Overview of Planetesimal Accretiondips/DIPS/talks/ormel.pdfOverview of Planetesimal Accretion German-Japanese Workshop Jena, 01.10.2010 Chris W. Ormel Max-Planck-Institute for Astronomy,
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Overview of Planetesimal Accretion
German-Japanese WorkshopJena, 01.10.2010
Chris W. OrmelMax-Planck-Institute for Astronomy, Heidelberg, Germany
with
Kees Dullemond, Hubert Klahr, Marco Spaans
MPIA + U. of Heidelberg || U. of Groningen
Chris Ormel: Overview of planetary accretion models || Jena 01.10.2010 || 2/24
Contents
● Planet formation in context
● Gravitational focusing
● Runaway & oligarchy growth
● Timescales
● Role of debris
Chris Ormel: Overview of planetary accretion models || Jena 01.10.2010 || 3/24
Observational constraints, context
● Timescales for (gas) disk disappearance Several Myr
● Abundance of gas-giants in exo-systems
Mamajek (2009)
Chris Ormel: planetesimal accretion || Jena 01.10.2010 || 4/24
Solar system
Allende, ©Carsten Münker
Eros © NASA
Chris Ormel: Overview of planetary accretion models || Jena 01.10.2010 || 5/24
Particle motions
cg~105 cm/s
1mm 1cm 1m
radial drift
10 m/s
1 m/s
─ Gas moves subkeplerian
─ Radial drift ~10 m/s
─ “m-size barrier”
Chris Ormel: planetesimal accretion || Jena 01.10.2010 || 6/24
Time
Mass
Plts. form
Oligarchic growth/Core formation
Runaway growth
Runaway bodies separate
km??
100 km
Gas runaway accretion
10 ME
mm/cmm?
Plts. Source peters out @isolation mass
Chris Ormel: Overview of planetary accretion models || Jena 01.10.2010 || 7/24
Planet formation
● Disk Instability─ Gas in the disk collapses
e.g. Boss papers
● Core accretion─ Dust to planetesimals
─ Planetesimals to protoplanets
─ Protoplanet growth/migration
─ (protoplanet interactions)
Chris Ormel: Overview of planetary accretion models || Jena 01.10.2010 || 8/24
Accretion unknowns
● How are planetesimal bodies formed?─ Several barriers: bouncing,
radial drift, fragmentation, charge (Güttler/Zsom et al. 2010; Brauer et al
2007; Birnstiel et al. 2009; Okuzumi 2009)
─ Particle concentration through turbulence Johansen et al. 2007, 2009; Cuzzi et al
2008, 2010
─ Initial size, formation timescale
Chris Ormel: planetesimal accretion || Jena 01.10.2010 || 9/24
Gravitational focusing
col=R2
Chris Ormel: planetesimal accretion || Jena 01.10.2010 || 10/24
Viscous stirring (VS)Total motion
Low random motions
Total motion
Large random motions
vtot = vkepl + vran; vran ~evkepl
● Feedback effects growth
– Increase vesc
– Increase VS
Chris Ormel: planetesimal accretion || Jena 01.10.2010 || 11/24
GF – velocity regimes
Random velocity, v (eccentricity)
Approach velocity, va
~vhva=
v
Hill velocity Escape velocity, vesc
Rgeo
Rcol,max Collision radius
Intermediate/”Classical”Low v Large v
Chris Ormel: Overview of planetary accretion models || Jena 01.10.2010 || 12/24
Runaway and oligarchic growth
Log (mass)
Log (mass)
(Color indicates eccentricity/random motion)
massSem
i m
ajo
r axis
Normal growth
Runaway growth
2 component distribution of
oligarchs & Planetesimals
(Kokubo & Ida 1998)
Oligarchic growth
Chris Ormel: Overview of planetary accretion models || Jena 01.10.2010 || 13/24
RG/Oligarchy: physical processes
● Dynamical─ Viscous stirring;
─ Dynamical friction;
─ Gas drag;
─ Scattering;
● Physical─ Accretion
─ Fragmentation
─ Growth: increases vesc
─ Stirring: increases vran
Ormel et al. (2010a):
─ Runaway growth: GFF increase
─ Oligarchy: GFF decrease/stabilize
Chris Ormel: Overview of planetary accretion models || Jena 01.10.2010 || 14/24
1 AU simulation (Ormel et al 2010b)
● Indicated are:─ Radius plts. (X)
─ Position plts. (Y)
─ Group total mass: Area dot~m1/3tot; mtot = Ng midv
─ Grav. focusing factor w.r.t. biggest particle (v/vh, color)
Single body(Ng = 1)
Hill radius vh: Hill velocity of largest body, vh~R1
Chris Ormel: Overview of planetary accretion models || Jena 01.10.2010 || 15/24
Analysis
● Preconditions
─ Dynamically cold disk
─ All mass in planetesimals
● Runaway Growth
─ GFF increases
─ Growth timescales ~ same (fast!)
