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

MAGNETIC INHIBITION OF ACCRETION AND OBSERVABILITY OF ISOLATED OLD NEUTRON STARS

Authors: Toropina O.D., Romanova M.M., Toropin Yu.M., Lovelace R.V.E.

	The figure shows an example of the calculated accretion flow onto a star with a dipole magnetic field. The run is characterized by the dimensionless parameters b=10^-7, parameter b₀=20, and dimensionless magnetic diffusivity h_m=10^-5. The background color scale represents the density of the flow and the solid lines the poloidal magnetic field lines. The length of the arrows is proportional to flow speed. The bold solid line shows the Alfven surface. The dashed line shows the sonic surface. The flow becomes strongly anisotropic close to the dipole. The simulations were done on a grid with 513 x 513 cells. The accretion rate to the star is (dM/dt / dM_B/dt) ~ 0.21.
	Enlarged view of the inner region of Figure 1. The anisotropy of the flow is evident. The bold solid line shows the Alfven surface. The dotted line shows the surface, corresponding to Lundquist number S=R_Av_A/h_m=1; inside this surface S>1 while outside it S<1.
	Differential mass accretion rate per unit solid angle d(dM/dt)/dW as a function of the polar angle Q for different magnetic moments m at a radius 2R_*. The dashed line corresponds to Bondi accretion onto non-magnetized star.
	Dependence of the accretion rate on the magnetic moment m.
[return]	Dependence of the accretion rate on the ratio of the Alfven radius to the radius of the star R_A/R_*.