1. Jets from Viscous Accretion Disks
Gareth Murphy
Claudio Zanni
Jonathan Ferreira

2. 02/07/08 Rhodes
Introduction
● To accrete material, need to lose angular
momentum efficiently.
● Either by upward transport (jet), or anomalous
viscous transport (disk)
● ● Equipartition Magnetic field?

3. 02/07/08 Rhodes
JEDs and SADs
● Jet Emitting Disk (JED) has equipartition
magnetic field -> large lever arm as far as
Alfven surface
● Standard Accretion Disk (SAD) has small lever
arm (~αH) disk height

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How to get large scale field into
disk?
● Dynamo action? (Pudritz 1981, Brandenburg et
al 05) May not be able to produce a large
steady axisymmetric field
● Magnetic field dragging?
● From star?

5. 02/07/08 Rhodes
Inward dragging of ISM field?
● No! Turbulent motions can
cause magnetic field to
diffuse radially outward
(van Ballegooijen 1989,
Lubow, Papaloizou &
Pringle 1994)
● Yes! Non-turbulent surface
layer can advect field inwards
(Bisnovyati-Kogan &
Lovelace 2007, Rothstein &
Lovelace 2008a,b)

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Standard Accretion Disk Model
● Kluzniak-Kita (2000) - perturbative solution for
steady accretion flow in thin disk
● α > α_crit for accretion at all heights during the
simulation
● Superimpose an ordered, weak magnetic field
on the disk.
● Include viscosity, resistivity
● Prandtl number=1
● Damps MRI

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Field redistribution

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Accretion Timescale @ r=6
● After 2 accretion
timescales, mu
has broken power
law distribution
● -0.5 init
● -2 inner region
● -7 outer region

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● Super-fast
magnetosonic jet
● Fast SM & Alfven
critical surfaces

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Questions
● Stable outflow?
● Powerful?
● Magnetically or Pressure-driven outflow?
● Effect of jet torque on disk?

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Accretion & Ejection Power
Total Power Mechanical Poynting
Jet 1.7 1.03 0.69
Disk (r =1.2) 35.4 35.1 0.3
Mechanical = (Thermal + Gravitational + Kinetic)
Total jet power is a small fraction of accretion power

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Specific Energy (Bernoulli) Invariant

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Poloidal forces projected along a magnetic
surface

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Poloidal forces projected along field lines

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Effect of Mag Torque vs Visc Torque
Mag torque is small compared with viscous – accretion
mainly viscous

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Perspectives
● Weak, large scale field threading a thin disk can
be advected inward.
● Super fast magnetosonic jet launched from
weakly magnetised accretion disk.
● Jet appears to be thermally launched –
numerical biases present!
● Magnetic torque negligibly affects disk

17. 02/07/08 Rhodes
Extra slides: Field structure

18. 02/07/08 Rhodes
Extra slides:
Poloidal
Current
contours
I=rBФ