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Protoplanetary Disks
David J. Wilner (CfA)
“protoplanetary”
properties
Aug 22, 2006
grain
growth
disk gaps &
protoplanets
1
Collaborators
U. Michigan
E. Bergin
N. Calvet
L. Hartmann
NRAO
IfA
J. Williams
S. Andrews
M. Claussen
C. Chandler
Leiden
CfA
C. Qi
T. Bourke
M. Hughes
M. Hogerheijde
Heidelberg
J. Rodmann
T. Henning
Arcetri
A. Natta
L. Testi
UNAM
P. D’Alessio
2
Molecular Clouds  Disks
Taurus Dark Cloud
Barnard (1906)
L1527
Protostars
2000 AU
Benson & Myers 1989
Spitzer c2d T. Bourke
Dense Cores
Copernicus 1543
3
Schematic Solar System Evolution
10
105 yr
protostar
+ primordial
disk
Lstar
planet building
protoplanetary disk
107 yr
109 yr
1
104 yr
planetary system
+ debris disk
100 AU
cloud collapse
8,000
5,000
2,000
Tstar (K)
adapted from Beckwith & Sargent 1996, Nature, 383, 139
4
Schematic Solar System Evolution
10
Lstar
planet building
protoplanetary disk
107 yr
1
100 AU
8,000
5,000
2,000
Tstar (K)
adapted from Beckwith & Sargent 1996, Nature, 383, 139
5
Observational Challenges
• bulk of disk mass is “cold” (and dark) H2
– probed only through minor constituents
– solids: thermal emission, scattered light
– gas: trace species, subject to excitation and chemistry
• angular scales are small, difficult to image
– nearest regions with large samples at d=140 pc,
e.g. Taurus, Ophiucus, Lupus, Chamaeleon, ...
R (AU)
 (arcsec)
outer disk
200
~ 1.4
Kuiper Belt
40
~ 0.3
disk gap
0.4
~ 0.003
6
Disk n,T,... f(r,z): Panchromatic
x-ray
uv optical
hot gas/accretion
starlight
mid-ir
far-ir
submm mm cm
warm gas & dust
cool gas & dust
star
dust
(1% of disk mass)
4’’
HST
TW Hya K8V d=56 pc
Calvet et al. 2002
7
Characterizing Large Samples: SEDs
• easy to detect warm
(~900 K) mm size dust
in near-ir (t ~1 for ~MCeres)
• no confusion
Barnard (1906)
Spitzer Space Telescope
Hartmann et al. 2005
8
Disk Frequency and Lifetime
• most (all?) stars born
with circumstellar disks,
e.g. 3.4 mm excess
(Haisch, Lada & Lada 2001)
~ 50% gone by 3 Myr
~ 90% gone by 5 Myr
large dispersion
• Spitzer (e.g. FEPS):
most of the action in
protoplanetary disk
evolution is < 5 Myr
Hillenbrand 2005
(Meyer et al.)
log(age) 
9
Protoplanetary Disk Masses
• millimeter l’s: dust emission has low opacity
– dFn = Bn(T) knS dA
– millimeter flux ~ mass, weighted by temperature
Mdisk
Fn
= 0.03 M 1 Jy
(
2
D
100 pc
) (
3
50 K
l
T
1.3 mm
)
0.02 cm2 gm-1
k1.3mm
– Mdisk<0.001 - 0.1 M (Beckwith et al. 1990)
- 0.1 M
disk
mass
~F850mm
- 0.001 M
log(star mass)
log(age)
Andrews & Williams 2005
10
Environment: the Orion “Proplyds”
• clusters are the common
star formation environment
• proplyds ionized by
1 Ori C  evaporating
• SMA observations:
4 proplyds: M > 0.01 M
18 upper limits (<M> ~0.001 M)
• disks truncated?
Williams, Andrews, Wilner 2005
11
Resolved Dust Emission
• routine imaging of classical T-Tauri, Herbig Ae stars:
 > 0.7 arcsec, mass limit ~ 0.001 M (~40 sources)
• e.g. IRAM Taurus 2.7/1.3 mm (Dutrey et al. 2001)
–
–
–
–
resolve disk elongations
simple model: S ~r-p, T~r-q
“large” disk sizes: R>150 AU
“shallow” surface density
distributions: p+q ~ 1.5
12
Physical Models of Disk Structure
• replace arbitrary
power-laws with selfconsistent radiative and
hydrostatic equilibrium
• accretion < 10-8 M/yr
 irradiated, flared
Kenyon & Hartmann 1987,
Chiang & Goldreich 1997, 99
D’Alessio et al. 1997, 98, 01, ...
