Kelvin-Helmholtz Instability of Counter-Rotating Discs

Observations of galaxies and models of accreting systems point to the occurrence of counter-rotating discs where the inner part of the disc ($r<r_0$) is co-rotating and the outer part is counter-rotating. This work analyzes the linear stability of radially separated co- and counter-rotating thin discs. The strong instability found is the supersonic Kelvin-Helmholtz instability. The growth rates are of the order of or larger than the angular rotation rate at the interface. The instability is absent if there is no vertical dependence of the perturbation. That is, the instability is essentially three-dimensional. The nonlinear evolution of the instability is predicted to lead to a mixing of the two components, strong heating of the mixed gas, and vertical expansion of the gas, and annihilation of the angular momenta of the two components. As a result the heated gas will free-fall towards the disc's center over the surface of the inner disc

Jets and Disk-Winds from Pulsar Magnetospheres

We discuss axisymmetric force-free pulsar magnetospheres with magnetically collimated jets and a disk-wind obtained by numerical solution of the pulsar equation. This solution represents an alternative to the quasi-spherical wind solutions where a major part of the current flow is in a current sheet which is unstable to magnetic field annihilation.Comment: 6 figures, accepted for publication in the Ap

Propeller outflows from an MRI disc

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.Comment: 13 pages, 10 figures, submitted to MNRA