Debris Disks:Seeing Dust,
Thinking of Planetesimals and Planets
Alexander V. Krivov
Astrophysical Institute and University Observatory
Friedrich Schiller University Jena
Germany
1st CPS International School of Planetary Sciences

Kobe, January 8, 20091

Outline Concept of debris disks
Part I Observations of debris disks
 Basic theory of debris disks
 Debris disks: seeing dust

Part II Debris disks: thinking of planetesimals
 Debris disks: thinking of planets
 Debris disks: thinking of planetary systems
 Summary

1st CPS International School of Planetary Sciences

Kobe, January 8, 20092

Outline Concept of debris disks
Part I Observations of debris disks
 Basic theory of debris disks
 Debris disks: seeing dust

Part II Debris disks: thinking of planetesimals
 Debris disks: thinking of planets
 Debris disks: thinking of planetary systems
 Summary

1st CPS International School of Planetary Sciences

Kobe, January 8, 20093

The name of the game

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Our solar system
KB dustKB

AB ZodiGiant planets

Terrestrial planets

1st CPS International School of Planetary Sciences

Debris disk
Kobe, January 8, 20095

Other planetary systems
KB dustKB

Giant planets

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Debris disk
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Protoplanetary phase and debris disk phase
Standard formation scenario:• A disk of gas and dust
• From dust to planetesimals• From planetesimals to embryos(~1Myr)(closer to the star, otherwise takes too long)• Gas accretion onto large embryos(< 10 Myr)
• Disk clearing• Formation of terrestrial planets(~100 Myr)
• A planetesimal belt on the periphery (KB), producing dust

Evolutionary phases of planetary systems
10Myr

10Gyr

Debris disk phase
PP phase

1st CPS International School of Planetary Sciences

Kobe, January 8, 20097

Outline Concept of debris disks
Part I Observations of debris disks
 Basic theory of debris disks
 Debris disks: seeing dust

Part II Debris disks: thinking of planetesimals
 Debris disks: thinking of planets
 Debris disks: thinking of planetary systems
 Summary

1st CPS International School of Planetary Sciences

Kobe, January 8, 20098

(1) Infrared excesses (“Vega phenomenon”)

The first discovery of an infraredexcess over stellar photosphericemission of a main-sequence star:
Vega. Evidence for dust!

Aumann et al., ApJ 278, L23-L27 (1984)

1st CPS International School of Planetary Sciences

Kobe, January 8, 20099

(1) Infrared excesses (“Vega phenomenon”)

Since then:thanks to IRAS, ISO, ..., Spitzer,
the Vega phenomenon has been
observed around ~1000 nearby
stars
About 15% of main-sequence stars
have dust around them. Almost thesame incidence around AFGK stars,
may be slightly higher for early types,
M unclear
Near future: Herschel-> poster by Jens Rodmann
Zuckerman & Song, ApJ 603, 738-743 (2004)
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(2) Images

The first image of a dust
disk around a
main-sequence star:β Pictoris
Dust is present in the formof a structured disk

ESO Press photo (1997)
Smith & Terrile., Science 226, 1421-1424 (1984)

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(2) Images

Vis

NIR

FIR

sub-mm~10 Myr age

β Pic

Since then: imaging
with many ground-based
instruments and HST,
both in scattered lightand thermal emission

AU Micα PsA

By now, about 20 disks
imaged~1 Gyr age
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α Lyr

ε Eri
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Observed dust is evidence for planetesimals

Ages of systems: 10 Myr ... 10 Gyr
But: estimated lifetime of dust particles: < 1Myr
Dust cannot be primordial andneeds continuous replenishment
As growth is impossible,
dust must stem from parent bodies, planetesimals
A conceivable mechanism:
collisions of planetesimals

1st CPS International School of Planetary Sciences

Kobe, January 8, 200913

Outline Concept of debris disks
Part I Observations of debris disks
 Basic theory of debris disks
 Debris disks: seeing dust

Part II Debris disks: thinking of planetesimals
 Debris disks: thinking of planets
 Debris disks: thinking of planetary systems
 Summary

1st CPS International School of Planetary Sciences

Kobe, January 8, 200914

Physical processes: (a) collisions
Collisional cascade

Streng
th

G
ra
v it
y

re

gi

m

e

Individual collisions

regimeplanetesimals... boulders ... dust
Benz & Asphaug, Icarus 142, 5-20 (1999)

