Authors: R. V. E. Lovelace, M. M.
Romanova, G. S. Bisnovatyi-Kogan
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 Alfven radius (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 Alfven radius 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