Propeller-driven Outflows and Disk Oscillations

Abstract

We report the discovery of propeller-driven outflows in axisymmetric magnetohydrodynamic simulations of disk accretion to rapidly rotating magnetized stars. Matter outflows in a wide cone and is centrifugally ejected from the inner regions of the disk. Closer to the axis there is a strong, collimated, magnetically dominated outflow of energy and angular momentum carried by the open magnetic field lines from the star. The "efficiency" of the propeller may be very high in the respect that most of the incoming disk matter is expelled from the system in winds. The star spins down rapidly due to the magnetic interaction with the disk through closed field lines and with the corona through open field lines. Diffusive and viscous interaction between the magnetosphere and the disk are important: no outflows were observed for very small values of the diffusivity and viscosity. These simulation results are applicable to the early stages of evolution of classical T Tauri stars (CTTSs) and to different stages of evolution of cataclysmic variables and neutron stars in binary systems. As an example, we show that young rapidly rotating magnetized CTTSs spin down to their present slow rotation in less than 10 6 yr.

The Reflares and Outburst Evolution in the Accreting Millisecond Pulsar Sax J1808.4–3658: A Disk Truncated Near Co-Rotation?

“…To enter a strong propeller regime with matter expelled from the system, r m needs to be larger than r co by at least a factor 1.3 » so that the gas can gain enough kinetic energy to reach the escape velocity (Spruit & Taam 1993). If a strong propeller (Illarionov & Sunyaev 1975;Romanova et al 2005;Ustyugova et al 2006) is operating in the system then the very-low luminosities we observe are not due to an extremely low mass flow in the disk, but instead reflect the fact that only a tiny (poorly constrained) fraction of the mass actually falls onto the neutron star surface, generating X-rays (see also Lasota et al 1999 where a similar scenario was proposed for the dwarf nova WZ-Sge). In this case, the flow itself must likely be radiatively inefficient since otherwise the much-higher accretion rate in the disc will likely dominate the X-ray emission.…”

Order in the chaos? The strange case of accreting millisecond pulsars

“…We note that results of MHD simulations on the interaction between the accretion disc and the magnetosphere of a NS in the propeller regime (i.e. when the accretion radius is larger than corotation radius) presented by [17]; see also [18]) show that the spin down torque resulting from this interaction may decrease with decreasing accretion rate, with the material torque owing to the accreted matter relegated to a marginal role in building the overall torque. This behavior is related to the weaker coupling between the magnetosphere and the disc matter corresponding to a lower accretion rate, which has the effect to weaken the toroidal component of the magnetic field in the magnetosphere, which is the one responsible for the spinning down of the pulsar.…”

Flow Structure in Magnetic CVs

“…This regime is known as the "propeller regime". In this regime, the accretion process is very non-stationary (Romanova et al 2004b(Romanova et al , 2005Ustyugova et al 2006). In the equilibrium rotation, the condition r c = r m is satisfied.…”