Abstract
Accreting magnetized stars may be in the propeller regime of disc accretion in which the angular velocity of the stellar magnetosphere exceeds that of the inner disc. In these systems, the stellar magnetosphere acts as a centrifugal barrier and inhibits matter accretion onto the rapidly rotating star. Instead, the matter accreting through the disc accumulates at the disc-magnetosphere interface where it picks up angular momentum and is ejected from the system as a wide-angled outflow which gradually collimates at larger distances from the star. If the ejection rate is lower than the accretion rate, the matter will accumulate at the boundary faster than it can be ejected; in this case, accretion onto the star proceeds through an episodic accretion instability in which the episodes of matter accumulation are followed by a brief episode of simultaneous ejection and accretion of matter onto the star. In addition to the matter dominated wind component, the propeller outflow also exhibits a well-collimated, magnetically-dominated Poynting jet which transports energy and angular momentum away from the star. The propeller mechanism may explain some of the weakly-collimated jets and winds observed around some T Tauri stars as well as the episodic variability present in their light curves. It may also explain some of the quasi-periodic variability observed in cataclysmic variables, millisecond pulsars and other magnetized stars.
Highlights
An accreting magnetized star in spin equilibrium with its disc will enter the “propeller regime” of accretion if the accretion rate in the disc declines on a timescale shorter than the star’s spin reequilibration timescale
The propeller regime is characterized by three main features: firstly, the accretion of matter from the disc onto the star is inhibited by the centrifugal barrier; secondly, the centrifugal force may accelerate matter into an outflow; and lastly, the rapidly rotating star spins down due to the spindown torque exerted on the star
No viscosity or diffusivity terms have been included in the MHD equations and we investigate only accretion driven by the MHD turbulence associated with the magnetorotational instability (MRI) [see tests and other details in 15]
Summary
An accreting magnetized star in spin equilibrium with its disc will enter the “propeller regime” of accretion if the accretion rate in the disc declines on a timescale shorter than the star’s spin reequilibration timescale. When the accretion rate declines, the magnetosphere of the star expands outward due to the decrease in the ram pressure from the disc. In this new configuration, the angular velocity of the expanded stellar magnetosphere (which was previously in equilibrium) exceeds the Keplerian angular velocity of the matter in the inner disc, creating a centrifugal barrier at the disc-magnetosphere boundary. We summarize the results of a recent numerical study of the propeller regime utilizing a highresolution magnetohydrodynamics (MHD) code in cylindrical coordinates
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
