1. ASTR112 The Galaxy Lecture 8 Prof. John Hearnshaw 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

2. ASTR112 The Galaxy Lecture 8 Prof. John Hearnshaw Types of IS gas cloud H II ; hot ionized hydrogen (p,e) H I ; neutral atomic hydrogen (H) H 2 ; dense molecular cloud (H 2, CO etc.) H II clouds are hot (T ~ 9000 K), and most easily observed, emitting visible light and radio waves. Typical density n ~ 10 8 particles/m 3 H I clouds are most numerous ( ~1% visible mass of Galaxy); T ~ 90 K, n ~ 10 6 – 10 7 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. ASTR112 The Galaxy Lecture 8 Prof. John Hearnshaw Dense molecular clouds contain mainly H 2, but are most easily observed by radio emission lines from other molecules such as CO, OH, NH 3, H 2 O etc.; n ~ 10 9 – 10 12 molecules/m 3 ; 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 H 2 CO found in many clouds. Intercloud medium H I intercloud medium T ~ 5000 K, H largely neutral n ~ 3 × 10 5 m -3 coronal gas (T ~ 10 6 K; n < 10 4 m -3 ), highly ionized and very low density

4. ASTR112 The Galaxy Lecture 8 Prof. John Hearnshaw Five gaseous phases of the interstellar medium

5. ASTR112 The Galaxy Lecture 8 Prof. John Hearnshaw H II 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. H II regions invariably are red, being dominated by the strong Balmer Hα line of hydrogen.

6. ASTR112 The Galaxy Lecture 8 Prof. John Hearnshaw Some famous H II 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. ASTR112 The Galaxy Lecture 8 Prof. John Hearnshaw Above: Trifid nebula, M20, in Sagittarius Below: Lagoon nebula M8 in Sagittarius Right: Rosette nebula in Monoceros

8. ASTR112 The Galaxy Lecture 8 Prof. John Hearnshaw Above: Orion nebula, M42 Below, Tarantula nebula, 30 Doradus in the Large Magellanic Cloud Right: η Carinae nebula, in southern Milky Way

9. ASTR112 The Galaxy Lecture 8 Prof. John Hearnshaw Hubble Space Telescope images of the Orion nebula Right: detail of centre

10. ASTR112 The Galaxy Lecture 8 Prof. John Hearnshaw Eagle nebula M17

11. ASTR112 The Galaxy Lecture 8 Prof. John Hearnshaw Physical processes in H II 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. ASTR112 The Galaxy Lecture 8 Prof. John Hearnshaw Typical radius and mass of H II 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 10 3 M ⊙

13. ASTR112 The Galaxy Lecture 8 Prof. John Hearnshaw Brightest optical emission lines in H II 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. ASTR112 The Galaxy Lecture 8 Prof. John Hearnshaw 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. ASTR112 The Galaxy Lecture 8 Prof. John Hearnshaw Emission lines in a typical gaseous nebula. The strolngest lines are Hα in red, [O III ] in green and [O II ] in ultraviolet

16. ASTR112 The Galaxy Lecture 8 Prof. John Hearnshaw Spectrum of NGC7009, a planetary nebula, but similar to a typical diffuse gaseous nebula spectrum. Diagram of spectrum of the Orion nebula

17. ASTR112 The Galaxy Lecture 8 Prof. John Hearnshaw Chemical composition of HII nebulae element log 10 N H 12.0 He 11.0 C 8.5 N 8.0 O 8.8 All other elements have log 10 N < 8.0

18. ASTR112 The Galaxy Lecture 8 Prof. John Hearnshaw Radio emission from H II 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. ASTR112 The Galaxy Lecture 8 Prof. John Hearnshaw Evolution of H II regions H II regions are surrounded by H I gas, but being much hotter, they are high pressure regions which therefore expand. The expansion is supersonic, and creates shock waves in the surrounding H I gas. Usually hot stars disappear in a few × 10 6 years, before pressure equilibrium can be achieved, and so the H II region also dies out, reverting to H I condition.

20. ASTR112 The Galaxy Lecture 8 Prof. John Hearnshaw End of lecture 8 30 Doradus, the Tarantula nebula, a giant H II nebula in the LMC