Magnetically Driven and Collimated Jets from the Disc-Magnetosphere Boundary of Rotating Stars

TL;DR

This paper reviews recent MHD simulation studies of magnetically driven, collimated jets from the disc-magnetosphere boundary of rotating stars, revealing new wind types and jet structures in different rotational regimes.

Contribution

It introduces the concept of conical winds in slowly rotating stars and describes a two-component outflow in rapidly rotating stars, advancing understanding of jet launching mechanisms.

Findings

Discovery of conical winds with a 30° opening angle.

Magnetic forces drive winds more than centrifugal forces.

Identification of a two-component outflow in the propeller regime.

Abstract

We discuss recent progress in understanding the launching of outflows/jets from the disc-magnetosphere boundary of slowly and rapidly rotating magnetized stars. In most of the discussed models the interior of the disc is assumed to have a turbulent viscosity and magnetic diffusivity (as described by two ``alpha'' parameters), whereas the coronal region outside of the disc is treated using ideal magnetohydrodynamics (MHD). Extensive MHD simulations have established the occurrence of long-lasting outflows in both the cases of slowly and rapidly rotating stars. In the case of {\it slowly rotating stars}, a new type of outflow, {\it a conical wind}, is found and studied in simulations. The conical winds appear in cases where the magnetic flux of the star is bunched up by the inward motion of the accretion disc. Near their region of origin, the winds have the shape of a thin conical shell with a half-opening angle of $ \sim 30^\circ$. At large distances these outflows become magnetically collimated by their toroidal magnetic field and form matter dominated jets. That is, the jets are current carrying. The predominant driving force for the conical winds is the magnetic force proportional to the negative gradient of the square of the toroidal magnetic field and not the centrifugal force. In the case of {\it rapidly rotating stars} in the ``propeller regime,'' a two-component outflow is observed. The first component is similar to the matter dominated conical winds. A large fraction of the disc matter may be ejected into the winds in this regime. The second component is a high-velocity, low-density magnetically dominated {\it axial jet} where matter flows along the opened polar field lines of the star.