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8: Composition and Physicalstate of Interstellar Dust

James Graham

UC, Berkeley

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Reading

Tielens, Interstellar Medium, Ch. 5

Mathis, J. S. 1990, AARA, 28, 37

Draine, B. T., 2003, AARA, 41, 241

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Nature of Interstellar Dust

Grain heating and cooling

Grain size distribution

Tiny grains, temperaturefluctuations and PAH

Carbon in the ISM

Interstellar ices

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The Galaxy in the Near-IR

The sky in the near-IR COBE maps the sky between 1.3 m and 4 mm The near-IR (J, K & L) shows mostly stars & reduced ISM absorption The disk-like nature of our Galaxy with its bulge is evident

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The Galaxy in the Far-Infrared

COBE 100, 140 & 240 m No ordinary stars, only a few with circumstellar dust shells are weakly

detected

The bulk majority of emission is from clouds of cool dust ( 20 K)

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Grain Heating and Cooling

Possible sources of grain heating: Absorption of starlight Collisions with atoms, e, cosmic rays, other grains Chemical reactions on grain surface

Possible mechanisms of grain cooling: Radiative cooling (emission of photons) Collisions with cold atoms and molecules Sublimation of atoms/molecules from grain surface

Under many circumstances, radiative heating andcooling dominate

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Radiative Heating of Grains Absorption of photon

Grain left in excited state Probability A ~ 107 s1 for spontaneous emission

Complex molecules with many energy levels can convertpart of electronic energy into vibrational energy on timescale t 1012 s This energy is quickly distributed over all internal degrees of

freedom Grains are heated

At 105

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Equilibrium Grain Temperature Large grains

Heating is by the IS radiation field

Flux on a grain surface is !J

for an isotropic radiation field

Heating rate for one grain of radius a is

F = cos I d d = 2 sind = 2 dsurface

= 2 I d01 = I = J

4 a2 JQabs(a,)d0

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Grain Heating

Most of the heating is by UV photonswhere Qabs ~ 1 Define JUV

weakly dependent on a for large grains

The heating rate

JUV JQabs(a,)d0

4 a2 JQabs(a,)d0

= 4 a2JUV

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Grain Emission

The emissivity of a grain is given byKirchoffs law In thermodynamic equilibrium absorption at per unit grain area

hence this must be the emission rate

The total power radiated by a grain is

B(T)Qabs(a,)

4 a2 B(Tgr )Qabs(a,)d0

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Equilibrium

Balance between absorption andradiation is expressed at

Qabs is the Planck-average emissivity

4 a2JUV = 4 a2 B(Tgr)Qabs(a,)d0

JUV = B(Tgr)Qabs(a,)d0

= Qabs(a,Tgr)Tgr

4

Qabs(a,T) =B(T)Qabs(a,)d0

B(T)d0

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Equilibrium In the diffuse ISM grains are cold (~ 20 K)

Need Qabs in the far-IR For constant m =n-ik we have Qabs ~ a/, but m=m()

Typically for real materials Qabs ~ 1/2 at long wavelength

More generally Qabs ~ a/1+

Thus Qabs ~ T1+ and the equilibrium dust temperature is

JUV 2h2

1eh / kTgr 1

a1+

d0

ahkTgrh

5+x 4+

ex 1dx

0

Tgr JUVa

1/(5+ )

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Draine & Lee Graphite

Draine & Lee 1984 ApJ 285 89

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Draine & Lee Silicate

-2, =1

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Planck Average Emissivity

T2, =1

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Equilibrium Temperatures

Grains heated by themean IS radiation field T* 5000K

W 1.5 x 10-13

Equilibrium temperaturefor grains 0.1 m isabout 20 K Graphite grains are

hotter because ofstronger UV absorption

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Absorption & Grain Size

Qext =Qsca

Qext

Qsca

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Interpretation of the Continuum Absorption

Continuum opacityshows absorption overa broad range ofwavelengths (Mathis1990 AARA 28 37)

Mie curves show asteep rise thenflattening

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Decomposing Interstellar Extinction

