Small scale dissipative structures of the diffuse ISM I– CO...
Transcript of Small scale dissipative structures of the diffuse ISM I– CO...
Small scale dissipative
structures of the diffuse ISM
I– CO diagnostics
P. Hily-Blant . . . . . . . IRAM, Grenoble, France
E. Falgarone. . . . . . . . LERMA/ENS, Paris, France
J. Pety . . . . . . . . . . . . IRAM, Grenoble, France
Diffuse molecular gas
• low extinction : AV ≤ 1mag
• radiation field : interstellar mean field
• magnetized : B ≈ 1 − 30µG
• tracer : mostly 12CO(1 − 0), generally assumed optically thick
• turbulent : linewidths ≈ 10cs
• structured
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• IRIS dataMiville-Deschenes & Lagache
ApJS 2005
• crosses : YSO from IRAS faint sources
catalog log10(I12/I25) < −0.4 I60 < I100
• circles : Lynds objects• diffuse gas : I < 7MJy/sr
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Aims and method
−→ structured at which scales ?−→ seeds of star forming structures ?
• observe large quiescent regions : cover large scales high angular resolutions high sensitivity
• statistical analysis of the velocity field high spectral resolutions large number of spectra
• correlation density and velocity fieldsgood tracers of velocity field and mass :13CO, 12CO
• a good telescope : IRAM-30m (knowm beam pat-tern)
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Morphology : integrated intensity
8000 spectra, δx = 10′′ = 0.0075 pc, δv = 0.05km s−1
IRAM-30m
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13CO(1 − 0) : dense 12CO(1 − 0) : tenuous
Filaments : ∅ ≈ 0.03 pc ≈ 6000 AU
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12CO(1−0) line wings
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Physical conditions : LVG
• 13CO filaments : 12CO and 13CO(1−0), (2−1) N(13CO)/∆v = 2 × 1015cm−2 (km s−1)−1
nH2 ≈ 1 − 2 × 103cm−3, Tkin = 8 − 9 K
• thin-12CO filaments : 12CO(1 − 0), (2 − 1) τ(1−0) ≤ 0.6 N(12CO)/∆v = 3 × 1015cm−2 (km s−1)−1
nH2 ≈ 8 − 1 × 102cm−3, Tkin = 20 − 400 K
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1000
1000
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Tkin [K]
100
thin−12CO filaments
13CO filaments
n [cm−3]
?
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Magnetic fields
Morphology : spatial coherence
thin-12CO filaments
Polarization angle ofBlos :
P.A. = 110 ± 18
13CO filaments
Orientation of filaments is not random : role of ~B ?
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What generates non-random structure
in molecular clouds ?
Possibly turbulence and its fundamental property ofintermittency :
• at a given small scale l, rare and large local excur-sions of the velocity create timescales much shor-ter than the corresponding turnover timeτl ∼ l/vl ∼ l2/3
• these large excursions are rare but significantlymore numerous than in a Gaussian distribution
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Tracers of intermittency
• Incompressible turbulence non-Gaussian distributions of velocity incre-
ments, shear, vorticity,...
the smaller the lag, the larger the devia-
tion from Gaussian
coherent intense vortices, seen in the laboratory
• Compressible turbulence Existence of shocks : same statistical properties
as small scale vortices (Pety & Falgarone 2000)
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Pety & Falgarone 2000
Shocks versus intense vortices
Simulations of supersonic turbulence
(Porter, Pouquet Woodward, 1992)5123, decaying, rms Mach ∼ 1
• full cube (left)• shocks (center)• vortices (right)
⊲ both have non Gaussian statistics⊲ fv = 3 × 10−2 but fS ∼ 1
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Is gas vorticity accessible to observations ?
Subset of largest line Centroid Velocity increments (CVIs)Subset of largest 〈Ωsky〉los
Lis et al 1996
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Velocity shear statistics
Hily-Blant et al 2006
Lag : 18 pixelsLag : 3 pixels
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The regions of largest velocity shear
Hily-Blant et al 2006
Extrema of CVI
• are not correlated with 12CO or 13CO integrated emission :
pure velocity structures
• are associated with the thin-12CO filaments
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The regions of largest velocity shear
– Small scales –
IRAM-PdB
0.015 pc
0.4 pc
IRAM-30m
Hily-Blant et al 2006
Extrema of CVI
• contain structures at sizes down to 1000 AU
• large velocity shears at 1000 AU scale :
≈ 1km s−1/1000AU = 200km s−1 pc−1
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The regions of largest velocity shear : large scales
0.015 pc
IRAM PdB
IRAM 30m
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Conclusions
⊲ Diffuse molecular gas is structured in space andvelocity
⊲ Optically thin 12CO is not distributed uniformly,but organized into filaments
⊲ These filaments are not randomly distributed, butfollow the orientation of magnetic fields
⊲ The molecular gas velocity field is intermittent
⊲ Dissipative structures of turbulence may have beenmapped for the first time : warm and tenuous12CO filaments
‡ Cold and dense 13CO filaments may proceed fromdissipation that take place into 12CO filaments
‡ These dissipative structures would provide the gaswith a heating source, independant of photodisso-ciation
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Model vs observations
MHD numerical simulations :
Padoan et al 1998
⊲ dynamical role of magnetic
fields
⊲ multifluid approach
Position-velocity cuts
Vlsr
Position
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Morphology : channel maps13CO(1 − 0) 12CO(1 − 0)
Far 12CO(1 − 0) wings : T12/T13 ≥ 35
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Dynamics into filamentary clouds
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