Under debris disk conditions:- Disruptive collisions- Cratering collisions
1st CPS International School of Planetary Sciences

Collisional cascade grinds
planetesimals to ever-smaller
fragments, down to dust sizes.
Kobe, January 8, 200915

Physical processes: (b) stellar “photogravity”
Stellar “photogravity”

planetesimals in nearly-circular orbits,
dust grains in elliptic ones,
fine dust leave the system in hyperbolas

Krivov et al., Icarus 445, 509-519 (2006)
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Physical processes: (c) drag forces
Poynting-Robertson force

dust orbits slowly circularize
and shrink toward the star
Wyatt & Whipple, ApJ 111, 134 (1950)
Breiter & Jackson, MNRAS 299, 237 (1998)

Stellar wind drag force“corpuscular analog” to the P-R effect(strong for late-type stars)
Burns et al. (1979), Plavchan et al. (2005)

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Physical processes: (d) planetary perturbations
Gravity of planets

confines planetesimals,
stirs them up,
exerts secular and resonant perturbations on themsecular offset (e, ϖ)secular warp (i, Ω)

Original Schematics

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Physical processes: (e) others

Sublimation and sputtering

eliminate dust close to the star,
gradually reduce their size
(important for early-type stars)

Lorentz force

important for small grains only
and only if appreciable MF is present... and many others
that are usually not included in debris disk models,
but may be important...
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Modeling methods
 Celestial mechanics / N-body simulations- N-body + “inflated spheres” + local PiaB (Thébault, Wyatt)
Accurate dynamics but inaccurate collisions (if any)

 Kinetic theory / Statistical codes- Multiannulus PiaB (Kenyon & Bromley, Thébault)- Kinetics in orbital elements (Dell’Oro, Krivov)
Accurate collisions but simplified dynamics

 Hybrid methods- “Superparticles” (Grigorieva)
Share (dis)advantages of two previous methods

Main challenge: how to combine accurate dynamics with accurate collisions?
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Collisions and photogravity at work: size distribution

Blowout limit

Krivov, Löhne, & Sremčević, AAp 455, 509 (2006)• Tiny blowout grains are present, but in smaller amounts• Grains just above blowout limit dominate cross section• Size distribution is wavy
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Kobe, January 8, 200921

Collisions and photogravity at work: size distribution

Blowout limit

Thébault & Augereau,AAp 472, 169 (2007)• Cratering collisions are important• They substantially enhance the “main” maximum
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Planetesimal belt

Collisions and photogravity at work: radial profiles

Krivov, Löhne, & Sremčević, AAp 455, 509-519 (2006)

Surface density index ~-1.5, brightness index ~-3.5
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From dust distributions to observables
Need to calculate light scattering and thermal emission of dust

The thermal emission
exampleKrivov, Müller, Löhne,& Mutschke,
ApJ 687 (2008)

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Kobe, January 8, 200924

Outline Concept of debris disks
Part I Observations of debris disks
 Basic theory of debris disks
 Debris disks: seeing dust

Part II Debris disks: thinking of planetesimals
 Debris disks: thinking of planets
 Debris disks: thinking of planetary systems
 Summary

1st CPS International School of Planetary Sciences

Kobe, January 8, 200925

What do infrared spectra tell us about dust grain properties?

HD 69830

Beichman et al.,ApJ 626, 1061-1069 (2005)

At a first glance:
looks like cometary dust!

Lisse et al., ApJ 658, 584-592 (2007)

A closer look: HD69830 50% crystalline,
Hale-Bopp only 7-30%. Asteroidal dust?
-> many posters!

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What do infrared spectra tell us about dust grain properties?
HD 181327

Chen et al., ApJS 166, 351-377 (2006)

Chen et al.,ApJ 689, 539-544 (2008)

10 mu Si feature is often observed.
When not, no Si? Or large grains?

Water ice around at 90 AU froman F5 star despite photodesorption?

