Debris Disks around Nearby Stars

David J. Wilner (Harvard-Smithsonian CfA)

• What are Debris Disks? dust requires replenishment• Interest in Resolved Morphologies holes, blobs as planet signatures• Imaging Examples: Vega, Eridani, Fomalhaut, ...• Future Prospects Spitzer Space Telescope, SMA, ALMA

SUNY Stony Brook, April 14, 2004

collaborators: M. Holman (CfA), C.D. Dowell (Caltech), M. Kuchner (Princeton)

Introduction to Debris Disks

• Vega infrared excess discovered serendipitously during IRAS calibration (Aumann et al. 1984) thermal emission from cold dust

• Orbiting dust particles subject to gravity, wind/radiation pressure (ejection) and Poynting-Roberston drag (inspiral to star)

• tP-R = (400/)(Mo/M*)(r/AU)2 yr << stellar age (~350 Myr) dust particles must be replenished

• other nearby Vega-excess stars found by IRAS include Pic, Fomalhaut, Eri (the “Fantastic Four”)

far-ir

optical

• Pic disk geometry confirmed by images of visible scattered light (Smith & Terrile 1984)

• ISO 60 m survey finds 14/84 nearby main-sequence stars (17%) with excess emission (Habing et al. 2001)

• debris disks are cool (T<100 K), Kuiper Belt size (R>50 AU) tenuous (L/L* ~ 10-5 to 10-2, M ~ Mmoon), gas poor

• various analyses of IRAS and ISO databases show: - 100+ candidates resembling Fantastic Four in T, L/L*

- no strong dependence with stellar type (M, L*) - dust may decline with age (gradually? abruptly?) few x 100 Myr ~ Solar System heavy bombardment

Stages of Disk Evolution/Planet Formation

1. embedded protostar 104-105 yr

2. HAe/Be Star 105-106 yr

3. transition phase ~107 yr

4. debris disk >> 107 yr

Malfait et al. 1998

F

scattered light

Observational Probes of Disk Structureemitted light

< spatial resolution << temperature dependence <

> contrast with star, dynamic range >

optical/near-ir far-ir/submmmid-ir

J band Coronagraph+ AO 850 m: 14 arcsec beam

p i

c

HST/NICMOS Scattered Light: Gaps and Rings

JCMT 850 m SCUBA Images

• First moderate resolution submm images (14 arcsec) of Fantastic Four show disk and ring morphologies, also emission peaks offset from stellar photospheres

• Submm emission hints at sculpting by planets: cleared interior cavities, persistent dust features

Planet Detection Parameter Space

Kepler mission

What Creates the Dust Blobs?

• background galaxies: unlikely given the source counts

• dust generated in situ by collisions of large planetesimals: would have to be recent (disperse in ~10 to 100 orbital periods) and likely rare (massive enough to release Mmoon) • dust directly associated with orbiting bodies, e.g. remnants of circumplanetary disks?

• dust spiralling starward trapped in resonances with planet (cf. zodiacal dust trapped by Earth, Dermott et al. 1994)

Plutinos are in 3:2 Mean MotionResonance with Neptune

Jewitt Kuiper Belt Page

CfA Minor Planet Center

Dust in our Solar System from Afar

• numerical simulations suggest Solar System would be recognized to harbor at least two planets: Neptune, Jupiter

• note: Solar System dust emission at 850 m at 10 pc only ~ 1 Jy (<< solar photosphere)

Face-on view of the brightness from a numerical simulation of the column density of 23 mm dust particles from Liou & Zook (1999). The signatures of the planets are (1) deviation from a monotonic radial brightness profile, (2) ring along Neptune orbit, (3) variation along ring, (4) relative lack of particles within 10 AU

(Liou & Zook 1999)

Trapping by a low M, low e Planet

• for Neptune, Earth: first order resonances substantial trapping

• example: 3:2

• each orbit has j=3 longitudes of libration for trapped particle

(a) Several particle orbits with different’s (longitudes of pericenter). (b) Libration centers of the 3j-2o- term for two of these orbits. (c) Locus of all libration centers. (d) The density wave follows the motion of the planet at the same angular frequency as the planet.

