 1, Postfach 1523, D-85740 Garching bei München, Germany;Department of Mathematics, University of Newcastle upon Tyne, Newcastle NE1 7RU; Nordita, Blegdamsvej 17, DK-2100 Copenhagen Ø, Denmark;Theoretical Astrophysics Center, Juliane Maries Vej 30, DK-2100 Copenhagen Ø, Denmark;Copenhagen University Observatory, Juliane Maries Vej 30, DK-2100 Copenhagen Ø, Denmark;Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824, USA |
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Abstract: | We excite an epicyclic motion, the amplitude of which depends on the vertical position, z , in a simulation of a turbulent accretion disc. An epicyclic motion of this kind may be caused by a warping of the disc. By studying how the epicyclic motion decays, we can obtain information about the interaction between the warp and the disc turbulence. A high-amplitude epicyclic motion decays first by exciting inertial waves through a parametric instability, but its subsequent exponential damping may be reproduced by a turbulent viscosity. We estimate the effective viscosity parameter, α v, pertaining to such a vertical shear. We also gain new information on the properties of the disc turbulence in general, and measure the usual viscosity parameter, α h, pertaining to a horizontal (Keplerian) shear. We find that, as is often assumed in theoretical studies, α v is approximately equal to α h and both are much less than unity, for the field strengths achieved in our local box calculations of turbulence. In view of the smallness (~0.01) of α v and α h we conclude that for β p gas p mag~10 the time-scale for diffusion or damping of a warp is much shorter than the usual viscous time-scale. Finally, we review the astrophysical implications. |
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Keywords: | accretion accretion discs instabilities MHD turbulence |
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