FROM UNIVERSE TO PLANETS€¦ · OLIGARCHIC GROWTH ‣ Runaway growth continues until the feedback...
Transcript of FROM UNIVERSE TO PLANETS€¦ · OLIGARCHIC GROWTH ‣ Runaway growth continues until the feedback...
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TO PLANETSLECTURE 4
FROM UNIVERSE
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Samples
Observations: IR and (sub-)mm
semimajor axis [AU]M
ass [
]M
jup
?
!
"#
$
%&
'(
10!2 10!1 100 101 102 103
10!4
10!3
10!2
10!1
100
101
102
Lab & IDPs (interplanetary dust particles) Meteorites
Exoplanets
μm cm m km
10−15 g 1027 g
REVIEW
only theory
Condensation
Collisional growth and fragmentation
Planetesmial/core formation and
runaway growth
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σ/2
σ/2
impact parameter bm
m
min. distance Rcmax. velocity vmax
σ/2
σ/2
impact parameter bm
m
min. distance Rcmax. velocity vmax
REVIEW
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horseshoe
too far away
too far away
just right
just right
r
ϕ
2Δa
ΔaΔa2
Δa2
Δa ≈ 2.3rH
σ /2
σ /2
impact parameter bm
m
min. distance Rcmax. velocity vmax
REVIEW
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σ /2
σ /2
impact parameter bm
m
min. distance Rcmax. velocity vmax
horseshoe
too far away
too far away
just right
just right
r
ϕ
2Δa
ΔaΔa2
Δa2
Δa ≈ 2.3rH
REVIEW
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REVIEWwidth scales with hill radius:
Mass in the feeding zone grows with planet mass:
in the terrestrial regionin the giant planet region
REVIEW
Miso ≈ 0.07 M⊕
rH = ap3
GMp
3M*
Miso =8
3π3/2C3/2M−1/2
* Σ3/2a3p
Δamax ≈ CrH
Mfz ≈ 2πap ⋅ 2Δamax ⋅ Σp ∝ M1/3
Miso ≈ 9 M⊕
REVIEW
Isolation mass grows with cylindrical radius:
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TO PLANETSLECTURE 4.1: PLANETESIMAL INTERACTIONS
FROM UNIVERSE
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‣ Large number of planetesimals: statistical treatment
‣ Similar to gas molecules and kinetic gas theory
‣ “Hot” (many collisions, high velocity) vs. “cold” distributions
‣ “Heating”: mutual gravitational scattering
‣ “Cooling”: collisions, gas drag, ejection
‣ Treat eccentricity , inclination , and mass using a distribution
e i mf(m, e, i)
→
RANDOM VELOCITIES
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‣ Kepler motion is characterised by 6 orbital elements: ‣ Semi-major axis: ‣ Eccentricity: ‣ Inclination: ‣ Longitude of ascending node: ‣ Longitude of perihelion: ‣ Time of passage at perihelion:
‣ Hill coordinates ( ) are rotating coordinates with the Keplerian angular velocity. In this frame, Keplerian motion is a superposition of shear, epicyclic, and vertical oscillations.
‣ For random the oscillations are randomly distributed about the systematic shear flow:
ape
iΩ
ωT
x, y, z
Ω, ω, Tv ≈ e2 + i2vK
RANDOM VELOCITIES
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‣ Numerical simulations give Rayleigh distribution for and :
‣ Gaussian distribution of the random components , , and with velocity dispersions:
‣ Equipartition of energy between eccentric and inclined motions (isotropic distribution with an equilibrium between “heating” and “cooling”):
e i
vR vzδvϕ = vϕ − vK
f(e, i) =4e sin i
⟨e2⟩⟨sin2 i⟩exp [−
e2
⟨e2⟩−
sin2 i⟨sin2 i⟩ ]
σ2R =
12
⟨e2⟩v2K σ2
z =12
⟨i2⟩v2K
for small angles: sin i ∼ i
RMS values: ⟨ ⋅ ⟩
⟨e2⟩ = 2 ⟨i2⟩
RANDOM VELOCITIES
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‣ The Jacobi integral is the only conserved quantity for the circular restricted three-body problem (the energy and momentum of the system are not separately conserved). For that are not too close:
‣ The region defines the feeding zone of the larger mass:
EJ
m ≪ Mp ≪ M*
EJ > 0
e =eh
i =ih b =
baph
EJ =EJ
a2ph2Ω2
Kh = ( m
3M* )1/3
EJ =12 (e2 + i2) −
38
b2 +92 bmax,circ = 2 3
2bmax = 2 [ 43 (e2 + i2) + 12]
1/2
RANDOM VELOCITIES
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‣ The random velocities set the growth regime: orderly, runaway, or oligarchic. The key ingredients are:
‣ Viscous stirring (increase of random velocities) through collisions or gravitational scattering between planetesimals and protoplanets.
