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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 RAb -0.3 , which is close to theoretical prediction RA ~ b -2/7 (Davidson & Ostriker 1973).

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

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From our simulations we observe that a star with dipole magnetic field accretes matter only at specific Mdip  rate which is less than the Bondi rate MB. That is,  Mdip = kMB  with k< 1

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This accretion rate Mdip is smaller when  b ~  M / m is smaller, that is, when the star's magnetic field  is larger. Also, Mdip  increases as the magnetic diffusivity  hm increases; 

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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 Rcr =  (Gm/ W * 2)1/3  is significantly larger than the Alfven radius RA is similar to accretion to a non-rotating star.  However, the rate of accretion Mdip is smaller than in the corresponding non-rotating case. For a rapidly rotating star, where Rcr <  RA , 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.  

 

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2000-2011, last updated on 19.03.11