Magnetic Propeller Outflows 3

A model is developed for magnetic 'propeller'-driven outflows which 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 A (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 transistion from spin-down to spin-up states the Alfvén radius R A decreases from a value larger than the corotation radius to one which is smaller. In this transistion 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.