Selected results - Accretion to dipole

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

Spherical Bondi accretion onto a magnetic dipole

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

We have developed a method for MHD simulation of spherical Bondi-type accretion flow to a rotating star with an aligned dipole magnetic field. Using this method we have made a detailed study of the accretion to a non-rotating star for different accretion rates, stellar magnetic moments, and magnetic diffusivities. We also include an illustrative case of accretion to a rapidly rotating star. The simulation study confirms some of the predictions of the analytical models (Davidson & Ostriker 1973; Lamb et al. 1973; Arons & Lea 1976). However, the simulated flows show a different behavior from the models in important respect summarized below.

Our results for accretion to a non-rotating star agree qualitatively with some of the early theoretical predicutions. In particular, (1) A shock wave forms around the dipole which acts as an obstacle for the accreting matter; (2) A closed inner magnetosphere forms where the magnetic energy-density is larger than the matter energy density; (3) The outer dipole magnetic field is strongly compressed by the incoming matter. (4) The flow is spherically symmetric at large distances, but becomes anisotropic near and within the Alfven surface. Closer to the star the accretion flow becomes highly anisotropic. Matter moves along the polar magnetic field lines forming funnel flows (Davidson & Ostriker 1973). (5) The Alfven radius varies with b ~ M / m² as R_A ~ b ^-0.3 , which is close to theoretical prediction R_A ~ b ^-2/7 (Davidson & Ostriker 1973).

The new features observed in our simulations of accretion to a non-rotating star include the following:

	From our simulations we observe that a star with dipole magnetic field accretes matter only at specific M_dip rate which is less than the Bondi rate M_B. That is, M_dip = kM_B with k< 1;
	This accretion rate M_dipis smaller when b ~ M / m ² is smaller, that is, when the star's magnetic field is larger. Also, M_dip increases as the magnetic diffusivity h_m increases;
	We observe that the shock wave which initially forms around the magnetosphere is not stationary but rather expands outward in all of our simulation runs. This is contrary to the theoretical models which assume a stationary or standing shock wave (Arons & Lea 1976).

We are presently making a systematic study of accretion to a rotating star with dipole field. In this work we give only sample results which illustrate the new behavior resulting from the star's rotation. Accretion to a slowly rotating star, where the corotation radius R_cr = (Gm/ W _* ²)^1/3 is significantly larger than the Alfven radius R_A is similar to accretion to a non-rotating star. However, the rate of accretion M_dipis smaller than in the corresponding non-rotating case. For a rapidly rotating star, where R_cr< R_A, propeller outflows form in the outer parts of magnetosphere and outside magnetosphere as proposed by Illarionov and Sunyaev (1975). These outflows result in a major change in accretion flow and field configuration.