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

100

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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