Dullemond et al. 2000, 02, ...
S~r-1
Tm~r-0.5
D’Alessio et al. 2001
H~r1.25
SED
.
steady a accretion: S ~ m/3pn ~ (r3/2Tm) -1
13
Testing Disk Structure Models
• TW Hya: d=56 pc (unique)
• irradiated accretion disk model
– fits complete SED
– matches resolved mm data,
size scales ~10 to 200 AU
SMA 870 mm
Qi et al. 2004
VLA 7 mm
data
SED
residual
Calvet et al. 2002
14
Resolved Line Observations
• CO is the most abundant gas tracer of H2
• CO lowest J rot. lines collisionally excited, thermalized
– very optically thick: Tk(r) ~ r-q  q = 0.5 - 0.6 (flared)
– dn/n > 106: detailed kinematics (e.g. Simon et al. 2000)
SMA: TW Hya
HD163296
HD169142
15
Keplerian: v(r/D)= (GM*/r)0.5 sin i
SMA
HD 163296
C
B
A
D
A
B
C
D
16
TW Hya Line Modeling
• analyze with 2D radiative transfer and 2 minimization
n,T: f(r,z)
v(r)
+

R = 17210 AU,
i = 6 1 deg,
M*, dvturb, X(CO), ...
SMA
Model
Doppler Shift
17
Gas vs. Dust Temperature
• CO J=3-2, 6-5 stronger
than SED model predicts
SMA
TW Hya
• higher gas temperatures
required in upper disk
atmosphere to match
– effect of x-rays?
(Glassgold & Najita 2001)
– mechanical heating?
Qi et al. 2006
18
Towards Nebular Chemistry
• simple species detected in a handful of disks
(e.g. Kastner et al. 1997, Dutrey et al. 1998, Thi et al. 2003)
– (global) depletions 5 to >100x,
at limits of current sensitivity
– ion-molecule reactions (HCO+)
– photochemistry (HCN/CN)
– deuteration (DCO+)
Aikawa 2006
• rich chemistry
– organics (H2CO)
– starting to image, e.g. TW Hya HCO+(3-2)
SMA
19
Protoplanetary Disk Properties
• Plenty of Dust (SEDs, silicate features)
• Gas rich (accretion, flared shapes)
• Masses: 0.001 to 0.1 M (mm dust emission)
• Sizes: 10’s- 100’s AU (CO, dust, scattered light)
• Kinematics: Keplerian rotation (CO lines)
• Lifetimes: few Myr (near-ir/mid-ir/mm statistics)
 all we need to form planets
20
NASA Disk Evolution Movie
QuickTime™ and a
MPEG-4 Video decompressor
are needed to see this picture.
21
The Rocky Road to Planethood
22
The Beginning: Particles Stick
QuickTime™ and a YUV420 codec decompressor are needed to see this picture.
SiO2
C. Dominik
Blum et al. 1998, 2000
• collisional growth
– sub-mm to cm size dust sticks at <1 m/s relative velocities
23
Spatially Resolved Scattered Light
TW Hya
HST/STIS
• gray scattered light,
r ~ 40 -150 AU
• “large” scattering
particles >> l ~1 mm
Roberge et al. 2005
24
Millimeter Spectral Signatures
•
abs. coeff: k ~ l-b
• diagnostic of dust size
(see Draine 2006)
– a << l, b = 2 (ism dust)
– a >> l, b = 0 (pebbles)
b~1
b>1
• if low opacity and R-J,
then Fl~ k l-2~ l-(b+2)
• at mm l’s, find b ~1
– grain growth?
– partly optically thick?
Beckwith & Sargent 1991
25
Grain Growth or Optical Depth?
e.g. UX Ori, CQ Tau:
a1.1-7mm~ 2.0, 2.6
b ~ 0 and large RD ?
any b and small RD ?
