Calculating optical properties ofirregular grains(and perhaps regoliths…)
B. T. Draine
Princeton University
2009 January 9
CPS School, Kobe
2009.01.09

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Tadashi Mukai led the way…

CPS School, Kobe
2009.01.09

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Plan

CPS School, Kobe
2009.01.09

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Grain Geometry?
• Modeling usually assumes sphericalgrains (because calculations are easy).
• Polarization of starlight and polarizationof infrared emission: real grains areNOT spherical.
• What is actual grain geometry?

CPS School, Kobe
2009.01.09

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Dust Grain Geometry•

Interplanetary Dust Particles (IDPs)Bradley (2003)

Bradley, J. P. 2003, Treatise on Geochemistry, vol.1, 689-711

CPS School, Kobe
2009.01.09

5Shen, Draine & Johnson 2008, ApJ, 689, 260

BA, N=128

BA, N=256

BA clusters≡ BPCA clusters:
monomers arrive
on random traject.
and stick at pointof first contact

BA, N=512
CPS School, Kobe
2009.01.09

BA, N=1024
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Two New Classes of Random Aggregates: BAM1 and BAM2CPS School, Kobe
2009.01.09

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BA, N=256

BAM1, N=256

BAM2, N=256
Shen, Draine & Johnson 2008, ApJ, 689, 260

CPS School, Kobe
2009.01.09

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How to measure “porosity”?

Shen, Draine & Johnson 2008, ApJ, 689, 260

CPS School, Kobe
2009.01.09

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Porosity vs. N for BA, BAM1, BAM2CPS School, Kobe
2009.01.09

Shen,Draine Draine& Johnson
ApJ, 689,260 10(Shen,& 2008,Johnson
2008)

Asymmetry of BA, BAM1, BAM2 clusters
Equivalent ellipsoid withsemi-axes a,b,c
Asymmetry decreases
asBA⇒BAM1⇒BAM2
Asymmetry decreases
as N increases
N=256: median c/a=
1.56 for BA
1.54 for BAM1
1.47 for BAM2CPS School, Kobe
2009.01.09(Shen,Draine& Johnson2008)
Shen,Draine& Johnson2008, ApJ,689, 260
11How to calculate absorption and scattering
properties of irregular clusters?

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2009.01.09

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DDA Accuracy for
N=256 BA cluster, 50% graphite+50% silicate
aeff=0.127µm, P=0.853 ⇒ R=0.241µm
ndip = # dipoles in one sphere
X=11.3
9.0
7.2
5.7
4.3
2.4
0.95
0.38
Shen, Draine& Johnson
2008, ApJ,689, 260(Shen,Draine& Johnson
2008)Effect of varying
cluster size R

CPS School, Kobe
2009.01.09

Shen, Draine& Johnson
2009(to be acceptedin ApJ)
(Shen,Draine,& Johnson2009)14

Dependence of polarization on wavelength
λ=0.501µm

λ=0.631µm

a0=0.02→0.08µm
R=0.17→ 0.68µm
X= 2.1 → 8.5(for λ=0.50µm)

P(0.63µm)>P(0.50µm)

CPS School, Kobe
2009.01.09

pol<0Shen, Draine & Johnson 2009 (to be accepted in 15ApJ)

Effect of Varying Porosity P

Shen, Draine & Johnson 2009 (to be accepted in ApJ)CPS School, Kobe
2009.01.09

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Effect ofVarying
Composition

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Shen, Draine & Johnson 2009 (to be accepted in ApJ)

Graham, Kalas & Mathews 2007,ApJ,654,595

Debris Disk
Around AU Mic(M1 dwarf, d=9.9pc)
Graham et al (2007) used HST
ACS to obtain polarization map at
λ=0.61µm
Model to reproduce I(b) and pol(b)Based on EMT-Mie theory,
Graham et al. claim water-ice
model with porosity P=0.91±0.09
fits data.
Are lower porosity clusters alsoable to reproduce observations?

CPS School, Kobe
2009.01.09

Graham,Kalas& Mathews
2007,ApJ,654,595(Graham,Kalas& Matthews
2007)

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BAM2 N=256 clusters, P=0.58

a0=0.02→0.08µm
R=0.17→ 0.68µm
X= 1.7 → 6.8

solid line: “HG”
param. inferred byGraham et aldashed: averagefor dn/dR ∝ R-3.5 for
0.13<R<0.55µm
good fit obtained for
50%sil+50%graph.
with P=0.58(<< 0.91)

Shen, Draine & Johnson 2009 (to be accepted in ApJ)CPS School, Kobe
2009.01.09

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Cometary dust

Levasseur-Regourd& Hadamcik
2001,ESASP.495,587L(Levasseur-Regourd& Hadamcik
2001)

BAM2 clusters with P≈0.63
and R ≈1.25µm appear to reproduce
High-polarization comets.
Mix of 100% sil + 50/50 sil/gra clusters
would reproduce neg. pol. branch.
Might need even larger R…
CPS School, Kobe
2009.01.09

X=8.1,10.0
Shen, Draine& Johnson2009(to be accepted(Shen,Draine& Johnson2009)in ApJ)

Can DDA be used to model
dust layer = nano-regolith?
DDA can be used for periodic array of “Target Unit Cells” (TUCs)(Draine & Flatau 2008, JOSA A25, 2693).
TUC could be an irregular aggregate.
Near-field interactions/multiple scattering are taken into account.
DDSCAT 7.0 includes this capability.

CPS School, Kobe
2009.01.09

Draine & Flatau 2008, JOSA, A25, 2693

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SummaryCPS School, Kobe
2009.01.0922

Thank You

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2009.01.09

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Referencesz Bradley, J. P., Treatise on Geochemistry vol.1, 689-711(2003)z Shen, Draine & Johnson, ApJ 689, 260 (2008)
URL: http://www.astro.princeton.edu/~draine/agglom.html
z Shen, Draine & Johnson, (to be accepted in ApJ, 2009)arXiv0901.2177
URL: http://www.astro.princeton.edu/~draine/SDJ2009.html
z Levasseur-Regourd & Hadamcik, ESASP 495, 587L (2001)
z Draine & Flatau, JOSA A25, 2693 (2008)z Spitzer M101
URL:http://gallery.spitzer.caltech.edu/Imagegallery/image.php?image_name=ssc2005-11a