Polarized Thermal Dust Emission in the Interstellar Medium

1
Polarized Thermal Dust Emission in the
Interstellar Medium

• John Vaillancourt
• California Institute of Technology

U. Chicago Roger Hildebrand Larry
Kirby Northwestern Giles Novak Megan Krejny
Caltech Darren Dowell Hiroko Shinnaga Harvard /
CfA Hua-bai Li
U. Western Ontario M. Houde M. Attard U. Western
Australia Jackie Davidson
CMB component separation and the physics of
foregrounds Pasadena, 2008 June 15
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Too many dust Topics...

• Included Polarized emission from
• thermal dust,
• in dense molecular clouds,
• at ????60 ?m - 1 mm (? 300 - 5000 GHz).
• How do we extrapolate to diffuse ISM and ? gt 1 mm
  ?
• This talk does not discuss
• Starlight polarization and the diffuse ISM
• Galactic magnetic fields (Carl Heiles, Monday
  afternoon)
• Grain alignment (Alex Lazarian, later today)
• Anomalous microwave emission (Rod Davies, later
  today)

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Polarized Emission from I.S. Dust

• Polarization by scattering grain size
  wavelength
• Polarization by selective extinction / absorption
• Diffuse ISM AV few magnitudes
• Observed at near-optical wavelengths (UV - NIR)
• Polarization from emission
• Dense ISM AV gt 20 mag., currently 30 mag.
• Observed at FIR - MM ? 50 ?m - 1 mm
• CMB foregrounds require understanding of
• Long wavelength emission ? gt 3 mm (? lt 100 GHz)
• in the diffuse ISM

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Extinction vs. Emission Polarization
Polarization of extincted starlight Diffuse ISM,
UV-Vis-NIR
Polarization by emission Dense ISM
350 ?m (Hertz, few arcmin) (after D. Dowell)
850 ?m (Archeops few deg.) (Benoit et al. 2004)
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Polarized Emission vs. Wavelength
W51
100 ?m 350 ?m 850 ?m
Dotson et al. 2000, 2008 Chrysostomou 2002
Schleuning et al. 2000, Matthews et al. 2008 (350
?m grayscale/contours)
Angle differences contain interesting B-field
info. but ...
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Polarization Spectrum
Spectropolarimetry - Visible ?s
Multi-? polarimetry - FIR/MM ?s
Cloud Cores Schleuning 1998
extinction
graphite
Orion - KL
Orion - KHW
polarization
Whittet 2004
Cloud Envelopes Vaillancourt et al. 2008
H2O
Si-O
Orion - BN
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Predicted polarization Spectra (1)

• Dust emission from
• a single grain species at
• a single temperature
• (Hildebrand et al. 1999)

Observed Spectra
Does not match observations !
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Predicted polarization Spectra (2)
Tenuous Cloud
2.0

• Dust emission from
• a single grain species at
• a single temperature
• (Hildebrand et al. 1999)

TA gt TB, pA lt pB, ?A ?B OR TA TB, pA lt pB, ?A
gt ?B

• Dust emission from
• multiple grain species
• multiple temperatures or emissivities
• (Hildebrand et al. 1999)

Heterogeneous Cloud
TA gt TB, pA gt pB, ?A ?B
Hildebrand et al. 1999
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Predicted polarization Spectra (3)
Bethell et al. 2007
Hildebrand et al. 1999

• Radiative torque grain alignment model in
  starless clouds
• Nearly all grains exposed to same I.S. radiation
  field
• Large grains are more efficiently aligned (Cho
  Lazarian 2005)
• Large grains cool more efficiently
• Colder grains better aligned than warm grains

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Predicted polarization Spectra (4)
Observed Spectra

• Radiative torques embedded stars
• Grains near stars aligned
• Grains far from stars not well-aligned
• Warm grains better aligned than cool grains