Testi et al. 2001
26
Ambiguity Resolved
Rodmann et al. 2006
• VLA 7 mm: large R, low TB,
small corrections for plasma
 t < 1  large grains
– TW Hya: Calvet et al. 2002
– CQ Tau: Testi et al. 2003
– 11 Taurus disks (barely)
resolved, most b1.3-7mm ~ 1
27
Complexity in Interpretation of b
• b is an “average”
k1mm
for any dust model
• for size power law
n(a) ~a-q, amin<a< amax
b  0 for large amax
only if q < 3
• does agglomeration
result in a power law?
b1-7mm
q=4.0 2
1
q=2.5
0
amax
Natta et al. 2004
28
Grain Growth and Settling
decrease dust/gas in
upper layers
population of ~cm size
grains in midplane
t=0
Weidenschilling 1997
29
TW Hya lcm Emission
• dust disk model underpredicts 3.5 cm emission
• ionized protostellar wind?
– if Fcm dMacc/dt, then ~0.2 mJy
• spinning dust? (Rafikov 2006)
– requires unrealistic Carbon
fraction in nanoparticles/PAHs
• nonthermal?
– X-rays (Kastner et al. 1997)
– 3.5 cm apparently variable
(upper limit of Rucinski 1992)
30
TW Hya: X-rays from Accretion
• not scaled up solar-type magnetic activity
(Kastner et al. 2002, Stelzer & Schmitt 2004)
• plasma conditions unlike
stellar coronae
– n ~ 1013 cm-3, ~100x denser
– T ~ 3 MK, (comparatively) cool
• peculiar chemical abundances
– metals depleted
– N, Ne enhanced
 accreting, depleted gas,
decoupled from dust
31
TW Hya: Evidence for “Pebbles”
• 3.5 cm radio emission
– not variable: weeks to years
– resolved at ~ arcsec scale
– low brightness ~10 K
– steep spectrum to 6 cm
 dust emission from disk
small + ~cm size dust
Wilner et al. 2005
32
Remarks on Grain Growth
• compelling evidence for growth (and processing)
– most of original dust mass in mm/cm size particles
• no clear trends with any stellar properties
– mass ~5x, luminosity ~250x, age ~10x
– no “evolutionary” trends (early active phase?)
• mm/cm sizes persist for Myrs
– planetesimal formation not efficient as predicted?
– competition between agglomeration and collisions?
• are the disks we can study the ones that will
never form planets? (no: transitional objects)
PPV: Natta, Testi, Calvet, Henning, Waters & Wilner, astro-ph/0602041
33
Infrared Gaps: Disk Clearing?
• inner holes result in infrared flux deficits,
e.g. TW Hya, CoKu Tau 4, GM Aur, DM Tau, ...
r~4 AU wall at outer edge of gap
Bryden et al. 1999
“gap”
Calvet et al. 2002
inner disk with bit of ~mm dust
34
Photoevaporation Models
• evaporation at R>Rg, where gas is no longer bound
(Clarke et al. 2001, Matsuyama et al. 2003, Takeuchi et al. 2005,
Alexander et al. 2006a,b)
accretion rate
– Rg ~ few AU for T-Tauri stars
– open gap at Rg when evaporation rate ~ accretion (“uv switch”)
gap
time
Clarke et al. 2001
radius
35
Protoplanets and Gaps
• protoplanet-disk interaction transfers angular momentum,
opens radial gap around protoplanet orbit
– subsequent evolution: (mostly) cleared inner hole
• active area: viscosity not understood, disk structure not known
(and inbalance of torques leads to planet migration)
Takeuchi et al. 1996
36
A Gap in Saturn’s Rings
• Moon (S/2005 S1) in Keeler Gap, Cassini (May 1, 2005)
• zero pressure, low viscosity, clean gap
37
Skepticism about Disk Gaps
• complexity, degeneracy of geometry and opacity
– spectral gap need not imply physical gap
“... we remain skeptical of the
existence of such a large central
gap devoid of dust.”
- Chiang & Goldreich (1999)
“While wide gaps may exist in certain
[protoplanetary] systems, SEDS do not
provide convincing proof of their existence-perhaps millimeter wave imaging... may
someday provide vivid evidence for the
presence of disk gaps.”
- Boss & Yorke (1996)
38
The GM Aur Infrared Gap
Calvet et al. 2005
HST/NICMOS
Schneider et al. 2002
IRAM CO(2-1)
• ~ 2 Myr, M*=0.84 M
Rdisk ~ 525 AU
.
