Is there evidence of planets in debris disks?

Is there evidence of planets in debris disks? Mark Wyatt Institute of Astronomy University of Cambridge La planètmania frappe les astronomesKalas, P. 1998,La Recherche 314, 38

Is there evidence for planets in debris disks? Yes!  Eridani has both a dust disk (Greaves et al. 1998) and a planet detected by radial velocity surveys (Hatzes et al. 2000) But radial velocity planets and debris disks are at different locations and it is unclear to what extent the two phenomena are related (Greaves et al. 2004; Beichman et al. 2005)

Do debris disks contain evidence for planets? • What signature would a planet impose on a debris disk? • Have these signatures been observed? • Is there any other possible cause of these signatures? • Can we make further testable predictions?

Central cavities Central cavities were inferred from the lack of mid-IR emission in the SED: But imaging proved the existence of the inner holes: HR4796 Fomalhaut Log(F, Jy) Telesco et al. (2000) Kalas et al. (2005) 1 10 100 Wavelength, m Walker & Wolstencroft (1998)

Central cavities: without planets P-R drag would fill in the hole? Without planets to scatter or trap dust in resonance, P-R drag would fill in the inner hole in tpr = 400r2/M* years This was the model proposed to explain the hole in the  Pictoris disk Kuiper Belt dust distribution Number density Distance, AU With Without Planets Liou & Zook (1999) Roques et al. (1994)

Central cavities: no, P-R drag is insignificant Surface density distribution from balance of P-R drag and collisions (Wyatt 2005): Tenuous disks 0 < 1 flat density distribution P-R drag dominated Dense disks 0 > 1 dust confined to planetesimal belt collision dominated Surface density Distance from star All known debris disks have o>10 and so are collision dominated P-R drag is insignificant in all detectable debris disks

Origin of the inner holes? • Lack of mid-IR emission implies few colliding planetesimals in inner regions(Wyatt 2005) • Few planetesimals expected in middle of planetary systems as planets clear gaps along their orbits(Wisdom 1980) • Planet formation is faster closer to the star resulting in inner holes in the models (Kenyon & Bromley 2002) • But, could early circumstellar disk evolution also produce inner holes without planet formation (Clarke et al. 2001) ? Inner holes are weak, though credible, evidence of planets

Secular perturbations: warps A planet's gravity affects the orbits of planetesimals and dust in a debris disk. Perturbations from a planet can be secular or resonant (Murray & Dermott 1999). Secular perturbations are the long term effect of the planet’s gravity and act on all disk material over >0.1 Myr timescales • A planet aligns planetsimals to its orbital plane so that a disk iswarpedif • one planet is misaligned with the disk (Augereau et al. 2001) • two planets with different orbital planes Augereau et al. (2001)

Secular perturbations: spirals and offsets Planets on eccentric orbits impose eccentricities on nearby planetesimals causing: spiral structure offset centre of symmetry Wyatt et al. (1999) Wyatt (2005)

Star Star Resonant perturbations: clumpy rings Resonances affect material with orbital periods that are a ratio of two integers times that of planet • Resonances cover small regions of parameter space, but can be filled by: • Inward migration of dust • Dust spirals in due to P-R drag and • resonances halt inward migration • Outward migration of planet • Planet migrates out sweeping • planetesimals into its resonances • Resonant filling causes a clumpy ring to form along the planet’s orbit Resonance Pl Resonance Pl

Why resonances are clumpy

Sun Earth  Dust migration into resonance The structure expected when dust migrates into planetary resonances depends on the planet’s mass and eccentricity (Kuchner & Holman 2003): low mass high mass low eccentricity high eccentricity Ozernoy et al. (2000) Dermott et al. (1994) Quillen & Thorndike (2002) Wilner et al. (2002)

Resonant structures due to planet migration Wyatt (2003)

Have these signatures been observed? Warps Spirals Offsets Brightness asymmetries Clumpy rings Yes!!

