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Giant Planet Accretion and Migration:
Surviving the Type I Regime
Giant Planet Accretion and Migration:
Surviving the Type I Regime
Edward Thommes
Norm Murray
CITA, University of Toronto
Edward Thommes
Norm Murray
CITA, University of Toronto
The Western Workshop, UWO, May 19, 2006The Western Workshop, UWO, May 19, 2006JPL
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Gas giant formation: The core accretion model
Gas giant formation: The core accretion model
Gas disk lifetime sets upper limit on gas giant formation: ~1-10 Myrs from observations (e.g. Haisch, Lada & Lada 2001)
The core accretion model (Mizuno 1980, Pollack et al 1996):
1. Solid core grows, ~10 MEarth
2. Core accretes massive gas envelope, 100+ MEarth
Observational support for core accretion: planet-metallicity correlation
(Gonzalez 1997, Fischer & Valenti 2003)
HD 149026 planet (Saturn mass, ~70 MEarth core; Sato et al. 2005, Charbonneau et al 2006)
Gas disk lifetime sets upper limit on gas giant formation: ~1-10 Myrs from observations (e.g. Haisch, Lada & Lada 2001)
The core accretion model (Mizuno 1980, Pollack et al 1996):
1. Solid core grows, ~10 MEarth
2. Core accretes massive gas envelope, 100+ MEarth
Observational support for core accretion: planet-metallicity correlation
(Gonzalez 1997, Fischer & Valenti 2003)
HD 149026 planet (Saturn mass, ~70 MEarth core; Sato et al. 2005, Charbonneau et al 2006)
Marcy et al 2005
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Planet-disk interactionPlanet-disk interaction Presence of substantial gas disk
means planet-disk interactions important!
Bodies in gas disk launch density waves repulsive torque between body and inner, outer disk
Jupiter-mass planets open a gap, locked into viscous evolution of disk: “Type II” inward migration
Smaller bodies: no gap, outer torques stronger: “Type I” inward migration
Presence of substantial gas disk means planet-disk interactions important!
Bodies in gas disk launch density waves repulsive torque between body and inner, outer disk
Jupiter-mass planets open a gap, locked into viscous evolution of disk: “Type II” inward migration
Smaller bodies: no gap, outer torques stronger: “Type I” inward migration
Density of planetdisk torque
Ward 1997
Geoff Bryden
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Migration and accretion ratesMigration and accretion rates
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Comparing the timescalesComparing the timescales
Scary result! Thus people tend to ignore/greatly reduce Type I (e.g. Thommes, Duncan & Levison 2003, Ida & Lin 2004, Alibert et al. 2005)
But is there a way to make the worst-case scenario work...?
Scary result! Thus people tend to ignore/greatly reduce Type I (e.g. Thommes, Duncan & Levison 2003, Ida & Lin 2004, Alibert et al. 2005)
But is there a way to make the worst-case scenario work...?
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Accretion, no migration Accretion, no migration
Thommes & Murray 2006
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Accretion + MigrationAccretion + Migration
Thommes & Murray 2006
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A viscously evolving diskA viscously evolving disk
t=0
t=1 Myr
t=10 Myrs
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Accretion + Migration in a viscously evolving gas diskAccretion + Migration in a viscously evolving gas disk
Thommes & Murray 2006
=10-2
Mdisk
M100 AU
M30 AU
Disk gas mass
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Winners and losersWinners and losers Inner region:
growth too fast, cores lost onto star
Outer region: growth too slow relative to disk lifetime
In between: An annulus where the growth rate turns out just right
Inner region: growth too fast, cores lost onto star
Outer region: growth too slow relative to disk lifetime
In between: An annulus where the growth rate turns out just right
Thommes & Murray 2006
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Method: Vary disk mass, metallicity,
For each set (MD,
,[Fe/H]), compute largest protoplanet mass when 1 MJup of gas left inside 100 AU
Results Disks with higher MD,
[Fe/H] do better There is always an
“optimal” , ~10-2-10-3; consistent with fits to T Tauri disks (Hartmann et al 1998)
Method: Vary disk mass, metallicity,
For each set (MD,
,[Fe/H]), compute largest protoplanet mass when 1 MJup of gas left inside 100 AU
Results Disks with higher MD,
[Fe/H] do better There is always an
“optimal” , ~10-2-10-3; consistent with fits to T Tauri disks (Hartmann et al 1998) Thommes & Murray 2006
Disk properties and core formationDisk properties and core formation
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SummarySummary In the worst-case scenario of unmitigated Type I
migration: protoplanets in a young, massive gas disk fall onto
central star long before they can reach gas giant core size (~10 MEarth)...
