Enrique Vázquez-Semadeni. Instituto de Radioastronomía y Astrofísica, UNAM, México

Transcription

1 Enrique Vázquez-Semadeni Instituto de Radioastronomía y Astrofísica, UNAM, México 1

2 Collaborators: CRyA UNAM: Javier Ballesteros-Paredes Pedro Colín Gilberto Gómez Manuel Zamora-Avilés Abroad: Robi Banerjee (Obs. Hamburg) Former Postdoc: Jonathan Heiner 2

3 Outline: Formation and evolution of dense cold clouds. The case for globally collapsing and accreting MCs. Simple synthetic observations of simulations of the process. 3

4 A galaxy like ours contains ISM ~ M sun in stars ~ M sun in gas ~ 10 8 M sun in dust M sun sun = 2x g 65,000 light years (~ 20 kpc) (1 pc = 3.09 x cm) 4

5 Brief summary of ISM structure: The interstellar medium (ISM) contains gas in a wide range of conditions: Density Temperature Cold molecular (H 2 ) gas (clouds, clumps, cores) Cold atomic ( HI ) gas (diffuse clouds) Warm (atomic or ionized) gas (intercloud gas) Hot gas (supernova remnants) 10 2 >10 6 cm K ~ cm K ~ cm K ~ 10-2 cm K Note these are ranges, not single values (e.g., Dickey+77; Heiles 01). Possibly a density continuum. 5

6 A hierarchical (nested) structure: Engargiola et al. 2003: Study of M33 Color image: HI distribution Circles: Giant Molecular Clouds (GMCs) GMCs seem to be the tip of the iceberg of the density gas distribution. They conclude that GMCs form out of the HI. (See also Blitz et al. 2007, PPV.) 6

7 CLOUD FORMATION AND GLOBAL COLLAPSE

8 1. Velocity convergence: A density enhancement requires an accumulation of initially distant material into a more compact fluid parcel. dρ = ρ u dt Continuity equation i.e., need to have a convergence of the velocity field into the parcel. However, most models of clouds have relied on the notion of static equilibrium. 8

9 2. ISM thermodynamics. (See reviews in Vázquez-Semadeni+2003, LNP, 614, 213; Vázquez-Semadeni 2015, ASSL, 407, 401) A key property of the atomic ISM is that it is thermally unstable in some regimes (Field 1965). The internal energy equation (per unit mass) is de dt = ( γ 1) e u + Γ n Λ, where n = number density, in units of cm -3, (ρ = µ m H n) Γ is the (radiative) heating function, and Λ is the (radiative) cooling function. 9

10 The instability is easiest to understand using the thermal equilibrium condition ρ Γ( ρ, T ) = Λ( ρ, T µ ), to eliminate the temperature from the ideal gas equation of state, to write P = P ( ρ ) Peq ( ρ ). In the absence of local energetic events, the heating function to zeroeth order satisfies Γ cst. 10

11 The interstellar cooling function TI under the isobaric criterion. TI under the isochoric and the isobaric criteria. Dalgarno & McCray

12 Due to the forms of the cooling and heating, the behavior of P eq is: P eq, at which WNM heating Γ equals (stable) cooling nλ. Mean ISM thermal pressure CNM (stable) Field+1969; Wolfire et al The gas is unstable under the isobaric criterion where dp/dρ < 0: If ρ, P, and the fluid parcel is even further compressed. Runaway compression until dp/dρ > 0 again. Thermally unstable range The flow segregates into a cold (~100 K) dense and a warm (~10 4 K ) diffuse phase. 12

13 When a dense cloud forms out of a transonic compression in the WNM, it automatically continuously acquires (accretes) mass; acquires internal turbulence (through TI, NTSI, KHI? Vishniac 1994; Walder & Folini 1998, 2000; Koyama & Inutsuka 2002, 2004; Audit & Hennebelle 2005; Heitsch et al. 2005, 2006; Vázquez-Semadeni et al. 2006). WNM n, T, P, v 1 WNM n, T, P, -v 1 The compression may be driven by global, external turbulence, large-scale instabilities, etc. 13

14 May cold clouds be born gravitationally unstable? Because they form out of a transition from the warm/diffuse to the cold/dense phase, they quickly become Jeans-unstable. (If the colliding flows were not already driven by larger-scale gravitational instability.) ρ 10 2 ρ, T 10-2 T Jeans mass, ~ ρ -1/2 T 3/2, decreases by ~ 10 4 upon warm-cold transition. Contrary to standard interpretation of near virial equilibrium. 14

15 Simulation of cloud formation with Gadget 2 SPH code; 2.6x10 7 SPH particles: (Gómez & Vázquez-Semadeni 2014, ApJ, 791, 124) Converging inflow setup L = 256 pc <n> = 1 cm -3 v inflow ~ 7 km s -1 T ini = 5000 K R cyl = 32 pc R inf L box L inflow 15

