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

We present 2.5-dimensional time-dependent simulations of the nonlinear evolution of nonrelativistic outflows 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 fixed platform in these simulations) into a cold corona in stable equilibrium. The initial magnetic field lines are taken to be uniform and parallel to the disk axis (z-axis). Because of the gradient force in the nonlinear torsional Alfvén waves generated by the rotor at the footpoints of the field lines, the initial magnetic configuration opens up in a narrow region on the disk's surface located at ri < r < 2ri with ri being the innermost radius of the disk. Within this narrow region, a wind is ejected from the field lines that have opened to less than the critical angle (≃60°), as expected from the centrifugally driven wind theory. Our simulations show that the strong toroidal magnetic field generated recollimates the flow toward the disk's axis and, through magnetohydrodynamic (MHD) shocks, produces knots. The knot generation mechanism occurs at a distance of about z ≃ 8ri from the surface of the disk. Knots propagate down the length of the jet at speeds less than the diffuse component of the outflow. The knot generator is episodic and is inherent to the jet.