Dead zones and the growth of giant planets

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Dead zones and the growth ofgiant planets

• Ralph Pudritz
• (McMaster University)
• Soko Matsumura
• (Ph.D. McMaster PDF Northwestern)
• Ed Thommes
• (CITA Norwestern)

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Outline

• 1. Planet formation disks and gaps
• 2. Dead zones (DZs)
• 3. Gap opening masses in disks with DZs
• 4. Dead zones and planetary migration step 1
• The Point Dead zones (no MRI turbulence)
  expected from first principles they shape both
  planetary masses halt planetary migration

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Extrasolar Planets

• Several thousands of solar type stars surveyed
  5 20 have planets within 5 AU.
• More than 200 now known
• Question what halted migration in (some?)
  systems?

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HH 30 (from HST)
Protoplanetary disks from cores to planets
Gas Accretion Gap-formation
Protoplanet
http//www.astro.psu.edu/users/niel/astro1/slidesh
ows/class43/slides-43.html
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1. Planet Formation Disks and Gaps

• Giant planet formation two mechanisms under
  intense investigation
• 1. Core accretion model. Coagulation of
  planetesimals that when exceeding 10 Earth
  masses, gravitationally captures gaseous
  envelope (eg. Bodenheimer Pollack 1986)
• 2. Gravitational instability model . GI in
  Toomre unstable disk produces Jovian mass objects
  in one go (eg. Boss 1998).
• For either 1 or 2 final mass determined by
  gap opening in face of disk viscosity.

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When planets start to appear
Gap opens in a disk when Tidal Torque
Viscous Torque
Protoplanet
Tidal Torque
Disk
Viscous Torque
Disk
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(Matsumura Pudritz 2005, ApJL 2006, MNRAS)
- Gap-opening mass Final mass of a planet
- Two competing forces (Tidal vs Viscous) -
Smaller gap-opening masses in an inviscid disk
Need to know - disk flaring (h/a) -
viscosity
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Disk structure reprocessing stellar radiation
Submm
Infrared
Optical
Radiative resprocessing hydrostatic
equilibrium disk models 1 Disk Surface Tds 2
Disk Interior Ti
Chiang and Goldreich (1997)
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• - Most promising source of viscosity
• Magneto-rotational instability (MRI)
  turbulence
• (Balbus Hawley, 1991)
• Dead Zone where MRI is inactive (Gammie 1996)
• -gt In sufficiently poorly ionized region,
  Ohmic
• dissipation damps out MRI
• MRI active region Disk is well-ionized -gt MRI
  turbulence
• - Larger gap-opening mass for larger
  viscosity

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Dead Zone (Gammie, 1998) - Ionization rate is
very low - Magneto-rotational instability (MRI)
turbulence is inactive - .. So disks viscosity
is low there
Ionization X-rays cosmic rays
radioactive elements thermal collisions of
alkali ions
Recombination metal ions molecular
ions grains
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Our dead zones include entire pressure scale
height h of colder mid-plane (also include
critical column density ratio for excitation of
motion at midplane by turbulence in envelope).
13 AU
(Matsumura Pudritz 2005, 2006)
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Jupiter
Uranus or Neptune
Earth
Even a terrestrial mass planet opens a gap in a
DZ!!
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1. eg. Type I migration (before gap-opening)
? 10 MEarth (lt MUranus)
Dead Zone
Star
Protoplanet

• Numerical Technique
• We use a hybrid numerical code combining N-body
  symplectic integrator SYMBA (Duncan et al 1998)
  with evolution equation for gas (Thommes 2005)
• Allows us to follow evolution of planet and disk
  for disk lifetime 3 10 Million years.
• (Matsumura, Pudritz, Thommes 2006)

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Planetary migration planet disk interaction
(eg. Ward 1997)

• Planet exerts tidal torque at Lindblad resonances
  in disk.
• This excites spiral density waves - propagate
  away from resonances spread angular momentum
  throughout disk
• PROBLEM Migration too efficient lose planets
  in a million years!
• QUESTION What saves planetary systems?

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?10-2
?10-2
?10-5
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Evolution of disk column density during gap
opening
Note pile up of material at outer edge of dead
zone. This denstiy gradient deflects migration
of outer light planets.
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If planet forms within the DZhalt migration of
terrestrial planets by opening a gap in the DZ
10 M_E planet started in dead zone Left 2
million yrs Viscosity
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?10-3
?10-3
?10-5
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• Migration of a Jovian planet over 10 Myr.
• Note extent of gap opened by planet once inside
  dead zone. (But see Sokos following talk)
• Planet started at 20 AU settles into orbit at 4AU
  after 10 Myr

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• 10 ME opens gap at 3.5 AU in dead zone
• Also
• 1 ME opens gap near 0.1 AU
• - Look for this

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Summary

• DZs are inescapable (physics of MRI high
  column density of protostellar disks)
• DZs -gt sharp radius beyond which
• massive planets form (initially beyond 10 AU)
• DZs -gt terrestrial planets open gaps within
  them gt halt rapid loss of terrestrial planet
  cores. hope for Kepler mission?
• Outer edge of DZ very interesting place for GI
  instabilities?
• Question how do planets accrete as they migrate
  in evolving disks? See Sokos talk.