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DISK ACCRETION TO MAGNETIZED STARS

THE PROPELLER REGIME OF DISK ACCRETION TO A RAPIDLY ROTATING MAGNETIZED STAR

[abstract] [full text] [plots from the paper] [animation]

Axisymmetric MHD simulations have been done for the interaction of an accretion disk with a rapidly rotating magnetized star in the propeller regime. We performed multiple simulation runs for different angular velocities of the star w* (from 0.086 to  0.82) with the other parameters fixed. 

We observed that disk accretion to a star in the propeller regime exhibits a number of interesting new features: (1) The most remarkable one is the fact that the rate of spin-down increases as the accretion rate increases. This is opposite the dependence in the non-propeller regime. (2) The fact that accretion occurs in the strong propeller regime is also important. The accretion is quasi-periodic and occurs through elongated funnel streams which go around the region of centrifugally dominated magnetosphere. (3) About 1/3 of the starís magnetic flux goes to the magnetic tower which represents a region of opened field lines. However, the remainder of the flux goes in to a radially expanded closed magnetosphere, which connects the star and the disk. This magnetic field is responsible for the strong angular momentum transport between the star and the disk. (4) The propeller stage may be very efficient in spinning-down rapidly rotating accreting stars. 

It was recently suggested that the opening of the magnetic field lines disconnects the star and the disk so that the spin-down of the star should be significantly reduced. Our simulations show that most of the magnetic flux of the star is in a closed magnetosphere and is coupled to the accretion disk. The angular momentum of the star is magnetically transported to the disk. Thus, the propeller mechanism may explain the spinning-down of the Classical T Tauri stars. For much stronger magnetic fields, propeller-driven outflows were observed, which will be discussed in a subsequent paper (Ustyugova et al. 2004). Recently, we were able to perform full 3D simulations of the disk accretion to an inclined dipole (Koldoba et al. 2002, Romanova et al. 2003b, 2004). Next, we plan to investigate the propeller regime using fully three-dimensional MHD simulations.

 

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