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Planetesimal Forma.on
gas drag se1ling of dust turbulent diffusion damping and excita.on mechanisms for planetesimals embedded in disks minimum mass solar nebula par.cle growth core accre.on
Radial dri= of par.cles is unstable to streaming instability Johansen & Youdin (2007); Youdin & Johansen (2007)
Gas drag
Gas drag force
where s is radius of body, v is velocity difference • Stokes regime when Reynold’s number is less than 10
• High Reynolds number regime CD ~0.5 for a sphere • If body is smaller than the mean free path in the gas Epstein regime (note mean free path could be meter sized in a low density disk)
FD = CD⇥gas�s2v2/2
CD � 24Re�1
FD =4�
3⇥gass
2csvEssen.ally ballis.c except the cross sec.on can be integrated over angle
Drag Coefficient
cri.cal drop moves to the le= in main stream turbulence or if the surface is rough
Note: we do not used turbulent viscosity to calculate drag coefficients
Re =sv
�
Stopping .mescale
• Stopping .mescale, ts, is that for the par.cle to be coupled to gas mo.ons
• Smaller par.cles have short stopping .mescales
• Useful to consider a dimensionless number tsΩ which is approximately the Stokes number
F = Mdv/dt ts =Mv
FD
Se1ling .mescale for dust par.cles
• Use gravita.onal force in ver.cal direc.on, equate to drag force for a terminal velocity
• Timescale to fall to midplane
• Par.cles would se1le unless something is stopping them
• Turbulent diffusion via coupling to gas
Fz = Mdvz
dt�M
vz
tsFz = �M�2z
tsettle = z/z = z/vz = ��1(�ts)�1
vz = (ts�)z�
Turbulent diffusion
• Diffusion coefficient for gas • For a dust par.cle • Schmidt number Sc • Stokes, St, number is ra.o of stopping .me to eddy turn over
.me • Eddy sizes and veloci.es
– Eddy turnover .mes are of order t~Ω-‐1
• In Epstein regime
• When well coupled to gas, the Diffusion coefficient is the same as for the gas
• When less well coupled, the diffusion coefficient is smaller
D0 = �cshD = �csh/Sc
Sc = (1 + St)
St � 34�
M
s2
�⇥gcs
Following Dullemond & Dominik 04
l �⇥
�h dv �⇥
�cs
Mean height for different sized par.cles Diffusion vs se1ling
• Diffusion processes act like a random walk
• In the absence of se1ling • Diffusion .mescale
• To find mean z equate td to tse1le
• This gives
and so a predic.on for the height distribu.on as a func.on of par.cle size
z =�
Dt
z =�
< z2 >
td � z/ ˙z ˙zz =D
2td = 2z2/D
tsettle = ��1(�ts)�1
z2 =D
2�(�ts)�1
Equilibrium heights
Dullemond & Dominik 04
Effect of sedimenta.on on SED
Dullemond & Dominik 04
Minimum Mass Solar Nebula
• Many papers work with the MMSN, but what is it? Commonly used for references: Gas Dust
• Solids (ices) 3-‐4 .mes dust density • The above is 1.4 .mes minimum to make giant planets
with current spacing – Hyashi, C. 1981, Prog. Theor. Physics Supp. 70, 35
• However could be modified to take into account closer spacing as proposed by Nice model and reversal of Uranus + Neptune (e.g. recent paper by Steve Desch)
�g = 2400� r
1AU
⇥�3/2g cm�2
�d = 10� r
1AU
⇥�3/2g cm�2
Larger par.cles (~km and larger)
• Drag forces: – Gas drag, collisions, – excita.on of spiral density waves, (Tanaka & Ward) – dynamical fric.on – All damp eccentrici.es and inclina.ons
• Excita.on sources: – Gravita.onal s.rring – Density fluctua.ons in disk caused by turbulence (recently Ogihara et al. 07)
Damping via waves • In addi.on to migra.on both eccentricity and inclina.on on averaged damped for a planet embedded in a disk. Tanaka & Ward 2004
• Damping .mescale is short for earth mass objects but very long for km sized bodies
• Balance between wave damping and gravita.onal s.rring considered by Papaloizou & Larwood 2000
e/e = �0.78 t�1wave
i/i = �0.54 t�1wave
twave = ⇥�1p
�Mp
M⇥
⇥�1⇤
�g(rp)r2p
M⇥
⌅�1 �cs
VK
⇥4
Note more recent studies get much higher rates of eccentricity damping!
