Magnetohydrodynamic Simulations of Accretion onto 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 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 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 ∝ Ω −1.0 * µ −1.7 η 0.4 m . Predictions of this work are important for the observability of isolated old neutron stars and for wind fed pulsars in X-ray binaries.

A Continuum of Accretion Burst Behavior in Young Stars Observed by K2

“…Models (e.g., D'Angelo & Spruit 2012) predict a "trapped disk" regime in which relatively continuous accretion bursts occur on short timescales. When r m >> r c , material tends to be flung out azimuthally in what is known as the "strong propeller" regime (Romanova et al 2003a). …”

High-mass X-ray binaries in the Milky Way

“…However, these simulations could not include yet all relevant 3D magnetohydrodynamic instabilities that dramatically affect plasma entry into the NS magnetosphere and thus the mass accretion rate (see, e.g. Toropin et al, 1999;Romanova et al, 2003, and references therein).…”

Propeller-driven Outflows and Disk Oscillations

“…Different aspects of the propeller regime have been investigated analytically (Davies et al 1979;Li & Wickramasinghe 1997;Lovelace et al 1999;Ikhsanov 2002;Rappaport et al 2004;Ekşi et al 2005) and studied with computer simulations (Wang & Robertson 1985;Romanova et al 2003.…”