Numerical simulations of astrophysical jets from Keplerian accretion disks.

摘要

This thesis presents a series of magnetohydrodynamic (MHD) simulations which were designed to study the origin and evolution of astrophysical jets (galactic and extra-galactic). We developed and extended a version of the ZEUS-2D code which served as the numerical basis of our simulations and attached to it a complete analysis package.;With our version of the code, we established an initial state which consists of an accretion disk and its cold corona in stable equilibrium around a central object. No softening parameter was used to model the Newtonian gravitational potential of the central object. The corona and accretion disk are initially in pressure balance with one another. These initial states were constructed so as to be numerically stable.;In this thesis, we only considered magnetic configurations for which the Lorentz force is initially zero (J ;To test the theory of winds centrifugally driven from the surface of Keplerian accretion disks, we started with an open magnetic field line configuration. We found that a steady jet is quickly established, allowing direct comparison with the theory. We find the gas to be centrifugally accelerated through the Alfven and the fast magnetosonic surfaces and collimated into cylinders parallel to the disk's axis. The velocities achieved in our simulations are of the order of 250 km/s for our standard young stellar object (a 0.5 ;The second magnetic configuration we have studied consists of a uniform vertical structure wherein the magnetic field lines are parallel to the disk's axis. Here, the rotation of the disk twists the magnetic field lines. Because of the Keplerian scaling of the rotational velocity with the disk radius, the twisting of the field lines is higher in the inner parts of the disk. The strong magnetic gradient thus generated opens up the initial magnetic configuration in a narrow region. Within this narrow region, a wind is ejected from the field lines that have opened to less than the critical angle (;No special initial magnetic field structure is required in order to launch episodic outflows in our simulations. Rather, conditions favorable for the formation of an outflow set themselves up automatically through the production of a toroidal magnetic field whose pressure readjusts the structure of the field above the disk. The knot generator is episodic, and is inherent to the jet. (Abstract shortened by UMI.)