Magnetic activity in protoplanetary discs

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Magnetic activity in protoplanetary discs
Catherine Braiding (Macquarie) Arieh Königl
(Chicago) BP Pandey (Macquarie) Raquel Salmeron
(ANU)

• Mark Wardle
• Macquarie University
• Sydney, Australia

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• Magnetic fields
• Role of magnetic field is unclear
• MHD turbulence (magnetorotational instability)?
• disc-driven MHD winds?
• disc corona?
• dynamo activity?
• magnetic field strength?

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• Magnetic field strength
• Expect B gt 10 mG given the measured strength in
  cloud cores
• Compression during formation of disk and star
• Shear in disc may wind up field and/or drive MRI
• Equipartition field in the minimum mass solar
  nebula
• Evidence for 0.1 1 G fields in the solar nebula
  at 1AU

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• B 1G is required for angular momentum transport

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• Protostellar disks are poorly conducting
• high density implies low conductivity
• recombinations relatively rapid
• drag on charged particles
• deeper layers shielded from ionising radiation
  for r lt 5 AU
• x-ray attenuation column 10 g/cm2
• cosmic ray attenuation column 100 g/cm2
• dead zone near midplane (Gammie 1996)

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• Magnetic diffusion
• Essential ingredient in any theory
• large for YSO discs, so determines field
  evolution
• permits accretion of matter, not magnetic field
• energy dissipation
• turbulent scales
• boundary conditions for jet models
• determined by abundances of charged particles and
  their collision cross sections with neutrals

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• Magnetic diffusion regimes

fully ionized partially ionized
Ideal MHD ions and electrons tied to field ions, electrons and neutrals tied to field
Ambipolar neutrals decoupled
Hall ions decoupled ions and neutrals decoupled
Ohmic ions and electrons decoupled ions, electrons and neutrals decoupled
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• If the only charged species are ions and
  electrons,
• Three distinct diffusion regimes
• see Pandey Wardle (2008) for generalisation
• to all levels of ionisation

A
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Wardle 2007
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• Initial conditions large scale poloidal field

Mellon Li 2009
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Shu et al 2007
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• B drift due to ambipolar diffusion

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• Hall diffusion

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Braiding - thesis
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Braiding - thesis
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Braiding - thesis
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Magnetorotational instability
Wardle Salmeron in prep Pandey Wardle in prep
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• Ambipolar or ohmic diffusion (Bz gt 0)

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• Ambipolar or ohmic diffusion (Bz lt 0)

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• Hall diffusion (Bz lt 0)

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• Hall diffusion (Bz gt 0)

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Wardle Salmeron in prep
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Maximum growth rate and corresponding wavenumber
Wardle Salmeron in prep
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Abundances 1AU, no grains
e
M
m
C
He
log?z(s-1)
H
log n / nH
z / h
Wardle 2007
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Wardle 2007
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MRI growth rate (?)
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MRI growth rate (?)
ohmicambipolar diffusion
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MRI growth rate (?)
Full diffusion (Bz gt 0)
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MRI growth rate (?)
Full diffusion (Bz lt 0)
no Hall diffusion
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• Disc-wind launching at 1 AU

Salmeron Konigl 2009
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Abundances 1AU, 0.1mm grains
m
C
He
M
e
log?z(s-1)
0
H
log n / nH
1
-11
-4
2
-12
-3
3
-13
-2
-14
z / h
Wardle 2007
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MRI growth rate (?)
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MRI growth rate (?)
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• Summary
• Magnetically-driven accretion requires B 1
  gauss at 1 AU
• Molecular cloud core collapse calculations give B
  of this order
• magnetic braking too severe?
• magnetic flux problem?
• relevance of zero net flux MRI calculations?
• Magnetic diffusion varies quantitatively and
  qualitatively within the disk
• MRI-driven turbulence with strong hall diffusion
  relatively unexplored
• is wind launching possible across a range of
  radii?
• dead zones Bz lt 0 vs Bz gt 0
• Even a small residual population of grains
  increase magnetic diffusion
• in absence of grains, X-rays ? ?active 150 g
  cm2 at 1AU
• 1 AU 0.1 µm ?active 2 g cm2
• 3 µm ?active 80 g cm2
• require 1000-fold reduction in grain charge
  carrying capacity relative to 0.1µm grains
  (dustgas mass 0.01)

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