Structure in the Vega System

• Vega ( Lyrae): A0V main sequence star, d=7.76 pc• system viewed nearly pole-on: vsini, reddening

• JCMT 850 m SCUBA image (Holland et al. 1998) shows: - roughly circular boundary - an offset emission peak - asymmetry extended NE-SW - central cavity around the star

• interferometry allows imaging with factor > 10x higher angular resolution; need high sensitivity

(Wilner et al., ApJ, 569, L115)

see Koerner et al. 2001 for OVRO study

IRAM PdBI Observations

• compact D config baselines 15-80 m

• dry winter weather • 4 tracks : tint = 23 h

• 1.3 mm + 3.3 mm simultaneously

• rms: ~0.3 mJy at 1.3 mm, ~0.1 mJy at 3.3 mm

Images of Vega at =1.3 mm

2.8 x 2.1 arcsecstellar photosphere

5.3x4.6 arcsecand dust blobs

(low surface brightness)

Trapping by a high M, high e Planet• presence of two peaks

• different separations of peaks from star

• peaks not co-linear with star

• patterns from different principle resonances occur at same longitude, 3:1, 4:1, 5:1, ...

Libration centers of the 3 -o-o- term. (a) Several particle orbits with different e and . (b) The libration centers of two of these orbits when the planet is at pericenter. (c) All the libration centers. (d) Clumps formed by particles trapped in this term appear to rotate at half the angular frequency of the planet.

Modeling the Millimeter Emission

(left) A representative numerical simulation of 1.3mm dust emission from orbital dynamics that includes a Jupiter mass planet, radiation pressure, and P-R drag. The dust becomes temporarily entrained in mean motion resonances associated with the planet, producing a two-lobed structure. (right) Simulated observation of the numerical model, taking account the IRAM PdBI response for the Vega observations, and the IRAM PdBI image after subtraction of the stellar photosphere.

Vega Summary

Searching for Light from the Planet

(left) Composite H band mosaic of Vega region obtained with PALAO. Eight point sources are detected. (right) H band sensitivity of the deep images to faint objects as a function or radial distance from Vega (analyzed for the east field). Solid points represent individual measurements; the solid line delineates the azimuthal average. The area between the vertical dotted lines indicates the locus of the inferred planet.

(Metchev et al. 2002)

Searching for Light from the Planet

(left) Deep Keck NIRC2 K’ band image of Vega. All candidate companions are in this field. The dashed circle indicates a radius of 15 arcsec. (right) 5 sensitivity of the image. The dashed lines indicate the planet masses from the models of Burrows et al. (1997).

(Macintosh et al. 2003)

Is Vega like the Early Solar System?

The temporal evolution of one of Thommes et al.’s simulations of an unstable Jupiter-Core-Core-Saturn system. Shown are the semi-major axes (thick solid) as well as the instantaneous perihelion and aphelion distances ofthe orbits.

• Thommes et al. (1999) +Thommes et al. (2002) suggested Neptune was scattered into a highly eccentric orbit

• Malhotra (1995) suggested Neptune migrated (outwards) by 7 to 8 AU in ~10 Myr; see Wyatt (2003) for Vega model varient

Saturn?

Jupiter?

Uranus?

Neptune?aphelion

perihelion

The Eri Debris Disk• single K2V star, age 0.5-1.0 Gyr, d=3.22 pc (3rd closest

naked eye star), closest analog to young Solar System

• (controversial) ~ 1 MJup radial velocity/astrometric planet,

a=3.4 AU, e=0.6 (Hatzes et al. 2000)

• far-ir spectrum

fit by r~60 AU

ring-like disk

(Dent et al. 2000,

Li et al. 2003,

Sheret et al. 2003

Moran et al. 2004)

Structure in the Eri System

• Structure due to a planetary perturber? Liou et al. 1999, Ozernoy et al. 2000 Quillen & Thorndike 2002

• JCMT 850 m SCUBA image shows nearly face-on ~60 AU radius ring with azimuthal variations (Greaves et al. 1999)

Inner Peak?

Potential of Inner Dust Imaging

• interaction of inspiralling dust with eccentric planet should produce two dust peaks, like the Vega system

• follow motions of dust peaks to independently characterize planet

350 m Observations with SHARC II• SHARC II: Caltech Submillimeter Observatory

facility camera, 12x32 filled array of ‘pop-up’ bolometers, optimized for 350 m (9 arcsec beam)

• Observations made in Jan 2003 commissioning run,

• >16 hours on Eridani in excellent weather (225<0.037), image rms ~3 mJy

Image of Eri at =350 m Image of Eri at =350 m

350 m Imaging Results • confirm basic ~ 60 AU ring structure

• no evidence for central rise in flux density corresponding to inner “zodiacal” component;

central clearing bolsters planet scenario

• clumpy structure of ring resolved into two (nearly) symmetric arcuate features, brightest se and nw

• clumps outside ring consistent with background of high redshift galaxies >10 mJy ~1 arcmin-2 (Smail et al. 2002)