Excita.on via turbulence Stochas.c Migra.on
• Johnson et al. 2006, Ogihara et al. 07, Laughlin 04, Nelson et al. 2005
• Diffusion coefficient set by torque fluctua.ons divided by a .mescale for these fluctua.ons
• Gravita.onal force due to a density enhancement scales with
• Torque fluctua.ons • parameter γ depends on density fluctua.ons δΣ/Σ • γ~α though could depend on the nature of incompressible
turbulence • See recent papers by Hanno Rein, Ketchum et al. 2011
tfluc � ��1
Mp2�G�g
(⇥�)2 � (�Mp2⇤rG⇥g)2
Eccentricity diffusion because of turbulence
• We expect eccentricity evolu.on • with
• or in the absence of damping
• constant taken from es.mate by Ida et al. 08 and based on numerical work by Ogihara et al.07
• Independent of mass of par.cle • Ida et al 08 balanced this against gas drag to es.mate when
planetesimals would be below destruc.vity threshold
< e2 >= Dt
D � (⇥�)2
L2⇤ � �2
�⇥gr2
M�
⇥2
⇤
e � 100�
��gr2
M�
⇥(⇥t)1/2
In-‐spiral via headwinds
• For m sized par.cles headwinds can be large – possibly stopped by
clumping instabili.es (Johanse & Youdin) or spiral structure (Rice)
• For planetary embryos type I migra.on is problem – possibly reduced by
turbulent sca1ering and planetesimal growth (e.g., Johnson et al. 06)
Heading is important for m sized bodies, above from Weidenschilling 1977
Density peaks Pressure gradient trapping
• Pressure gradient caused by a density peak gives sub Keplerian veloci.es on outer side (leading to a headwind) and super Keplerian veloci.es on inner side (pushing par.cles outwards).
• Par.cles are pushed toward the density peak from both sides.
figures by Anders Johansen
Forma.on of gravita.onal bound clusters of boulders
points of high pressure are stable and collect par.cles
Johansen, Oishi, Mac Low, Klahr, Henning, & Youdin (2007)
The restructuring/compac4on growth regime (s1 ≈ s2 ≈ 1 mm…1 cm; v ≈ 10-‐2…10-‐1 m/s)
Collisions result in s4cking
Impact energy exceeds energy to overcome rolling fric4on (Dominik and Tielens 1997; Wada et al. 2007)
Dust aggregates become non-‐fractal (?) but are s4ll highly porous
Low impact energy: hit-‐and-‐s.ck collisions
Intermediate impact energy: compac.on Blum & Wurm 2000 Paszun & Dominik, pers. comm.
From a talk by Blum!
contour plot by Weidenschilling & Cuzzi 1993
1 AU
collisions bounce
collisions erode
from a talk by Blum
Diameter
Dia
met
er
1 µm
100 m
100 µm
1 cm
1 m
1 µm 100 m 100 µm 1 cm 1 m
Non-fractal Aggregate Growth (Hit-and-Stick) Erosion
Non
-fra
ctal
Agg
rega
te
Stic
king
+ C
ompa
ctio
n
Cra
teri
ng/F
ragm
en-
tati
on/A
ccre
tion
Cratering/ Fragmentation
Restructuring/ Compaction
Eros
ion
N
on-f
ract
al A
ggre
gate
Gro
wth
(H
it-a
nd-S
tick
)
Non-fractal Aggregate Sticking + Compaction
Cratering/Fragmen-tation/Accretion
Cra
teri
ng/
Fr
agm
enta
tion
Mass loss
Mass conservation
Mass gain
* *
* *
* for compact targets only
Blum & Wurm 2008
1 AU
Clumps
• In order to be self-‐gravita.ng clump must be inside its own Roche radius
• Concentra.ons above 1000 or so at AU for minimum solar mass nebula required in order for them to be self-‐gravita.ng
�s > M⇥/r3 � 6� 10�7g cm�3
⇤M⇥M⇤
⌅ � r
1AU
⇥�3
s < rH s < r
��ss3
M�
⇥1/3
Clump Weber number
• Cuzzi et al. 08 suggested that a clump with gravita.onal Weber number We< 1 would not be shredded by ram pressure associated turbulence
• We = ra.o of ram pressure to self-‐gravita.onal accelera.on at surface of clump
• Analogy to surface tension maintained stability for falling droplets
• Introduces a size-‐scale into the problem for a given Concentra.on C and velocity difference c. Cuzzi et al. used a headwind velocity for c, but one could also consider a turbulent velocity