-> many posters!
1st CPS International School of Planetary Sciences

Kobe, January 8, 200927

Outline Concept of debris disks
Part I Observations of debris disks
 Basic theory of debris disks
 Debris disks: seeing dust

Part II Debris disks: thinking of planetesimals
 Debris disks: thinking of planets
 Debris disks: thinking of planetary systems
 Summary

1st CPS International School of Planetary Sciences

Kobe, January 8, 200928

Seeing kids, thinking of parents

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Statistics of debris disks: a long-term decay

t-2

t-1

t-0.5

Moór et al. ApJ 644, 525-542 (2006)• Dust luminosity decay with system’s age, albeit with a large scatter• Reason: collisional depletion of a planetesimal belt
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Dominik-Decin (2003) model• Equal-sized planetesimals “feed” dust
• Dust has a single power-law size distribution
Dominik & Decin, AAp 598, 626-635 (2003)
1st CPS International School of Planetary Sciences

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Dominik-Decin (2003) model• For collision-dominated disks (usually the case),
total disk mass ~ dust mass ~ t -1
• “Delayed stirring” suggested to explain highluminosity in some of the old systems
Dominik & Decin, AAp 598, 626-635 (2003)
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Wyatt et al. (2007) model

Still, an assumption of a quasi-steady state(a single power-law size distribution)
Wyatt et al., ApJ 658, 569-583 (2007)
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Kobe, January 8, 200933

Wyatt et al. (2007) model• For any given age, there is a maximum possible amount of dust
• Disk evolution depends on r, e, I, Dc
Wyatt et al., ApJ 658, 569-583 (2007)

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Löhne et al. (2008) model

Krivov, Sremčević, & Spahn,
Icarus 174, 105-134 (2005)• Large planetesimals are not in collisional equilibrium• Strength-gravity transition plays a major role
Löhne, Krivov, & Rodmann, ApJ 673, 1123-1137 (2008)
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Löhne et al. (2008) model
Scaling rules

F (xM0 , r, t) = x F(M0, r, xt)
F (M0 , yr, t) ≈ F(M0, r, y

-4.3

t)

-ξ

F (M0 , r, zt) ≈ z F(M0, r, t),
ξ~0.3...0.4
Löhne, Krivov, & Rodmann, ApJ 673, 1123-1137 (2008)
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Löhne et al. (2008) model

Löhne, Krivov, & Rodmann, ApJ 673, 1123-1137 (2008)
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Kobe, January 8, 200937

Löhne et al. (2008) model• The dust mass decays as ~t ξ
• Index ξ depends on the “primordial” size distribution of planetesimals•Typical values: ξ ~ -0.3...-0.4 and not -1
Löhne, Krivov, & Rodmann, ApJ 673, 1123-1137 (2008)
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Löhne et al. (2008) model

A synthetic population of debris disks calculated with the modelis in a good agreement with the Spitzer 24 and 70µm statistics
Löhne, Krivov, & Rodmann, ApJ 673, 1123-1137 (2008)
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Individual systems: two approaches

Krivov, Müller, Löhne, & Mutschke, ApJ 687 (2008)
1st CPS International School of Planetary Sciences

Obs
erva
bles

Our approach

Thermal emission model

Dus
t dis
k

?
Collisional model

Plan

etes
i

mal
belt

Traditional approach

Kobe, January 8, 200940

Individual systems: the Vega disk

Su et al., ApJ (2005); Marsh et al., ApJ (2006)
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Individual systems: the Vega disk

Müller, Löhne, & Krivov, in prep. -> poster by Sebastian MüllerCollisional + thermal emission modeling
Parameters: stellar luminosity; disk location, extension, excitation;
dust composition; collisional outcome prescription
1st CPS International School of Planetary Sciences

Kobe, January 8, 200942

Outline Concept of debris disks
Part I Observations of debris disks
 Basic theory of debris disks
 Debris disks: seeing dust

Part II Debris disks: thinking of planetesimals
 Debris disks: thinking of planets
 Debris disks: thinking of planetary systems
 Summary

1st CPS International School of Planetary Sciences

Kobe, January 8, 200943

Inner gaps: footprints of planets?

Infrared excesses and images reveal EKB-sized disks,extending from several tens AU outward, with inner gaps
1st CPS International School of Planetary Sciences

Kobe, January 8, 200944

Inner gaps: footprints of planets?