‣ Dynamical friction: energy transfer from large to small bodies
‣ Establishes energy equipartition between bodies of different sizes.
‣ Damping due to dissipation in inelastic collisions and through gas drag.
RANDOM VELOCITIES
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‣ Distant fly-bys produce small angle deflections.
‣ Transforms some of the forward velocity (kinetic energy) to a random perpendicular velocity/energy.
‣ Integration over all encounters gives the increase of the random kinetic energy.
δE =2G2m3
b2σ2coulomb logarithm: factor by which small-angle collisions are more effective than large-angle collisions
swarm number density:
dEdt
=4πG2m3nsw
σln Λ
nsw ≈ΣswΩK
2mσgravitational force
m
b
m
e
VISCOUS STIRRING
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‣ The rate of increase of the random velocity
‣ Relaxation timescale: the time it takes for a particle to forget its initial velocity (or orbit).
‣ The viscous stirring rate is related to by:
‣ for typical values at and .
τrelax
τvs ∼ 6000 yrs 1 au m = 1019 g
dσdt
=2πG2m
σ3ΣswΩK ln Λ ∝
1σ3
⟶ σ ∝ t1/4
dEdt
=ddt ( 1
2mσ2) ⟶
4πG2m3 ( ΣswΩK
2mσ )σ
ln Λ = mσdσdt
VISCOUS STIRRING
τrelax =σ2
dσ2/dt=
2σ4
ΣswΩKπG2m ln Λ
τvs =d⟨e2⟩
dt∼
⟨e2⟩τrelax
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‣ On average, the increase of is larger than that of , but both relax into a Rayleigh distribution.
‣ The diffusion of planetesimals in semi-major axis is a result of a random walk due to two-body scattering.
‣ The growth rate of a protoplanet tends to decrease as it “heats” the neighbouring planetesimals.
e i
VISCOUS STIRRING
200 400 600 800TIME yr
0.001
0.002
0.003
0.004
1
2
3
4
5
e1 2 1 2
i1 2 1 2
N body3 bodySI00
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DYNAMICAL FRICTION‣ Same principle as before, but now with different masses.
dynamically “cold“ diskscattering
dynamically “hot“ disk
high σ
⟶hp ≈
σΩK
low σ
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DYNAMICAL FRICTION‣ Same principle as before, but now with different masses.
‣ Massive body heats up its environment Massive body is scattered less, smaller masses are scattered more. Many encounters lead to energy equipartition:
dynamically “cold“ diskscattering
dynamically “hot“ disk
low σhigh σ
⟶hp ≈
σΩK
12
mσ2m =
12
Mσ2m
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DYNAMICAL FRICTION
Kokubo & Ida (2012)
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FEpstein = −4π3
ρgasa2vthv
GAS DAMPING‣ Gas drag is similar to that of smaller particles, but sizes
are large enough to put them in the hydrodynamic (Stokes) regime.
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FEpstein = −4π3
ρgasa2vthvFStokes = −
CD
2πa2ρgasvv
GAS DAMPING‣ Gas drag is similar to that of smaller particles, but sizes
are large enough to put them in the hydrodynamic (Stokes) regime.
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FStokes = −CD
2πa2ρgasvv
GAS DAMPING‣ Gas drag is similar to that of smaller particles, but sizes
are large enough to put them in the hydrodynamic (Stokes) regime.
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‣ Gas drag is similar to that of smaller particles, but sizes are large enough to put them in the hydrodynamic (Stokes) regime.
FStokes = −CD
2πa2ρgasvv
τdamp,gas ≈mv
Fdrag=
2mCDπa2ρgasevK
∝ a
GAS DAMPING‣
‣ Damping acts against viscous stirring, thereby facilitating growth. For strong damping, the system evolves into a shear dominated regime (thin planetesimal disc). The 2D dynamics lead to high collisional probabilities and large focussing factors large growth rates.
‣ Assuming the random velocity is , the damping timescales is approximately:
⟶
v ∼ evK
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ORDERLY AND RUNAWAY GROWTH‣ Recall our mass accretion rate
including the focusing factor:
‣ Depending on the velocity dispersion, we get two different growth regimes:
‣ Viscous stirring increases , leading to orderly growth, with a power-law size distribution having most of the mass in the largest bodies.
