II- Molecular clouds

Transcription

1 2. II- Molecular clouds 3. Introduction 4. Observations of MC Pierre Hily-Blant (Master2) The ISM / 290

2 3. Introduction 3. Introduction Pierre Hily-Blant (Master2) The ISM / 290

3 3. Introduction Molecular clouds are concentrated close to the plane of the galaxy Pierre Hily-Blant (Master2) The ISM / 290

4 3. Introduction Molecular gas in the MW S147 S235 S212 Ursa Major Cam Per OB2 20 Tau-Per-Aur Complex W3 Polaris Flare CTA-1 Cas A NGC7538 Cepheus Flare G r Cyg OB7 Cyg X Lacerta Pegasus e a t Hercules R i W51 f W44 Aquila South t Aquila Rift Ophiuchus Galactic Center R CrA Lupus G317 4 Coal Sack Chamaeleon Carina Nebula Vela CMa OB1 Maddalena s Cloud Mon R2 Mon OB1 Rosette R λ O Ori A & B Orion Complex Galactic Longitude Gum Nebula S. Ori Filament i Gem OB1 r n i g S147 Galactic Longitude log T mb dv (K km s 1 ) FIG. 2. Velocity-integrated CO map of the Milky Way. The angular resolution is 9 over most of the map, including the entire Galactic plane, but is lower (15 or 30 ) in some regions out of the plane (see Fig. 1 & Table 1). The sensitivity varies somewhat from region to region, since each component survey was integrated individually using moment masking or clipping in order to display all statistically significant emission but little noise (see 2.2). A dotted line marks the sampling boundaries, given in more detail in Fig. 1. Beam Galactic Latitude Galactic Latitude Dame et al 2001 Pierre Hily-Blant (Master2) The ISM / 290

5 3. Introduction Molecular gas in the MW S147 S235 Cyg Lindblad Ring & Local Arm OB7 W3 Cas A Cyg X O u Sagittarius Tangent r Scutum Tangent W51 W50 A r W44 Aquila Rift Galactic Longitude Galactic Longitude 160 F a r C a Norma Tangent r i n a A Centaurus Tangent Vela Carina Tangent P e r s e u s CMa OB1 Maddalena s Cloud Mon OB1 m Gem OB1 S Galactic Longitude Resolution log T mb db (K arcdeg) FIG. 3. Longitude-velocity map of CO emission integrated over a strip ~4 wide in latitude centered on the Galactic plane (see 2.2) a latitude range adequate to include essentially all emission beyond the Local spiral arm (i.e., at v > 20 km s 1). The map has been smoothed in velocity to a resolution of 2 kms 1 and in longitude to a resolution of 12. The sensitivity varies somewhat over the map, since each component survey was integrated individually using moment masking at the 3-σ level (see 2.2) LSR radial velocity (km s 1 ) LSR radial velocity ( km s 1 ) P e r s e u s A r m t e M o l m 3 k p c E x p a n d i n g A r m N u c l e a r D i s k e c u l a r R i n g r m A r Dame et al 2001 Pierre Hily-Blant (Master2) The ISM / 290

6 3. Introduction Giant Molecular Clouds 50% mass of the neutral phase of the ISM is in few 1000 GMCs L = pc; 10 4 < M < M ; n(h 2 ) n(h) Non-negligible role in the dynamics of the galaxy Gas is cold: T kin < 100 K, density n 1000 cm 3 Gas in molecular clouds (MC) is neutral (x e 10 5 n 1/ ) Pierre Hily-Blant (Master2) The ISM / 290

7 3. Introduction M51 at 160 pc resolution Koda+09 Pierre Hily-Blant (Master2) The ISM / 290

8 3. Introduction The Taurus-Perseus-Auriga complex Ungerechts & Thaddeus 1987 Pierre Hily-Blant (Master2) The ISM / 290

9 3. Introduction The Orion A & B clouds Pierre Hily-Blant (Master2) The ISM / 290

10 4. Observations of MC Pierre Hily-Blant (Master2) The ISM / 290

11 Observations of MC H 2 is very difficult to observe: J = 1 level at 170 K J = 1 transitions strongly forbidden J = 2 level at 511 K CO is the primary tracer of molecular gas in the ISM small electric dipole small critical density: n crit = cm 3 most abundant molecule at low temperature after H 2 but 1-0 transition close to atmospheric O 2 line easy to excite and easily detected CO is detected from z = 0 to 6 12 CO(1 0) line at 115 GHz or 2.6 mm: radiotelescopes Observations at very large spectral resolution (ν 0 /δν ): provide us with precise kinematic information Pierre Hily-Blant (Master2) The ISM / 290