Signpost of Planet Formation

• bright (L ~ 10-4 L*) narrow (a/a ~ 0.1) ring of observed size explained by collisional cascade in planetesimal disk stirred by recent formation of bodies of radius >1000 km

• does not account for azimuthal variations

(Kenyon & Bromley 2002, 2004)

Sculpting by a Planet?• characterization of possible unseen planets requires

matching robust features using numerical simulations

0.2 MJup

e = 02:1, 3:2 w/ highlibration

< 0.3 MJup

e ~ 0.35:3, 3:2 w/ phase segregation

Ozernoy et al. 2000 Quillen & Thorndike 2002

• models that selectively populate particular resonances are not realistic unless additional factors invoked, e.g. parent bodies trapped by planet migration, encounter

Comments on Models

• structure depends on many parameters, e.g. planet mass, eccentricity, semi-major axis, orbital phase, inclination, dust properties, orbits of parent bodies

• prominent “two-blob” morphology, like Vega,

Jupiter mass planet in eccentric orbit traps dust in exterior principal mean motion resonances

• models have time dependence that can be tested by synoptic observations with sufficient sensitivity and angular resolution (submm interferometry)

The Closest (< 4 pc) F,G,K Stars

Ceti0.0005 ME

Eri 0.01 ME

Sun<0.0001 ME

Cenmultiple

Procyonbinary

61 Cygbinary

Ind multiple

Future Prospects• Spitzer: high sensitivity at far-infrared wavelengths not accessible from the ground will provide exquisite SEDs for a large sample and greatly improve statistics

Riecke GTO Projects,FEPS Legacy Project

M. Meyer

First Results from Spitzer

Fomalhaut

Young Solar Analog Debris Disk• HD107146, G2V, distance 28.5 pc, age ~100 Myr• discovered during ground based support for Legacy Project “Formation and Evolution of Planetary Systems”

(Williams et al. 2004)

undetected by IRAS at 25 m

• substantial population of cold disks, see Wyatt et al. (2003)

• Submillimeter Array: a collaborative project of the Smithsonian Astrophysical Observatory and the Academia Sinica (Taiwan), eight 6 meter diameter antennas on Mauna Kea for arcsecond imaging initially for 1300, 850, 450 mm atmospheric windows

• submm interferometry is challenging• official SMA dedication was November 22, 2003• look for first call for external proposals in mid-2004

Early SMA ImagesT

W H

ya C

O(3

-2)

Kep

leria

n D

isk

(Qi e

t al

. 20

04)

Mars Atmosphere CO(2-1) (Gurwell)

• ALMA: large array (64 x 12 m + 12 x 7 m) North America, Europe, and likely Japan high sensitivity, high resolution (10 mas)

• best possible site, Atacama at 5000 m, large bandwidth, high fidelity imaging, active compensation for atmosphere

full operation in 2012?

Debris Disks around Nearby Stars

• What are Debris Disks? dust requires replenishment

• Resolved Morphologies holes, blobs as planet signatures

• Imaging Examples dust structure plausibly due to resonances with planet

• Future Prospects Spitzer, SMA, ALMA

Dust can outshine Terrestrial Planets

Dust clumps in the zodiacal cloud from 10 pc: (a) Model of the brightest unresolved clump from collisions in the asteroid belt; the horizontal line indicates the flux from an Earth, the vertical line represents the beam size; (b) Model of the Earth’s resonant ring (Dermott et al. 1994) at 10 m with a 0.06 arcsec beam. The Earth’s emission would be at [+0.1,0]and would be 10 to 20 times brighter than the bright trailing clump in the ring (Wyatt 2001).

• ISO 25 m survey of nearby main-sequence stars shows that warm disks are rare (Laureijs et al. 2002).

• [25/60] impies T<120 K

• evacuated inner regions are common features of debris disk systems

Spectral Energy Distributions of Excess

Zodiacal LightClementine 1994

Fomalhaut: a Nearly Uniform Ring

• residuals reveal a “clump” with 5% of total flux

SCUBA 450 m images of the Fomalhaut disk (from Holland et al. 2002): (a) observation, (b) axisymmetric smooth disk model, and (c) the residuals, which show that the asymmetry could be explained by a clump embedded in a smooth disk. All contours are spaced at 1 = 13 mJy/beam. The dashed white oval in (b) shows the inner edge of the mid-plane of the disk, a 125 AU radius ring inclined 20 degrees to the line of sight. The stellar photosphere has been subtracted.

(Holland et al. 2002)

100 AU

JCMT 850 m SCUBA Images

19.3 pc 7.7 pc 7.8 pc 3.2 pc