Dust in the Interstellar Medium • Current picture of ISM from multi-wavelength imaging of gas and dust – multiwavelength spectroscopy, and polarization measurements 1 optical image of sky Mellinger PASP 2009 Kwok Chapters 11, 12, 13; Draine Chapters 21, 22, 23, 24
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Dust in the Interstellar Medium • Current picture of ISM from multi-wavelength imaging
of gas and dust – multiwavelength spectroscopy, and polarization measurements
Direct emission from ~ 1 µm grains at ~ 20K, in equilibrium with i/s radiation field
Grains absorb i/s photons and re-radiate as black bodies
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Observational Manifestations of Dust In ISM (1)
• Thermal emission from grains (sub-mm < λ < 2 µm) – grains in thermodynamic equilibrium with ism surroundings
• Extinction (depends on λ)
– grains modify incident e-m radiation by absorbing/scattering
– diminishes flux between UV (0.1 µm) to mid-IR (20 µm) – implies particle size ~ 0.1 - 0.2 µm – grain composition from spectral features in extinction
curves
• Polarization-dependent attentuation of starlight – light from reddened stars is polarized
• Pre-solar grains in meteorites
– i/s grains from solar nebula 4.5 Gyrs ago
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Observational Manifestations of Dust In ISM (2) • Anomalous heavy element abundances (low compared to solar)
– atoms stick to grains → varying depletion of heavy elements
– depletion scales with condensation temperature, local density
• Abundance of H2 in ISM
– implies formation through catalysis on grains – dust shields from UV radiation, prevents dissociation – column densities dust and HI, H2 correlated
• IR absorption lines
– from silicates, H2O and CO ices → dust composition – X-ray spectroscopy?
• Diffuse radiation in galaxy
– scattering by grains, not atoms or molecules 5
Extinction: absorption/scattering of starlight
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Herschel (1785) – dark areas in Milky Way – “holes in the heavens” Barnard (1919) – photographic survey – “stars dimmed by absorbing
medium” Wolf (1923), Bok (1931) quantified “extinction” from star counts.
Trumpler (1930) demonstrated λ-1 law
Stars behind cloud edges mostly redder than stars outside cloud. Scattering by particles smaller than wavelength of light; blue scattered more than red (λ-1 law) Transmitted light reddened (like ‘redder’ sunsets following forest fires/volcano eruptions)
Dark clouds due to extinction
[absorption/scattering] by dust in line of sight
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Infrared image: less extinction
Reflecting cloud results from star to side or in front of dust.
Reflected light slightly bluer
The apparent magnitude of a star at wavelength λ increases due to extinction Aλ
mλ = M + 5 logd -5+ Aλ mint = M + 5 logd -5
at λ, apparent magnitude mλ, intrinsic magnitude mint, d distance (pc)
change in magnitude due to extinction = Aλ = mλ - mint
Now I = I0e-τλ and mλ - mint = -2.5log Fλ/Finte-τλ,
∴mλ - mint = 2.5τλloge
Thus mλ -mint = 1.086τλ, and Aλ = 1.086τλ
i.e. change in magnitude due to extinction ≈ optical depth in line of sight
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Optical depth through a cloud of size s is: κλ - absorption coefficient, ρ - density Emission intensity I falls off by e-1 over distance l, where
n – number density of grains, σ - scattering cross-section
Galaxy also transparent in far UV (beyond 124Ǻ) as X-ray
energies pass 5 KeV
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Composition of grains Spectral features in extinction curves → graphite 2200Ǻ (circumstellar dust from carbon stars) silicates 9.7 and 18 µm (circumstellar dust from oxygen rich stars)
Also features due to water ice mantles on grains. Most recent results on solar system dust from “Stardust” mission to Comet Wilde (Don Burnett, GPS)
Spectra from ISO – SWS
9.7, 18 µm silicate lines prominent
Ice features (from grain mantles) much
stronger along line of sight to GC (Sgr*) than in diffuse ism
Very different from dust in meteorites C(diamond) 0.002 µm SiC 0.3 – 20 µm C(graphite) 1 -20 µm
Al2O3(corundum) 0.5– 3µm
Originate in SN, AGB stars, novae, red giant winds
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In ISM, dust well-mixed with gas
Dust-Gas (HI , H2, HII) correlations not unexpected In dense molecular clouds:
→ high dust column densities, high τ in visible,V → grains catalysts for H2
→ dense clouds shield molecules, prevent dissociation From 21 cm HI line observations: τHI ~ NH/T∆v, ∆v is full width of H line at half maximum (km/s)
For dust, Aλ ∝ τλ = σVND
Implies that ND ∝ NH i.e. gas and dust co-exist
Observations confirm good correlation
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NHI NHI+H2
Recall AV ∼ 3(B-V)
Savage & Mathis 1979 ARAA
Copernicus observations of HI (Lyman α) and H2 (UV lines)
Here NH = total hydrogen column density =NHI + 2NH2
For AV ≤ 3, NHI ∝ E(B-V)
NHI /E(B-V) = 4.8 x 1021 H atoms cm-2 mag-1
For AV > 3, NHI+H2 ∝ E(B-V) NHI +2NH2/E(B-V) = 5.8 x 1021 atoms cm-2 mag-1
Since R = AV /E(B-V) = 3.1 (NHI +2NH2)/AV = 1.87 x 1021 atoms cm-2 mag-1
and AV ∝ τV = σVND ∴ (NHI +2NH2)/AV ≡ gas to dust ratio
Substituting for mass of hydrogen atom and σV
→ Dust to gas mass ratio 1/100
Variations in R and gas to dust ratio seen in other galaxies e.g. LMC, SMC (Magellanic Clouds)
Differences in grain size, composition, metallicity/environment?