‣ Addition of Dynamical friction and gas drag tends to equalise kinetic energies and damp of the more massive bodies, leading to runaway growth of a small number of embryos.
σ
σ
dMp
dt=
32
ΣpΩK
Γ=focusing factor
πRs (1 +v2
esc
σ2 )
1Mp
dMp
dt∝ {
M−1/3p , σ ≫ vesc (orderly)
M1/3p , σ ≪ vesc (runaway)
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OLIGARCHIC GROWTH‣ Runaway growth continues until the feedback from the
big bodies stirs up the neighbouring planetesimals again.
‣ Growth rates slows (similar to orderly growth), but are still faster than planetesimals in their surroundings (similar to runaway growth).
‣ Transition from runaway to oligarchic growth occurs between 10−3−10−2 M⊕
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OLIGARCHIC GROWTH
Raymond et al. 2006
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GAS ACCRETION1. Core formation
2. Atmosphere formation
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GAS ACCRETION
2. Atmosphere formation 4. Exceed critical core mass
3. Hydrostatic growth
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GAS ACCRETION
6. Gas dispersal4. Exceed critical core mass
5. Runaway accretion
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GAS ACCRETION
6. Gas dispersal
7. Kelvin-Helmholtz contraction
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GAS ACCRETION
7. Kelvin-Helmholtz contraction
6. Gas dispersal
1. Core formation
2. Atmosphere formation
3. Hydrostatic growth
4. Exceed critical core mass
5. Runaway accretion
‣ Gas accretion begins very slowly because it is pressure supported. Prevents new gas from being accreted.
‣ Once a critical core mass is achieved, runaway gas accretion becomes very rapid.
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Mid-plane cross section
Npart ∼ 50 millionΣ = 1700 g cm−3 (R/au)−1
Disc properties
Rout = 0.625 auRin = 0.025 au
Mplanet = 15 M⊕
aplanet = 0.25 auRcore ≈ 0.00011 au
Planet properties
GAS ACCRETION: EMBEDDED PHASE
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Full disc
Npart ∼ 50 millionΣ = 1700 g cm−3 (R/au)−1
Disc properties
Rout = 0.625 auRin = 0.025 au
Mplanet = 15 M⊕
aplanet = 0.25 auRcore ≈ 0.00011 au
Planet properties
ρgrain = 3 g cm−3Ngrains = 6
Dust properties
smin = 0.5 μmsmax = 5 cm
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Vertical cross section
Npart ∼ 50 millionΣ = 1700 g cm−3 (R/au)−1
Disc properties
Rout = 0.625 auRin = 0.025 au
Mplanet = 15 M⊕
aplanet = 0.25 auRcore ≈ 0.00011 au
Planet properties
ρgrain = 3 g cm−3Ngrains = 6
Dust properties
smin = 0.5 μmsmax = 5 cm
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3D cube: rendered
RHill
4
RHill
RBondi
RHill
RBondi
RHill
4
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Isothermal Adiabatic
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3D cube: particles
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x
z
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xyz
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GAS ACCRETION: RUNAWAY GROWTH‣ The hydrostatic atmosphere is described by the structure
equations (conservation laws)
‣ Mass:
‣ Momentum (force balance):
‣ Energy: external energy injection ( ), volume work ( ), and internal energy ( ):
‣ Energy transport (radiative vs convective):
ϵ PdVdU
dMdr
= 4πr2ρ
dPdr
= −GMr2
ρ
dLdr
= 4πr2ρ (ϵ − T∂S∂t )
dTdr
=TP
dPdr
∇ where ∇ =d ln Td ln P
= min(∇rad, ∇ad)
∇rad =3
64πσGκPLT4M
∇ad =∂ ln T∂ ln P
S
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‣ Accounting for boundary conditions (attached vs detached) leads to a step-like growth behaviour:
GAS ACCRETION: RUNAWAY GROWTH
run-away/oligarchic growth until isolation mass
slow accretion feeding zone grows slowly
reaching critical mass run-away accretion (demand limited)
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attract atm
osphere
run-away accretion
gas giants
terrestrial planets
faile
d ga
s gia
nt
Core Accretion
Disk Instability
Dust Planetesimal
dust over-densities collapse
collisional growth
planetesimals → oligarchs gravity dominates
direct gravitational collapse of gas
strip away the gas?
ice giants
GAS ACCRETION: RUNAWAY GROWTH
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TO PLANETSLECTURE 4.2: MIGRATION
FROM UNIVERSE
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MIGRATION‣ Disc migration: interactions with the gas disc.
