Interstellar Medium: HI Clouds and Absorption Lines

1. 12. The interstellar medium: gas 12.3 HI clouds (and IS absorption lines) 12.4 Dense molecular clouds 12.5 Interstellar masers 12.6 Note on pressures in IS gas NGC1232

2. HI clouds and interstellar (IS) absorption lines • Wide distribution throughout galactic disk to R ~ 20 kpc • Greatest density of clouds for 4 kpc < R < 14 kpc • Number along a given lines of sight in glactic plane • ~ 7 or 8/kpc • Typical size a few pc to a few tens of parsecs • Typical mass MHI ~ 100 M⊙ • Temperature T ~ 90 K • Radio emission λ = 21 cm, frequency f = 1420.406 MHz

3. 21-cm emission in other spiral galaxies This image shows the HI emission in the face-on spiral M101 using the Westerbork radio telescope in Holland. The HI distribution is easier to determine than in the Milky Way, because we can observe this galaxy from an external vantage point.

4. HI and CO distribution with radius R in the Galaxy

5. The Magellanic Stream and HI high velocity clouds represent weak sources of 21-cm emission located well away from the galactic plane. They are the result of tidal interation of the Galaxy on the satellite galaxies, the Magellanic Clouds.

6. The 21-cm line The 21-cm line of neutral atomic hydrogen is known as a hyperfine structure transition. Metastable upper energy state has e and p spins parallel, lower energy state, antiparallel.

7. Lifetime of uper energy state ~ 11 million years, with spontaneous emission of a photon. The upper energy state is populated by collisions which are relatively frequent (one such excitation per H atom occurs about every 400 yr). Three quarters of all H atoms are on average in the upper state.

8. HI line formation

9. IS absorption lines star in galactic plane observer on Earth HI cloud Spectrum of a distant galactic plane star contains narrow IS absorption lines produced by heavy elements in IS gas clouds.

10. IS lines in a stellar spectrum

11. IS lines due to Na, Ca, Ti, K, Fe and molecules • CN, CH, CH+ are known in optical region • IS lines due to C, N, O, Mg, Si, P, S, Cl, Ar, • Mn, Fe and molecules H2, HD, CO are • observed in the ultraviolet

12. Above: narrow IS absorption lines in the spectrum of a distant galactic plane star differ markedly from the broader stellar line. Right: multiple components in the IS NaD line due to clouds at different velocities in the line of sight.

13. IS line strengths give information on chemical • composition of IS HI clouds. Some heavy • elements (e.g. Ca) are greatly depleted in IS • clouds (deficient by a factor ~ 2 × 10-4), while • others (e.g. C, N, O) are hardly changed • relative to solar composition. • Element depletion is by heavy element • accretion onto dust grains, thereby removing • some refractory elements from the gas.

14. The depletion of heavy elements in HI clouds as deduced by the strengths of IS absorption lines. There is no correlation of depletion factor with atomic weight A, but a good correlation with the element’s condensation temperature Tc.

15. Dense molecular clouds The most common molecules are H2, CO, CN, OH, H2CO. Most molecules (but not H2) give characteristic radio emission lines, which allow them to be identified. Over 50 have been detected. Absorption lines are usually seen for OH, always for H2CO. Molecule formula discovery λ number sources hydroxyl OH 1963 1.8 cm ~600 ammonia NH3 1968 1.3 cm 12 water H2O 1968 1.3 cm 35 formaldehyde H2CO 1969 6.2 cm ~150 carbon monoxide CO 1970 2.6 mm 60

16. Some interstellar molecules observed in the IS medium. The first 5 are found as optical/UV IS absorption lines in stellar spectra; the second set are seen as radio emission lines in dense molecular clouds, (or as radio absorption lines when distant sources are seen through dense molecular clouds).

17. Microwave spectrum of emission lines from a dense molecular cloud

18. Properties of dense molecular clouds • Temperature T ~ 10 to 30 K • Number densities n ~ 108– 1012 molecules m-3; • mass density ρ ~ 10-15 kg.m-3 • Cloud mass may be ~ 103M⊙ • Cloud size ~ 10 pc • Dense molecular clouds are often very dusty

19. Note that dust shields molecules from • UV radiation from stars, which would • dissociate most molecules. • Also dust surfaces provide a site for the • formation of the H2 molecule. Other • molecules can form from gas phase • reactions.

20. Dense molecular clouds are under • gravitational collapse because there is • enough mass for self gravity to pull them • together. • They are consequently sites of star • formation

21. Above: galactic distribution of CO in molecular clouds Below: CO cloud radial velocity vs galactic longitude

22. Some HII nebulae which are also associated with dense molecular clouds • η Car • M20 • Trifid nebula • 3. Orion nebula • 4. M16 Eagle nebula

23. Note on pressures in the IS gas P = n k T P pressure (Pa); n number density (m-3); T absolute temp. (K); k Boltzmann’s constant (J.K-1) Phase n (m-3) T (K) P (Pa) HII 108 9000 10-11 HI 107 90 10-14 dense molecular 109 to 1012 10 – 30 10-13 – 10-10 hot HI 3 × 105 5000 10-14 coronal gas 103 10610-14

24. HI clouds are in pressure equilibrium with the • hot HI and coronal gas intercloud medium • The pressure of HII clouds is much higher • than the surrounding medium (normally HI) • and they therefore expand supersonically • (~ 10 km/s) into the surrounding gas.

25. Dense molecular clouds also have much • higher pressures, but this is the result of their • high masses, causing them to collapse and be • compressed under their self gravity (they are • the only phase of the ISM where self-gravity • dominates over gas pressure) • Note IS gas pressures are always very low. • On Earth 1 atmosphere ≈ 105 Pa, much • higher than in ISM

26. Interstellar masers MASER: Microwave Amplification by Stimulated Emission of Radiation Observed in OH lines (λ ~ 18 cm) and sometimes in lines of H2O (1.35 mm) and SiO (6.95 mm, 3.47 mm) IR pumping from thermal IR from dust can cause a population inversion of OH in gas in a metastable upper level – this is a condition for maser action.

27. Stimulated emission can occur, resulting in a very intense emission line from a small region of space (generally a few tens of A.U. across). Maser sources are compact and probably occur in dusty regions associated with star formation or in circumstellar dust shells around M-type stars. There are several OH and H2O maser sources in the dense molecular cloud associated with the Orion nebula – possibly where new-born stars are still enshrouded in a coccoon of circumstellar dust grains.

28. END OF LECTURE 9