debris disks · 10‐02‐10 18.08 exoplanets for all ‐ lund 2010, february 8/9 13 these grains...

10‐02‐1018.04 ExoPlanetsForAll‐Lund2010,February8/9 1

Debris Disks Laplace


Debris Discs Laplace


Debris From Wikipedia, the free encyclopedia Jump to: navigation, search

Debris (pronounced /!de".bri#/, /d$!bri#/) is a word used to describe the remains of something that has been

otherwise destroyed. The singular form of debris is debris. Depending on context, debris can refer to a number

of different things.

René Liseau


Debris From Wikipedia, the free encyclopedia Jump to: navigation, search

Debris (pronounced /!de".bri#/, /d$!bri#/) is a word used to describe the remains of something that has been

otherwise destroyed. The singular form of debris is debris. Depending on context, debris can refer to a number

of different things.

René Liseau

Debris Discs for All

whatwedoknow

whatwedon’tknow

whattheproblemsare

…and how to resolve them


But why would we care ?


But why would we care ?

because of this ?


Is it about Planets ?

Maybe - in any case, it’s about Dust…

Dust as synonymous for Debris


The Planet Connection

Dust in the inner Solar System

The Zodiacal Dust/Gegenschein

Inner Solar System Asteroid Belts


Infrared Radiation – Wavelengths, Dust Sizes and Temperatures

€

λ ≈10µm ⇔T ≈ 300K

Leinert et al. 1998 A&AS 127, 1


grainradiusradialdistanceopacityindex


temperature depends on the distance from the central star

and on the grain size*

*In general, the grain temperature will also depend on its shape, chemical make-up and porosity.

atthesamedistancefromthestar

smallergrains(and/orwithβ>0)areho7erbigger(and/orwithβ=0)grainsarecooler


ExoDebris Disks

Excess over Photosphere

at long wavelengths

Cold Discs


These grains are situated relatively much farther from their star than their Earth-Sun-Zodi cousins

…but why would these grains need to be debris ?

ISM=Gas&Dust

ISMformsStar&Disc

DisccontainsISMDust

ISMDustdifferentfromDebrisDiscDust?

orisDustsimplyDust…?whichbitesanotherone…


These grains are situated relatively much farther from their star than their Earth-Sun-Zodi cousins

”situated” are they just sitting there ?

ForcesacUngonthegrains

ForceTimeScalesandLifeUmeofthegrains

Dynamicsofthegrains


€

Gravitational and Radiation Pressure forces act opposing each other. Their ratio

Frad

Fgrav

≡ b =3

16πcGL∗ M∗

ρgrainagrain

independent of the radial distance r

grains unbound when b ≥ 0.5, i.e.

a < ablow =3

8πcGL∗ M∗

ρgrain

mass - luminosity relation for Main - Sequence stars

example Sun : ablow ≤1µm (upper end ISM size), time scale a few ×103 years.


€

Poynting - Robertson drag∗ makes grains (b << 0.5) spiral inwards toward star

time scale tPR ∝agrainρgrain

Qrad

r2

L∗≈ 400 rAU

2

b years

example Sun : azodi ≈ 20 − 200 µm and tPR (1AU) ≈ 105 -106 years

Zodi dust is recently generated and not primordial ISM dust.

• c.f. inertial solar frame and rotating grain-frame; differences in absorbed and emitted photon momenta. Yarkovski force enhances PR, anisotropic absorption/emission by lage bodies’ day/night side.

sic !


€

Stellar winds/ms - mass loss∗

Ratio of time scales for stellar wind and radiation pressure forces

t ˙ M

tPR

≈ 3 Qrad

Q ˙ M

L ˙ M ( )star

L ˙ M ( )Sun

(Plavchan et al. 2009, ApJ 698, 1068)

both t ˙ M and tPR some million of years

<< main - sequence ages, which typically some 102 -103 million of years.

