lecture 5. interstellar dust: chemical & thermal...
TRANSCRIPT
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Lecture 5. Interstellar Dust: Chemical & Thermal Properties
!. Spectral Features2. Grain populations and Models3. Thermal Properties4. Small Grains and Large Molecules-------------------------------------------------5. Icy Grains
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1. Features in the Interstellar Extinction Curve
The interstellar extinction curve Aλ has the potential to reveal the nature of dust.
Aλ is remarkably smooth, which suggests many components (size & composition):
Size distribution: c.f. overall (broad) variation of Aλ with λ.
Composition: c.f. discrete features in Aλ , the 220 nm bump & 9.7 & 18 µm features.
a=260 nm
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Interstellar Extinction CurveThe shape of the interstellar
extinction curve does not look like a Mie Qext plot.
• The overall smoothnessimplies many components
• The breadth implies a distribution in sizes, with small grains more abundant than big ones
Illustrated here with “toy” water ice models with 50 nm & 250 nm grains; small grains 90% by number
a=50 nm
a=250 nm
50nm
250 nm
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Features in the Extinction Curve
220 nm
9.6 µm
2200 Å feature 10 µm silicate featureThe strongest dust spectral features occurs at 220 nm
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Role of Silicate MineralsSilicates generally have strong absorption resonances near 10 µm due to the Si-O bond stretch.– It is virtually certain that the interstellar 9.7 µm
feature is produced by interstellar silicates (absorption as well as emission is observed).
– The 10 µm emission feature is observed in outflows from cool O-rich stars
• Their atmospheres condense silicates• It is absent in the outflows from C-rich stars where
O, needed for silicates, is all locked up in CO • The broad feature at 18 µm can be identified
with the O-Si-O bending mode in silicates
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Vibrational Modes of Silicate MineralsSpecies Mode Wavelength
(µm)MgSiO3 Si-O stretch 9.7
O-Si-O bend 19.0Mg2SiO4 Si-O stretch 10.0
O-Si-O bend 19.5FeSiO3 Si-O stretch 9.5
O-Si-O bend 20.0Fe2SiO4 Si-O stretch 9.8
O-Si-O bend 20.0SiC SiC stretch 11.2
enstatite
fosterite
ferrosilite
fayalyte
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The 220 nm Feature• Ubiquitous in the Milky Way
– 217.5 ± 0.5 nm (fixed)– width varies (10%) as does
strength• Graphite has a strong UV
resonance due to π-orbital valence electrons– Why is the feature so uniform?
• 220 nm bump is weak in the Small Magellenic Cloud– weakness correlated with C/O
• Hydroxylated Mg2SiO4 (fosterite)also has a 220 nm feature. (Steel & Duley 1986)
The actual carrier of the 220 nm feature has not been identified.
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220 nm Feature in IDPs
JP Bradley et al. Science, 307, 244, 2005: Non-solar isotopic ratios indicate these IDPs are interstellar in origin.
A. Interstellar 220 nm featureB. Broad & narrow examplesC. lab: hydroxylated
amorphous silicateD. lab: Mg3Si4O10[OH]2E. IDP organic carbonF. IDP silicates
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2. Grain PopulationsThere are at least three populations: The optical
extinction, 220 nm bump, and FUV extinction manifest independent changes
1. Aλ rises from the NIR/optical to the near UVrequires a ~ 150 nm, but if only 150 nm grains were present, Aλ for λ < 200 nm would be approximately constant
2. Steep rise in FUV extinction down to 80 nm requiresa ~ λ/2π ~ 15 nm, otherwise Qext would be flat
3.The 220 nm bump implies a specific carrier– symmetry and constancy of λ0 imply absorption in the
small particle limit a ≤ 10 nm.– small graphite spheroids a ≈ 3 nm, b/a = 1.6 might
work, except for variation in central wavelength
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Dust Models: MRN Distribution• Grain size distribution is likely to be continuous
– Mathis, Rumpl & Nordseick (ApJ 217 425 1977) proposed power law distribution of graphite and silicate grains in approximately equal numbers
amax = 250 nm, set by NIR and visibleamin = 5 nm, set by FUV curve
• MRN power law has most mass in large particles, most area in small particles:
dnda
∝= AnH a−3.5 , amin < a < amax
M ∝ a3 dnda da∫ ∝ amax
0.5 − amin0.5
A ∝ a2 dnda da∫ ∝ amin
−0.5 − amax−0.5
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Draine & Lee Model
Drain & Lee 1984 ApJ 285 89Two 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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Draine & Lee: Silicate
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Draine & Lee: Graphite
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Weingartner & Draine Model
a4 times the size distribution
Weingartner & Draine, ApJ 598 246 2001
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3. Grain Thermal Properties
Heating Processes– Absorption of starlight– Collisions (warm gas, cosmic rays, other grains)– Chemical reactions on grain surface
Cooling Processes– Radiative (photon emission)– Collisions with cool gas – Sublimation from grain surface
Radiative heating and cooling often dominate.
