Lecture 5. Interstellar Dust: Chemical & Thermal Properties

!. Spectral Features2. Grain populations and Models3. Thermal Properties4. Small Grains and Large Molecules-------------------------------------------------5. Icy Grains

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

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

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

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

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

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.

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

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

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

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

Draine & Lee: Silicate

Draine & Lee: Graphite

Weingartner & Draine Model

a4 times the size distribution

Weingartner & Draine, ApJ 598 246 2001

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.

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)

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

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

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

∞∫

Planck Average Emissivity

Qa (a,T) =BλQa (a,λ)dλ0

∞∫Bλ dλ0

∞∫

Based on the Draine & Lee dust model

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 )

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.

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?

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

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

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.

PAHs & Astronomical Spectra

Orion Bar

Mid-IR ISO Spectral Features

Courtesy of AGGM Tielens

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.

Possible Forms of Carbon in the ISM

Too many possibilities?

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,

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

DIBs Associated with C60+

HD 183143

Circumstellar DiamondsISO spectra of two pre main-sequence stars

Lab spectra of nano-diamond crystals resemble astrophysical sources

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.

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.

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

Gas-Phase and Solid CO

Amorphous & Crystalline Solids