The Propeller Regime of Disk Accretion to a Rapidly Rotating Magnetized Star

Abstract

The propeller regime of disk accretion to a rapidly rotating magnetized star is investigated here for the first time by axisymmetric 2.5-dimensional magnetohydrodynamic simulations. An expanded, closed magnetosphere forms in which the magnetic field is predominantly toroidal. A smaller fraction of the star's poloidal magnetic flux inflates vertically, forming a magnetically dominated tower. Matter accumulates in the equatorial region outside magnetosphere and accretes to the star quasi-periodically through elongated funnel streams that cause the magnetic field to reconnect. The star spins down owing to the interaction of the closed magnetosphere with the disk. For the considered conditions, the spin-down torque varies with the angular velocity of the star (ω*) as ∼ - ω* 1.3 for a fixed mass accretion rate. The propeller stage may be important in the evolution of X-ray pulsars, cataclysmic variables, and young stars. In particular, it may explain the present slow rotation of the classical T Tauri stars.