Spherical Bondi Accretion onto a Magnetic Dipole

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Abstract
Spherical supersonic (Bondi-type) accretion to a star with a dipole magnetic field is investigated using resistive magnetohydrodynamic simulations. A systematic study is made of accretion to a nonrotating star, while sample results for a rotating star are also presented. We find that an approximately spherical shock wave forms around the dipole with an essential part of the star's initial magnetic flux compressed inside the shock wave. A new stationary subsonic accretion flow is established inside the shock wave with a steady rate of accretion to the star smaller than the Bondi accretion rate B. Matter accumulates between the star and the shock wave with the result that the shock wave expands. Accretion to the dipole is almost spherically symmetric at radii larger than 2RA, where RA is the Alfvén radius, but it is strongly anisotropic at distances comparable to the Alfvén radius and smaller. At these small distances matter flows along the magnetic field lines and accretes to the poles of the star along polar columns. The accretion flow becomes supersonic in the region of the polar columns. In a test case with an unmagnetized star, we observed spherically symmetric stationary Bondi accretion without a shock wave. The accretion rate to the dipole dip is found to depend on β∝B/μ2, where μ is the star's magnetic moment and ηm the magnetic diffusivity. Specifically, dip∝β0.5 and dip∝η0.38m. The equatorial Alfvén radius is found to depend on β as RA∝β-0.3, which is close to theoretical dependence ∝β-2/7. There is a weak dependence on magnetic diffusivity, RA∝η0.07m. Simulations of accretion to a rotating star with an aligned dipole magnetic field show that for slow rotation the accretion flow is similar to that in the nonrotating case with somewhat smaller values of dip. In the case of fast rotation the structure of the subsonic accretion flow is fundamentally different and includes a region of "propeller" outflow. The methods and results described here are of general interest and can be applied to systems where matter accretes with low angular momentum.
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