The mechanism of disc disruption by strongly magnetic accretors

A range of important astrophysical systems consist of a strongly magnetic star accreting material from a surrounding disc. These include young stars in T Tauri systems, magnetic neutron stars in X-ray binaries and strongly magnetic white dwarfs in some cataclysmic variables. In all these objects, the stellar magnetic field disrupts the inner part of the disc, and subsequently channels material on to the star's surface. The interaction between the disc, channelled flow and star affects the spin behaviour of the star and hence the properties and evolution of the system. This paper considers the fundamental problem of how the stellar magnetic field disrupts the disc, in a way consistent with subsequent magnetic channelling of the accretion flow. It is shown that magnetic dissipation, inside the corotation radius, leads to increased central pressure which cannot be balanced by the stellar vertical gravity. The disc expands vertically over a narrow radial region to form an accretion curtain of material channelled by the stellar magnetic field. The increased vertical pressure gradients allow material to surmount the potential barrier associated with reaching the sonic point of the channelled flow and matter can subsequently accrete on to the star. An expression is derived for the disc disruption radius as a function of the stellar rotation rate.