Dust in the Interstellar Medium • Current picture of ISM from multi-wavelength imaging of gas and dust – multiwavelength spectroscopy, and polarization measurements
optical image of sky Mellinger PASP 2009
Kwok Chapters 11, 12, 13; Draine Chapters 21, 22, 23, 24 1 Optical
2 IRAS: 4 bands 12 – 100 µms DIRBE: 10 bands 1.25 – 240 µms
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 2 1 = 3 ℎ휈 1 휈 2 ℎ휈 퐵 �푘푘 3 푐 푒 − 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 4
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
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) 6
Dark clouds due to extinction [absorption/scattering] by dust Reflecting cloud results from in line of sight star to side or in front of dust. Reflected light slightly bluer
Infrared image: less extinction
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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
-τ -τ Now I = I0e λ and mλ - mint = -2.5log Fλ/Finte λ,
∴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: s τ= κρds λλ∫ 0 κλ - absorption coefficient, ρ - density
Emission intensity I falls off by e-1 over distance l, where
11 l = = κρn σ λλ d n – number density of grains, σ - scattering cross-section ∴ = = , for constant σ 푠 푠 = where N is dust column density 휏휆 ∫0 푛푑 휎휆푑푑 휎d 휆 ∫0 푛푑 푑푑
휎휆푁푑 → Aλ ~ ~ Nd
extinction ∝ number of흉 흀grains in line of sight 9
Extincted objects appear redder; λ-1 law recall stars behind cloud edges
Extinction in wavelength bands AB, AV, AR … where B centered on 4500Ǻ, V on 5500Ǻ etc
Extinction to a star from color excess
e.g. EB-V = (B-V)obs – (B-V)int
(B-V)obs : observed color index (B-V)int : intrinsic color index
V = MV + 5logd -5 +Av, B = MB + 5logd – 5 +AB
∴ EB-V = MB +AB –MV –Av – MB + MV = AB –AV
→ color excess EB-V = AB –AV
For known spectral types Aλ can be determined
Wavelength dependence of extinction → interstellar extinction curve typically as function of but also ratio of퐴휆 color excesses versus1 �퐴푉 ⁄휆 1 휆 ⁄
(Indebetouw et al. 2005) Essentially comparing two stars of same spectral type & luminosity where one has negligible extinction
= = 훥푚휆 − 훥푚푉 퐴휆 − 퐴푉 퐸휆−푉
훥푚퐵 − 훥푚푉 퐴퐵 − 퐴푉 퐸퐵−푉 Define RV as ratio of total extinction to color excess RV = AV/EB-V R dimensionless; characterizes slope of curve; dependent on composition and size of grains
∴RV = - [Eλ-V/EB-V]λ→∞ assuming Aλ → 0 as λ → ∞
R from extrapolating extinction curves to long λ ↓
diffuse ISM, RV ~ 3 dense regions of molecular clouds RV ~ 5
And AV ~ 3-5 EB-V
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Extinction Curves Linear relation from visible regime to longer λ, Cardelli et al 1989 (parametrized fits see Draine AARA) extinction ∝ 1/λ
2175Ǻ (1/λ = 4.6 µm-1) “bump” → aromatic carbon feature; particles < 0.1 µm
Deviations from mean reddening law of increase beyond UV, steep slope. Consistent with Rayleigh -4 scattering Aλ ∝ λ Small particles ≤ 0.015 µm
Herschel 36 – exciting star M8; fits normal law but R=5.3? HD48099 close to standard BD +56 524 – low density ISM, diffuse dust → larger R for denser clouds? larger grains due to ice mantles? 13 Interstellar extinction tables give average values for extinction as a function of wavelength
From tables with R = 3.1: AV/AJ = 3.55, AK/AJ= 0.382
and AK/AV = 0.382/3.55
m Solar neighborhood :
5 µm extinction = 0.095/3.55 x 16 ~ 0m.4 Galaxy becomes transparent beyond 5 µm
Galaxy also transparent in far UV (beyond 124Ǻ) as X-ray energies pass 5 KeV 14
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) 15
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 16
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 17
Copernicus observations of HI (Lyman α) and H2 (UV lines)
N HI NHI+H2
Recall AV ∼ 3(B-V)
18 Savage & Mathis 1979 ARAA
Here NH = total hydrogen column density =NHI + 2NH2
For AV ≤ 3, NHI ∝ E(B-V) 21 -2 -1 NHI /E(B-V) = 4.8 x 10 H atoms cm mag
For AV > 3, NHI+H2 ∝ E(B-V) N +2N /E(B-V) = 5.8 x 1021 atoms cm-2 mag-1 HI H2
Since R = AV /E(B-V) = 3.1 21 -2 -1 (NHI +2NH2)/AV = 1.87 x 10 atoms cm mag
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?
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