Galactic dust in the Herschel and Planck era. François Boulanger Institut d Astrophysique Spatiale

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1 Galactic dust in the Herschel and Planck era François Boulanger Institut d Astrophysique Spatiale

2 Motivation Dust emission Dust models Dust life cycle Planck early results Dust polarisation Outline

3 Dust is essential An actor of star formation Heating and cooling agent Photo-electric effect on dust are the main heating processes of HI and H2 gas penetrated by UV light Dust radiation keeps gas cold in contracting proto-stellar condensations. Gas coupling to the magnetic field The ionization balance of dust grains controls the gas ionization and thus the ambipolar diffusion timescale. Pre-stellar chemistry Molecular chemistry in proto-stellar cores is controled by chemistry on molecular ices formed on dust surfaces. It depends on the surface available for molecule sticking.

4 A tracer of star formation Dust emission as a calorimeter Dust is routinely used to trace the early steps of star formation embedded in dense molecular clouds with high visible extinction. A gas tracer Dust extinction & emission trace the structuration of interstellar matter from the diffuse interstellar medium to the proto-stellar cores independently of the gas composition An interstellar compass Dust emission and extinction is polarized. The polarization indicates the local magnetic field direction. Much of the analysis of the Herschel and Planck data involves dust

5 DUST EMISSION

6 Planck 857 GHz IRAS Miville Deschênes et al Herschel

7 Dust Emission Herschel Spitzer Planck 8 µm DUSTEM Model Large Grains > 10 nm PAHs? VSGs (nano carbon grains)? Carbon + Silicates

8 Early Planck Paper on spinning dust emission Perseus MC Anomalous dust is an additional evidence of the existence of molecular-size dust particles The emission intensity and SED are critically dependent on the size distribution of PAHs, and the excitation conditions (radiation field & gas density) Oph PDR W Data will allow to characterize variations in SEDs and intensity with phsyical conditions Planck Collaboration 11

9 DUST MODELS

10 Dust Models (I) Amorphous silicates and carbon grains (PAHs, nano-grains, amophous carbon) In the diffuse ISM dust holds about half of the mass in heavy elements (dust/h mass ratio 1%). A few elements (O, C, Fe, Si, Mg) contribute most of the dust mass Model parameters for size distribution and dust-to-gas mass ratio Parameters fitted on observations of dust in the diffuse ISM: element depletions, extinction curve, scattering properties and dust emission Existing models (Cloudy, Draine & Li, DUSTEM, Zubko+ 04) are very similar in their assumptions Dust models tie observations together across the electro-magnetic spectrum and interstellar environments

11 Dust Models (II) Difficulties Fitting of observations does not yield a unique solution Laboratory analogs do not provide the actual properties of interstellar dust Dust evolution Modeling is a learning process. Comparison with data contributes to define the properties of interstellar dust Herschel and Planck data are key to constrain emission properties at far- IR to mm wavelengths.

12 How to model dust emission The Dust SED depends on the intensity of the radiation field, and the dust composition and size distribution. Data interpretation must be supported by a model. Model inputs: Local radiation field Abundances and size distribution of dust components Dust optical and physical properties Model outputs Dust spectral energy distribution Extinction curve. The DUSTEM and Cloudy codes can be coupled to a PDR code to model dust and gas emission in a consistent way. Radiative transfer calculation needed to model molecular clouds

13 DUSTEM +PDR Models Dust spectral energy distribution versus radiation field intensity from 0.1 to 10 4 the Solar Neighborhood field Compiegne +11 Dust spectral energy distribution and temperature as a function of depth into a Av=50 cloud PhD Manuel Gonzalez Garcia G=10 4

14 Modeling needed to interpret variations of SED across PDR and disentangle excitation, dust properties & cloud structure

15 DUST LIFE CYCLE EVIDENCE & PROCESSES

16 Dust Life-Cycle Turbulence & Gravity Diffuse ISM Dust destruction Grain shattering & sputtering SF Molecular Clouds Dust Growth Mantle formation & Grain coagulation Star dust renewal τ R ~ several 10 9 yrs Radiation SN explosions WindsDust grinding HII PDR Massive Star Forming Regions Star Formation τ SF ~ 10 9 yrs

17 Dust Evolution Processes Guillard 2009 (PhD)

18 Element Depletions The depletion factor (effective value averaged over all elements) is observed to increase with the mean gas density and H 2 gas fraction (Jenkins 2009). Measured dust-to-h mass ratio range from 0.4 (Warm gas) to 1% (Diffuse molecular clouds). This result suggests that refractory elements accrete on grains in the cold neutral medium, and are sputtered out of grains in the warm teneous gas phases.