─ Size distribution
● Oligarchy
─ GFF increases (levels off)
─ Slower growth
─ 2 component
Radius of biggest body(evolutionary parameter)
(inverse)Gravitational FF
Myr
Runaway growth Oligarchy
Equate timescales to solve Rtr (Ormel et al 2010a)...
Chris Ormel: Overview of planetary accretion models || Jena 01.10.2010 || 16/24
Ormel et al. 2010a
For R0 = 10 km
Chris Ormel: planetesimal accretion || Jena 01.10.2010 || 17/24
Time
Mass
Plts. form
Oligarchic growth/Core formation
Runaway growth
Runaway bodies separate
km??
100 km
Gas runaway accretion
10 ME
mm/cmm?
Plts. Source peters out @isolation mass
Chris Ormel: Overview of planetary accretion models || Jena 01.10.2010 || 18/24
Growth timescales in oligarchic reg.
● Oligarchic growth is slow─ Eccentricities (random motions) strongly increase
“Protoplanets heat food, before eating” (Goldreich ea 2004)
─ Gas damping → equilibrium (large) GFF e.g., Kokubo & Ida (2002)
Depends slightly on planetesimal size
Chris Ormel: planetesimal accretion || Jena 01.10.2010 || 19/24
Σ ~ 10 g/cm^2 (surface density plts)
1 Mearth
5 AU
Veq, Tgrowth from Kokubo & Ida (2002)
Core formation
Chris Ormel: Overview of planetary accretion models || Jena 01.10.2010 || 20/24
Growth timescales in oligarchic reg.
● Oligarchic growth is slow─ Eccentricities (random motions) strongly increase
“Protoplanets heat food first, before eating”
─ Gas damping → equilibrium (large) GFF e.g., Kokubo & Ida 2001
─ Scattering, gap formation Levison et al. (2010)
─ Expect planetesimals to fragment
→ Study accretion behavior of (small) fragments Paardekooper (2007); Johansen & Lacerda (2010); Kobayashi et al. (2010);
Ormel & Klahr (2010)
Chris Ormel: Overview of planetary accretion models || Jena 01.10.2010 || 21/24
Accretion of debris/fragments
Factors:─ Pre-planetesimal population is small
─ Fragments settle in thin plane May speed up growth (Kenyon & Bromley 2009)
─ Radial drift Removes fragments
─ Large gravitational focusing factors (???) Not necessarily
Chris Ormel: planetesimal accretion || Jena 01.10.2010 || 22/24
Gas flow around small protoplanet
F2b Fdrag
– Drag force large for small particles
– Small particles coupled to (head)wind
– Fdrag ~ F2b
– No energy conservation (orbital decay)
Fsun
x (radial)
y (azimuthal)
Chris Ormel: Overview of planetary accretion models || Jena 01.10.2010 || 23/24
Interactions w/ gas friction
St: size of test particles, sζw: size of planets, Rp
eg Rp=10km; s=1cm @1au Rp=103km; s=1cm @1au Rp=103km; s=1m @1au
Chris Ormel: Overview of planetary accretion models || Jena 01.10.2010 || 24/24
Fragment accretion timescales
● Accretion timescales─ 5 AU, MMSN
─ “cold debris”
─ No depletion fragments
─ No trapping particles
─ No atmospheres
─ No turbulence (wake)
Chris Ormel: Overview of planetary accretion models || Jena 01.10.2010 || 25/24
Summary/Neglected effects
● Identified the runaway growth & oligarchy stages
● Formation 10 Mearth core remains difficult
─ Fragmentation; gap formation/resonances; trapping
─ Planetary atmosphere; migration of solids/planet
● Gravitational focusing boost growth─ Requires low random motions
● Role of the debris & gas-solid interaction
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