We =�gc2
G�2ds
2= C�2
⇤c
vK
⌅2 �r
s
⇥2⇤
M⇥/r3
�g
⌅
Growth rates of planetesimals by collisions
• With gravita.onal focusing
• Density of planetsimal disk ρ, • Dispersion of planetesimals σ • Σ=ρh, σ=hΩ • Ignoring gravita.onal focusing • With solu.on
dM
dt= �s2⇥⇤
�1 +
v2esc
⇤2
⇥
dM
dt⇥�1 � �(M/�d)2/3
s � t
Growth rate including focusing
• If focusing is large
• with solu.on quickly reaches infinity • As largest bodies are ones where gravita.onal focusing is important, largest bodies tend to get most of the mass
Fg =�
1 +v2
esc
�2
⇥� M
s
dM
dt��1 �M4/3
s(t) ⇥ 1C � �t
Isola.on mass
• Body can keep growing un.l it has swept out an annulus of width rH (hill radius)
• Isola.on mass of order
• Of order 10 earth masses in Jovian region for solids le= in a minimum mass solar nebula
M = 2�a�rH rH = a(M/3M�)1/3
Miso � a3�3/2M�1/2⇥
Self-‐similar coagula.on
• Coefficients dependent on s.cking probability as a func.on of mass ra.o
• Simple cases leading to a power-‐ law form for the mass distribu.on but with cutoff on lower mass end and increasingly dominated by larger bodies
• dN(M)/dt = – rate smaller bodies combine to make mass M
– subtracted by rate M mass bodies combine to make larger mass bodies
Core accre.on (Earth mass cores)
• Planetesimals raining down on a core • Energy gained leads to a hydrosta.c envelope • Energy loss via radia.on through opaque envelope
• Maximum limit to core mass that is dependent on accre.on rate sezng atmosphere opacity
• Possibly a1rac.ve way to account for different core masses in Jovian planets
Giant planet forma.on Core accre.on vs Gravita.onal Instability
• Two compe.ng models for giant planet forma.on championed by
– Pollack (core accre.on) – Alan Boss (gravita.onal instability of en.re disk)
• Gravita.onal instability: Clumps will not form in a disk via gravita.onal instability if the cooling .me is longer than the rota.on period (Gammie 2001)
Where U is the thermal energy per unit area
– Applied by Rafikov to argue that fragmenta.on in a gaseous circumstellar disk is impossible. Applied by Murray-‐Clay and collaborators to suggest that gravita.onal instability is likely in dense outer disks (HR8799A)
• Gas accre.on on to a core. Either accre.on limited by gap opening or accre.on con.nues but inefficiently a=er gap opening
tcool =U
�T 4� ��1
Connec.on to observa.ons
• Chondrules • Composi.on of different solar system bodies
• Disk deple.on life.mes
• Disk velocity dispersion as seen from edge on disks
• Disk structure, composi.on
• Binary sta.s.cs
Reading • Weidenschilling, S. J. 1977, Areodynamics of Solid bodies in the solar nebula,
MNRAS, 180, 57 • Dullemond, C.P. & Dominik, C. 2004, The effect of dust se1ling on the appearance
of protoplanetary disks, A&A, 421, 1075 • Tanaka, H. & Ward, W. R. 2004, Three-‐dimensional Interac.on Between A Planet
And An Isothermal Gaseous Disk. II. Eccentricity waves and bending waves, ApJ, 602, 388
• Johnson, E. T., Goodman, J, & Menou, K. 2006, Diffusive Migra.on Of Low-‐mass Protoplanets In Turbulent Disks, ApJ, 647, 1413
• Johansen, A., Oishi, J. S., Mac-‐Low, M. M., Klahr, H., Henning, T. & Youdin, A. 2007, Rapid Planetesimal forma.on in Turbulent circumstellar disks, Nature, 448, 1022
• Cuzzi, J. N., Hogan, R. C., & Shariff, K. 2008, Toward Planetesimals: Dense Chondrule Clumps In The Protoplanetary Nebula, ApJ, 687, 143
• Blum, J. & Wurm, G. 2008, ARA&A, 46, 21, The Growth Mechanisms of Macroscopic Bodies in Protoplanetary Disks
• Armitage, P. 2007 review • Ketchum et al. 2011 • Rein, H. 2012