‣ Type I: low mass planet.
‣ Type II: massive planet in a gap.
‣ Type III: turbulent migration (intermediate special case for massive disks).
‣ Tidal migration: planets whose orbit (or partial orbit) takes it close to the star causing a bulge on the star.
‣ Gravitational scattering: interactions with other planets (particularly giant planets) or large number of planetesimals.
‣ Kozai cycles and tidal friction: planets that are inclined relative to the plane of a binary star (or other planets).
→
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DISC MIGRATION‣ Planets create non-axisymmetric, time-dependent gravitational
potentials that make density waves in the gas disc (spiral waves).
‣ These density waves feedback onto the planet through gravitational torques, leading to angular momentum transport and migration.
‣ Orbital decay due to direct gas drag is negligible at planetary masses.
‣ Helps explain how hot Jupiters can exist so close to the host star when they preferentially form beyond the snow line.
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DISC MIGRATION
‣ The leading density enhancement pulls the planet forward (leading to outward migration), while the trailing density enhancement pulls the planet backwards (leading to inward migration).
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‣ Assume there is no co-rotating material (Type II migration)
‣ does not change the angular momentum, but conservation of energy implies that a change in will alter .
‣ Assume the deflection is small:
δv⊥δv⊥ δv∥
[Δv2, δv2∥] ∼ 0
DISC MIGRATION: IMPULSE APPROXIMATION
|δv⊥ | = ∫∞
−∞
F⊥
mdt =
2GmbΔv
m
b
m
e
Δv
δv∥ ∼1
2Δv (2GMp
bΔv )2
Δv2 = |δv⊥ |2 + (Δv − δv∥)2
Δj = apδv∥ =2G2M2
pap
b2Δv3
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‣ Gas interior overtakes the planet net gain in angular momentum. Gas exterior to the planet is overtaken by the planet net loss.
‣ The interaction is frictional with the net direction of migration depending on the difference between the interior and exterior torques.
‣ We are assuming linear trajectories (approximately true to circular orbits). Deflections cause departures from this geometry, so we assume disc viscosity is able to restore the geometry by the subsequent pass.
→
→
DISC MIGRATION: IMPULSE APPROXIMATION
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‣ Net torque requires integrating over gas within small annuli on either side of the planet (i.e. from to ) with a mass:
‣ Differences in orbital frequencies:
‣ For small displacements ( ) a first order expansion of the frequencies gives:
‣ We get the total temporal change in angular momentum by integrating over the exchange between all interacting gas parcels per unit time:
b b + db
b ≪ ap
db dbΔt =
2π|Ω − Ωp |
|Ω − Ωp | ≈dΩK
dab =
3ΩK
2apb
dJdt
= − ∫Δj dm
Δt
dm ≈ 2πapΣdb
DISC MIGRATION: IMPULSE APPROXIMATION
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‣ Assuming and constant surface density, we can then integrate:
‣ The migration time scale varies as: (massive planets migrate faster)
‣ More rigorous derivations require looking at resonances:
‣ Co-rotation: (if we neglect gas pressure and self gravity)
‣ Lindblad: where and is the epicyclic frequency (frequency at which a radially
displaced fluid parcel will oscillate):
Δv ≈ ap |Ω − Ωp | ≈ (3/2)Ωpb
Ω = Ωp
m(Ω − Ωp) = ± κ m = 1, 2, 3, . . .κ
dJdt
= − ∫∞
bmin
8G2M2pΣap
9Ω2Kb4
db = −8G2M2
pΣap
27Ω2Kb3
min
τmig =J
dJ/dt∝
1MP
DISC MIGRATION: IMPULSE APPROXIMATION
rLR = ap ( m ± 1m )
2/3
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‣ After a loooong derivation of the torques, the total torque can be derived by a summation of all resonances:
‣ Skipping to the end…
‣ Which gives a migration timescale of:
‣ The same scaling as our simple derivation.
DISC MIGRATION: TYPE I
(resonance strength) × (gas mass at resonance)
Γ =∞
∑m=1
ΓmOLR +
∞
∑m=1
ΓmILR + ΓCR
Γtot = − (1.36 + 0.54p)(Mp
M* )2 Σa4
pΩ2K
h2where Σ ∝ (R/R0)−p
τI = (2.7 + 1.1p)−1Mp
M*
M*
Σa2pΩK
∝1
Mp
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‣ Recall the outer disc extracts angular momentum (moves out) and the inner disc loses it (moves in) gap.