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The Galaxy in the Far-Infrared
3.3
6.27.7 11.3
Bulk of emission c.f.≈ 18 K dust (140-µm peak in FIR spectrum of Lec 1)
Significant 3-25 µm emission c.f. warmer grains
Distinctive features at 3.3, 6.2, 7.7, 8.6 & 11.3 µm
Mean spectrum of Milky Way IS Dust(Synthesis of balloon & satellite data)
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Radiative Heating of Grains• On absorption of photon, grain is left in an excited
state. The probability for spontaneous emission is large A ~ 107 s–1.
• Complex grains as well molecules with many energy levels can rapidly convert part of this electronic energy into vibrational energy on time scales ∆t ≈ 10–12 s• This energy is quickly distributed over all
internal degrees of freedom, and the grains are heated.
• Most photon absorptions heat the grain sinceA⋅∆t ≈ 10–5 << 1
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Heating of Large Grains• Heating by IS radiation, whose flux for an
isotropic radiation field, is πJλ
• Heating rate for one grain of radius a is
where JUV is defined as
Juv is insensitive to a for large grains. Most of the heating of large grains c.f. UV photons for which Qa ~ 1
Fλ = µIλ dΩsurface∫ = 2π Iλ µdµ
0
1∫ = π Iλ = π Jλ
JUV ≡ JλQa (λ)dλ0
∞∫
4π a2 π JλQa (λ)dλ0
∞∫ = 4π a2 JUV
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Steady Thermal Balance Using Kirchoff’s Law, jν(T) = Bν(T) κν (T), the radiative cooling rate is
∫∞
0
2 )(4 λλππ λ dQBa a
The balance between absorption and radiation is
where ‹Qa› is the Planck-averaged emissivity
4π a2 JUV = 4π a2 π BλQa (λ)dλ0
∞∫
JUV = BλQa (λ)dλ0
∞∫ = Qa (T) σT 4
π
Qa (a,T) =BλQa (a,λ)dλ0
∞∫Bλ dλ0
∞∫
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Planck Average Emissivity
Qa (a,T) =BλQa (a,λ)dλ0
∞∫Bλ dλ0
∞∫
Based on the Draine & Lee dust model
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The Temperature of Large IS GrainsGrains in the diffuse ISM are cold, ~ 20 K.To calculate Td, we need Qa in the far-IR Recall that for constant m =n-ik, Qa ~ a/λ, but in factm=m(λ) and typically for real materials Qa ~ 1/λ2
at long wavelength. More generally we parameterize the efficiency as Qa ~ a/λ1+β
The equilibrium dust temperature is
JUV ∝2hυλ2
1ehυ / kTd −1
⎛ ⎝ ⎜
⎞ ⎠ ⎟
aλ1+β dυ
0
∞∫ ∝ ah kTd
h⎛ ⎝ ⎜
⎞ ⎠ ⎟
5+β x 4 +β
ex −1dx
0
∞∫
Td ∝JUV
a⎛ ⎝ ⎜
⎞ ⎠ ⎟
1/(5+β )
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Calculated Grain Temperatures
Specify the mean IS radiation field by a BB color temperatureT* ≈ 5000K and a dilution fatcorW ≈ 1.5 x 10-13
For 0.1 µm grains,Td ~ 20 K
Graphite grains are hotter because they are more efficient UV absorbers.
