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12. The interstellar medium (ISM): gas 12.1 Types of IS gas cloud 12.2 H II regions (diffuse gaseous ne

12. The interstellar medium (ISM): gas 12.1 Types of IS gas cloud 12.2 H II regions (diffuse gaseous nebulae). The Rosette nebula in Monoceros. Types of IS gas cloud H II ; hot ionized hydrogen (p,e) H I ; neutral atomic hydrogen (H)

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12. The interstellar medium (ISM): gas 12.1 Types of IS gas cloud 12.2 H II regions (diffuse gaseous ne

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  1. 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

  2. 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

  3. 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– 1012molecules/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

  4. Five gaseous phases of the interstellar medium

  5. 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 Hα line of hydrogen.

  6. Some famous HII nebulae Orion nebula M42 NGC1976 η Carinae nebula NGC3372 30 Doradus (in LMC) NGC2070 Lagoon nebula M8 NGC6523 Rosette nebula NGC2237 Trifid nebula M20 NGC6514

  7. Below: Lagoon nebula M8 in Sagittarius Above: Trifid nebula, M20, in Sagittarius Right: Rosette nebula in Monoceros

  8. Below, Tarantula nebula, 30 Doradus in the Large Magellanic Cloud Above: Orion nebula, M42 Right: η Carinae nebula, in southern Milky Way

  9. Hubble Space Telescope images of the Orion nebula Right: detail of centre

  10. Eagle nebula M17

  11. 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)

  12. 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 types O to B0 (T* ~ 50 000 K to 25 000 K) Mass: 0.1 to 103M⊙

  13. 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

  14. 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.

  15. Emission lines in a typical gaseous nebula. The strolngest lines are Hα in red, [OIII] in green and [OII] in ultraviolet

  16. Spectrum of NGC7009, a planetary nebula, but similar to a typical diffuse gaseous nebula spectrum. Diagram of spectrum of the Orion nebula

  17. 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 log10N < 8.0

  18. 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

  19. 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.

  20. End of lecture 8 30 Doradus, the Tarantula nebula, a giant HII nebula in the LMC

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