Magnetic propeller outflows

A model is developed for magnetic, "propeller"-driven outflows that cause a rapidly rotating magnetized star accreting from a disk to spin-down. Energ y and angular momentum lost by the star goes into expelling most of the acc reting disk matter. The theory gives an expression for the effective Alfven radius R-A (where the inflowing matter is effectively stopped), which depe nds on the mass accretion rate, the star's mass and magnetic moment, and th e 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 Alfven ra dius R-A decreases from a value larger than the corotation radius to one th at is smaller. In this transition the "propeller" goes from being "on" to b eing "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.