Simulations and animations: Alessandro Morbidelli

Inner gaps can be naturally explained with two or more migrated planets
Alternatively, with two or more planets after encounters

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First attempts to find these planets

8 young systems
with cold excesses
searched with
VLT/NACO
no planets foundApai et al.,ApJ 672,1196-1201(2008)

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Another attempt: Fomalhaut
Elliptic ring with inner gap: a planet was expected...

Kalas et al., Nature (2005)

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Another attempt: Fomalhaut...and is now discovered

Kalas et al., Science 322, 1345 (2008)

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There is more structure in resolved disks

Radial,azimuthal,verticalstructure:rings,●gaps●clumps●spirals●offsets●warps...
●

HD32297 (Kalas 2005 )β Pic (press photo)τAU Mic (Liu 2004)

ε Eri (Greaves et al. 1998)

HD32297 (Kalas 2005 )α PsA (Holland et al. 2003)

Vega (Su et al. 2005 )1st CPS International School of Planetary Sciencesτ Cet(Greaves et al.2004)

HD139664 (Kalas 2006 )Kobe, January 8, 200949

Clumps of ε Eri

Greaves et al., ApJ 619, L187-L190 (2005)

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Clumps due to resonances with a planet

Simulations and animations: Mark Wyatt
Wyatt, ApJ (2003); Wyatt, ApJ (2006);
Krivov, Queck, Löhne, & Sremcevic, AAp (2007);
Reche, Beust, Augereau, & Absil, AAp (2008)
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Not only clumps, but also rings
Dependence on planetary mass and eccentricity

re
n
No

nt
a
n
so

s
e
r
u
ct
u
r
t
s

Rings

Clumpy disks

Kuchner & Holman, ApJ 588,
1110-1120 (2003)
1st CPS International School of Planetary Sciences

Reche et al., AAp 480,
551-561 (2008)
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Dust can also form spirals
Wyatt, ApJ 639,
1153-1165 (2006)

Large grains:stay in clumps

Medium-sized grains:form a ring

Small grains:
spirals emanatingfrom clumps
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Caution: alternative explanations for clumps and spirals• Major collision
betweenplanetesimals• “Supercomet”• Passage ofa nearby star

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Caution: alternative explanations for clumps and spirals• Major collision
betweenplanetesimals• “Supercomet”• Passage ofa nearby star
Grigorieva,Thébault, & Artymowicz, AAp 461, 537-549 (2007)• A spiral-ilke pattern for ~1000 yrs• Avalanches possible, but only for dustiest disks
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β PicPlanets were expected:(1) offset (2) warp (3) FEBs (tens of papers)(4) several rings (Okamoto et al., Nature 2005)(5) “stream” (Baggaley, JGR 1999; Krivov et al., AAp 2004)

Freistetter, Krivov, & Löhne, AAp 466, 389-393 (2007)
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β Pic...and “planet 1” now seems to have been discovered

Lagrange et al., A&A (in press)
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Kobe, January 8, 200957

Outline Concept of debris disks
Part I Observations of debris disks
 Basic theory of debris disks
 Debris disks: seeing dust

Part II Debris disks: thinking of planetesimals
 Debris disks: thinking of planets
 Debris disks: thinking of planetary systems
 Summary

1st CPS International School of Planetary Sciences

Kobe, January 8, 200958

The hot dust problem
Example:HD 69830
Only 2% of FGK stars
exhibit hot dust detections(< 24 µm or < 10 AU)
Most of these systems with
hot dust (4 / 7) have dust
luminosities larger than“maximum for a given age”
allowed by stationary
collisional models
Wyatt et al., ApJ 658, 569-583 (2007)

Possible parent
body locations

1AU

Beichman et al., ApJ 626, 1061-1069 (2005),
Lovis et al., Nature 441, 305-309 (2006)
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The hot dust problem: exozodis

Vega system:
Discovered a “zodiacal cloud” within 1 AU from the star
Derived 10-7 Mearth in small dust grains
Requires dust production rate of ~0.01 Mearth / Myr
Incompatible with steady-state evolution: an unrealisticallylarge mass of the “asteroid belt”
Absil et al., AAp 452, 237-244 (2006)

Later on, several other exozodis with similar problems
Di Folco et al., AAp 475, 243-250 (2007); Absil et al., ApJ, in press (2008)
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The planetary system of Vega: transport of planetesimals?

