ASTR112 The Galaxy Lecture 8 Prof. John Hearnshaw 12. The interstellar medium (ISM): gas 12.1 Types...
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Transcript of ASTR112 The Galaxy Lecture 8 Prof. John Hearnshaw 12. The interstellar medium (ISM): gas 12.1 Types...
ASTR112 The GalaxyLecture 8
Pro
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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
Ros
ette
neb
ula
in M
onoc
eros
ASTR112 The GalaxyLecture 8
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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 densityn ~ 108 particles/m3
HI clouds are most numerous ( ~1% visible mass of Galaxy); T ~ 90 K, n ~ 106 – 107 m-3; emit 21-cm radioradiation, and the heavier elements they contain give IS absorption lines in spectra of distant stars at low gal. latitude
ASTR112 The GalaxyLecture 8
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Dense molecular clouds contain mainly H2 , but are mosteasily 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 < 104 m-3), highly ionized and very low density
ASTR112 The GalaxyLecture 8
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HII regions (diffuse gaseous nebulae)
Free p, e, plus a few ions of heavier elements. Ionizationby 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 giveemission lines in spectra. HII regions invariably arered, being dominated by the strong Balmer Hα lineof hydrogen.
ASTR112 The GalaxyLecture 8
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Some famous HII nebulae
Orion nebula M42 NGC1976η Carinae nebula NGC337230 Doradus (in LMC) NGC2070Lagoon nebula M8 NGC6523Rosette nebula NGC2237Trifid nebula M20 NGC6514
ASTR112 The GalaxyLecture 8
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Above: Trifidnebula, M20, inSagittarius
Below: Lagoonnebula M8 in Sagittarius
Right: Rosette nebula in Monoceros
ASTR112 The GalaxyLecture 8
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Above: Orion nebula, M42
Below, Tarantula nebula, 30 Doradusin the Large Magellanic Cloud
Right: η Carinae nebula, in southern Milky Way
ASTR112 The GalaxyLecture 8
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Hubble Space Telescopeimages of the Orion nebulaRight: detail of centre
ASTR112 The GalaxyLecture 8
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Physical processes in HII regions
H + hν (λ < 912 nm) → p + e (photoionization)p + e → H* + hν (recombination)H* → H + hν (cascading)
O++ + e → (O++)* + e (collisional excitation)(O++)* → O++ + hν (radiative deexcitation)
ASTR112 The GalaxyLecture 8
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Typical radius and mass of HII regions
Spectral type of star radius of nebula (pc) O5 70–200 B0 20 A0 0.5
They can only readily be observed around stars of typesO to B0 (T* ~ 50 000 K to 25 000 K)
Mass: 0.1 to 103 M⊙
ASTR112 The GalaxyLecture 8
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Brightest optical emission lines in HII spectra
Balmer lines: Hα (656.3 nm), Hβ (486.1 nm), Hγ (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
ASTR112 The GalaxyLecture 8
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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.
ASTR112 The GalaxyLecture 8
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Emission lines in a typical gaseous nebula. The strolngestlines are Hα in red, [OIII] in green and [OII] in ultraviolet
ASTR112 The GalaxyLecture 8
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awSpectrum of NGC7009, a planetary nebula, but similarto a typical diffuse gaseousnebula spectrum.
Diagram of spectrumof the Orion nebula
ASTR112 The GalaxyLecture 8
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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 < 8.0
ASTR112 The GalaxyLecture 8
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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. 109α line is H transition from n = 110 to n = 109, λ ~ 6 cm, ν = 5050 MHz
ASTR112 The GalaxyLecture 8
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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 inthe surrounding HI gas. Usually hot stars disappear in a few × 106 years, before pressure equilibrium can beachieved, and so the HII region also dies out, revertingto HI condition.