Author

Rachid Ouyed

Date of Award

12-1996

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Astrophysics

Supervisor

Ralph E. Pudritz

Abstract

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). \Ve 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 that was developed in order to make contact with the theory and observations
of jets.

With our version of the code, we managed to establish an initial state which
consists of an accretion disk and its cold corona in stable eqailibrium around a central object.
No softening paranleter 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. The corona is Inagnetized with the magmetic field lines extending
smoothly into the disk without kinks or discontinuities, avoiding, in this way, any
undesired currents in the initial set up. The disk is set in Kepler rotation and gas is
continuously injected into the corona above at the very snlalll speed of 10⁻³ times the
Kepler velocity.

In this thesis, we only considered magnetic configurations for which the Lorentz
force is initially zero (J x B = 0). In particular initial J = 0 configurations are
studied. \Ve carefully set the boundary conditions to be open conditions so as to avoid any collimation due to grid reflection effects.

To test the theory of winds centrifugally driven from the surface of Keplerian accretion
disks, we started with an open magnetic field line configuration. The magnetic
field lines have opening angles (with respect to the disk surface) less than the critical
angle (≃ 60°), as required for a centrifugally driven wind to start. 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 Alfvén and the fast magnetosonic
surfaces and collimated into cylinders parallel to the disk's axis. The collimation is
due to the pinch force exerted by the dominant toroidal magnetic field generated by
the outflow itself. The velocities achieved in our simulations are of the order of 250
km/s for our standard young stellar object (a 0.5 M proto-star) and of the order
or 10⁵ km/s for our standard active galactic nuclei (a 10⁸M black hole). Our jet
solutions are very efficient in magnetically extract.ing angular momentum and energy
fronl the disk.

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 and generates a toroidal field
component. 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 lnagnetic gradient thus generated opens up the initial magnetic configuration
in a narrow region located at 1rᵢ < r < 8rᵢ, 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 towards the disk's axis and, through MHD shocks,
produces knots. The knot generation mechanism occurs at a distance of about z ≃ 8rᵢ from the surface of the disk.

We have discovered that 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. Thus, jets
are apparently capable of producing the variability that leads to episodic events, independently of the underlying source.