The shape of the interstellar extinctioncurve Does not look like a Mie efficiency plot Overall smoothness of A implies multi-

component

Size distribution of grain Breadth of curve imples particle size

distribution Small grains more abundant than big

ones Toy water ice model

50nm & 250nm grains 90% by number are small grains

a=50 nm

a=250 nm

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Grain Populations

There are at least three populations The optical extinction, 220 nm bump, and the FUV extinction

each change without affecting the other

Steep rise in FUV extinction up 80 nm Requires a ~ /2$ = 15 nm Otherwise Qext would be flat

220 nm bump implies a specific carrier Symmetry and constancy of 0 imply absorption in the small

particle limit a 10 nm Small graphite spheroids a 3 nm, b/a = 1.6

A() rises through the near-IR/opticalnear UV a ~ 150 nm If only 150 nm grains were present A() at < 200 nm would

be approximately constant

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Dust Models: MRN Grain size distribution is likely continuous

Mathis Rumpl & Nordseik (1977 ApJ 217 425) proposed a powerlaw size distribution of graphite and silicate grains; approximatelyequal numbers

amax = 250 nm, set by fit to near-IR and visibleamin = 5 nm, set by fit to FUV curve

MRN power law has most mass in large particles, mostarea in small particles:

dnda

= AnHa3.5 , amin < a < amax

M a3 dnda da amax0.5 amin0.5

A a2 dnda da amin0.5 amax0.5

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Draine & Lee Model

Drain & Lee 1984 ApJ 285 89 Two component MRN model: 5 < a/nm < 250 Graphite: 60% of C Astronomical silicate: 90% of Si, 95 % Mg, 94%

of Fe & 16% of O

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Grain Size Determines Spectral Properties

Mie calculation for a = 0.1,1, & 10 m sphericalgrains

Optical constants from

ww

w.astro

.uni-jena.de/Laboratory/S

peclab/labor.htm

l

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PAHs & Tiny Grains

Many nebulae HII regions

Planetary nebulae

Reflection nebulae

show emission in the 315 m region farstronger than expected from grains inthermal equilibrium with the ambientradiation field

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NGC 7023

NGC 7023 is a reflection nebulae excitedby a B3 star

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PAHs & Astronomical Spectra

Orion Bar

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The Galaxy in the Infrared Mean spectrum of

the Galactic ISMdust Synthesized from

balloon & satelliteobservatories

Bulk of emissionfrom 18 K dust

Significant 3-25 memission from fromhotter grains

Distinctive featuresat 3.3, 6.2, 7.7, 8.6& 11.3 m

3.3

6.27.7 11.3

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Polycyclic Aromatic Hydrocarbons

Small (< 1.5 nm) graphitic particles mayoccur as large molecules known as PAH(Polycyclic Aromatic Hydrocarbons)

Fragments of graphite sheets withhydrogen atoms at the edge

Lab spectra of PAHs show characteristicemission at 3.3, 6.2, 7.7, 8.6 & 11.3 mobserved in spectra of reflection nebulaeetc.

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Small PAHs

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PAH Modes C-H stretching at 3.3 m C-C stretching at 6.2 m C-C stretching at 7.7 m C-H in-plane bending at 8.6 m C-H out-of-plane bending

wavelength depends on thenumber of neighboring H atoms:

11.3 m for mono no adjacent H 12.0 m for 2 contiguous H 12.7 m for 3 contiguous H 13.55 m for 4 contiguous H

Mid-IR spectrum depends sizespectrum and degree ofhydrogenation

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Tiny Grains

Tiny grains have small heat capacity A 10 nm grain at 20 K has ~ 1.7 eV of

internal energy

Heat capacity is small Grains are small

Grains are cold cv ~ T3

Absorption of starlight photons leads totemperature spikes

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Temperature Fluctuations

Heating of a small grain (5 nm) by individual photonsabsorbed form the mean IS radiation field (Purcell 1976 ApJ206 685)

Cooling by many IR photons Time between spikes is ~ 1 hr

eV

eV

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A Day in the Life of a C Grain

A day in the lifeof carbonaceousgrains, heatedby the localinterstellarradiation field

abs is the meantime betweenphotonabsorptions(Draine & Li2001 ApJ 551807)