€

Particle populations

e ≈ b1− b

small particles, b >>1, e→1, ≈ always in outer regionsbig particles, b <<1, e→ 0, in inner region for time tPR

⇒ inner region depleted⇒ distribution changes with time t

*Aberration angle is ~ υorb/c for PR-drag, ~υorb/υsw for stellar wind drag. B. Gustafsson (1994, ”Florida-Bo Gustafsson”, Ann.Rev.Earth Planet Sci., 22, 553). Stellar wind drag reduces PR-drag. Most important for young K, M stars. Potentially explains paucity of detected Debris Disks around M-stars.


Debris production by means of collisions

€

Simple estimate of time scale for (destructive) grain- grain collisions

tcoll =agrainρgrain

Qcoll

r7

Mdisc2 Mstar

(Minato et al. 2006, A & A 452, 701)

or

tcoll =1

τ⊥Ω, τ⊥ is vertical optical depth and Ω is angular velocity

tcoll ∝1

LIR Lstar

, a few ×103 years (typically)

€

The equilibrium size distribution tends toward a power law

dN da∝ a−3.5 (Dohnanyi 1969, JGR 74, 2531)


€

The equilibrium size distribution tends toward a power law

dN da∝ a−3.5 (Dohnanyi 1969, JGR 74, 2531)

NumericalsimulaUons:sizedistribu=onsrarelyeverpowerlaws(Thébault & Augereau 2007, A&A 472, 169)

- tcoll not necessarily lifetime (allowance for multiple fragmenting collisions)

- cratering included (different collision/erosion outcomes)

- code follows dynamical evolution

- minimum survival size, a > a(Mdisc, tcoll > tstar) are collisionally unevolved, i.e. primordial


Thébault & Augereau 2007 A&A, 472, 169 Krivov et al. 2006, A&A, 455, 509

…observational test with Herschel

wavy opacities – not power laws RJ-slope of κλ corresponds to β ≈ 0.8

embargoed

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Rayleigh - limit of Lorentz - Mie theory x <<1, where the size parameter

x ≡ 2π aλ

i.e. the particle size, a is much smaller than the wavelength of the radiation, λ

greysubmmopaciUesβ0 : normalization?

€

q1 Eridani ring - belt β = 0 β Pictoris disc has β = 0.8

sca7eringabsorp=on€

dN da∝ a−3.5


embargoed

Belt and Ring Structure or Continuous Discs with and Planets ? non-power law size distribution


Belt and Ring Structure or Continuous Discs with and Planets ? non-power law size distribution

Kalas et al. 2008, Sci. 322, 1345 Olofsson et al. 2001, ApJ 563, L77


What’s the role of Planets in this Debris business ?


Q: What is the source of the observed debris ?

A: Generally non-observed km-size bodies…

€

Collisional steady state requires a total minimum mass of "debris parents"

Mplanetesimal > 4 LIR c 2( ) tstar (Chen & Jura 2001, ApJ 560, L171)

example Sun∗ : present Kuiper Belt mass ≈ 0.03− 0.1M⊕ : initially 30M⊕

[according to solar system formation models⇔ (a lot of) a missing mass problem]

*The mass of the Minimum Mass Solar Nebula (MMSN) corresponds to about 50 000 MMoon.

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Edgeworth-Kuiper-Belt shaped by planets

ΔR = 30-55 AU and dynamically stable (2:3 39.5 AU, 1:2 48 AU)

N(D>100 km) > 103 (known to exist Jan 2010)

N(D>100 km) > 7×104 (believed to exist)

Eris D ≈ 2500 km

€

Size distribution close to power lawdN dD∝D−q , q = 4 ± 0.5 (Bernstein et al. 2004, AJ 128, 1364)

ChemicalcomposiUon:icesCH4,CO,H2O,NH3


But these km-size bodies are NOT the ones we see (a < cm)

€

At submm/mm wavelengths ,

the mass of the radiating dust is obtained from

Mdust =Fν D2

κν Bν Tdust( ) i.e. one needs to know

the temperature Tdust at the radial distance rdust

rdust ≈1

2 Rstar

TeffTdust

2

and the opacity κν , often expressed as a power law

κν ∝ vβ

β = 0 ⇔ blackbody/greybody emission, Iν , submm ∝ λ−2

β = 2 ⇔ ISM grains (sub - micron size), Iν , submm ∝ λ−4


FomalhautBeltKuiperBelt

ExoDebris Disks: how many Spatially resolved ?

