Magnetic activity in accretion disc boundary layers |
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Authors: | Philip J.Armitage |
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Affiliation: | School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife KY16 9SS |
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Abstract: | We use three-dimensional magnetohydrodynamic simulations to study the structure of the boundary layer between an accretion disc and a non-rotating, unmagnetized star. Under the assumption that cooling is efficient, we obtain a narrow but highly variable transition region in which the radial velocity is only a small fraction of the sound speed. A large fraction of the energy dissipation occurs in high-density gas adjacent to the hydrostatic stellar envelope, and may therefore be reprocessed and largely hidden from view of the observer. As suggested by Pringle , the magnetic field energy in the boundary layer is strongly amplified by shear, and exceeds that in the disc by an order of magnitude. These fields may play a role in generating the magnetic activity, X-ray emission and outflows in disc systems where the accretion rate is high enough to overwhelm the stellar magnetosphere. |
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Keywords: | accretion, accretion discs magnetic fields MHD novae, cataclysmic variables stars: pre-main-sequence stars: winds, outflows |
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