Dust cycle through the ISM
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Dust cycle through the ISM
Francois Boulanger
Institut d ’Astrophysique Spatiale
• Global cycle and interstellar processing
• Evidence for evolution
• Sub-mm perspective
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Dust Cycling in Galaxies
Diffuse ISM Molecular Clouds
• CNM
• WNM
• WIM
Star Formation
SN
109 yrs
a few 107 yrs
3 109 yrs
3 106 yrs
Massive stars
Low mass stars
Giants
• cloud enveloppes
• dense cores
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Dust Evolution: Physical Processes
• Photo-processing/destruction
• Amorphization by cosmic rays
• Grain shattering/sputtering in fast supernovae shocks (WNM, WIM)
• Grain shattering in turbulent clouds (CNM)
• Grain coagulation (CNM)
Act on time scales shorter than replenishment time by dying stars (a few 109 yrs)
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Dust Spectral Energy Distribution
•Comp. Power Mass
•PAH 18% 6%
•VSG 15% 6%
•BG 67% 88%
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Dust Evolution: Evidence
• Variations in PAH abundance in the diffuse ISM and PDRs
• Enhanced VSG abundance in low density gas: the Spica HII region
• Cold dust associated with dense molecular gas: lower temperature, larger far-IR emissivity and no small grains
=> From the diffuse ISM to molecular clouds, PAHs to large grains
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Variations in PAH abundance
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Dust in the Spica HII region
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Dust SED/Composition
=> Enhanced VSG abundance factor ~ 5 : shock processing ?
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L1780 translucent cloud (Av~ 2)
• Gradual change in grain size distribution
• Not a systematic edge to center effect
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PRONAOS Cut
Sub-mm Observations Taurus FilamentSub-mm Observations Taurus Filament
Av=3.5 mag, D=140 pc
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Model with dust evolutionChange of dust properties at :- r < 4’ ± 1’- Av = 2.1 ± 0.5
• Small grainabundance :
0.1±0.1• Submm
emissivity :
3.4 +0.3-0.7
• Tcentre = 12.0 K
(Stepnik et al. 2002)
Similar conclusion reached for translucent molecular cirrus(e.g. Polaris flare: Bernard et al. 1999, A&A 347, 640)
J.P. Bernard, Herschel GP KP, Paris, June 16th 04
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Av = 0.2 mag
Av = 0.8 mag
(Bernard et al. 1999, A&A 347, 640)
Standard cirrus: 17.5 K, =2.0 Polaris cirrus:
13.0 K, =2.2
PRONAOS observations in the Polaris flare
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FIR / AV Ratio
Cambresy et al. 2002
Diffuse ISM Schlegel et al.
Cold Dust Polaris
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Enhancing the FIR/mm dust emissvity
FIR/NH = Md/NH * 1/g * <3*Qext/4a>
=> For a fixed dust to gas ratio (Md/NH ), higher values of FIR/NH for fluffy grains (lower g ) and/or higher < Qext/a> (composite grains?)
Dwek 1997
carbonsilicates
composite
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FIR to mm dust emissivity in the diffuse ISM
(250m)/NHI(250m)/NHI
<NHI>
Boulanger et al. 1996
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mm dust emissivityin the diffuse ISM
(1.2mm) / NH = 5.0 +/- 0.9 10-27 cm2
•Error bar = contribution from dust in H2 and HII (WIM) gas
•Corresponds to a opacity per dust mass : (1.2mm) = 0.30 cm2/g
•Comparable to the value in Draine and Lee (1984) model and observations of the Bok globule B68 (Bianchi et al. Assuming a diffuse ISM Av/NH ratio)
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FIR to mm emissivity in the diffuse ISM
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Structure and temperature effect
Agladze et al. (1996)
Silicates amorphous crystalline
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Summary
• Interstellar dust nature and evolution
• Its role as as a tracer of the
> ISM structure (disks, protostellar cores but also HI-H2 transition in the diffuse ISM and PDRs)
> Magnetic Field (polarisation)
• and an actor of ISM evolution
Interstellar dust observations are bringing an original perpspective on the ISM:
More talks on Friday morning ...
Sub-mm wavelengths still at exploration stage => Herschel