KB dust

KB with clumps(migrated) “Neptune”

Centaurs Zodi

“Jupiter”

Absil, Beust, Reche, Augereau, Krivov, Müller, & Löhne (in prep.)
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61

The planetary system of ε Eri: transport of dust?

KB dust

KB with clumps(migrated) “Neptune”

Dust transport Warm dust, but no zodi

a known RV planet

Reidemeister, Krivov, Müller, Löhne, Augereau, & Absil (in prep.)
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Alternatively, transport by dynamical instabilities?
Late Heavy Bombardment in the solar system 3.8 Gyr ago

Simulation and animation: Alessandro Morbidelli

Nobody knows whether LHB-type events are common
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Outline Concept of debris disks
Part I Observations of debris disks
 Basic theory of debris disks
 Debris disks: seeing dust

Part II Debris disks: thinking of planetesimals
 Debris disks: thinking of planets
 Debris disks: thinking of planetary systems
 Summary

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Summary• Debris disks are optically-thin, dusty, gas-poor disks
around main-sequence stars• Debris disks are common around those stars• Debris disk observations reveal emission of dust
• Debris disks are direct evidence for planetesimals• Debris disks are indirect evidence for planets• Debris disks give clues to the formation, evolution, andarchitecture of planetary systems

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References• Aumann et al., ApJ 278, L23-L27 (1984)• Zuckerman & Song, ApJ 603, 738-743 (2004)• ESO Press photo (1997)• URL:http://astro.berkeley.edu/~kalas/disksite/pages/gallery.html• Benz & Asphaug, Icarus 142, 5-20 (1999)• Krivov et al., Icarus 445, 509-519 (2006)• p.18: Original by Krivov
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References• Krivov, Löhne, & Sremčević, AAp 455, 509 (2006)• Thébault & Augereau, AAp 472, 169 (2007)• Krivov, Löhne, & Sremčević, AAp 455, 509-519(2006)• Krivov, Müller, Löhne, & Mutschke, ApJ 687 (2008)• Beichman et al., ApJ 626, 1061-1069 (2005)• Lisse et al., ApJ 658, 584-592 (2007)• Chen et al., ApJS 166, 351-377 (2006)• Chen et al., ApJ 689, 539-544 (2008)

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References

•
•
•
•
•
•
•
•
•
•

Moór et al., ApJ 644, 525-542 (2006)
Wyatt et al., ApJ 658, 569-583 (2007)
Löhne, Krivov, & Rodmann, ApJ 673, 1123-1137 (2008)
Löhne, Krivov, & Rodmann, ApJ 673, 1123-1137 (2008)
Löhne, Krivov, & Rodmann, ApJ 673, 1123-1137 (2008)
Löhne, Krivov, & Rodmann, ApJ 673, 1123-1137 (2008)
Su et al., ApJ (2005)
Marsh et al., ApJ (2006)
Müller, Löhne, & Krivov, in prep.
Simulations and animations: Alessandro Morbidelli,
R. Gomes, H.F. Levison, K. Tsiganis, & A. Morbidelli,
Nature, 435,466 (2005)

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References• Apai et al., ApJ 672, 1196-1201 (2008)• Kalas et al., Nature 435.1067 (2005)• Kalas et al., Science 322, 1345 (2008)• Kalas, ApJ 635, L169 (2005)• Greaves et al., MNRAS 351, L54 (2004)• Liu, Science 305, 1442 (2004)• Greaves et al., ApJ 506, L133 (1998)• Su et al., ApJ 628, 487 (2005)• Kalas et al., ApJ 637, L57 (2006)
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References• Greaves et al., ApJ 619, L187-L190 (2005)• Simulations and animations: Mark Wyatt• Kuchner & Holman, ApJ 588, 1110-1120(2003)• Reche et al., AAp 480, 551-561 (2008)• Wyatt, ApJ 639, 1153-1165 (2006)• p.54 --• Grigorieva,Thébault, & Artymowicz, AAp 461,
537-549 (2007)
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References• Freistetter, Krivov, & Löhne, AAp 466, 389-393(2007)• Lagrange et al., A&A (in press)• Su et al., ApJ (2005)• Marsh et al., ApJ (2006)• Absil et al., AAp 452, 237-244 (2006)• Simulation and animation: Alessandro
Morbidelli

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