Other causes of signatures? collisions Could this be the cause of the clumps? Telesco et al. (2005) • No for clumps seen in the sub-mm (e.g., Fomalhaut): • the collision would have to involve two >1400 km objects • too few can coexist in the disk for this to be likely • Yes for clumps seen in the mid-IR around young systems (e.g.,  Pictoris): • smaller colliding objects, ~100km • witnessed at special point in time Wyatt & Dent (2002)

Other causes of signatures? ISM sandblasting If ISM sandblasting of a debris disk is important, substantial asymmetries can arise… Motion relative to the ISM Artymowicz & Clampin (1997) … however, the ISM contribution is only important >400 AU from the star

Other causes of signatures? binary companions As well as truncating disks, binary companions can also impose spiral structure and asymmetries… Tidal perturbations cause open two armed structure Secular perturbations cause asymmetric extended structure Quillen et al. (2005) Augereau & Papaloizou (2003) … but the binary companions cannot explain all the spiral structure in the HD141569A disk (Wyatt 2005)

Other causes of signatures? stellar flybys Stellar flybys induce perturbations which excite eccentricities which cause spiral structure which collapses into nested eccentric rings Such an event may explain clumps seen in the NE of the  Pictoris disk Kalas et al. (2000) However, flyby encounters of field stars at an appropriate distance to perturb the disk (<1000 AU) are extremely rare Larwood & Kalas (2001)

Debris disk planet predictions • Detect planet itself directly or indirectly: hard • Multi-epoch imaging: • Resonant structures • orbit with planet • decade timescales • 2 detection of rotation in  Eri (Greaves et al. 2005) • Secular structures • >0.1Myr timescales • Multi-wavelength imaging: • can be done now!

Summary of the Vega planet migration model Assuming the dust has same distribution as planetesimals, Vega’s two asymmetric clumps seen in the sub-mm can be explained by the migration of a 17Mearth planet from 40-65AU in 56 Myr Observed Model Orbit Distribution Spatial Distribution Emission Distribution Wyatt (2003)

Dynamics of small bound grains • Radiation pressure alters orbital period of dust and so its relation to resonance • Small grains have higher libration widths than planetesimals • Particles smaller than 200m (L*/M*)-0.5 fall out of resonance  = 0.002 0.005 0.01 0.02 3:2 2:1 Smallest bound grains have axisymmetric distribution Resonant argument,  Time Wyatt (submitted)

Dynamics of small unbound grains 3:2 2:1 • Radiation pressure puts small (>0.5) grains on hyperbolic trajectories • The collision rate of resonant planetesimals is higher in the clumps Collision rate Collision rate Planet Planet Longitude relative to planet Longitude relative to planet Blow-out grains exhibit spiral structure if created from resonant planetesimals Wyatt (submitted)

Particle populations in a resonant disk 3:2 2:1 Population Spatial distribution I Same clumpy distribution as planetesimals II Axisymmetric distribution III   r-1 distribution IIIa Spiral structure emanating from resonant clumps IIIb Axisymmetric distribution Wyatt (submitted)

Application to Vega SED modelling used to convert Su et al. 3 component model into continuous size distribution… • … then used to assess contribution of grain sizes to observations: • Sub-mm samples pop I • Mid- and far-IR sample pop III Flux, Jy Jy / logD Wavelength, m Particle diameter, m Observations in different wavebands sample different grain sizes and so populations, thus multi-wavelength images should show different structures and can be used to test models Wyatt (submitted)

Application to Vega Size distributionCollisional lifetime AU2 / logD yr Particle diameter, m Particle diameter, m Wyatt (submitted) • Conclusions • Pop. II reduced by collisions with blow-out grains (Krivov et al. 2000) • Pop. III grains removed at 2M/Myr • Pop. II destroyed at 0.02M/Myr • Pop. I processed at >2M/Myr • Thus pop. III is type IIIa, so… … mid- to far-IR images should exhibit spiral structure emanating from clumps

Debris disk exoplanetology When/if debris disk exoplanets are confirmed we will want to compare them to other exoplanets, and use them to inform planet formation models, so why not start now? Number Semimajor axis, AU • Debris disk planets occupy a region of parameter space inaccessible to other techniques • Continuous distribution of orbital radii to 100s of AU? • Confirmation vital! Planet mass, MJupiter Semimajor axis, AU

Conclusions • Planets would impose structures on debris disks ranging from clumps to warps, offsets, brightness asymmetries and spirals • All of these structures have been observed in debris disks and (in most cases) there is no other explanation • Planets also cause holes, but this is weak evidence of planets • We need to confirm planetary interpretation through • multi-epoch imaging • multi-wavelength imaging • This is a credible and extremely valuable exoplanet detection technique

Is there evidence of planets in debris disks?