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4. Small Grains and Large Particles
• Small grains have small heat capacity and small radiating area.
• Absorption of starlight photons leads to temperature spikes.
A 10 nm grain at 20 K has 1.7 eV of internal energy.Since Cv ~ mgrT3, the grain compensates for itssmall size by getting hot before cooling down.
Where do these statements come from?
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Temperature FluctuationseV
eV
• Heating of a small 5-nm grain by individual photons absorbed from the mean IS radiation field (Purcell 1976 ApJ 206 685)
• Cooling by many IR photons• Time between spikes is ~ 1 hr
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Temperature Fluctuations
IR emission from tiny grains occurs at shorter wavelengths than expected from equilibrium.
For a grain rising to Tmax, and the emitted radiation peaks at hv ≈ 5 Tmax
• emission at 60 µm requires Tmax ≈ 50 K, or a 10 eV photon absorbed by a 7 nm grain
• emission at 12 µm requires Tmax ≈ 250 K or a 10 eV photon absorbed by a 1.5 nm grain
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Tiny GrainsVery small grains are more abundant than suggested by MRN size distribution: The diffuse IR emission of reflection nebulae from 2 - 25 µm is hard to understand unless grains are hotter than expected from equilibrium considerations (temperature fluctuations ofvery small grains!).
PAHs
Polycyclic Aromatic Hydrocarbons
PAH molecules are fragments of graphite sheets with edge H atoms; they show characteristic emission at 3.3, 6.2, 7.7, 8.6 & 11.3 µm observed in warm dust.
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PAHs & Astronomical Spectra
Orion Bar
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Mid-IR ISO Spectral Features
Courtesy of AGGM Tielens
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Early Composite Dust ModelDesert, Boulanger,
& Puget AA 237 215 1990
– Big silicate grains15 < a/nm < 110
ρdust/ρgas = 0.0064– Very small
graphitic grains1.2 < a/nm < 15
ρdust/ρgas = 0.00047– PAHs
0.4 < a/nm < 1.2ρdust/ρgas = 0.00043 See Draine ARAA 41 241 2003 for update.
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Possible Forms of Carbon in the ISM
Too many possibilities?
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Structure of C60
• Each C atom connected by one double and two single bonds
• Soccer ball• Closed-shell
electronic structure
Introduced by Kroto as proposed origin of polyacetylene chains observed in C-rich AGB stars; 1996 Nobel Prize awarded for lab discovery,
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Diffuse Interstellar Bands
• ~ 200 DIBs known• Most DIBs are unidentified• Some DIBs may be due to large carbon-bearing
molecules• C60
+ was a candidate for λλ 9577, 9632 bands
BD+63o1964
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DIBs Associated with C60+
HD 183143
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Circumstellar DiamondsISO spectra of two pre main-sequence stars
Lab spectra of nano-diamond crystals resemble astrophysical sources
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5. Icy Grains in Cold Regions
In addition to radiative processes, grains can interact with one another and with the gas, especially in dense regions. Examples are:
• Coagulation leading to grain growth and changes in the size distribution, manifested by variation of RV along different lines of sight & especially its increase in dense regions.
• Cold grains acquire mantles of molecular ices,consisting of mix of H2O, CO2, CO2, CH3OH, etc.
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Interstellar Ices• 3.1 µm:
amorphous, dirty H2O ice
• 4.27 µm: CO2stretching
• 4.6 µm: CN stretch (XCN, OCN– ?)
• 4.67 µm: CO• 6.0 µm: H2O
bending• 6.8 µm: ? • 15 µm: CO2
bendingAbsorption bands due to solid-state features in dense clouds towards embedded IR sources.
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Spectroscopic Differences BetweenSolid & Gas Phase
• Suppression of rotational structure– Molecules cannot rotate freely in ice
• P, Q, R branches collapse into one broad vibrational band
• Line shifting– Interaction of molecules with surroundings
modifies bond force constants• Line broadening
– Interact with ice environment: each molecule is located at a slightly different site• Broadening depends on species
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Gas-Phase and Solid CO
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Amorphous & Crystalline Solids