13. the interstellar medium: dust

ASTR112 The GalaxyLecture 10

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13. The interstellar medium: dust

IRA

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Dark clouds, reflection nebulae and Bok globules

• Dust was first found in form of large dark clouds (e.g. Coalsack, Horsehead etc) which are silhouetted against bright backgrounds of stars or HII regions.• Named ‘holes in the heavens’ by Wm Herschel (1785)• Identified as obscuring clouds by E.E.Barnard in early years of the 20th century.


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Dark clouds:• Typical size ~10 pc across• Typical mass ~ 2000 M⊙

• Number known in Galaxy ~2600• Galactic latitude nearly always |b| < 10º

Distribution of darkclouds in the galacticplane near the Sun


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Distribution of dark clouds in the Milky WayMost dark clouds are found near the galactic equator


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Also seen are small very dense dark globules of dust,known as Bok globules (after Bart Bok, who first drew attention to them).

Bok globules:• Size 0.05 to 1 pc• Mass 0.2 to 60 M⊙

• Often seen against a bright HII background• Globules may be individual proto-stars condensing from a dense molecular cloud


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Bok globules in the nebula IC2944


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Reflection nebulae:• Light from a nearby star is scattered by dust grains into the line of sight• Colour is blue, as blue light is the most readily scattered• Scattering of light from blue stars, usually type B; spectrum is also of this type, i.e. absorption lines• Light is often highly polarized (20 – 30 per cent)• Amongst best known examples are the reflection nebulae from circumstellar dust surrounding brightest members of the Pleiades star cluster; also the reflection nebula which is part of M20, the Trifid nebula


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Reflection nebulae:above: Pleiadescentre: M20 Trifid nebularight: NGC1999


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Other places where interstellar dust is found:• General diffuse layer between dark clouds in plane of Galaxy. • This layer causes (i) interstellar reddening of stars near the gal. equator, (ii) interstellar polarization of starlight, and (iii) diffuse galactic light (DGL).• Also the infrared cirrus: low density whispy filaments of dust seen by emission in IR, occurring very near Sun and hence seen at fairly high galactic latitudes.


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Wolf diagrams

Max Wolf (Heidelberg, 1923) analysed star counts indirection towards a dark cloud to obtain the clouddistance and estimate the amount of absorption (which depends on cloud mass of dust).


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5log5 dMm

3dNm

constlog3log dNm

const)(6.0log MmNm

For transparent space

The number of stars brighter than magnitude m and within distance d is:

Hence:

and so


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If a dark cloud intervenes along the line of sight, thenstars behind the cloud go from magnitude m0 to m = (m0 + A), where A is the extinction caused by the cloud.

Both m0, a measure of cloud distance through

and A, a measure of the amount of dust in the cloud, can be measured from the resulting step in the Wolf diagram.

5)pc(log50

dMm


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Left: a schematic Wolf diagramRight: actual Wolf diagram for the dark cloud NGC 6960The vertical axis is the logarithm of the number of starsper square degree brighter than a given apparent magnitude


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The general dust layer: IS extinction and reddening

• General dust layer demonstrated by Robert Trumpler (1930)• Dust layer causes more distant low latitude stars to be (a) fainter (IS extinction), and also (b) redder (IS reddening).• Extinction

•Reddening

VVVAdMm 5log5

0obs)()( VBVBE

VB


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• Both extinction AV and reddening EB-V are proportional to the amount of dust along the line of sight• In general extinction A(λ) is a function of wavelength, λ • Whitford extinction law is:

valid from near ultraviolet to the infrared• Ratio of extinction to reddening is roughly constant for all stars affected by dust, irrespective of their distance

1A

2.3 VB

V

EA

R


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Extinction and reddening by IS dust grains


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Reddening of starlight by interstellar dust


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Dust observed by IRAS (1984)

• λ = 12, 25, 60, 100 μm• Dust often occurs in dense molecular clouds, T ~10 K which therefore emits most strongly at 100 μm• But IRAS found many warmer discrete sources in molecular clouds, corresponding to solar mass proto-stars inside dusty shells• IRAS also discovered the infrared cirrus


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Above: IRAS all-skyimage of the dust layer in the Galaxy from IRthermal emission fromdust grains.

Below: a detail of theGalaxy’s dust layer as revealed by IRAS


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IRAS infrared cirrusat the north galactic pole.Image constructed from12, 60 and 100 μmwavelengths.


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Statistics for galactic dust

• Total dust mass is ~1 per cent of mass of ISM (remainder is gas)• Mean dust density in the galactic disk is ndust ~ 10-6 grains/m3

Compare this to mean gas density of ngas ~ 10+6 gas atoms/m3

• Mean visual extinction in galactic plane (b = 0º) is AV ~ 1 to 2 mag. for each kpc of distance but the distribution is very patchy.


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Calculation example for IS extinction

Photometry of a star gives mV = 14.61, (B – V) = 1.1;spectroscopy indicates the spectral type is G0 V.For G0 V stars, (B – V)0 = 0.60 and MV = 5.0.

Hence EB-V = (B-V)obs – (B-V)0 = 0.50Therefore AV = 3.2 EB-V = 1.60 giving mV0 = mV – AV = 14.61 – 1.60 = 13.01

Distance modulus = mV0 – MV = 5logd – 5so 5logd – 5 = 13.01 – 5.0 = 8.01 or logd = 2.602Thus d = 400 pc


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Extinction in ultraviolet (UV)

• Satellite observations used for UV stellar photometry (λ < 300 nm) allow the extinction law A(λ) to be measured in UV.• Results show that Whitford law (A(λ) 1/λ) is not valid in UV.• Maximum extinction at about 220 nm• Broad minimum in extinction from λ < 200 nm down to λ = 125 nm• The extinction rises steeply in far UV for λ < 125 nm


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UV extinction plotversus wavelengthshowing the 220 μmgraphite peak.


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Extinction in infrared

• Extinction is small in infrared• However some M giant stars have dust shells around them giving large circumstellar extinction • These circumstellar grains probably form in the atmosphere of the M star itself• Such stars generally show a broad dip in spectrum at λ ~ 9.7 μm, presumed to be caused by silicate dust grains• Silicate dust grains are also thought to be the major component of interstellar dust grains


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Broad IR absorptionfeatures in the spectrumof an IR source arebands produced bysolid grains, such asices and silicates. Theparticles are probablycircumstellar.


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Nature of interstellar dust grains

• No single grain composition or size fits all the data• Various possible models include: ice grains, graphite, silicates, silicates plus ice mantle, polycyclic aromatic hydrocarbons (PAHs), dirty ice grains (H2O plus H,C,N,O compounds), metallic grains• Visual extinction is best explained by silicate cores, ice mantles, particle size ~ 100 nm • Graphite grains explain the 220 nm extinction peak; size ~ 50 nm• Far UV extinction from silicates, size 5 – 20 nm; also silicates explain 9.7 μm circumstellar extinction in IR


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A typical dust grain


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End of lecture 10

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13. the interstellar medium: dust

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