Heterogeneous Cloud
Combining 2 models produces a minimum in P vs ?.
TA gt TB, pA gt pB, ?A ?B
Hildebrand et al. 1999
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SEDs the polarization spectrum
OMC-1
Polarization
Models
Observation
SED
28 K
polarization (), Flux (Jy/beam)
52 K
45 K ?1 P10
17 K ?2 P0
Polarization

• Observed cloud SEDs indicate wide
  dust-temperature distribution
• polarization ?-minimum constrains SED models
• function of components temperature T, and
  spectral index ?
• independent of relative total column densities

45 K P4
SED
25 K P0
10 K P4
Wavelength (?m)
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IR Cirrus - High-latitude dust

• All grains likely exposed to same environment
• Finkbeiner, Davis, Schlegel (FDS99) -- high
  latitude dust
• T 9.5 K, ? 1.7 (silicate ?)
• T 16 K, ? 2.7 (graphite ?)
• If silicate is polarized and graphite unpolarized
  then
• TC gt TSi, pC lt pSi
• Prediction
• thermal dust polarization is constant for ? gt 1 mm

2.0
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The future of Dust Polarimetry

• Need new instruments which
• Cover wide spectral range
• New environments, other than dense clouds
• Better sampled polarization total intensity
  SEDs
• Instruments like...
• HAWC / SOFIA
• SHARP / CSO
• SCUBA-2 / JCMT
• CCAT
• ALMA
• Planck
• ...
• ...

Planck ?
CSO / JCMT
SOFIA
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The future of Dust Polarimetry
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Polarized Emission from I.S. Dust

• Current observations
• ? 60 - 1000 ?m (? 300 - 5000 GHz)
• Dense ISM only, AV gt 30
• Grain alignment cloud models consistent with
  P-spectrum
• Near future observations ( 10 years)
• ? 50 - 3000 ?m (? 100 - 6000 GHz)
• Slightly diffuse ISM, AV gt 5
• Push tests for alignment and cloud models to
  different environments and wavelengths
• Planck
• Most interesting dust bands ? 0.85, 1.4, 2.1,
  3.0 mm
• AV 5-10 at 5 arcmin resolution (IRAS in
  polarization)

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Extra Slides
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Optical polarization

• Traces B-field in diffuse ISM
• 9300 stars covers most of Galaxy (Heiles 2000)

P gt 5, d gt 1 kpc, b lt 30?
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Multi-scale fields in Molecular Clouds
NGC 6334
Novak et al. 2007
IRAS 100 ?m
25 arcmin
SPARO 450 ?m 300 FWHM
Hertz 350 ?m 20 FWHM
SHARC-2 grayscale
(other sources, e.g. posters/talks by F.
Poidevin, H. Li)
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Relating Poln by Absorption Emission
other definitions
Definition of Polarization
Absorption of Background Source
? ltlt 1
Emission self-absorption

• Assumptions
• Observing same grains
• Observing at same wavelength
• Scattering is negligible

• Solutions
• Use for only very low ? regions
• Measure ?(?) in total intensity
• Measure albedo, or use only for ? gtgt a

See Hildebrand Dragovan (1995, ApJ, 450, 663)
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Where are dust grains aligned?
Diffuse Clouds (Av few)
Dense Clouds (Av gt 30)
Polarization ()
Arce et al. 1998
P-efficiency, P / Av
Multiple clouds (Vaillancourt in prep.)
Arce et al. 1998

• Pabs ? ? Pemis ?
• Yes (Hildebrand Dragovan 1995)

Visible extinction, AV (mag)
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Stellar Locations P-spectrum

• Radiative torques ?
• correlation between P-spectrum and stellar
  locations
• Spitzer / IRAC finds stars in dense clouds
• Existing SMM observations (20 arcsec)
  insufficient to resolve stars
• SHARP (10 arcsec) or SCUBA-2 (7 _at_ 450 ?m) may
  resolve stars, but a FIR polarimeter will be more
  sensitive to warm dust near stars

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Why is dust Polarized?
Polarization by Absorption
Polarization by Emission
? FIR - MM
? UV - NIR
Diagrams after A. Goodman http//cfa-www.harvard.
edu/agoodman/ppiv/