Angular momentum transport through magnetic accretion curtains inside disrupted discs




The structure of accretion curtain flows, which form due to disc disruption by a strongly magnetic star, is considered. It is shown that a sub-Alfvénic, magnetically channelled flow is consistent with matching the magnetic field across the curtain base where it meets the disrupted inner region of the disc. The resulting angular velocity distribution in the curtain flow is calculated, together with the consequent angular momentum transfer rate to the star.

It is shown that the transition of material from the diffusive disc flow to the channelled curtain flow results in some angular momentum being fed back into the disc. This is consistent with the total angular momentum balance, and can result in a significantly smaller accretion torque acting on the star than that given by the standard model which assumes no angular momentum feedback to the disc. The sonic point coordinates are found and a critical stellar rotation rate results below which the sonic point merges with the curtain flow base, due to a reduced centrifugal barrier to the flow. Hence, at these lower rotation rates, little thermal assistance is required for matter to make the transition to a supersonic accretion flow.