Modeling Interstellar Dust:
Observational Constraints
and a Dust Model
Based on Compact Grains
B. T. Draine
Princeton University
CPS School, Kobe
2009.01.08

2009 January 8

1Why weStudy dust…

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2009.01.08

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Modeling Interstellar DustCPS School, Kobe
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Some references

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“Direct” Observations of Interstellar DustCPS School, Kobe
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Observational Constraints onDust: Observed Extinction•

10µm feature: amorphoussilicate< 2% of silicate material iscrystalline•

2200A “bump”: probably
aromatic C (prob. PAHs)•

3.4µm feature: C-H stretch.
Aromatic/aliphatic ratio isuncertain:~85/15 (Pendleton &Extinction per H nucleon

Allamandola 2002)<15/85 (Dartois et al 2004)•λ dependence requires
size distribution froma<0.01µm to a>0.3µm
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2009.01.08

Draine, B.T. 2008, Space Science Reviews, in press(arXiv:0809.5233)

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Indirect constraints on interstellar dust

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One Tentative Model(Draine & Li 2007; Draine & Fraisse 2009)

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Polycyclic Aromatic Hydrocarbons (PAHs)
Draine, B.T., 2003, ARAA, 41, 214

Cesarsky et al. 1996 A&A 315 L305
Emission features at• 3.3 µm (C-H stretch)• 6.2, 7.6 µm(C-C stretch)• 8.6 µm (C-H in-plane bend)• 11.3, 12.7 µm (C-H o-o-p bend)• 16.4, 17.0, 17.4 µm (C-C-C… stretching/bending?)

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Size Distributions(models are not unique…)
dust mass per logarithmic interval in radius

PAHs

Draine & Fraisse, submitted to ApJ 2008 (arXiv:0809.2094)

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(zodi-subtracted) 100µm sky seen by IRAS and COBE
(processed by Schlegel, Finkbeiner & Davis 1998)

the Milky Way seen by DUSTGLOWCPS School, Kobe
2009.01.08

11Schlegel, Finkbeiner & Davis, 1998 ApJ 500, 525

Modeling IR Emission•

Physical grain model: for eachgrain, calculate
Cabs(ν) , heat capacity•

Stochastic heating:
Find p(T; comp,size) for eachcomposition, size•

Time-averaged IR emission:•Sum over compositions, size
distribution

A day in the life…

Draine, B.T., 2003, ARAA, 41, 214

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Modeling IR EmissionCPS School, Kobe
2009.01.08

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IR Emission Calculated for Model
Dust model heated by various starlight intensities
Galaxy spectrum: weighted sum of such spectra

qPAH=4.7%
qPAH=fraction oftotal dust massin PAHs
Nearly invariant
Draine & Li (2007)
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2009.01.08

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Draine & Li 2007 ApJ, 657, 810-837

Spitzer Space
Telescope
85 cm, LHe-cooled optics
Launched: Aug. 2003
LHe exhaustion:~April 2009
Imaging: 3.6 - 160µm
Spectra: 5 - 38µm

CPS School, Kobe
2009.01.08
http://ipac.jpl.nasa.gov/media_images/screen_jpg/sirtf/SIRTF_ir_1small.jpg

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IR View
NGC 7331(SINGS)
3.6µm: starlight
8.0µm: dustglow

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http://gallery.spitzer.caltech.edu/Imagegallery/image.php?image_name=ssc2004-12a

How well does the dust model do?
Example: photometry of NGC7331 (SINGS data)

Draine et al. 2007, ApJ, 663, 866-89417http://gallery.spitzer.caltech.edu/Imagegallery/image.php?image_name=ssc2003-06d

NGC 3031(=M81)
DUST
MAP

Starlight
Power/AreaCPS School,Kobe
2009.01.08

Dust IR Power/Area
unpublished

Dust Mass/Area18

NGC 3031 (=M81)

qPAH≡(PAH mass)/(total dust mass)