the answer is: 20 November2,2009CatalogofResolvedCircumstellarDisksCaerMcCabe&Carlo_aPham


SpitzerObserva=onsofFGKstars

Trilling et al.(The Astrophysical Journal, 674,1086, 2008 February 20 )

Summary/compilation of Spitzer Observations of Sun-like Stars

>200 systems observed (mostly spatially unresolved)

distance ≤ 60 pc age ≤ 12 Gyr ⅓ × solar ≤heavy elements≤3× solar

Spitzer Observations of FGK stars

Trilling et al. (2008, ApJ 674,1086)


Spitzer Observations of FGK stars

Trilling et al. (2008, ApJ 674,1086)

Summary/compilation of Spitzer Observations of Sun-like Stars

>200 systems observed detecUonsat24µm:4% detecUonsat70µm:15%

excess rates for AFGK stars indistinguishable excess decline with age is slow

exo-discs are cool: < 100 K (Zodi) exo-discs are large: > 10 AU (EKB)


Nilsson et al. (2009, A&A 508, 1057)

excess rates for AFGK stars indistinguishable excess decline with age is slow

!

noclearstellarmassdependencenostrongstellaragedependence

DebrisDiscIncidence

sameage:10Myrdifferentdiscmasses(MMoon)

differentstellarmasses:3.5–>0.2M

combinedwithPleiades’age:100Myr

€

LIR Lstar ∝ t−p , p > 0.8

…butagesofMS‐starsnotoriouslydifficult…

€

cf . LIR Lstar ∝ t−1, collisions


AretheobservedIRexcessesduetoDebrisDiscs:formed,maintainedandshapedby

Planets?

Hypothesis/Paradigm

- dust in Debris Discs due to collisions among planetesimals

- planetesimals due to Planet forming process

Kóspáletal.(2009,ApJ700,L73)


AretheobservedIRexcessesduetoDebrisDiscs:formed,maintainedandshapedby

Planets?

Spitzer Observations of Stars with Planets and without Planets (24 µm and 70 µm)

150 Planet Stars (RV-planets) 118 no-Planet Stars (searched but not detected)

Results:Kóspáletal.(2009,ApJ700,L73)conclude

”thepresenceofaplanetthathasbeendetectedviacurrentradialvelocitytechniquesis

notagoodpredictorofthepresenceofadebrisdiskdetectedatinfraredwavelengths.”

LimitaUons/biases:JupiterΔV≈12sini(ms‐1),P≈12years;LIR/L≤10-7

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ISM0.005to<<1µm

Starformingclouds<<10µm

ProtoplanetaryDiscs<<10cm

in situ Interplanetary Space

Planetary Discs: µmcmdmmkm103km

GasGiants

RockyPlanets

critical size

Grains Grow* to Planets (and shrink to debris again…) ?

*Herbstetal.(2008,Nature452,194)‐Güttler et al. (arxiv.0910.4251) - Zsom et al. (arxiv.1001.0488)

Debris Discs?

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What is known ? How many are there ? How is their existence inferred ?

Are debris discs discs ? How continuous are they ? How smooth are they ? How many resolved discs are known ? How symmetric are they ?

Are debris discs tracers of planets ?

Do they form planets ? Do they mark out planets ? Do they diagnose planetary systems ? How do debris discs evolve in time ?

How gas-free are debris discs? How fine is the debris ? What is the debris made of ? What is the dynamics of debris discs ?

We still like to think so…


35

Planets are invoked/needed to explain Debris Disc features/structure

rings clumps

disc warps asymmetric brightness

eccentric offsets spiral wave

belts


ceterum censeo darvinum esse volandum

debris disks · 10‐02‐10 18.08 exoplanets for all ‐ lund 2010, february 8/9 13 these grains...

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