‣ Two conditions that need to be met for a gap opening:
‣ Thermal: (prevents accretion above/below the mid-plane). This implies a mass ratio:
‣ Viscous: (viscosity refills the gap; usually the criterion controlling gap opening). Assuming that
and :
→
rH ≳ H
τclose ≳ τopen
bmin = rH ν = αcsH
DISC MIGRATION: GAP OPENING
q =Mp
M*≥ 3h3 (typically : Mp ≳ 0.13MJupiter)
( dJdt )
LR= −
8G2M2pΣap
27Ω2Kb3
min≥ 3πνΣa2
pΩK = ( dJdt )
visc⟶ q ≥
243π8
αh2
(typically : Mp ≳ 2.5MJupiter)
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‣ Once the gap is opened, the gas is being pushed away from the planet and the torques diminish (slows down).
‣ The planet is kept in the middle of the gap and migration happens on viscous timescales:
‣ Only depends on the mass of the star and characteristics of the disc (independent of ). However, this is only valid if the planet is not too massive. Two regimes:
‣ Disc dominated
‣ Planet dominated
Mp
DISC MIGRATION: TYPE II
τII =a2
p
ν=
1α
h2Ω−1K
τII =τvisc, if B ≫ 1
τviscB, if B ≪ 1B =
3πΣ0R20
Mp
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‣ Type I migration timescales are very short ( ).
‣ Type II migration is 1-2 orders of magnitude longer.
‣ Suggests that planets should all fall into the star within the lifetime of the disc.
∼ 104 yrs
DISC MIGRATION: SUMMARY
disc lifetimes
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‣ Type I migration timescales are very short ( ).
‣ Type II migration is 1-2 orders of magnitude longer.
‣ Suggests that planets should all fall into the star within the lifetime of the disc.
‣ Not as effective in gravito-turbulent discs or for asymmetric gaps (Type III migration).
∼ 104 yrs
DISC MIGRATION: SUMMARY
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‣ Type I migration timescales are very short ( ).
‣ Type II migration is 1-2 orders of magnitude longer.
‣ Suggests that planets should all fall into the star within the lifetime of the disc.
‣ Not as effective in gravito-turbulent discs or for asymmetric gaps (Type III migration).
∼ 104 yrs
DISC MIGRATION: SUMMARY
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‣ Type I migration timescales are very short ( ).
‣ Type II migration is 1-2 orders of magnitude longer.
‣ Suggests that planets should all fall into the star within the lifetime of the disc.
‣ Not as effective in gravito-turbulent discs or for asymmetric gaps (Type III migration).
‣ Thermodynamics is an important factor
∼ 104 yrs
outwardinward
DISC MIGRATION: SUMMARY
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DUSTGASEFFECTS ON DUST
‣ Plant gaps prevent large dust grains from migrating past the planet. Only small grains that are well coupled to the gas can still cross. Can affect the grain size distribution in the inner disc.
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2π/Ωp
2π/Ωq=
Pp
Pq=
nm
where [n, m] = 1, 2, 3, . . .
MEAN MOTION RESONANCES (MMR)
‣ Multiple planet systems often form resonant chains as inner planets catch migrating planets behind them.
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‣ Similar to finding the natural frequency on a swing. Pushing at the right time is important for stable motion.
Stable
Unstable
MEAN MOTION RESONANCES (MMR)
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MEAN MOTION RESONANCES (MMR)‣ Structure in the astroid belt revealed when plotting the
mean orbits (instantaneous snapshots don’t show this because of the random eccentricities of the astroids). Similar effect in Saturn’s rings.
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TO PLANETSLECTURE 4.3: PHOTOEVAPORATION
FROM UNIVERSE
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VISCOUS EVOLUTION‣ Viscous evolution theory predicts:
‣ Long disc lifetimes (~10–100 of Myr).
‣ Discs should go progressively optically thin at all radii due to viscous accretion and spreading.
‣ We should see many discs in “transition” phase.
‣ Observations show:
‣ Discs are dispersed in ~1–10 Myr, with a median of 3–5 Myr.
‣ Very few transition discs (~10%).
‣ Clearing must be fast (~0.5 Myrs).
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VISCOUS EVOLUTION
high mass
Viscous evolution predicts….
Observations instead show….
