Three-Dimensional MHD Simulation of Astrophysical Jet by CIP-MOCCT Method

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Three-Dimensional MHD Simulation of Astrophysical
Jet by CIP-MOCCT Method
Hiromitsu Kigure (Kyoto U.), Kazunari Shibata
(Kyoto U.), Seiichi Kato (Osaka U.)
Abstract The acceleration and collimation
mechanisms of astrophysical jets are still not
made clear and various models have been proposed.
One of the most promising models is
magnetohydorodynamic (MHD) acceleration from
accretion disks. We develop the CIP-MOCCT scheme
to three-dimensional cylindrical code and solve
the interaction between an accretion disk and a
large-scale magnetic field. To investigate the
stability of the jet, a non-axisymmetric
perturbation is imposed on the rotational
velocity of the disk. The jet launched from the
disk has a non-axisymmetric structure but the
dependences of the jet velocity, mass outflow
rate, and mass accretion rate on the magnetic
field strength are similar to those in
axisymmetric case.
Introduction Astrophysical Jets and MHD Model
Astrophysical Jet Plasma flow with very high
velocity from, e.g., AGNs, YSOs, XRBs, etc. One
of the most promising model for jet launching and
collimation is the MHD model. The magnetic field
penetrating the accretion disk is twisted by the
rotation of accretion disk. The toroidal magnetic
field propagates as the torsional Alfven waves
(TAWs), making naturally a helical field. The
collimated shape of the jets is explained by the
hoop-stress of the helical field. To investigate
the stability of the disk and jet system, we
perform the three-dimensional non-axisymmetric
ideal MHD simulation with solving the disk
self-consistently (e.g., Matsumoto Shibata
1997, Steinacker Henning 2001).
Initial Conditions
As an initial condition, we assume that an
equilibrium disk rotates in a central point-mass
gravitational potential (e.g., Matsumoto et al.
1996, Kudoh et al. 1998). It is also assumed that
there exists a corona outside the disk with
uniformly high temperature. The corona is in
hydrostatic equilibrium without rotation. The
initial magnetic field is assumed to be uniform
and parallel to the rotation axis of the disk
(Br, Bf, Bz) (0, 0, B0).
Non-axisymmetric Structure in the Jets
Lobanov Zensus (2001) found that the 3C273 jet
has a double helical structure and it can be
fitted by two surface modes and three body modes
of K-H instability. On the other hand, the jet
launched from the disk in our simulation has a
non-axisymmetric (m2 like) structure in both
perturbation cases.
The stability condition for non-axisymmetric K-H
surface modes is
Two figures below show the distribution of
logarithmic density on the z2.0 plane at t7.0.
We check the above-mentioned stability condition
between the point 1 and 2, and between the point
3 and 4. In the sinusoidal perturbation case,
In the
random perturbation case,
K-H body modes become unstable if
, or,
The jets satisfy the former unstable condition
for a little time but after that the jets become
stable for that condition. Therefore, neither
surface modes nor body modes of K-H instability
can explain the production of this
non-axisymmetric structure.
The dependences of the maximum velocities, the
maximum mass outflow rates, and the maximum mass
accretion rates of jets on the magnetic energy.
(A) Axisymmetric cases (no perturbation), (B)
Sinusoidal perturbation cases, (C) Random
perturbation cases. The broken line shows the
Vz?Emg1/6, dMw/dt? Emg0.5, or, dMa/dt? Emg0.7.
Sinusoidal perturbation case
Random perturbation case