IV. Molecular Clouds. 1. Molecular Cloud Spectra

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1 IV. Molecular Clouds Dark structures in the ISM emit molecular lines. Dense gas cools, Metals combine to form molecules, Molecular clouds form. 1. Molecular Cloud Spectra 1

2 Molecular Lines emerge in absorption: optically thick in emission: optically thin.. Interstellar Molecules es 1934 Detection of diffuse IS bands in the optical 1935 Russell assumed origine from molecules 1937 Identification of IS absorption lines: CO (Swings) CH, OH, NH, CN, C (Swings & Rosenfeld) 1941 CH +, CH, CN (Adams, Mc Kellar) 1951 Bates & Spitzer: Investigating the formation and destruction of diatomic mol OH lines in the radio range 1968 Detections of IS NH 3, H O (Townes, Cheung) since 1965 OH-, H O-, CO-, H CO- Maser detection since 1973 organic ion-molecule chemistry at 1989 more than 100 IS molecules identified

lines: CO (Swings) CH, OH, NH, CN, C (Swings & Rosenfeld) 1941 CH +, CH, CN (Adams, Mc Kellar) 1951 Bates & Spitzer: Investigating the formation and

3 A list of more than 100 interstellar molecules can be found at: C 7 H HC 11 N One molecule that might be out there is C 60. It is a member of a family of molecules called fullerenes after the American architect R. Buckminster Fuller ( ), who advocated that we live in houses in the shape of geodesic domes. 3

It is a member of a family of molecules called fullerenes after the American architect R.

4 3. Molecular Lines Molecular Line Transitions: Electronic transitions at 1000Å 1μ Vibrational trans. V at 1 10 μ Rotational trans. J at mm 4. Interstellar Molecule e Formation Molecules are sensitive to UV radiation Molecules form in dense regions shielded against IS radiation: cool stellar atmospheres cool stellar winds in shock fronts (s.figure) Dust particles are easily charged Ions are adhered to their surfaces Molecules form Hollenbach & McKee

Interstellar Molecule e Formation Molecules are sensitive to UV radiation Molecules form in dense regions shielded

5 5. Molecular Cloud Structure Brightness and velocity-integrated contours reflect hierarchical clumpy structure of molecular clouds visible in different species! 5

6 5.1. Clouds? Star formation Turbulence HII regions Heat conduction etc. form filaments McKee & Ostriker (007) ARAA, 45 Coincidence of dense structures in IR, non-thermal radio and CO 6

7 5.. Clumps 7

8 8 n ~ r - Isolated gas and dense particle systems achieve Virial equilibrium: T + V 0 T: kinetic (+ thermal) energy V: potential energy assumptions: spherical cloud, cloud mass M c ρ(r) r -n Larson , 0 5/3, n n n n L M G M E R M G n n E c c th c c pot σ σ

energy assumptions: spherical cloud, cloud mass M c ρ(r) r -n

9 5.3. Clump mass spectrum Orion B: First GMC systematically surveyed for dense gas and embedded YSOs by E. Lada 1990 Survey of gas clumps Clumps in range M M dn dm M 1.6 dn d ln M M 0.6 MdN d ln M M 0.4 Most of mass in massive clumps For a video see: 9

.500 M dn dm M 1.6 dn d ln M M 0.6 MdN d ln M M 0.

10 Hierarchical Structure Clump picture: hierarchical structure Clouds ( 10 pc) Clumps (~1 pc) Precursors of stellar clusters Cores (~0.1 pc) High density regions which form individual stars or binaries Fractal picture: clouds are scale-free V A D / D 1.4 fractal dimension 5.4. Mol.Cloud Parameters Larson 1981 The larger and more massive the (sub-)clouds the larger the velocity dispersion (~T). 10

$scale-free V A D / D 1.4 fractal dimension 5.4. Mol.$

11 Larson The Jeans Mass The random motion σ of clumps in molecular clouds are at or even larger than sound speed Critical length to allow the decoupling from homogeneous background ρ 0 : a density perturbation ρ 1 grows with t Instability (see sect. VI) Jeans Length π cs l lj G Jeans Mass M J σ s ρ 4π lj ρ γ P T ρ 3 T ρ

larger than sound speed Critical length to allow the decoupling from homogeneous

12 6. Giant Molecular Clouds Typical characteristics of GMCs: Mass M Distance to nearest GMC 140 pc (Taurus) Typical size pc Size on the sky of near GMCs 5..0 x full moon Average temperature (in cold parts) K Typical density mol./cm 3 Typical (estimated) life time ~10 7 year Star formation efficiency ~1%..10% Composition of material: 99% gas, 1% solid sub-micron particles ( dust ) (by mass) Gas: 0.9 H /H, 0.1 He, 10-4 CO, 10-5 other molecules (by number) Dust: Mostly silicates + carbonaceous (< µm in size) Properties of the gas: Gas mostly in molecular form: hydrogen in H, carbon in CO, oxygen in O (O?), nitrogen in N (?). At the edges of molecular clouds: transition to atomic species. Photo-Dissociation Regions (PDRs). H cannot be easily observed. Therefore CO often used as tracer. 1

.10% Composition of material: 99% gas, 1% solid sub-micron particles ( dust ) (by mass) Gas: 0.9 H /H, 0.

13 Nearby well-studied GMCs: Taurus (dist 140 pc, size 30 pc, mass 10 4 M ): Only low mass stars (~105), quiet slow star formation, mostly isolated star formation. Ophiuchus (dist 140 pc, size 6 pc, mass 10 4 M ): Low mass stars (~78), strongly clustered in western core (stellar density 50 stars/pc), high star formation efficiency Orion (dist 400 pc, size 60 pc, mass 10 6 M ): Cluster of O-stars at center, strongly ionized GMC, O-stars strongly affect the low-mass star formation Chamaeleon... Serpens... 13

Ophiuchus (dist 140 pc, size 6 pc, mass 10 4 M ): Low mass stars (~78), strongly clustered in western core (stellar density 50

Lecture 3 Properties and Evolution of Molecular Clouds. Spitzer space telescope image of Snake molecular cloud (IRDC G11.11-0.11

Lecture 3 Properties and Evolution of Molecular Clouds Spitzer space telescope image of Snake molecular cloud (IRDC G11.11-0.11 From slide from Annie Hughes Review CO t in clouds HI: Atomic Hydrogen http://www.atnf.csiro.au/research/lvmeeting/magsys_pres/