Propeller outflows from an MRI disc

TL;DR

This study uses axisymmetric simulations to explore how rapidly rotating, magnetized stars in the propeller regime eject matter through episodic outflows, leading to stellar spin-down over million-year timescales.

Contribution

It provides the first detailed simulation of MRI-driven propeller outflows, revealing episodic ejection, accretion-instability, and stellar spin-down mechanisms.

Findings

Outflows are launched as discrete plasmoids due to magnetosphere interaction.

Episodic accretion and ejection occur when matter accumulates faster than it is expelled.

Stars experience significant spin-down over approximately 1 million years.

Abstract

We present the results of axisymmetric simulations of MRI-driven accretion onto a rapidly rotating, magnetized star accreting in the propeller regime. The stellar magnetosphere corotates with the star, forming a centrifugal barrier at the disc-magnetosphere boundary which inhibits matter accretion onto the star. Instead, the disc matter accumulates at the disc-magnetosphere interface and slowly diffuses into the inner magnetosphere where it picks up angular momentum and is quickly ejected from the system as an outflow. Due to the interaction of the matter with the magnetosphere, this wind is discontinuous and is launched as discrete plasmoids. If the ejection rate is lower than the disc accretion rate, the matter accumulates at the disc-magnetosphere boundary faster than it can be ejected. In this case, accretion onto the star proceeds through the episodic accretion instability in which episodes of matter accumulation are followed by simultaneous accretion and ejection. During the accretion phase of this instability in which matter flows onto the star in funnel streams, we observe a corresponding rise in the outflow rate. Both the accretion and ejection processes observed in our simulations are highly non-stationary. The stars undergo strong spin-down due to the coupling of the stellar field with the disc and corona and we measure the spin-down timescales of around 1 Myr for a typical CTTS in the propeller regime.