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Grain Growth in Protoplanetary Disks:
the (Sub)Millimeter
Sep 11, 2006 From Dust to Planetesimals, Ringberg
David J. WilnerHarvard-Smithsonian Center for Astrophysics
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Relevance of (Sub)Millimeter
• “vibrational” dust emission is dominant mechanism (thermal fluctuations in charge distribution)
• longest observable ’s for dust: 0.35 to 35 mm
• sensitive to cold dust, T<10’s of K
• low opacity, sample emission at all disk depths dependence of opacity diagnostic of dust properties
(e.g. growth to millimeter size)
• no contrast issue with stellar photosphere
• major new facilities under construction: ALMA, eVLA
PPV: Natta, Testi, Calvet, Henning, Waters & Wilner, astro-ph/0602041
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ISM Protoplanetary Disks
0.85 mm
Johnstone & Bally 1999
Williams, Andrews, Wilner 2005
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• T Tauri, Herbig Ae disks (d≤150 pc, 1-10 Myr)– integrated flux vs. (single dish bolometer)
• observe power law form, F ~ -, 2 < < 3
– spatially resolved brightness (interferometer)
HD169142Dent et al. 2006
stardust
F~-2.5
(Sub)Millimeter Observables
SMA Raman et al. 2006
300 AU
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• mass opacity ( > 0.1 mm) power law form
• normalization, power law index , depend on dust properties:– composition– size distribution– geometry– …
(see Draine 2006)
Basics of “”
Adams et al. 1988, following Draine & Lee 1984
~-2
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• flux density emitted by an element dA
• if <<1 and h<<kT, then
and simply related to
F ~-(2+)
From to
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Beckwith & Sargent (1991)
Disk Dust appears Different
• early (sub)mm obs: disk <>~1 vs. ISM ~1.7 (e.g Weintraub et al. 1989, Adams et al. 1990, Beckwith et al.
1990, Beckwith & Sargent 1991, Mannings & Emerson 1994)
d=(-2)(1+)
0 1 2
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~1 Interpretations
1. changes in dust properties: – grain growth
small, a << /2 =2
large, a >> /2 =0 mm size, ~1
~ -1 due to dust composition particle geometry
2. optically thick emission: – F ~ -2 (in part) > ( - 2)
Pollack et al. 1994 mixture, compact, segregated spheres, n(a) ~ a-q, q=3.5
Calvet & D’Alessio 2001
amax=1 mm
amax=10 cm
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Testi et al. (2001)
Dust Properties or Optical Depth?
• e.g. Herbig Ae stars UX Ori, CQ Tau: 1.1-7mm~ 2.0±0.3, 2.65±0.1
~ 0 and large disk? any and small disk?
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Resolve Ambiguity
• observe spatial distribution of sub(mm) brightness• arcsecond scales require interferometry
– 1.3, 3 mm: BIMA, OVRO, PdBI, NMA; ATCA, SMA– 7 mm: VLA (thanks to CONACyT, MPIfR, NSF)
longer : lever minimizes uncertainty, probes larger dust; more concern about ionized gas
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• combine fluxes, images, improved disk models: – TW Hya– CQ Tau– 7 (2) Herbig Ae stars– 14 (10) Taurus PMS stars– 10 (5) southern PMS stars– 24 (20) Taurus/Oph PMS stars
Interferometer Studies
Calvet et al. 2002
Testi et al. 2003
Natta et al. 2004
Rodmann et al. 2006
Lommen et al. 2006
Andrews & Williams 2007
TBvs. disk radius at 0.4, 3, and 7 mm, from two dust models ofD’Alessio et al. 2001
Calvet & D’Alessio 2001
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=0.70.1
Calvet et al. 2002
Grain Growth in TW Hya
• irradiated accretion disk model matches SED and VLA (and SMA) intensities from 10’s to Rout~ 200 AU
• shallow (sub)mm slope requires amax >> 1 mm
• observed 7 mm low brightness requires << 1
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Many (Barely) Resolved Disks
VLA 7mm Rodmann et al. 2006
ATCA 3mm Lommen et al. 2006
VLA/PdBI/OVRO
Natta et al. 2004
SMA 0.87/1.3mm Andrews & Williams
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• ≤1 for many/most resolved disks
Many More Determinations
solid: Lommen et al. 2006dashed: Rodmann et al. 2006dotted: Natta et al. 2004
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is an average, for any dust model– cannot disentangle all properties <1: hard to avoid substantial mass fraction a~O()
Limitations/Complexity of
Natta & Testi 2004
1mm
0
2
1
1-7mm
Natta & Testi 2004amax
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TW Hya at 3.5 cm?
• disk model underpredicts 3.5 cm emission
• emission mechanism?– ionized protostellar wind
• if FcmdMacc/dt, low by 103x
– spinning dust (Rafikov 2006)
• requires high (unrealistic) C fraction in nanoparticles/PAHs
– synchrotron • X-rays not stellar activity: dense,
cool, and depleted accretion (Stelzer & Schmitt 2004)
– thermal dust, const
F ~ -2.60.1
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Weidenschilling 1997 Dullemond & Dominik 2005
Grain Size Evolution
• theory: growth, settling, destruction, … – depart from simple power law size distribution– create midplane population of ~cm size (timescale?)
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TW Hya: Pebble Population
toy model: small + ~cm size grains
Wilner et al. 2005
• 3.5 cm disk dust emission 1. not variable: weeks to years
2. resolved at arcsec scale, brightness only ~10 K
3. steep spectrum to 6 cm
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• no trend of with stellar luminosity, mass, age• tantalizing trends of with mid-ir growth, settling indicators
Any Correlations?
PPV: Natta et al. 2006 Lommen et al. 2006
Acke et al. 2004
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Remarks
• (sub)mm <1: compelling evidence for growth – most of original dust mass in mm size particles
• no clear trends with stellar properties• mm/cm sizes persist for Myrs
– competition between agglomeration and collisions
• are the disks we can study in the (sub)mm the ones that will never form planets? – probably not: transition disks
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Transition Disks: Inner HolesSpitzer IRS implies r~24 AU hole
“... we remain skeptical of the existence of such a large centralgap [5 AU] devoid of dust.” - Chiang & Goldreich (1999)
Calvet et al. 2005
Wilner et al. 2006
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Transition Disks: Inner Holesmid-ir implies r~4 AU hole
Calvet et al. 2002
Hughes et al., in prep
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Next Generation (Sub)mm Facilities• 10 to 100x better sensitivity, resolution, image quality• dust emission structure at 0.1 to 0.01 arcsec• precision (sub)mm spectral index maps
at the limits of ALMAWolf & D’Angelo 2005
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End