Selected results - MHD simulations of spherical accretion to a star in the propeller regime

Recent papers on astro-ph

Wind Accretion to Dipole
- Bondi accretion
- Isolated old NS
- Propeller stage
- Magneto t a i l s

Disk Accretion to Dipole
- Inclined   rotator
- F u n n e l flows
- Propeller   stage
- Hot spots on star
- Radiative   shock

The Origin of Jets

Accretion Disks Theory
- Counterrotating
- ADAF theory

Extrasolar Planets

WIND ACCRETION TO MAGNETIZED STARS

MHD SIMULATION OF SPHERICAL ACCRETION TO A STAR IN THE "PROPELLER" REGIME

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

This work investigates spherical accretion to a rotating magnetized star in the "propeller" regime using axisymmetric resistive, magnetohydrodynamic simulations. In this regime accreting matter tends to be expelled from the equatorial region of the magnetosphere where the centrifugal force on matter rotating with the star exceeds the gravitational force. Axisymmetric magnetohydrodynamic simulations of spherical Bondi accretion to a rotating magnetized star in the propeller regime of accretion have shown that:

1. A new regime of matter flow forms around a rotating star. Matter falls down along the axis, but only a small fraction of the incoming matter accretes to the surface of the star. Most of the matter is expelled radially in the equatorial plane by the rotating magnetosphere of the star. A low-density torus forms in the equatorial region which rotates with velocity significantly larger than the radial velocity. Large scale vortices form above and below the equatorial plane.

2. The star is spun-down by the magnetic torque and to a lesser extent the matter torque at its surface. The rate of loss of angular momentum dL/dt proportional to -W_*^1.3 m ^0.8 $ and it is approximately independent of h_m. The rotational energy lost by the star goes into the directed and thermal energy of plasma.

3. The accretion rate to the star is much less than the Bondi accretion rate and decreases as

(a) the star's rotation rate increases ~ W_*^-1.0 ,
(b) as the star's magnetic moment increases ~ m ^-^2.¹ ,
(c) as the magnetic diffusivity decreases ~ h_m ^0.7.

4. Because the accretion rate to the star is less than the Bondi rate, a shock wave forms in our simulations and propagates outward. It has the shape of an ellipsoid flattened along the rotation axis of the star.