What is wrong with steady accretion discs?

C. J. Nixon, J. E. Pringle
2019 Astronomy and Astrophysics  
In a standard, steady, thin accretion disc, the radial distribution of the dissipation of the accretion energy is determined simply by energy considerations. Here we draw attention to the fact that while the (quasi-)steady discs in dwarf novae in outburst are in agreement with the expected emission distribution, the steady discs in the nova-like variables are not. We note that essentially the only difference between these two sets of discs is the time for which they have been in the high
more » ... ty, high accretion rate state. In such discs, the major process by which angular momentum is transported outwards is MHD turbulence. We speculate that such turbulence gives rise to corona-like structures (here called magnetically controlled zones, or MCZs) which are also able to provide non-negligible angular momentum transport, the magnitude of which depends on the spatial scale L of the magnetic field structures in such zones. For short-lived, high accretion rate discs (such as those in dwarf novae) we expect L ∼ H and the MCZ to have little effect. But, with time (such as in the nova-like variables) an inverse cascade in the MHD turbulence enables L, and the net effect of the MCZ, to grow. We present a simple toy model which demonstrates that such ideas can provide an explanation for the difference between the dwarf novae and the nova-like variable discs.
doi:10.1051/0004-6361/201935852 fatcat:h4fzloboebh3dgpn6krgsf23g4