Dust cycle through the ISM

Francois Boulanger

Institut d ’Astrophysique Spatiale

• Global cycle and interstellar processing

• Evidence for evolution

• Sub-mm perspective

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

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)

Dust Spectral Energy Distribution

•Comp. Power Mass

•PAH 18% 6%

•VSG 15% 6%

•BG 67% 88%

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

Variations in PAH abundance

Dust in the Spica HII region

Dust SED/Composition

=> Enhanced VSG abundance factor ~ 5 : shock processing ?

L1780 translucent cloud (Av~ 2)

• Gradual change in grain size distribution

• Not a systematic edge to center effect

PRONAOS Cut

Sub-mm Observations Taurus FilamentSub-mm Observations Taurus Filament

Av=3.5 mag, D=140 pc

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

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

FIR / AV Ratio

Cambresy et al. 2002

Diffuse ISM Schlegel et al.

Cold Dust Polaris

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

FIR to mm dust emissivity in the diffuse ISM

(250m)/NHI(250m)/NHI

<NHI>

Boulanger et al. 1996

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)

FIR to mm emissivity in the diffuse ISM

Structure and temperature effect

Agladze et al. (1996)

Silicates amorphous crystalline

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