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Chapter 13.Debris disks

Debris disk phasePP phase

10Myr 10GyrEvolutionary Phases of Planetary Systems

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Outline Observations of debris disks

Physics of debris disks

Size distribution

Radial structure

Azimuthal structure

Long-term evolution

Short-term evolution

Summary

How do we know they exist?

How do they work?

How large are the dust grains?

Where is the material located?

Why are the disks clumpy?

Why do the disks fade away?

Aren� t they all exceptions?..

What was all this about?

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Observations of

debris disks

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1984

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(1) Infrared excesses (� Vega phenomenon� )

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

What is observed?

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

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Zuckerman & Song, ApJ 603, 738-743 (2004)

What is observed?

Since then: thanks to IRAS, ISO, ..., Spitzer,the Vega phenomenon has beenobserved around ~1000 nearbystars

About 15% of main-sequence starshave dust around them

Almost the same incidence aroundA,F,G,K stars, may be slightlyhigher for early types, M unclear

(1) Infrared excesses (� Vega phenomenon� )

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What is observed?

The first image of a dustdisk around amain-sequence star:β Pictoris

Dust is present in the formof a structured disk

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

(2) Images

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(2) Images

What is observed?

Since then: imagingwith many ground-basedinstruments and HST,both in scattered lightand thermal emission

By now, about 20 disks imaged

Vis NIR FIR sub-mm

~1 Gyr age

~10 Myr age

β Pic

ε Eri

α PsA

α Lyr

AU Mic

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What is the origin of dust?

Ages of systems: 10 Myr ... 10 GyrBut: 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

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What is a � debris disk� ?

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What is a � debris disk� ?

Dust

Planets

Star

Planetesimals

Thus there are dust and planetesimals around a star.Solids have a continuous size distribution. They all are referred to as a debris disk.

The star may also harbor planets.The debris disk is genetically connected to, and interacts with, them

Debris disk

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Why should we care?

Dust observed in debris disks tells us about:planetesimals that supply dust

and planets that affect dust (and planetesimals)

Debris disk studies can give insight into

origin and evolution of planetary systems

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Does our solar system have a debris disk?

EKB dust disk

Zodiacal cloud

The inner part of our debris disk,known for centuries

The main part of our debris disk,yet to be discovered...

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Physicsof

debris disks

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Comparison with other disk systems

noyesyesyesCollisions

lowmediumlowhighDyn. temperature

yesnoyesnoEmbedded in gas

--~1015~1010~1030Mass distrib. width

~0.1~10-6~0.1~0.1Thickness, h / r

~1~1..21...10>104Toomre Q

~0~10-310-3...-2(gas)~0Shakura’s α

--2...10104...105 104...105Size / Rprimary

~10...100~1012<106106...109Age / orbits

(solids)

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Physical processes: (a) collisional cascade

Net result

Collisional cascade grinds planetesimals to ever-smaller fragments, down to dust sizes.

Individual collisions

Critical specific energy for fragmentation:strength and gravity regimes

Under debris disk conditions:-- Disruptive collisions- Cratering collisions

W.K. Hartmann

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Physical processes: (b) gravity and � photogravity�

Stellar � photogravity� planetesimals move in nearly-circular orbits,dust grains in elliptic ones,fine dust leave the system in

hyperbolas

Gravity of planets confines planetesimals and stirs them up, exerts secular and resonant perturbations on planetesimals and dust

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Physical processes: (c) drag forces

Poynting-Robertson force dust orbits slowly circularize and shrink toward the star (important for early- and solar-type stars)

Stellar wind drag force � corpuscular analog� to the P-R effect (strong for late-type stars)

Wyatt & Whipple, ApJ 111, 134 (1950)Breiter & Jackson, MNRAS 299, 237 (1998)

Burns et al. (1979), Plavchan et al. (2005)

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(d) Other processes

Sublimation and sputtering eliminate dust close to the star, gradually reduce their size

(important for early-type stars)

Lorentz force (important for small grains onlyand 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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Transport-dominated vs collision-dominated disks

Less dusty disks (here: our Zodi) Dustier disks (here: β Pic)

Krivov et al., AAp 362, 1127 (2000)Leinert & Grün (1990)

Collisional timescale can be <> transport timescales:

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Theoretical 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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Size distribution

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Krivov, Löhne, & Sremčevi , ć AAp 455, 509 (2006)

Size distribution

" Tiny blowout grains are present, but in smaller amounts" Grains just above blowout limit dominate cross section" Size distribution is wavy

Blowout limit

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Size distribution

Thébault & Augereau,AAp 472, 169 (2007)

" Cratering collisions are important" They substantially enhance the � main� maximum

Blowout limit

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Radial structure

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Temperature calculations: Sebastian Müller

Distances from IR excesses

A stars: 24-70µm excesses -> FGK stars: 33-70 µm excesses ->(color) T< ~ 90K (Su et al. 2006) (color) T<~ 45-85K (Hillenbrand et al. 2008)

Compatible with EKB-size disks,extending from several tens AU outward

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Distances from images

A star (Fomalhaut) K star (e Eri) M star (AU Mic)

150 AU 60 AU

30 AU

Same result: compatible with EKB-size disks,extending from several tens AU outward

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Intermediate conclusion

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Planetesimal belts and radial profiles

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

Surface density index ~-1.5, brightness index ~-3.5

Models

Pla

nete

sim

al b

elt

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Planetesimal belts and radial profiles

Surface density index ~-1...-2, brightness index ~-3...-4

Liu et al. (2004), Liu (2004), Strubbe & Chiang (2006),Augereau & Beust (2006)

Observations (AU Mic example)

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Intermediate conclusion

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Inner gaps: footprints of planets?

