MHD Simulations of Bondi Accretion to a Star in the "Propeller" Regime

Abstract

This work investigates Bondi accretion to 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 rotating with the star exceeds the gravitational force. The regime is predicted to occur if the magnetospheric radius larger than the corotation radius and less than the light cylinder radius. The simulations show that accreting matters 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 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 $\dot{L}$ proportional to $-Ω_*^{1.3}μ^{0.8}$, where $Ω_*$ is the star's rotation rate and $μ$ is its magnetic moment. Further, we find that $\dot{L}$ is approximately independent of the magnetic diffusivity of the plasma $η_m$. The fraction of the Bondi accretion rate which accretes to the surface of the star is found to be $\proptoΩ_*^{-1.0}μ^{-1.7}η_m^{0.4}$. Predictions of this work are important for the observability of isolated old neutron stars and for wind fed pulsars in X-ray binaries.