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DISK ACCRETION TO MAGNETIZED STARS

MHD SIMULATIONS OF DISK - MAGNETIZED STAR INTERACTIONS IN QUIESCENT REGIME: FUNNEL FLOWS AND ANGULAR MOMENTUM TRANSPORT

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

We have done a wide range of MHD simulations of disk accretion to a rotating aligned dipole in order to understand the different accretion phenomena. The simulations show that funnel flows (FF), where matter flows out of the disk plane and essentially free-falls along the stellar magnetic field lines, are a robust feature of disk accretion to a dipole. Specifically, we find that 

Funnel Flow:

1. The disk truncates and a funnel flow forms near the magnetosphere radius rm, where magnetic pressure of the dipole is comparable to the kinetic plus thermal pressure of the disk matter.

2. The velocity of matter in the FF is much smaller than the Alfven velocity, [v] ~ (0.05 0.3) vA, so that matter flows along the magnetic field lines. The funnel flow accelerates and become supersonic. The Mach number is M ~ 3-4 at the surface of the star. At the star velocity is close to the free-fall velocity: vp ~ 0.7 vff .

3. The angular velocity of the FF gradually varies from its value at the inner edge of the disk to the angular velocity of the star. The `twist' of the magnetic field lines in the FF is small, B
f /Bp < 0.1, and it has a maximum  approximately in the middle of the FF.

4. The main forces which are responsible for dragging matter to the FF are matter pressure gradient force (near the disk) and gravitational force in the rest of the FF. Magnetic force is negligibly small.

5. About 1/3 of the magnetic flux responsible for the spin-up / spin-down the star goes through the FF, while the remainder is above the FF.

Disk - Star Interaction:

1. The magnetic field of the star in uences the nearby regions of the disk inside a radius rbr, while viscosity dominates at larger radii. The radius rbr depends on magnetic moment  of the star m and density in the disk.

2. Inside the radius rbr  the disk is strongly inhomogeneous. The density is 2-3 times smaller than in the disk without magnetic field. Magnetically braked matter accumulates near magnetosphere and forms a dense ring and funnel flow.

3. The star may spin-up or spin-down depending on the ratio of its rotation rate to the rotation rate at the inner radius of the disk. We find that "torqueless" accretion is possible when rcr / rm ~1.5, where rcr is the corotation radius.

4. At the star's surface, the angular momentum flux is transported mainly by the "twist" of the magnetic field. Angular momentum carried by matter in the disk at ~ rm is transferred almost completely to the magnetic field at the star's surface.

5. The coronal magnetic field is observed to open and close, but strong out flows were not observed for the considered parameters and quasi-equilibrium initial conditions. In the area of the disk where the field is strong, r < r
f  ~ 0.5 rbr, the magnetic field lines tend to decelerate / accelerate rotation of the disk instead of being opened. Besides, opening of magnetic field loops is supporessed by matter dominated corona compared to GBW99 who accepted dependence r ~ R-4 .

6. Strong outflows may be associated with strongly non-stationary accretion in the disk or when the disk is very dense and magnetic field lines are inclined to the disk. Sporadic out ows can arise from a global magnetic instability of the disk (e.g., Lovelace et al. 1994, 1997, 2002), but they may be absent during the quiescent evolution of the disk-star system.

 

created by O. Toropina, 2000-2004 Your comments are welcome
2000-2011, last updated on 19.03.11