Rare transition disk
high accretion rate
time
low mass low accretion rate
t ∼ 106 yrs t ∼ 107 yrs
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TRANSITION DISCS
AA49CH03-Williams ARI 14 July 2011 19:21
!F! (
erg
s–1 c
m–2
)
10–10
10–9
10–10
10–9
10 0 101 102 10 0 101 102
10–9
10–10
10–11
10–10
10–11
a b
c d
FQ Tau CoKu Tau/4
GM Aur UX Tau
Weak-excess, anemic,or homologously
depleted disk
Classic transitionor cold disk
Cold disk Cold disk orpretransition disk
Photometry from optical to mid-IR wavelengthsSpitzer IR spectra
Stellar photosphereRange of SEDs for typical accreting T Tauri stars
!(µm)
Figure 9The diversity of transition disk spectral energy distributions (SEDs). (a) A weak-excess, anemic, or homologously depleted disk has asignificant flux decrement at all mid-IR wavelengths relative to the T Tauri SED. (b) A cold disk or classical transition disk displaysexcess emission above the photosphere only at mid-IR wavelengths and beyond. (c) A cold disk with little near-IR emission and a strong10-µm silicate feature. (d ) A cold disk with near-IR excess emission. This can also be considered a pretransition disk because its SEDcan be modeled with an optically thin gap separating optically thick inner and outer disk components. Figure adapted from Najita,Strom & Muzerolle (2007).
luminosity effect rather than an evolutionary one. Because M-type stars are much fainter andcooler than solar-type stars, they may present weak mid-IR excess emission even if the disk extendsinto the dust sublimation radius (Ercolano, Clarke & Robitaille 2009).
The relatively small number of objects seen in a transition stage suggests that the evolutionarypath through a transitional disk is either uncommon or rapid. However, observations show thatan IR excess at a given wavelength is always accompanied by a larger excess at longer wavelengths,out to !100 µm. This implies that, unless some disks manage to lose the near-, mid-, and far-IRexcess at exactly the same time, the near-IR excess always dissipates before the mid-IR and far-IRexcesses do. No known process can remove the circumstellar dust at all radii simultaneously, andeven if grain growth or dynamical clearing does not produce an inner hole, photoevaporation bythe central star will do so once the accretion rate through the disk falls below the photoevaporationrate. Therefore, it is reasonable to conclude that transition disks represent a common but relatively
www.annualreviews.org • Protoplanetary Disks 103
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TRANSITION DISCS
K[8][24]
0.01 1 100 au
K[8][24] [8] [24]
E ∼ T ∼ ν ∼ 1/λT[8] ≈ 1800 KT[24] ≈ 600 K
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Luhman et al. (2010)Ks - [24]
K s - [8
]
<K4 K5-M4
M3-M4 M6
Two-colour diagram of young sources in TaurusTRANSITION DISCSHomogeneous draining
‣ Equal decrease in K-[8] and K-[24]
K-[8]
K-[24]
Inside-out draining
K-[8]
K-[24]
‣ K-[8] decreases fast ‣ K-[24] decreases slow
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TRANSITION DISCS
Prim
ordia
l
Dis
cles
s
Viscously Draining
Ultra-settled
Inside
-out
Clea
ring
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TRANSITION DISCS
IC348Age 4-5Myr (Mayne et al 2007)
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PHOTOEVAPORATION: EUV (13.6–100 V)e‣ Thermal winds are produced by absorption of high
energy radiation. Gas can escape if .
‣ Naively:
‣ Bernoulli flow:
cs > vesc30 Philip J. Armitage
r / AU
normalized massloss rate r2 Σw
r = rg
EUVonly
EUV + X-ray ionizing flux
hot, bound diskatmosphere
photoevaporative flow,
v = few - 10 km s
-1
diffuse field
FUVirradiation
Figure 9: Mass loss by photoevaporation occurs when FUV, EUV or X-ray radiation heats the disksurface to a temperature such that the gas is unbound. The normalized mass loss rate r2⌃w isshown as a function of radius for three models: (1) EUV only, using an analytic fit to simulationresults by Font et al. (2004) for M⇤ = 0.7 M�, (2) an EUV plus X-ray model (Owen et al. 2010),also for M⇤ = 0.7 M�, and (3) an FUV model for M⇤ = M� (using data smoothed from Figure 5of Gorti, Dullemond & Hollenbach 2009). The EUV component drives mass loss primarily neara characteristic radius r ⇠ rg = GM⇤/c2s (where cs is the sound speed in the ionized gas), whileX-rays, produced in a magnetized stellar corona, or FUV irradiation produce a component of massloss weighted to larger radii. The absolute mass loss rates depend upon the stellar spectrum. Notethat di↵erent assumptions and computational methods were used to produce each of the three massloss curves, which are therefore illustrative rather than strictly comparable.
