Numerical Simulations of Astrophysical Jets from Keplerian Disks. II. Episodic Outflows

Abstract

We present 2.5-dimensional time-dependent simulations of the nonlinear evolution of nonrelativistic outNows from Keplerian accretion disks orbiting low-mass protostars or black holes accreting at sub- Eddington rates. The gas is ejected from the surface of the disk (which is a -xed platform in these simulations) into a cold corona in stable equilibrium. The initial magnetic -eld lines are taken to be uniform and parallel to the disk axis (z-axis). Because of the gradient force in the nonlinear torsional Alfvec n waves generated by the rotor at the footpoints of the -eld lines, the initial magnetic con-guration opens up in a narrow region on the diskIs surface located at with being the innermost r i r 2r i r i radius of the disk. Within this narrow region, a wind is ejected from the -eld lines that have opened to less than the critical angle (^60i), as expected from the centrifugally driven wind theory. Our simula- tions show that the strong toroidal magnetic -eld generated recollimates the Now toward the diskIs axis and, through magnetohydrodynamic (MHD) shocks, produces knots. The knot generation mechanism occurs at a distance of about from the surface of the disk. Knots propagate down the length of z ^ 8r i the jet at speeds less than the di†use component of the outNow. The knot generator is episodic and is inherent to the jet. Subject headings: accretion, accretion disks E galaxies: jets E ISM: jets and outNows E MHD