Dust in the Interstellar Medium • Current picture of ISM from multi-wavelength imaging

of gas and dust – multiwavelength spectroscopy, and polarization measurements

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optical image of sky Mellinger PASP 2009

Kwok Chapters 11, 12, 13; Draine Chapters 21, 22, 23, 24

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Optical

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

𝐵𝜈 =2ℎ𝜈3

𝑐21

𝑒ℎ𝜈

𝑘𝑘� − 1

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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

∴ 𝜏𝜆 = ∫ 𝑛𝑑𝑠0 𝜎𝜆𝑑𝑑 = 𝜎𝜆 ∫ 𝑛𝑑

𝑠0 𝑑𝑑, for constant σ

= 𝜎𝜆𝑁𝑑 where Nd is dust column density

→ Aλ ~𝝉𝝀 ~ Nd

extinction ∝ number of grains in line of sight

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0

s

dsλ λτ κ ρ= ∫

1 1

d

lnλ λκ ρ σ

= =

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 𝐴𝜆 𝐴𝑉� as function of 1 𝜆⁄

but also ratio of color excesses versus 1 𝜆⁄

(Indebetouw et al. 2005)

Essentially comparing two stars of same spectral type & luminosity where one has negligible extinction

𝛥𝑚𝜆 − 𝛥𝑚𝑉𝛥𝑚𝐵 − 𝛥𝑚𝑉

=𝐴𝜆 − 𝐴𝑉𝐴𝐵 − 𝐴𝑉

=𝐸𝜆−𝑉𝐸𝐵−𝑉

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,

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

scattering Aλ ∝ λ-4

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?

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Cardelli et al 1989 (parametrized fits see Draine AARA)

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

Solar neighborhood : <AV> = 2m/kpc

∴ Visual extinction to Galactic center = 16m ∴K-band (2.2µm) extinction to GC = 1.6 x 3.82/3.55 ~ 1.7m

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

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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?

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