12' The interstellar medium ISM: gas

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12. The interstellar medium (ISM) gas
12.1 Types of IS gas cloud 12.2 HII
regions (diffuse gaseous nebulae)
The Rosette nebula in Monoceros
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• Types of IS gas cloud
• HII hot ionized hydrogen (p,e)
• HI neutral atomic hydrogen (H)
• H2 dense molecular cloud (H2, CO etc.)
• HII clouds are hot (T 9000 K), and most easily
  observed,
• emitting visible light and radio waves. Typical
  density
• n 108 particles/m3
• HI clouds are most numerous ( 1 visible mass of
  
• Galaxy) T 90 K, n 106 107 m-3 emit 21-cm
  radio
• radiation, and the heavier elements they contain
  give IS
• absorption lines in spectra of distant stars at
  low gal. latitude

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• Dense molecular clouds contain mainly H2 , but
  are most
• easily observed by radio emission lines from
  other
• molecules such as CO, OH, NH3, H2O etc. n
• 109 1012 molecules/m3 T 10 to 30 K. Some
  complex
• polyatomic organic molecules found in a few
  clouds
• but OH has a widespread distribution in galactic
  plane,
• and CO and H2CO found in many clouds.
• Intercloud medium
• HI intercloud medium T 5000 K, H largely
  neutral
• n 3 105 m-3
• coronal gas (T 106 K n lt 104 m-3), highly
  ionized
• and very low density

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Five gaseous phases of the interstellar medium
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HII regions (diffuse gaseous nebulae) Free p, e,
plus a few ions of heavier elements.
Ionization by UV photons from massive O, B stars
within nebula. p, e occasionally recombine
giving H emission lines. Other elements also
ionized (e.g. O, O) and give emission lines in
spectra. HII regions invariably are red, being
dominated by the strong Balmer Ha line of
hydrogen.
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Some famous HII nebulae Orion nebula
M42 NGC1976 ? Carinae nebula
NGC3372 30 Doradus
(in LMC) NGC2070 Lagoon
nebula M8
NGC6523 Rosette nebula
NGC2237 Trifid nebula M20
NGC6514
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Below Lagoon nebula M8 in Sagittarius
Above Trifid nebula, M20, in Sagittarius
Right Rosette nebula in Monoceros
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Below, Tarantula nebula, 30 Doradus in the Large
Magellanic Cloud
Above Orion nebula, M42
Right ? Carinae nebula, in southern Milky Way
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Hubble Space Telescope images of the Orion
nebula Right detail of centre
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Eagle nebula M17
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Physical processes in HII regions H h? (? lt
912 nm) ? p e (photoionization) p e ?
H h? (recombination) H ? H
h? (cascading) O e ? (O)
e (collisional
excitation) (O)
? O h? (radiative

deexcitation)
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Typical radius and mass of HII regions
Spectral type of star radius of
nebula (pc) O5
70200 B0

20 A0
0.5 They can only
readily be observed around stars of types O to B0
(T 50 000 K to 25 000 K) Mass 0.1 to 103 M?
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Brightest optical emission lines in HII
spectra Balmer lines Ha (656.3 nm), Hß (486.1
nm), H? (434.0 nm) Ionized oxygen lines
(nebulium) OIII forbidden O
500.7 and 495.9 nm (green) OII
forbidden O 372.9 and 372.6 nm (UV) Other lines
of light atoms and ions such as C, N, Ne, S, He

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Formation of the bright forbidden nebular lines
in gaseous nebulae. The upper energy level is
metastable and populated by collisions. In low
density gas the metastable levels depopulate
radiatively and emit photons in the OIII and
OII lines for O and O respectively.
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Emission lines in a typical gaseous nebula. The
strolngest lines are Ha in red, OIII in green
and OII in ultraviolet
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Spectrum of NGC7009, a planetary nebula, but
similar to a typical diffuse gaseous nebula
spectrum.
Diagram of spectrum of the Orion nebula
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Chemical composition of HII nebulae
element log10N
H 12.0
He 11.0
C 8.5
N
8.0 O
8.8 All other elements have log10 N lt 8.0
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• Radio emission from HII regions
• thermal continuum radiation, also known as
• free-free radiation or thermal Bremsstrahlung,
• due to e colliding with p or other electrons
• radio emission lines from H cascading in very
• high electron orbitals, e.g. 109a line is H
• transition from n 110 to n 109, ? 6 cm,
• ? 5050 MHz

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Evolution of HII regions HII regions are
surrounded by HI gas, but being much hotter,
they are high pressure regions which therefore
expand. The expansion is supersonic, and
creates shock waves in the surrounding HI gas.
Usually hot stars disappear in a few 106
years, before pressure equilibrium can
be achieved, and so the HII region also dies out,
reverting to HI condition.
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End of lecture 8
30 Doradus, the Tarantula nebula, a giant HII
nebula in the LMC