Protoplanetary Disks as Accretion Disks

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Protoplanetary Disks as Accretion Disks
Roman Rafikov (Princeton)
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Outline

• Origin of protoplanetary disks
• Observational properties
• Spectra and their formation
• Angular momentum transport
• Emphasize differences and similarities with disks
  around compact objects

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Origin
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Origin
Collapse of Jeans-unstable dense clumps of
molecular gas. A single time event disk is not
fed externally for a long time.
leads to
Typical accretion rate and time scale
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Rotational Support
Collapsing cloud slowly rotates at
Conservation of angular momentum leads to disk
formation
Likely that most of the stellar mass has been
processed through the disk. B fields may have
been important.
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Observational properties
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Observational properties
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Observational properties sizes

• Determined via
• Hi-res imaging in the visible of scattered (by
  dust) stellar light
• IR, submm or mm imaging of disks own thermal
  emission
• IR interferometry can resolve
  sub-AU details
• SED modelling

Disks sizes range between tens to thousands of
AU, consistent with expectations
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Observational properties disk lifetimes

• Disk age stellar age
• Determine average disk lifetimes by looking at
  fraction of stars with disks in groups of
  different ages
• This fraction decays with age
• Typical lifetimes are of order 1-10 Myrs.
  Disappear due to photoevaporation.

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Observational properties spectra

• Protoplanetary disks are usually passive their
  own accretion luminosity is small compared to the
  irradiation by the central star

at r gt 1 AU

• Irradiated disk is flared

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Observational properties spectra
Disk flaring plays very important role in shaping
disk spectrum
Spectrum of a flat disk
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Observational properties masses
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Minimum Mass Solar Nebula
Protoplanetary Disks
Based on smearing out the refractory content in
SS planets
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Angular momentum transport
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Possible angular momentum transport mechanisms

• Accretion implies outward angular momentum
  transport need some kind of viscosity
• Keplerian disks are hydrodynamically stable
• Convection does not provide outward angular
  momentum
• Magneto-rotational Instability (MRI) is the most
  likely agent, BUT
• - Unlike the disks around compact objects
  protostellar disks are poorly conducting
• - MRI gets modified by resistivity in important
  ways (especially at small scales)

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MRI with resistivity
Lundquist Number
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Neal Turner

• Protoplanetary disks around 1 AU are too cold
  for thermal ionization
• External sources are shielded

Mark Wardle
This gives rise to a dead zone near the disk
midplane (Gammie 1996)
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Dead zone
Fleming Stone 2003
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Dead zone
Fleming Stone 2003

• While magnetic stress virtually dies out in the
  dead zone, Reynolds stress
  gets transmitted
  
  (albeit at low levels)
  into this zone
  
  maintaining some
  transport there.

• Accretion rate is not constant in the dead zone
  - long term steady state is not possible

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Other things to worry about

• Other non-ideal MRI effects
• Ambipolar diffusion
• Hall effect

• Dust
• Small dust grains are efficient charge absorbers
• Abundance of small dust grains is poorly known
• Dust can grow and sediment towards midplane
• This can lead to streaming instabilities and
  turbulence

• Planets
• Density waves lead to outward angular momentum
  transport

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Comparison with disks around compact objects
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Conclusions

• Protoplanetary disks are cold, massive accretion
  disks surrounding young stars
• Stars likely form via fast accretion in the
  initial phases of the disk life
• They are likely transient objects lifetimes
  1-10 Myrs
• They are passive, heated mainly by their central
  stars, emit mainly in the IR and sub-mm range
• Accretion is likely due to MRI, which is
  significantly modified by the non-ideal effects
• Low ionization makes resistivity very low and
  damps MRI in some parts of the disks creating
  dead zones