detailed hydrodynamic models show that gas can escape at a reduced rate from radii as small asr ' 0.1 rg (Begelman, McKee & Shields 1983; Font et al. 2004; Li↵man 2003). Even with thisboost, however, photoevaporation requires surface temperatures high enough that they can onlybe attained as a result of disk irradiation by energetic photons,
1. Far-ultraviolet (FUV) radiation, with 6 eV < h⌫ < 13.6 eV, that is able to dissociate hydrogenmolecules but not ionize hydrogen atoms.
2. Extreme-ultraviolet (EUV) radiation, with 13.6 eV < h⌫ < 100 eV, capable of ionizinghydrogen.
3. X-rays, defined by convention as photons with h⌫ > 0.1 keV.
The rate of mass loss depends upon the original spectrum of the radiation, upon how much atten-uation takes place along the line of sight to the disk surface, and upon how each type of radiation
Rg =GM*
c2s
Rc ≈Rg
5
∼ 5−10 au
∼ 1−2 au
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PHOTOEVAPORATION: EUV (13.6–100 V)e‣ Ionises H at the disc surface ( ,
; negligible disc heating).
‣ Large absorption cross-section: peaks at 13.6 eV (drops as ).
‣ Probably blocked at early times by accretion columns or (partially-) neutral X-ray/FUV winds.
‣ Dominated by diffuse recombination (~1/3 of H recombinations produce another ionising photon).
‣ Mass loss is localised near .
‣ Dispersal times are largely dependent on viscous accretion.
T ∼ 104 Kcs ∼ 10 km/s
ν−3
Rg
·Mwind ≈ 4 × 10−10 ( Φ1041 s−1 ) ( M*
M⊙ )1/2
M⊙ yr−1
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PHOTOEVAPORATION: FUV (6–13.6 V)e‣ Drive significant heating in the disc surface (~100-1000s K) and can
penetrate ~2 orders of magnitude deeper than EUV. Mostly attenuated by dust and PAHs.
‣ Mass-loss rate peaks at ~5–10 au, but only dominant beyond 100 au. Important for truncating the disc.
‣ Many heating mechanisms complicated chemical networks. ‣ Photoelectric emission from dust grains. ‣ Neutral C ionisation. ‣ H2 photodissociation. ‣ FUV pumping and collisional de-excitation of vibrationally excited H2. ‣ Reformation of H2 from FUV dissociated H atoms. ‣ Gas-grain collisions (dominated by surface area of small grains).
‣ Gas cooling is also important and occurs through fine-structure lines in atoms and low-lying rotational lines from molecules.
→
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PHOTOEVAPORATION: X-RAY (0.1–1 V)ke‣ Young solar type stars emit 10-3 of their bolometric luminosity in X-
ray (main sequence stars only 10-7).
‣ Similar penetration to FUV. Base flows have and are larger than EUV rates ( ; harder X-rays penetrate too deep to drive winds, but important for heating/ionising mid-plane).
‣ Spectral range and high energies ionisation of inner (K-shell) electrons in heavier elements (primarily in O, but also C and Fe).
‣ Gas primary heated by subsequent release of photo/Auger electrons that collisionally ionise and/or heat the lighter atoms/molecules in the disc.
‣ Cooling mainly through metal line emission and collisions with colder dust grains.
T ∼ 3000−5000 K∼ 40 × ≳ 0.3 − 0.4 keV
→
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PHOTOEVAPORATION‣ X-EUV radiation ionises and heats the disc atmosphere:
‣ Bound atmosphere at .
‣ Thermal wind at (large portion of the disc). Total mass loss rates (can be comparable to viscous accretion).
‣ Once accretion rates drop below the wind mass-loss rate at a given radius, a gap opens (typically near ).
‣ The outer and the inner disc become decoupled. The inner disc is starved.
‣ Inner disc (viscously) drains rapidly onto the star producing a transition disc. Direct EUV and X-ray flux photoevaporate the outer disc from the inside out.
R < Rg
R > Rg∼ 10−10−10−8 M⊙ yr−1
Rg
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PHOTOEVAPORATION
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PHOTOEVAPORATION
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PHOTOEVAPORATION
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PHOTOEVAPORATION
76%
75% disc lifetime
77%78%
79%80%
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PHOTOEVAPORATION‣ Struggles to produce transition discs with large gaps and
high accretion rates. By the time that the disc gap is large enough the accretion rates in the inner disc have slowed or even quenched.