...but as the gas disk dissipates, a window may open for cores to form and survive
endgame: gas envelope accretion plays large role in cleaning up rest of disk (cf. Lecar & Sasselov 2003)
Predictions Favourable disk properties: high M(0), high [Fe/H], and
~10-2 - 10-3
no giant planets (i.e. for ALMA, no gaps) in very young, massive disks
In the worst-case scenario of unmitigated Type I migration: protoplanets in a young, massive gas disk fall onto
central star long before they can reach gas giant core size (~10 MEarth)...
...but as the gas disk dissipates, a window may open for cores to form and survive
endgame: gas envelope accretion plays large role in cleaning up rest of disk (cf. Lecar & Sasselov 2003)
Predictions Favourable disk properties: high M(0), high [Fe/H], and
~10-2 - 10-3
no giant planets (i.e. for ALMA, no gaps) in very young, massive disks
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“Dead zones” in disks“Dead zones” in disks Magnetorotational
instability (MRI) (Balbus & Hawley 1991) leading candidate for disk viscosity
MRI requires ionized disk, to couple it to magnetic field cosmic rays, stellar X-rays (near star)
When the full vertical column not ionized, dead zone forms (Gammie 1996, Matsumura & Pudritz 2003)
Magnetorotational instability (MRI) (Balbus & Hawley 1991) leading candidate for disk viscosity
MRI requires ionized disk, to couple it to magnetic field cosmic rays, stellar X-rays (near star)
When the full vertical column not ionized, dead zone forms (Gammie 1996, Matsumura & Pudritz 2003)
Gammie 1996
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Disk evolution with a dead zoneDisk evolution with a dead zone
Dead zone: lower viscosityslower accretionpile-up of gas
Steep jumps in surface density can result
How does this affect migration...?
Dead zone: lower viscosityslower accretionpile-up of gas
Steep jumps in surface density can result
How does this affect migration...?
?
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Disk torques at a surface density jump
Disk torques at a surface density jump
Type I migration: inner < outer, gas
Introducing jump in gas can reverse the torque imbalance
outer edge of a dead zone can completely stop Type I migration!
Type I migration: inner < outer, gas
Introducing jump in gas can reverse the torque imbalance
outer edge of a dead zone can completely stop Type I migration!
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Matsumura, Thommes & Pudritz, in prep.
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Thommes
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A “hybrid” code: N-body+gas diskA “hybrid” code: N-body+gas disk The N-body part: SyMBA (Duncan, Levison & Lee 1998)
uses Wisdom-Holman (1991) symplectic method fast for near-Keplerian systems bounded energy error
resolves close encounters The disk-evolution part: 1-D (azimuthally, vertically
averaged) Keplerian disk, Σ evolves according to
The N-body part: SyMBA (Duncan, Levison & Lee 1998) uses Wisdom-Holman (1991) symplectic method
fast for near-Keplerian systems bounded energy error
resolves close encounters The disk-evolution part: 1-D (azimuthally, vertically
averaged) Keplerian disk, Σ evolves according to
(Goldreich & Tremaine 1980, Ward 1997)
--∫(dT/dr)dr applied to planet∫(dT/dr)dr applied to planet ...Fast! Can simulate 10...Fast! Can simulate 1077 yrs in ~2 days yrs in ~2 days
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Thommes 2005
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Resonant exoplanetsResonant exoplanets
Marcy et al. 2005
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The “standard model” of core accretion
The “standard model” of core accretion
Pollack et al (1996): 3 stages:1. solid core accretion
2. slow gas accretion until Mgas ~ Mcore
3. runaway gas accretion Corrections to the standard
model: Stage 1 simplified, actually
takes longer (e.g. Thommes et al. 2003)
Stage 2 HAS to be a lot shorter (can be done by lowering envelope opacity)
Pollack et al (1996): 3 stages:1. solid core accretion
2. slow gas accretion until Mgas ~ Mcore
3. runaway gas accretion Corrections to the standard
model: Stage 1 simplified, actually
takes longer (e.g. Thommes et al. 2003)
Stage 2 HAS to be a lot shorter (can be done by lowering envelope opacity)
Pollack et al. 1996
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OutlineOutline Background
giant planet formation by core accretion migration by planet-disk interaction
The timescale problem Calculations of concurrent core accretion and migration in
an evolving disk A way around the timescale problem
Disk properties and the prospects for planet formation Summary
Background giant planet formation by core accretion migration by planet-disk interaction
The timescale problem Calculations of concurrent core accretion and migration in
an evolving disk A way around the timescale problem
Disk properties and the prospects for planet formation Summary