“Propeller” Regime of Disk Accretion to Rapidly Rotating Stars

Abstract

We present results of axisymmetric magnetohydrodynamic simulations of the interaction of a rapidly rotating, magnetized star with an accretion disk. The disk is considered to have a finite viscosity and magnetic diffusivity. The main parameters of the system are the star's angular velocity and magnetic moment, and the disk's viscosity and diffusivity. We focus on the "propeller" regime where the inner radius of the disk is larger than the corotation radius. Two types of magnetohydrodynamic flows have been found as a result of simulations: "weak" and "strong" propellers. The strong propellers are characterized by a powerful disk wind and a collimated magnetically dominated outflow or jet from the star. The weak propellers have only weak outflows. We investigated the time-averaged characteristics of the interaction between the main elements of the system, the star, the disk, the wind from the disk, and the jet. Rates of exchange of mass and angular momentum between the elements of the system are derived as a function of the main parameters. The propeller mechanism may be responsible for the fast spinningdown of the classical T Tauri stars in the initial stages of their evolution, and for the spinning-down of accreting millisecond pulsars.

Accretion funnels onto weakly magnetized young stars

“…This is the "propeller" regime as studied e.g. by Ustyugova et al (2006) and references therein. Accretion thus implies r t < r co , with stellar magnetic field lines as leading spirals.…”

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.…”

“…Such an outflow would expel the majority of gas before it radiated energy in X-rays, which would make the discrepancy between optical and X-rays much larger. However, it is still possible that some (not too strong) outflow of material is present in the system, which could be driven either by a magnetospheric RIAF (like the strong propeller; Ustyugova et al 2006) or by an accretion driven RIAF (like a wind; Blandford & Begelman 1999), is present in the system. Finally, almost identical conclusions can be reached for the scenario B (trapped disk).…”

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.…”