‣ Probably caused by giant planets, but grain growth can partially mimic this effect too.
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PHOTOEVAPORATION‣ Struggles to produce transition discs with large gaps and
high accretion rates. By the time that the disc gap is large enough the accretion rates in the inner disc have slowed or even quenched.
‣ Probably caused by giant planets, but grain growth can partially mimic this effect too.
‣ Combination of the two: planet-induced photoevaporation
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TO PLANETSLECTURE 4.4: DEBRIS DISCS & COLLISIONS
FROM UNIVERSE
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DEBRIS DISCS
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AU Microscopii
DEBRIS DISCS
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Beta Pictoris
DEBRIS DISCS
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DEBRIS DISCS
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DEBRIS DISCS
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DEBRIS DISCS‣ Debris discs are made from collisional grinding of
leftover km-sized planetesimals.
‣ Collisions are destructive, producing a collisional cascade that repopulates small grain sizes (observable).
‣
‣
‣ Dust and gas dynamics decoupled ( ).
‣ Lifetimes depend on the stability of the system, size of the remnant disc, and the amount of stirring (Myr—Gyr).
Mdisc ≪ 0.01M*
Ldisc ≪ L*
Mgas < 10 Mdust
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DEBRIS DISCS‣ Poynting–Robertson drag: stellar radiation causes a dust
grain orbiting a star to lose angular momentum radial drift. This is related to radiation pressure tangential to the grain's motion.
‣ (a) Stellar radiation comes from forward direction, but grain radiates isotropically.
‣ (b) Stellar radiation hits the grain laterally, but the grain appears to radiate more in the forward direction.
→
Co-moving observer Observer at rest w/star
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DEBRIS DISCS‣ PR drag most pronounced for small objects close to the star:
‣ Larger particles may not have constant surface temperature so radiation is not isotropic.
‣ Radiation pressure affects the effective force of gravity on the particle. Blows small particles away from the star and reduces the effective gravity of larger grains.
‣ For and : particles are blown out
‣ : spiral in/out depending on size (eccentric orbits).
‣ : spiral in (lifetime roughly ; 10000 yrs at 1 au).
a ≲ 1 μm β ≳ 0.5
0.1 < β < 0.5
β ≈ 0.1 ∝ β−1
FPR =vc2
W =a2L*
4c2
GM*
R5
β =Fr
Fg=
3L*QPR
16πGM*cρgraina
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DEBRIS DISCS‣ Large grains and planetesimals contain most of the mass,
but contribute little to no flux. ‣ Must characterise the invisible population of eroding
parent bodies through modelling.
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GIANT IMPACTS
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GIANT IMPACTS‣ The late stages of the collisional accretion of planets involves
collisions between planetary-sized bodies. These giant impacts involve enormous amounts of energy and are probably responsible for a number of particularities in the solar system:
‣ Anomalous density of Mercury
‣ Earth-Moon similarities
‣ The topography of Mars
‣ Tilt of Uranus' rotation axis
‣ Existence of Charon (Pluto's moon).
‣ It is the last giant impact that leaves traces. The geological clock is reset in the impact region (molten surface).
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crust
Relative sizes
6378 km
2440 km
EARTH
MERCURY
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EARTH
MOONOxygen isotopes
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MARS
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PLUTO
CHARON URANUS
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LONG TERM EVOLUTION
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SUMMARY 1/2‣ Random velocities are the result of viscous stirring, dynamical
friction, damping. Together they determine the growth timescales of planetary cores.
‣ Largest cores experience runaway growth followed by slower oligarchic growth, limited by stirring and the isolation mass.
‣ Runaway gas accretion begins once the core reaches a critical mass of .
‣ Embedded planets migrate rapidly through the disc (type I migration). Once the planet is large enough, it opens a gap and the migration slows (type II migration).
‣ Result of torques exerted by spiral density waves.
10−20 M⊕
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SUMMARY 2/2‣ Disc lifetimes are shortened by thermal winds
(photoevaporation).
‣ Once the viscous accretion rate is comparable to the wind mass loss rate, a gap in the inner disc forms. The inner disc drains onto the star and the outer disc is quickly removed from the inside out.
‣ The remaining debris disc collisionally evolves for long time periods. Planets continue to sweep up and eject material.
‣ Giant impacts likely occur early on. Planets stir up debris from the outer disc and send them on eccentric orbits. Important water delivery mechanism.
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