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Temperature Fluctuations

IR emission from tiny grains occurs atshorter wavelengths than expected fromequilibrium For grains achieving Tmax

Radiation peaks at hv 5 Tmax Emission at 60 m needs Tmax 50 K grain

10 eV photon absorbed by a 7 nm grain

Emission at 12 m needs Tmax 250 K grain 10 eV photon absorbed by a 1.5 nm grain

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The 220 nm Feature The 220 nm feature is

ubiquitous in the Milky Way Strongest discrete feature in

the extinction curve Only C, O, Mg, Si or Fe is

abundant enough to give sucha strong feature

Central wavelength is almostconstant 217.5 0.5 nm Significant variation in the width

(10%) & strength Weakness correlated with

metal abundance Weak 2175 in the LMC Missing in the SMC

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The 220 nm Feature C atoms four valence

electrons 2s2 2p2 Three make up a orbital The remaining p electron is

shared or delocalized amongall the C-C bonds

Individual graphite sheets areheld together by weak van derWaals forces

Graphite has a strong UVresonance due to these$-orbital valence electrons

Need 25% of the cosmic Cabundance in small graphitespheres to explain thestrength

All six p orbitals are parallel to oneanother, and each contains oneelectron. Therefore there are three $bonds. Since there is no reason toprefer one form of $ interaction overthe other those three $ bonds aredelocalized over the whole molecule.

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The 220 nm Feature Why is the feature so uniform?

The width of the feature depends on the shape of theparticlesbut tuning the shape shifts the central wavelength

Polycyclic aromatic hydrocarbons (PAHs) have similarstructures to graphite sheets, with similar electronicwavefunctions PAHs generally have strong $ $ * absorption at 200-250 nm

PAHs are seen in emission in the IR

Large (up to 105 C atoms) PAH molecules may be the carrier ofthe interstellar 2175 (Weingartner & Draine 2001 ApJ 548 296)

Laboratory spectra are unavailable for PAH molecules of thesizes characteristic of the ISM

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Desert Boulanger & Puget (1990)Desert Boulanger &Puget (1990 AA 237215) Big silicate grains

15 < a/nm < 110dust/gas = 0.0064

Very smallgraphitic grains

1.2 < a/nm < 15dust/gas = 0.00047

PAHs

0.4 < a/nm < 1.2dust/gas = 0.00043

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Forms of Carbon in the ISM

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Diffuse Interstellar Bands

~ 200 DIBs known Most DIBs are unidentified Some DIBs may be due to large carbon-bearing

moleculesC60+ is a candidate for 9577, 9632 bands

BD+63o1964

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DIBs Associated with C60+

HD 183143

Foing &

Ehrenfreund 1994, N

ature, 369, 296;1997, A

&A

, 317, L59

?

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Circumstellar Diamonds ISO spectra of two

pre-main-sequencestars Lab spectra of

nano-diamondcrystals resembleastrophysicalsource

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Grains in Cold, Dark Clouds

Grains maycoagulate and altersize distribution Variation of R along

different lines ofsight

If A ~ -

Cardelli et al. 1989

ApJ 345 245

1.82

2.51.5

41

0

R

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Grains in Cold, Dark Clouds

Grains may acquire mantles ofmolecular ices consisting of mix ofH2O, CO2, CO2, CH3OH, etc. Absorption bands due to solid-state

features in dense clouds towardsembedded IR sources

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Solid State vs. Gas Phase

Suppression of rotational structure Molecules cannot rotate freely in ices

P, Q, R branches collapse into one broad vibrational band

Line broadening Molecules in ice interact with environment; each is

located at slightly different site Band is broadened Amount of broadening depends on species

Line shifting Interaction of molecules with surroundings modify bond

force constants Shift vibrational frequency

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Gas-Phase and Solid CO

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Interstellar Ices 3.1 m: amorphous,

dirty H2O ice 4.27 m: CO2

stretching 4.6 m: CN stretch

(XCN, OCN ?) 4.67 m: CO

6.0 m: H2O bending

6.8 m: ?

15 m: CO2 bending