Post on 07-Feb-2018
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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