Dead zones in protoplanetary discs

Transcription

1 Dead zones in protoplanetary discs Mark Wardle Macquarie University Raquel Salmeron (ANU) BP Pandey (Macquarie) Protoplanetary discs Magnetic fields Dead zones MRI and diffusion Modification to dead zones

2 Magnetic fields Magnetic fields are present in PPDs measured B in molecular clouds ==> B>1 mg in PPDs Zeeman, submm polarization Weak magnetic fields create turbulence subequipartition B and rotational shear ==> MHD turbulent torque magnetorotational instability (MRI) accretion, heating, stirring (chemistry, dust grains) disc evolution observational signatures planet formation planet migration (!) Accretion rates require B ~.1 1G at 1AU

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4 Magnetorotational instability (MRI)! magnetic field couples different radii in disc" tension transfers angular momentum outwards" kh > 1 required to fit in disc, i.e. v A /c s < 1! resulting turbulence transports angular momentum outwards!

5 Flux freezing breaks down in PPDs high density and low ionisation drag on charged particles deeper layers shielded from ionising radiation for r < 5 AU x-ray attenuation column ~1 g/cm 2 cosmic ray attenuation column ~1 g/cm 2 dead zone near midplane (Gammie 1996)

6 MRI with! dead zone! Turner & Sano 28!

7 Magnetic diffusion Ideal MHD electrons, ions and neutrals tied to magnetic field Ambipolar diffusion neutrals decoupled Hall diffusion ions and neutrals decoupled Ohmic diffusion electrons, ions and neutrals decoupled

8 Ambipolar Hall

9 Wardle 27

10 2!! I ohmic + ambipolar diffusion!!! " II η P Ω / v A 2 1 stable I k k c k!! " k III k II III s η H Ω / v A 2 k ideal MHD Wardle & Sameron 212

11 Charged particle abundances 6 M dust / M gas = M dust / M gas = 1 2 log n / n H M + e m + C + H 3 + He+ H + 7 Z 61 6 Z 51 8 Z Z 1 Z = 11 Z 2 M + e m + C + H He+ + 3 H + 7 Z 61 6 Z 51 8 Z 71 5 Z Z Z 81 6 M dust / M gas = 1 M +, e log n / n H 14 m + 18 log ζ H (s 1 ) C + H 3 + He+ H z / h

12 Max growth rate: no dust 2.5 B z < M dust / M gas = B z > 5 z (1 12 cm) z (1 12 cm) ohmic only z / h z / h ! / Ω B (G) Wardle & Sameron 212

13 Max growth rate: Mdust / Mgas = B z < 2.5 M dust / M gas = 1 2 B z > 5 z (1 12 cm) z / h ! / Ω ohmic only z (1 12 cm) z / h B (G) Wardle & Sameron 212

14 Column density of active layer! Wardle & Sameron 212

15 Column density of active layer! Wardle & Sameron 212

16 Summary & Discussion Ionisation levels determine extent of magnetically turbulent regions in protoplanetary discs dead zones: ~.3 3 AU from central star topped by magnetically active layers Hall diffusion modifies thickness of active layer by an order of magnitude dead zones: Bz < vs Bz > ; depends on magnitude of B can sustain accretion rates in these layers across radial extent of dead zone may drive undead zones more general geometries destabilise disk (Pandey & Wardle submitted) MHD simulations? linear analysis such as used here appears to be a good predictor of dead zone extent no MHD simulations with strong hall diffusion (yet) PS: simulations with non net flux are unrealistic