Magnetic Propeller Outflows

A model is developed for magnetic `propeller'-driven outflows which cause arapidly rotating magnetized star accreting from a disk to spin-down. Energy andangular momentum lost by the star goes into expelling most of the accretingdisk matter. The theory gives an expression for the effective Alfven radius$R_A$ (where the inflowing matter is effectively stopped) which depends on themass accretion rate, the star's mass and magnetic moment, and the star'srotation rate. The model points to a mechanism for `jumps' between spin-downand spin-up evolution and for the reverse transition, which are changes betweentwo possible equilibrium configurations of the system. In for example thetransistion from spin-down to spin-up states the Alfven radius $R_A$ decreasesfrom a value larger than the corotation radius to one which is smaller. In thistransistion the `propeller' goes from being ``on'' to being ``off.'' The ratioof the spin-down to spin-up torque (or the ratio for the reverse change) in ajump is shown to be of order unity.Comment: 5 pages, 2 figures, emulapj.sty Replace: new Figure