Magnetohydrodynamic Simulations of Accretion onto a Star in the “Propeller” Regime

Abstract

This work investigates spherical accretion onto a rotating magnetized star in the propeller regime using axisymmetric resistive magnetohydrodynamic simulations. In this regime accreting matter tends to be expelled from the equatorial region of the magnetosphere where the centrifugal force on matter corotating with the star exceeds the gravitational force. The regime is predicted to occur if the magnetospheric radius is larger than the corotation radius and less than the light cylinder radius. The simulations show that accreting matter is expelled from the equatorial region of the magnetosphere and that it moves away from the star in a supersonic, disk-shaped outflow. At larger radial distances the outflow slows down and becomes subsonic. The equatorial matter outflow is initially driven by the centrifugal force, but at larger distances the pressure gradient force becomes significant. We find that the star is spun down mainly by the magnetic torques at its surface with the rate of loss of angular momentum proportional to -Ωμ0.8, where Ω* is the star's rotation rate and μ is its magnetic moment. Further, we find that is approximately independent of the magnetic diffusivity of the plasma ηm for a factor ~30 range of this parameter, which corresponds to a range of magnetic Reynolds numbers ~1 to 1. The fraction of the Bondi accretion rate that accretes to the surface of the star is found to be ∝ Ωμ-2.1η. Predictions of this work are important for the observability of isolated old neutron stars and for wind-fed pulsars in X-ray binaries.