Abstract
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 viscosity, 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.
Highlights
In terms of the energy budget a steady accretion disc should be a relatively simple object
We have seen that in order to explain the properties of the steady accretion discs in both the outbursting dwarf novae and the nova-like variables some modification of standard accretion disc theory is required
We assume that each parcel of mass δm that is lost from radius R takes with it an angular momentum corresponding to2Ω(R) – we assume that the magnetic field line along which the mass δm flows is co-rotating with its footpoint at radius R until it reaches a radius kR
Summary
In terms of the energy budget a steady accretion disc should be a relatively simple object. The accretion discs about which we have the most detailed information in terms of spatial detail and time-dependent behaviour are the discs in cataclysmic variables These are short period semidetached binary systems in which a low mass quasimain-sequence star fills its Roche lobe and transfers mass via an accretion disc to a companion white dwarf (see, for example, the review by Warner 1995). Most closely approximating steady discs are those to be found in the dwarf novae in outburst (and super-outburst) and in the nova-like variables2 In these objects most of the emission in the optical and near ultraviolet (say, 600–120 nm) comes from the accretion disc (see, for example, the review by Bath & Pringle, in Pringle & Wade 1985).
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