Dust Surface Density

Global qPAH =4.0%
qPAH ImageCPS School, Kobe
2009.01.08
unpublishedunpublished19

IR Spectroscopy
Centers of galaxies:

red: observations
black: model
model = dust mix
heated by suitable
mix of starlightintensities(BTD, R. Reyes, J.D. Smith,& SINGS team…)

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unpublishedPolarization of Starlight: Aligned Dust Grains

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Whittet.D.C.B., 2003, "Dust in the Galactic Environment", (Bristol, Institute of Physics)
2009.01.08

Polarization of starlight

Polarization-dependent
attenuation by aligned grains
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2009.01.08

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original schematics

Polarization of Starlight in Milky Way(BTD & Fraisse 2009)

Different models: different axial ratios for oblate spheroids
Adopted alignment f(a)CPS School, Kobe
2009.01.08

Resulting polarization cross section

Draine & Fraisse, submitted to ApJ 2009 (arXiv:0809.2094)
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Draine & Fraisse, submitted to ApJ 2009 (arXiv:0809.2094)

Total Emission% Polarization(sightline ⊥ B0 )IR-submm polarization: 1. Model-dependent
2. Frequency-dependent∴TEST of models
CPS School, Kobe
2009.01.08
Planck to be launched April 12

Spinning DustCPS School, Kobe
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Spinning DustCPS School, Kobe
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Microwave Emission
Draine, B.T., 2003, ARAA, 41, 214•

Dust-correlated microwave
emission discovered by CMB
studies.

•Likely due to spinning dust
(Draine & Lazarian 1998) -same ultrasmall dust grainsneeded to explain 3-15um“PAH” emission.

Draine 2003
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Microwave Emission from Perseus
Molecular Cloud
Watson et al 2005,
ApJ 624, L89Watson et al 2005, ApJ 624, L89

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Summary of interstellar dust
model requirements

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2009.01.08

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Model for interstellar dust:
tests and predictions

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2009.01.08

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starlightCPS School, Kobe
2009.01.08

dustglow31
http://gallery.spitzer.caltech.edu/Imagegallery/image.php?image_name=ssc2004-19a

Draine et al. 2007, ApJ, 663, 866-894

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2009.01.08

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Draine et al. 2007, ApJ, 663, 866-894

PAH index qPAH=
fraction of total
dust mass in
PAHs with NC<103

PAH fractioncorrelateswith metallicity:
galaxies withAO<8.0
have qPAH < 1.2%
CPS School, Kobe
2009.01.0833

Referencesz Draine, B.T., Space Science Reviews, in press(arXiv:0809.5233) (2008)z Draine, B.T., ARAA, 41, 214 (2003)
z Cesarsky et al., A&A 315, L305 (1996)z Draine & Fraisse, submitted to ApJ (arXiv:0809.2094)(2008)z Schlegel, Finkbeiner & Davis, ApJ 500, 525 (1998)z Draine, B.T., ARAA 41, 214 (2003)
z Draine & Li, ApJ 657, 810-837 (2007)
z Spitzer Space Telescope
URL: http://ipac.jpl.nasa.gov/media_images/screen_jpg/
sirtf/SIRTF_ir_1small.jpg

Referencesz Spitzer NGC7331
URL: http://gallery.spitzer.caltech.edu/Imagegallery/image.php?image_name=ssc2004-12a
z Draine et al., ApJ 663, 866-894 (2007)
z Spitzer MIPS/IRAC M81
URL: http://gallery.spitzer.caltech.edu/Imagegallery/image.php?image_name=ssc2003-06
z Whittet.D.C.B., "Dust in the Galactic Environment",(Bristol, Institute of Physics) (2003)
z Heiles C., AJ 119, 923 (2000)
z Draine & Fraisse, submitted to ApJ (2009)(arXiv:0809.2094)

Referencesz Draine & Fraisse, submitted to ApJ (arXiv:0809.2094)(2009)z Draine, B.T., ARAA 41, 214 (2003)
z Watson et al., ApJ 624, L89 (2005)
z Spitzer M51
URL: http://gallery.spitzer.caltech.edu/Imagegallery/image.php?image_name=ssc2004-19a
z Draine et al., ApJ 663, 866-894 (2007)