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

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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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