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A weakly non-linear theory for spiral density waves excited by accretion disc turbulence

Authors

  • T. Heinemann,

    Corresponding author
    1. Institute for Advanced Study, Einstein Drive, Princeton, NJ 08540, USA
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  • J. C. B. Papaloizou

    Corresponding author
    1. Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Centre for Mathematical Sciences, Wilberforce Road, Cambridge CB3 0WA
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E-mail: tobi@ias.edu (TH); J.C.B.Papaloizou@damtp.cam.ac.uk (JCBP)

ABSTRACT

We develop an analytic theory to describe spiral density waves propagating in a shearing disc in the weakly non-linear regime. Such waves are generically found to be excited in simulations of turbulent accretion discs, in particular if the said turbulence arises from the magneto-rotational instability (MRI). We derive a modified Burgers equation governing their dynamics, which includes the effects of non-linear steepening, dispersion and a bulk viscosity to support shocks. We solve this equation approximately to obtain non-linear sawtooth solutions that are asymptotically valid at late times. In this limit, the presence of shocks is found to cause the wave amplitude to decrease with time as t−2. The validity of the analytic description is confirmed by direct numerical solution of the full non-linear equations of motion. The asymptotic forms of the wave profiles of the state variables are also found to occur in MRI simulations, indicating that dissipation due to shocks plays a significant role apart from any effects arising from direct coupling to the turbulence.

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