Location of the Inner Radius of a Magnetically Threaded Accretion Disk

Abstract

In models for disk accretion onto magnetized objects, the inner radius R0 ofthe Keplerian disk is conventionally expressed in the form R0 = xirA, where rA is the Alfven radius for spherical accretion and theparameter xi is usually taken to be 0.5. We point out that the valueof xi in fact depends on the fraction of the star's magnetic fluxthreading the disk, which is poorly known from theory: in general, xi~= 1.35 eta 4/7, where eta