“Propeller” Regime of Disk Accretion to Rapidly Rotating Stars

We present results of axisymmetic magnetohydrodynamic simulations of theinteraction of a rapidly-rotating, magnetized star with an accretion disk. Thedisk is considered to have a finite viscosity and magnetic diffusivity. Themain parameters of the system are the star's angular velocity and magneticmoment, and the disk's viscosity, diffusivity. We focus on the "propeller"regime where the inner radius of the disk is larger than the corotation radius.Two types of magnetohydrodynamic flows have been found as a result ofsimulations: "weak" and "strong" propellers. The strong propeller ischaracterized by a powerful disk wind and a collimated magnetically dominatedoutflow or jet from the star. The weak propeller have only weak outflows. Weinvestigated the time-averaged characteristics of the interaction between themain elements of the system, the star, the disk, the wind from the disk, andthe jet. Rates of exchange of mass and angular momentum between the elements ofthe system are derived as a function of the main parameters. The propellermechanism may be responsible for the fast spinning-down of the classical TTauri stars in the initial stages of their evolution, and for the spinning-downof accreting millisecond pulsars.