13. The interstellar medium: dust

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13. The interstellar medium dust
IRAS view of warm dust in plane of the Galaxy
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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 lt 10º

Distribution of dark clouds in the galactic plane
near the Sun
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Distribution of dark clouds in the Milky Way Most
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 Pleiades centre M20
Trifid nebula right 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 in direction towards a dark
cloud to obtain the cloud distance and estimate
the amount of absorption (which depends on cloud
mass of dust).
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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, then stars 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.
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Left a schematic Wolf diagram Right actual Wolf
diagram for the dark cloud NGC 6960 The vertical
axis is the logarithm of the number of stars per
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

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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(?) is a function of
  wavelength, ?
• 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

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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)
• ? 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-sky image of the dust layer in
the Galaxy from IR thermal emission from dust
grains. Below a detail of the Galaxys dust
layer as revealed by IRAS
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IRAS infrared cirrus at the north galactic
pole. Image constructed from 12, 60 and 100
µm wavelengths.
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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 106 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
0.50 Therefore AV 3.2 EB-V 1.60 giving mV0
mV AV 14.61 1.60 13.01 Distance modulus
mV0 MV 5logd 5 so 5logd 5 13.01 5.0
8.01 or logd 2.602 Thus d
400 pc
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• Extinction in ultraviolet (UV)
• Satellite observations used for UV stellar
  photometry
• (? lt 300 nm) allow the extinction law A(?) to
  be
• measured in UV.
• Results show that Whitford law (A(?) ? 1/?) is
  not
• valid in UV.
• Maximum extinction at about 220 nm
• Broad minimum in extinction from ? lt 200 nm
• down to ? 125 nm
• The extinction rises steeply in far UV for ? lt
  125 nm

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UV extinction plot versus wavelength showing the
220 µm graphite 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 ? 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 absorption features in the spectrum of
an IR source are bands produced by solid grains,
such as ices and silicates. The particles are
probably circumstellar.
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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
IRAS satellite whole sky image of IS dust in
the Galaxy