• Macc ~ 10-8 M/yr
• Spitzer IRS 5-40 mm
requires inner hole
radius ~ 24 AU !
39
GM Aur: Submm Imaging
HST NICMOS
Wilner et al. 2006
40
Next Generation lmm-cm Facilities
• 100x better sensitivity, resolution, image quality
• global partnerships to fund ~$1B construction
• Atacama Large Millimeter Array (l 300 mm-1 cm)
– Key Science Goal: I. Image protoplanetary disks,
to study their physical, chemical, and magnetic
field structures, and to detect tidal gaps...
– early science 2010? full operation 2013?
• Square Kilometer Array (l 1 cm-3 m)
– Key Science Project I. Cradle of Life:
(a) thermal imaging at 0.001 arcsec to
study terrestrial planet formation and
disk evolution on sub-AU scales, (b) SETI
– technologies in development  2020?
41
At the Limits of ALMA: Protoplanets
• hypothetical planet in TW Hya disk
simulated ALMA 900 GHz image
(Wolf & D’Angelo 2005)
5 AU
model density distribution
42
New Territory for SKA
• thermal emission at ~1 mas resolution with
short l cm capability and ~1000 km baselines
• track grain growth in inner disk to pebbles
– observe radial gradients, settling
– concentrations of pebbles? vortices?
• unprecedented sub-AU views
– low opacity, even in terrestrial zone
– direct detection of tidal gaps
– orbital timescales of inner disk ~ 1 year
 track secular changes (“movies”)
HST NICMOS
43
Summary
• observed disk properties are “protoplanetary”
–
–
–
–
–
Rdisk to ~ 100’s of AU
typical Mdisk ~ 0.01 M ,(wide range)
Keplerian velocity fields
SEDs & structure match irradiated accretion models
inner/outer disk lifetime ~ few Myr
• starting to observe aspects of planet building
– grain growth leading to small rocks
– constituents similar to proto-Solar nebula
– firm evidence for gaseous inner disk holes
• major new facilities on the horizon
– amazing prospects with 100101 mas resolution
44
End
45
The Whirlpools of Descartes
Rene Decartes
1596- 1650
Principia Philosophiae (1644)
stars and Sun are the same,
existing in rotating “vortices”
(tourbillons)
46
Environment: the Proplyds
• “proplyds” = disks around low mass stars in Orion Nebula
Cluster dramatically imaged by HST (O’Dell et al. 1993)
UKIRT
HST
• clusters are the common
star formation environment
• proplyds ionized by
1 Ori C  evaporating
• optically opaque:
lower limits on mass
• are they viable sites of
Solar System formation?
47
Proplyd Masses
• previous nondetections
(BIMA Mundy et al. 1995;
OVRO Bally et al. 1998)
• SMA observations:
4 proplyds: M > 0.01 M
18 upper limits (<M> ~0.001 M)
• disks truncated?
Williams, Andrews, Wilner 2005
48
Effects of Stellar Multiplicity
• millimeter fluxes lower for binary systems
• disk masses lower
• tidal truncation: disks within Roche lobes (Jensen et al. 1996)
• e.g. UZ Tau quadruple
– UZ Tau East
0.03
AU asin i binary: circumbinary
emission
(typical of single
star)
– UZ Tau West
50
AU binary:
weak circumstellar emission
– are disks aligned? coplanar?
OVRO Mathieu et al. 2000
49
Gas: Accretion vs. Stellar Age
measuring disk accretion rates
evolution of accretion
Muzerolle et al. 2000
50
Basic Questions
• How do disks form? What affects disk properties?
• How is angular momentum transported in disks?
• How and when do planets form in disks?
• How does environment influence disk evolution?
• Observables: size, mass, density, temperature,
ionization, composition, gas chemistry, dust
mineralogy, structure (flaring, warps, gaps), ...
51
Solar System Characteristics
• fossil record  disk origin
– planet orbits lie in a plane
– planet orbits nearly circular
– Sun’s rotational equator
coincides with this plane
– planets and Sun revolve in
same west-east direction
• What are initial conditions?
• How do planets form?
Copernicus 1543
• What accounts for diversity
of planetary systems?
