Abstract:

1
A new study of magnetohydrodynamic jets
Anthony Moraghan Michael D.Smith Alexandar Rosen
Armagh Observatory and Trinity College Dublin
Armagh Observatory
Dublin City University
Questions How does a magnetic field influence
the jet structure? Can fragile molecules survive
under the extreme conditions? Are extragalactic
jets, stellar jets and protostellar jets connate?
Abstract We examine the physics of astrophysical
jets under some extreme conditions. We present
preliminary results of simulations of atomic and
molecular MHD jets with very small plasma beta
(high magnetic field), the condition appropriate
for protostellar jets. The simulations are
performed with a specially modified version of
the ZEUS-3D code. We demonstrate that the outflow
structure is very sensitive to the field
strength. First of all we investigate molecule
survival in gas impacted by jets of various
speeds. A comparison with recent observations
suggests that molecular physics and MHD conspire
to generate the observed structures.
100 km/s
200 km/s
300 km/s
Molecular Cooling vs Atomic Cooling
What patterns are visible in the grid to the
right?
It is a comparison between molecular and atomic
jets at different velocities.
In the molecular case, as the speed is raised
more molecules get destroyed. In the atomic case,
as the speed is raised the structure becomes more
collimated. At low speed the molecular and atomic
structures are different whereas at high speed
they become similar.
Molecular Cooling
Resolution Study
Axisymmetric adiabatic jets at various resolutions
1000 x 200
2000 x 400
Atomic Cooling
3000 x 600
v1 axial velocity n 2n(H ) n(H)
2
Which do we choose?
Now Molecular MHD jets
?
Strong toroidal magnetic field of 832 Gauss
The grid size chosen for a simulation is always a
compromise between structure detail and computing
power/memory/time.
4000 x 800
?
Plasma 0.0005 Density 1000cm Speed
100km/s Pulsing 60 year period /- 20
-3
The 4000 x 800 run took 36 hours using 6
processors running in parallel.
As the total number of zones increase, small
shock and eddy details become evident and this
can alter the overall structure.
Conclusions
This poster shows the complexity of jet
simulations and the intricateness of all the
chosen parameters, not only the physical
parameters but also the simulation size
parameters. At high resolution in 2D we can
improve on the physics at the expense of the
fluid dynamics. Molecular cooling has a large
effect on the jet structure e.g. maintaining
collimation. The effects of MHD and molecular
cooling is now being studied.
email ajm_at_arm.ac.uk