1999ApJ...514..368L

A model is developed for magnetic, ``propeller''-driven outflows that cause a rapidly rotating magnetized star accreting from a disk to spin-down. Energy and angular momentum lost by the star goes into expelling most of the accreting disk matter. The theory gives an expression for the effective Alfvén radius R<sub>A</sub> (where the inflowing matter is effectively stopped), which depends on the mass accretion rate, the star's mass and magnetic moment, and the star's rotation rate. The model points to a mechanism for ``jumps'' between spin-down and spin-up evolution and for the reverse transition, which are changes between two possible equilibrium configurations of the system. In, for example, the transition from spin-down to spin-up states, the Alfvén radius R_A decreases from a value larger than the corotation radius to one that is smaller. In this transition the ``propeller'' goes from being ``on'' to being ``off.'' The ratio of the spin-down to spin-up torque (or the ratio for the reverse change) in a jump is shown to be of order unity.