52
Observational Challenges
• bulk of disk mass is “cold” (and dark) H2
– probed only through minor constituents
• dust: thermal emission, scattered light
• trace molecules: frozen out, dissociated, chemistry
• angular scales are small, difficult to image
– nearest regions with large samples at 140 pc,
e.g. Taurus, Ophiucus, Lupus, ...
• r ~ 400 AU disk
• r ~ 40 AU Kuiper Belt
• dr ~
0.4 AU disk gap
~
~
~
SMA
(e)VLA
3.0 arcsec
0.3 arcsec
0.003 arcsec
CARMA
(=OVRO+BIMA)
HST
53
Observational Challenges
• bulk of disk mass is “cold” (and dark) H2
– probed only through minor constituents
– solids: thermal emission, scattered light
– gas: trace species, subject to excitation and chemistry
• angular scales are small, difficult to image
– nearest regions with large samples at d=140 pc,
e.g. Taurus, Ophiucus, Lupus, Chamaeleon, ...
HST
R (AU)
 (arcsec)
outer disk
200
~ 1.4
SMA
Kuiper Belt
40
~ 0.3
disk gap
0.4
~ 0.003
54
Towards Nebular Chemistry
•
13CO
less thick and sensitive to structure
SMA
R=R(12CO)
smaller R?
depletion?
55
Selective Photodissociation
•
13CO
J=2-1 Rdisk << 12CO J=2-1 Rdisk
Data
172 AU
110 ± 5 AU
cf. DM Tau, IRAM PdBI, Dartois et al. 2003
56
Achromatic Optical Extinction
114-426
• observations at l probe
particle sizes ~ O(l)
• in silhouette proplyd, sub-mm
ISM dust has grown, i.e. a >> l
HST
Throop et al. 2001
57
Infrared Spectroscopy
• shape of 10 mm silicate
feature depends on grain size
0.1 mm
2.0 mm
van Boekel et al. 2003
58
CQ Tau Ambiguity Resolved
• CQ Tau
– M* ~ 1.5 M
age ~ 10 Myr
• 7 mm resolved
– dust emission
– disk models show
t < 1 for r > 8 AU
– for p =0.5 - 1.5,
RD~100 - 300 AU,
b=0.5 - 0.7
•
kd ~ l-0.60.1
large grains
VLA
Testi et al. 2003
59
Disk Frequency and Lifetime
• most (all?) stars born
with circumstellar disks,
e.g. 3.4 mm excess
~ 50% gone by 3 Myr
~ 90% gone by 5 Myr
and large dispersion
• Spitzer (e.g. FEPS):
most of the action in
protoplanetary disk
evolution is < 5 Myr
(Meyer, Hillenbrand et al.)
Haisch, Lada, Lada 2001
60
Muzerolle et al. 2000
Other lcm Emission Processes
• ionized jet/wind?
– if Fcm dMacc/dt, then ~0.2 mJy,
103x lower than observed
– spectral index 3.5-6 cm >0.3
– not likely
STIS
• nonthermal emission?
– X-rays (Kastner et al. 1997,...)
– 3.5 cm apparently variable
• < 84 mJy (3s, Rucinski 1992)
• 200 ± 28 mJy (Wilner et al. 2000)
– plausible, but...
Stelzer & Schmitt 2004
61
Disk n,T,... f(r,z): Panchromatic
x-ray uv optical mid/far-ir submm cm
hot gas/accr. starlight warm gas/dust cool gas/dust
star
dust
(1% of disk mass)4’’
SMA
HST
TW Hya (K7) Weinberger et al. 2002
400 AU
62
Analogous Tidal Structures
IRAM GG Tau
Cassini: moon in Keeler Gap
(zero pressure, low viscosity)
GG Tau: 1.3 mm dust
circumbinary disk, cleared interior
Guilloteau et al. 1999
63
Molecular Clouds  Disks
Taurus Dark Cloud
Barnard (1906)
Benson & Myers 1989
900 AU
Protostars
Padgett et al. 1999
Dense Cores
Copernicus 1543
64
Disk n,T,... f(r,z): Panchromatic
x-ray
uv optical
hot gas/accretion
mid-ir
starlight
far-ir
submm mm cm
warm gas & dust
cool gas & dust
star
dust
(1% of disk mass)
4’’
SMA
HST
TW Hya d=56 pc Calvet et al. 2002
65
TW Hya lcm Emission
• dust disk model underpredicts 3.5 cm emission
should we be
interested in
this difference?
66