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WIND ACCRETION TO MAGNETIZED STARS

MAGNETIC INHIBITION OF ACCRETION AND OBSERVABILITY OF  IONS  

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

Isolated old neutron stars (IONS) moving through the interstellar medium capture matter gravitationally. If the star is unmagnetized the captured matter  accretes to the surface of the star. However, the stars are expected to be magnetized. Moreover, some of the stars may be in the "propeller" stage of evolution. Both the magnetic field and the rotation act to decrease the accretion rate to the surface of the star. Here, we consider stars which are past the propeller stage so that rotation is unimportant. The influence of the magnetic field on the accretion rate to the star's surface is investigated using axisymmetric, resistive magnetohydrodynamic (MHD) simulations.  Matter is taken to inflow at the Bondi rate for a nonmagnetized star, and we verify that stationary Bondi accretion flows occur in the absence of a magnetic field.

The main conclusions of this work are: 

1. For matter accreting spherically to a magnetized star with the Bondi accretion rate dMB/dt, an outward propagating shock wave forms.  Inside this shock wave a new stationary, subsonic accretion flow is established. Accretion to the surface of the star occurs along two columns along the magnetic axis of the star. 

Outside of the Alfven radius the flow is approximately spherical while inside this radius the flow is collimated by the star's magnetic field. The flow is subsonic except at the ``throat'' or critical point of the flow, near the surface of the star, where the flow becomes sonic. 

2. Only a fraction of the initial Bondi flux dMB/dt accretes to the surface of the star, dM/dt = k dMB/dt, where k < 1. The empirical dependences we find are (dM/dt / dMB/dt) ~ (R* / RA)5 for RA / R* ~ 6-10,  where RA is the Alfven radius. In terms of the star's magnetic moment m, we find (dM/dt / dMB/dt) ~m-3. 

3. The accretion rate to the surface of the star decreases as the magnetic diffusivity of the plasma hm decreases,  dM/dt ~hm 0.6

4. Even if the residual magnetic field of IONS is very small, we still have R* / RA >> 1 so that accretion to the surfaces of the stars is greatly reduced from the Bondi rate. 

 The luminosities will be correspondingly reduced and may be undetectable with present instruments. The R* / RA  values in our simulations correspond approximately to the case of neutron stars with a very small surface magnetic fields, B ~ 106 G. Thus our simulation results indicate a strong inhibition of accretion to even very weakly magnetized neutron stars. The inhibition increases as the star's surface field increases.

The results presented here are also applicable to the wind-fed X-ray pulsars. However, the angular momentum of incoming matter in the binary system may be larger in X-ray pulsars, and disk accretion is more probable than in the case of IONS.

 

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