Simulations and animations: Alessandro Morbidelli

Inner voids can be naturally explained with two or more planets

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Intermediate conclusion

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First attempts to find these of planets

8 young systemswith cold excessessearched with VLT/NACO

no planets found

Apai et al.,ApJ 672, 1196-1201(2008)

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Azimuthal structure

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AU Mic (Liu 2004)

β Pic (press photo)

HD32297 (Kalas 2005 )

α PsA (Holland et al. 2003)

ττ Cet(Greaves et al.2004)

All resolved disks are structured

ε Eri (Greaves et al. 1998)

Radial,azimuthal,

verticalstructure:

●rings,●gaps

●clumps●spirals●offsets●warps

...

Vega (Su et al. 2005 )

HD32297 (Kalas 2005 )

HD139664 (Kalas 2006 )

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

Example: ε Eri

Theory of clumps

Suggested scenario:resonant (clumpy!) family ofplanetesimals produce dust,which stays in the resonance

Wyatt, ApJ 598, 1321-1340 (2003)

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Wyatt, ApJ 598, 1321-1340 (2003)

Theory of clumps

Can be captured in resonances by migrating planet

Will dust stay in resonance?

Resonant populationof total mass M

Sizes* from smax (planetesimals)* down to smin (dust)

Collisional cascade

Loss at dust sizes* small grains (RP)* fast grains

Krivov et al., AAp 462, 199-210 (2007)

How to create resonant family?

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Clumps like ε Eri� s could be produced by a resonant planetesimal family of lunar to martian mass

α =0.833...α =0.87

Theory of clumps

Krivov, Queck, Löhne, & Sremčević, AAp 462, 199-210 (2007)

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Long-term evolution

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IR surveys indicate a long-term decay of debris disks

Moór et al. ApJ 644, 525-542 (2006)

" On the average, dust luminosity goes down with system� s age" A high dispersion for any given age though

t-1 t-0.5t-2

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Dominik-Decin (2003) model

Dominik & Decin, AAp 598, 626-635 (2003)

" Equal-sized planetesimals � feed� dust" Dust has a single power-law size distribution

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Dominik-Decin (2003) model

Dominik & Decin, AAp 598, 626-635 (2003)

" For collision-dominated disks (usually the case),

total disk mass ~ dust mass ~ t -1

" �Delayed stirring� suggested to explain high luminosity in some of the old systems

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Wyatt et al. (2007) model

Still, an assumption of a quasi-steady state and a single power-law size distribution

Wyatt et al., ApJ 658, 569-583 (2007)

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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, Krivov, & Rodmann, ApJ 673, 1123-1137 (2008)

Löhne et al. (2008) model

" Large planetesimals are not in collisional equilibrium" Strength-gravity transition plays a major role

Krivov, Sremčević, & Spahn,Icarus 174, 105-134 (2005)

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Löhne et al. (2008) model

Löhne, Krivov, & Rodmann, ApJ 673, 1123-1137 (2008)

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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

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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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Short-term evolution

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Possible transient events

• Major collision between planetesimals

• �Supercomet�

• Passage of a nearby star

• Dynamical instability caused by planets

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Possible transient events

Grigorieva,Thébault, & Artymowicz, AAp 461, 537-549 (2007)

" A spiral-ilke pattern for ~1000 yrs" Avalanches possible, but only for dustiest disks

• Major collision between planetesimals

• �Supercomet�

• Passage of a nearby star

• Dynamical instability caused by planets

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Possible transient events

Simulation and animation: Alessandro Morbidelli

Nobody knows whether LHB-type events are common

• Major collision between planetesimals

• �Supercomet�

• Passage of a nearby star

• Dynamical instability caused by planets

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Su et al., ApJ 628, 487-500 (2005)

Is Vega transient?

Fits to SED & radial brightness profiles @ 24, 70, 160 µm suggest that small blowout grains are overabundant. Resulting size distribution not dynamically justified...

Blowout limit

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Marsh et al., ApJ 646, L77-L80 (2006)

Is Vega transient?

...so we tried size distribution from our collisional model (Krivov, Löhne, & Sremčević, AAp 455, 509-519, 2006),

with a partial success (Müller, Krivov, Löhne, in prep.).

Blowout limit

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Is β Pic transient?

The 52 AU blob is warmer than the surrounding disk(T=190 K vs 140 K);gives grain size ~0.1µm;must be blown out on a 1000 yr timescale;therefore transient

Origin:major breakup?supercomet?nearby star?All unlikely...

Telesco et al., Nature 433, 133-136 (2005)

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Is HD 69830 transient?

Only 2% of FGK starsexhibit 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)

Beichman et al., ApJ 626, 1061-1069 (2005),Lovis et al., Nature 441, 305-309 (2006)

Example:HD 69830

1AUPossible parent body locations

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

Is HD 69830 transient?

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?

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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 disks are direct evidence for planetesimals

• Debris disks are indirect evidence for planets

• Debris disks give clues to the history of planetesimal and planet formation and evolution

• There is a bulk of observational data on debris disks; there are elegant theories of debris disks; these observations and theories do not necessarily agree with each other