16 (Gómez & Vázquez-Semadeni 2014, ApJ, 791, 124) 16

17 Gravitational collapse likely to start in the cold atomic phase. Molecular gas may form as a consequence of the gravitational contraction......which increases column density sufficiently for self-shielding. Molecular gas not essential for SF (e.g., Glover & Clark 2012), but just a byproduct of the collapse. Cold atomic gas should be flowing towards molecular clouds (except if the MCs are already in the process of dispersal by SF.) 17

18 Dense clump structure (Banerjee, VS, Hennebelle & Klessen, 2009, MNRAS, 398, 1082). Gas flows from diffuse medium into dense clumps. There is a net mass flux through clump boundaries. The boundaries are phase transition fronts, not rigid walls. 18

19 This scenario is consistent with the nearly ubiquitous existence of atomic envelopes of MCs (e.g., Andersson+91, 93; Wannier+93; Ballesteros- Paredes+99; Engargiola+03; Stanimirovic+10; etc.) Ballesteros-Paredes+99 compared HI and CO PV diagrams in observational data and simulations. HI data: Leiden-Dwingeloo (Hartmann & Burton 97) survey. CO data: Ungerechts & Thaddeus 87. In simulations: CO = gas with n > n CO ~ 30 cm

20 time in Myr Molecular gas (n > 30 cm -3 ). (Ballesteros-Paredes et al. 1999, ApJ, 520, 285.) 20

21 Grey scale: HI. Contours: CO Simulation Shared features (among observations and simulations): CO lines narrower than HI lines. At locations where CO is present, HI also present, with N ~ that necessary for self-shielding. CO and HI do not generally peak at same velocity. Both HI and CO are asymmetric in velocity axis. 21

22 SYNTHETIC OBSERVATIONS OF THE MC FORMATION PROCESS (Heiner, VS & BP 2015, MNRAS, 452, 1353)

23 Numerical simulation of thermally-unstable ISM with decaying turbulence in atomic ISM and self-gravity Code: SPH (Gadget 2) Box size: 256 pc # SPH parts: 2.6 x 10 7 <n> = 3 cm -3 23

24 24

25 Simple-minded atomic/molecular post-processing: Throw 100 rays in random directions from each grid cell. Compute average A V of all rays. Use simple classification scheme for gas in cell (Heitsch & Hartmann 2008): < 1: atomic <A V > >1, n<n crit : CO-free molecular >1, n>n crit : CO molecular 1/ T υ H cm s 2 1K 25

26 Limitations: Assumption of instantaneous molecular gas formation overestimation of molecular fraction. Limited resolution Underestimation of molecular fraction (may miss very dense gas at small scales) Molecular fractions must be considered 1 st approximation only. 26

27 12.5 Myr 25.0 Myr 33.7 Myr atomic, n>30 cm -3 molecular 128-pc sub-boxes of simulation, containing the most massive cloud complex. Molecular fraction increases in time. 27

28 12.5 Myr Onion-like layered structure: WNM CNM CO-free molecular CO-molecular. Density [M sun pc -3 ~ 30 cm -3 ] T and v 25.0 Myr 33.7 Myr Contours: atomic. Greyscale: molecular 28

29 HI gas CO-free molecular CO molecular none 12.5 Myr 25.0 Myr 33.7 Myr 29

30 v x HI line formation v x v x Colors: v x Black contours: N HI Purple contours: N mol (including H 2 ) 30

31 v x Are we seeing the converging flows in the HI profiles? v x v x : molecular gas : Average HI profile : 1-σ deviation 31

32 Synthetic HI profiles, showing HI self-absorption (HISA) 32

33 HISA CO-free molecular CO molecular none 12.5 Myr 25.0 Myr 33.7 Myr 33

34 However, HISA is an elusive quantity: 2 nd derivative method (Krco+2008) Matching line minimum and opacity maximum. Depends on the background brightness. Unclear object boundaries from line profile shape, Different practical definitions gives different HISA maps. 34

35 SUMMARY

36 1. Gas is the ISM is generally flowing CNM MCs clumps cores stars Not static at any location. 2. Molecular clouds (MCs) evolve! Formed by convergence of warm gas streams. Grow in mass and density. Are likely dominated by infall, not strong random turbulence. 3. Gravitational collapse probably starts in atomic phase. HI gas likely to be flowing towards MCs. 4. Synthetic observations of simulations of the process show: Large mass fractions of CO-free molecular gas Although with uncertainties. Need simulations with self-consistent chemistry. Common bimodal shapes of HI profiles around MCs may be showing HI inflows. HISA observed, although it is quite subjective. 36

37 THE END 37

38 Yet many kinds of structures are at roughly the same thermal pressure: Myers 1978 but not all... in particular, the molecular gas. Molecular clouds harbor all star formation in the Galaxy. 38