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					 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, e Eridani, Fomalhaut, ...
• Future Prospects
  Spitzer Space Telescope, SMA, ALMA
collaborators:
 M. Holman (CfA), C.D. Dowell (Caltech), M. Kuchner (Princeton)
                    SUNY Stony Brook, April 14, 2004
        Introduction to Debris Disks
• Vega infrared excess discovered
                                          optical
  serendipitously during IRAS
  calibration (Aumann et al. 1984)
  thermal emission from cold dust
                                                              far-ir

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

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

• other nearby Vega-excess stars found by IRAS include
  b Pic, Fomalhaut, e Eri (the “Fantastic Four”)
• b Pic disk geometry confirmed by
  images of visible scattered light
  (Smith & Terrile 1984)

• ISO 60 mm 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


lFl



             l
 1. embedded protostar        2. HAe/Be Star
      104-105 yr                 105-106 yr



 3. transition phase          4. debris disk
       ~107 yr                  >> 107 yr




                                         Malfait et al. 1998
Observational Probes of Disk Structure
   scattered light                      emitted light
   optical/near-ir         mid-ir       far-ir/submm
                  < spatial resolution <
              < temperature dependence <
          > contrast with star, dynamic range >




  J band Coronagraph+ AO             850 mm: 14 arcsec beam
HST/NICMOS Scattered Light: Gaps and Rings
        JCMT 850 mm 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 Motion
   Resonance with Neptune




                             CfA Minor Planet Center


   Jewitt Kuiper Belt Page
     Dust in our Solar System from Afar
                                                               (Liou & Zook 1999)

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

• note: Solar System dust
  emission at 850 mm at
  10 pc only ~ 1 mJy
  (<< 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
      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 3jl-2lo- 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
                                (Wilner et al., ApJ, 569, L115)
 • Vega (a Lyrae): A0V main sequence star, d=7.76 pc
 • system viewed nearly pole-on: vsini, reddening

 • JCMT 850 mm 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

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

• l=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 l=1.3 mm




  2.8 x 2.1 arcsec         5.3x4.6 arcsec
stellar photosphere        and 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 3l -lo-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
                                                            (Metchev et al. 2002)




(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.
     Searching for Light from the Planet
                                                           (Macintosh et al. 2003)




(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) 5s sensitivity of the image. The dashed
lines indicate the planet masses from the models of Burrows et al. (1997).
    Is Vega like the Early Solar System?
• Thommes et al. (1999)
  +Thommes et al. (2002)
  suggested Neptune
  was scattered into a                                                   Neptune?
                                          aphelion
  highly eccentric orbit

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


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.
              The e 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 e Eri System
• JCMT 850 mm SCUBA image shows nearly face-on ~60 AU
  radius ring with azimuthal variations (Greaves et al. 1999)
                                     • Structure due to a
                                       planetary perturber?
                                       Liou et al. 1999,
                                       Ozernoy et al. 2000
                                       Quillen & Thorndike 2002




                                         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 mm Observations with SHARC II
• SHARC II: Caltech Submillimeter Observatory
  facility camera, 12x32 filled array of ‘pop-up’ bolometers,
  optimized for 350 mm (9 arcsec beam)




 • Observations made in Jan 2003 commissioning run,
 • >16 hours on e Eridani in excellent weather (t225<0.037),
   image rms ~3 mJy
Image of eeEri at l=350 mm
 Image of Eri at l=350 mm
        350 mm 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 (Da/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    (Kenyon & Bromley 2002, 2004)
  azimuthal variations
            Sculpting by a Planet?
• characterization of possible unseen planets requires
  matching robust features using numerical simulations


0.2 MJup                                                 < 0.3 MJup
e=0                                                      e ~ 0.3
2:1, 3:2                                                 5:3, 3:2
w/ high                                                  w/ phase
libration                                                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



         61 Cyg           Procyon
         binary           binary

                                       e Eri
                 Sun                 0.01 ME
             <0.0001 ME

          a Cen                     t Ceti
          multiple                  0.0005 ME

                       e 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”


                                              undetected by
                                              IRAS at 25 mm




                                        (Williams et al. 2004)
• 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 Images
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)




                                               full operation
                                                  in 2012?


• best possible site, Atacama at 5000 m, large bandwidth,
  high fidelity imaging, active compensation for atmosphere
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 mm 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).
     Spectral Energy Distributions of Excess


• ISO 25 mm 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
 Zodiacal Light
Clementine 1994
     Fomalhaut: a Nearly Uniform Ring
                                                             (Holland et al. 2002)




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

SCUBA 450 mm 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 1s = 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.
JCMT 850 mm SCUBA Images




            100 AU

19.3 pc    7.7 pc    7.8 pc   3.2 pc

				
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posted:3/10/2013
language:English
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