19 Dust evolution in translucent clouds Spitzer observations: low extinction area in Taurus (Flagey et al. 2009) Modeling work by Vincent Guillet (IAS) What is the evolutionary timescale? Start End Start End

20 Dark Cores Pagani+ 10 Scattering in the near-ir is evidence for growth of dust grains to µm size grains Additional evidence from variations in near-ir extinction curve and the strength of the 9.7 µm silicate feature (Flaherty +07, Chiar +11) Potentially a significant impact on dust far-ir/mm dust emissivity (Ormel +11)

21 Massive star forming regions Spitzer 4, 8 & 24 µm Flagey+10 The Eagle nebula Herschel PACS 70 µm Billot (NHSC, PACS private communication)

22 Global modeling Pioneering work by Dwek (1998) and Zhukovska et al. (2008) applied to the Milky Way. Dust cycles between diffuse ISM where it is destroyed and Molecular clouds where it is rebuilt on time scales much shorter than dust renewal time by stars Dust-to-Gas ratio and size distribution depends on the relative values of destruction, growth, and cycling timescales Interstellar dust is molecular dust

23 Is interstellar dust molecular cloud dust? Critical review of this disputed concept (Jones & Nuth 2011) Main Difficulties Some stellar dust is identified in meteroites How to form silicate grains in the ISM separate from carbon dust? Possible solutions The ISM structure and turbulence may make dust destruction not as effective as computed in global models ISM cycling may mainly impact carbon dust

24 PLANCK EARLY RESULTS INFRARED CIRRUS & MOLECULAR CLOUDS

25 Planck Collaboration 2011 Slides prepared by Marc-Antoine Miville Deschênes

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27 Residual emission traces H2 gas

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31 E is the bolometric dust emission In the local ISM, the variations of the dust temperature seem to be due to changes in dust emissivity In IVCs, evidence for grain shattering, and HVCs of a lower dust-to-gas mass ratio

32 Taurus Molecular Cloud Fit with a grey-body, IRAS 100µm, HFI 857, 550 & 353 GHz: Temperature map Planck collaboration CO, 12CO Spectral index & no CO: same but increasing Tdust

33 The -Tdust relation Taurus Molecular Cloud Archeops Desert+08 All pixels PRONAOS Dupac+03 Many publications but still no clear evidence that it is measuring an intrinsic property of dust. Necessary to take into account noise (Shetty et al. 2009a) and temperature variations along the line of sight (Shetty et al. 2009b) Taurus pixels with 12 CO Dust opacity depends on whih frequency are being used to do the fit. There is a lack of consistency in published studies Modelling is required to discuss results (Schnee et al. 2010)

34 Far-IR/sub-mm dust emissivity Diffuse ISM Models DUSTEM Present dust models are fitted on diffuse ISM SED from COBE Opacity law is close to -2 sil > carbon What about molecular clouds and prestellar cores? What is the impact of dust evolution? Taurus Molecular Cloud Draine & Li 07 Comparison of Planck based opacity with near-ir reddening yields an emissivity a factor 2 larger than that measured in the diffuse ISM with the same method (Planck Collaboration 11) Does it result from grain growth?

35 DUST POLARISATION

36 Coherent magnetic field along the molecular cloud filament. Musca Dark Cloud Peyrera & Magalhaes 2003

37 Polarisation Observations A rapidly growing body of stellar extinction data Sub-mm ground based data from SCUBA Balloon experiments (BLASTPOL & PILOT) will soon be providing data in the far-ir Planck is providing the first sky map of dust polarized emission (frequencies 353, 217, 143 & 100 GHz) Publication of first Planck papers on dust polarization is planned for 2012

38 Science Topics Dust polarisation properties Input data towards a Planck model of dust polarisation Which dust components are polarised? To what degree? Does the polarisation properties depend on local physical conditions, via the grain alignment dependence on the radiation field & the gas density or due to dust evolution? Magnetic field structure Large scale magnetic field as traced by dust. Comparison with what is known from synchrotron data. Turbulent component within interstellar matter. Interplay with gas turbulence and density structure, from the diffuse ISM to cold clumps

39 Summary Herschel and Planck have opened a new era for dust studies. Early results extend evidence for dust evolution to sub-mm dust properties of large dust grains Data modeling is required to interpret the observations Modeling tools exist, but present models do not provide a straightforward account of what is observed Most of the data analysis is still empirical (grey body fits). This is a necessary step but be careful to caveats Upcoming polarization data are about to open a new dimension in our understanding of Galactic dust