12 4. Observations of MC Pierre Hily-Blant (Master2) The ISM / 290

13 mm and sub-mm astronomy Few fw (1e-15 W) Receiver cabin Gain ~ 120 db Few mw Backends Frontends Pierre Hily-Blant (Master2) The ISM / 290

14 The Taurus MC: 12 CO(1 0) Pierre Hily-Blant (Master2) The ISM / 290

15 The Taurus MC: 12 CO(1 0) Pierre Hily-Blant (Master2) The ISM / 290

16 The Taurus MC: 12 CO(1 0) Pierre Hily-Blant (Master2) The ISM / 290

17 The Taurus MC: 13 CO(1 0) Pierre Hily-Blant (Master2) The ISM / 290

18 Characteristics of MCs Structure: not homogeneous but lacunary with holes and cavities striations (e.g. upper left) filamentary (see the 13 CO map) can not speak about the surface of a cloud Existence of molecular gas Photodissociation needs extinction of (F)UV photons Extinction is due to small dust particles Molecular clouds may be characterized by regions where the visual extinction A V is larger than 0.5 mag Pierre Hily-Blant (Master2) The ISM / 290

19 Structure of MC Structure is observed at all scales reminiscing of the turbulent cascade Falgarone et al. (1991) Pierre Hily-Blant (Master2) The ISM / 290

20 MC are close to virialization Solomon et al 1987 Solomon et al 1987 Pierre Hily-Blant (Master2) The ISM / 290

21 Formation of MC 1 Thermal instability: WNM fragments into a multi-phase medium: CNM + WNM clump statistics similar to CO clump statistics significant fraction of thermally unstable gas the various phases are tightly interspersed CNM in supersonic in the CNM and subsonic in the WNM 2 Gravity: creates more concentrated structures 3 UV-driven chemistry influences the cooling function, and temperatures are Pierre Hily-Blant (Master2) The ISM / 290

22 Formation of MC Pierre Hily-Blant (Master2) The ISM / 290

23 Energy balance in Molecular Clouds Self-gravity Virial theorem for unmagnetized cloud E grav = 3 5 αgm 2 /R, 3(P P ext ) + 2E kin + E grav + (M M 0 ) = 0 E kin = 1/2Mσ 2 = 3 2 Mσ2 1D M vir = 5σ2 1D R G γ = 5/3: P = V p dv = 3 2 MkT/µm H, P ext = p ext V p ext = 1 { 4πR 3 3Mc 2 s 3α 5 GM 2 } R Pierre Hily-Blant (Master2) The ISM / 290

24 p ext = 1 { 4πR 3 3Mc 2 s 3α 5 GM 2 } R Pierre Hily-Blant (Master2) The ISM / 290

25 For a fixed mass M, max. pressure sustainable corresponds to a maximum radius R T 1/2 and p max 0.68c 8 s/(g 3 M 2 ) p max /k = 10 5 (T/10) 4 (M/1 M ) 2 cm 3 K 1 For a given pressure p 0 : if R > R J, contraction, until R = R J, and this equilibrium is unstable and leads to collapse. M be 0.25[T/10] 2 [1e6/(p 0 /k)] 1/2 M Thermal pressure can not work against gravity for M > 0.25 M Pierre Hily-Blant (Master2) The ISM / 290

26 Free-fall time: ( ) 3π 1/2 ( 10 t ff = = cm 3 ) 1/2 year 32G ρ MCs should collapse in a free-fall time 10 6 yrs or star formation rate 10 7 / M /yr Higher star formation rate than observed ( 3 M /yr) What prevents clouds to collapse? n H Pierre Hily-Blant (Master2) The ISM / 290

27 Magnetic fields Total pressure: thermal non thermal (turbulent) magnetic (30% of turbulent) Basu (2000) and Crutcher (1999): σ ρ 2/1 Consequence: v A = B/ 4πρ B 6 µg σ 1 n 1/2 2 M A = σ v A 1.1 Magnetic fields: ambipolar diffusion t ad x e yrs t ff Mass to flux ratio: M cr = 0.13Φ/G 1/2 = 1000 M (B/30 µg) (R/2 pc) 2 Pierre Hily-Blant (Master2) The ISM / 290

28 Magnetic fields Pierre Hily-Blant (Master2) The ISM / 290

29 Pierre Hily-Blant (Master2) The ISM / 290