2 Dust processing in the ISM

2.1 Dust evolution

More than any previous infrared space mission, SIRTF will greatly enhance our understanding of the life-cycle of dust in the ISM. The ISM encompasses a huge range of environments that can differ in their physical parameters (e.g. density, temperature and radiation field) by many orders of magnitude. Interstellar environments include: (1) the hot tenuous gas that occupies most of the Galactic volume, (2) the Warm ISM (WISM), both neutral and ionized, with densities of a few 0.1 H/cm^3, (3) the Cold ISM (CISM) including diffuse interstellar clouds (Av < 0.1), the translucent cirrus (Av < 1) and the poorly shielded regions of molecular clouds (Av < 3), and (4) dense and opaque molecular cloud cores (Av > 3). On galactic scales most of the ISM mass is in the CISM and the WISM. Som,e of the structures in the CISM are gravitationally bound while others are more transient. The SEDI observations will therefore focus on the dust evolution within all these components.

Figure 1.
Comparison between the sensitivity of the SIRTF SEDI observations and the average ISM spectrum normalized to a column density of 4x10E20 H/cm^2 (Av=0.2). The ISOCAM and FIRAS spectra are shown as black curves and the DIRBE broad band measurements as black triangles. The sensitivities achieved using the AORs described in Sect.6 are shown at the full instrument resolution (upper boundary) and at the 160 micron MIPS resolution (38 arcsec: lower boundary). The IRS and MIPS SED sensitivities are shown by the red curve and blue line respectively. The range of known variations in observed SEDs is illustrated by the spectrum of cold dust from a cirrus cloud in Polaris (blue) and of a bright mid-IR halo cloud in Chamaeleon (green) scaled to Av=0.2.

After being released from the outflows of evolved stars, dust is subject to processing in the ISM through gas-grain, grain-grain and photon-grain interactions. The degree and nature of the processing depends on the rate and the energy of these interactions both of which are related to the density structure and dynamics of the ISM. Dust grains can decouple from the gas motions where their friction time (time for a grain to have collided with its own mass of gas) is larger than the timescales over which the gas velocity changes significantly. This applies to shocks in the WISM where grains do not follow the immediate velocity change of the gas but also to turbulent flows within the CISM.

Interstellar turbulence, because it is supersonic, generates a myriad of shocks and localized intense vortices which might affect the dust evolution more frequently and more deeply on average than the faster supernovae shock waves. It also continuously cycles dust grains through a variety of physical conditions triggering one or more of the following processes. High energy gas-grain collisions lead to the erosion of some of the dust mass into the gas phase by sputtering, while low energy collisions lead to the reverse process of gas accretion onto dust. For grain-grain collisions above some velocity threshold the dust is shattered into smaller fragments, while at lower velocities coagulation occurs. Finally, UV photons can alter dust by inducing photon-driven physical and chemical changes. As illustrated in Fig. 2, the physical processes acting on dust grains affect the dust size distribution and SED in specific ways. The SEDI observations will identify the processes regulating dust evolution and indicate where and on what size scales they are active.

Figure 2.
The impact of various physical processes on the SIRTF dust SEDs. The dust size distributions in the left panel have been computed using shock (green 50 km/s, red 200 km/s), accretion (purple) and coagulation (blue) models starting from a standard power law distribution n(a)~a^-3.5 (black). The resulting SEDs observed by SIRTF are shown on the right panel and have been computed using the model of Desert et al. (1990).

Current observations characterize only disconnected pieces of the puzzle of the evolution of dust in the diffuse ISM. Examples include: (1) The large variations in the elemental depletions observed with gas density (Savage & Sembach 1996) are evidence for the effectiveness of dust erosion in the WISM and accretion in the CISM. (2) The IR emission from dense molecular clouds (cold spectrum in Fig. 1) is characterized by the absence of small grain emission and lower large grain temperatures than in the atomic cirrus. These characteristics can be accounted for by the coagulation of small grains on and into larger particles, in agreement with the interpretation of the variations in the extinction curves in diffuse and molecular clouds. (3) The IRAS and ISO data provide evidence for the role of stellar photons in the dust processing in H II regions and Photo-Dissociation Regions (PDRs).

A major goal of the SEDI program is to provide a full picture of the dust evolution in the ISM, from just after dust formation in stellar outflows until incorporation into the dense regions of proto-stellar cores. The SEDI program can achieve this goal thanks to the unique characteristics of SIRTF, namely, the combination of wavelength coverage, brightness sensitivity, angular resolution and mapping efficiency. The physical processes acting on dust, mentioned above, leave specific signatures on the dust size distribution which SIRTF can trace thanks to its wavelength coverage (Fig. 2). High sensitivity is essential to observe dust in diffuse clouds and the WISM. The gain in angular resolution provided by SIRTF will allow us to probe a new range of size-scales in dust evolution. The SIRTF mapping capabilities open up a unique opportunity, within a Legacy Program, to sample the full variety of interstellar environments contributing to the Galactic infrared emission.

2.2 The SEDI perspective

To meet the goals of the SEDI program we have selected 16 fields in the nearby ISM (Table 1) in well characterized regions encompassing the full diversity of diffuse interstellar media through which the dust cycles. This selection takes into account the facts that: (1) SIRTF data towards extremely low surface brightness cirrus (Av <0.1) will be a by-product of cosmology surveys; (2) SIRTF data towards star-forming molecular clouds will be available from observations of key regions such as Ophiucus and Orion which are planned in the guaranteed time. This ensemble of data will provide the first complete and unbiased infrared view of dust emission properties over the ISM. In the following, we detail the questions which we will address in relation to the list of sources in Table 1.

$\bgroup\color{bluesedi}$\triangleright$\egroup$ Shock-processed dust

Supernova-generated shock waves are thought to dominate the overall evolution of interstellar dust in the WISM. The effects of shock waves on the interstellar dust size distribution are primarily the results of sputtering and shattering (see Fig.2). SIRTF has the brightness sensitivity to provide the first observational evidence of the impact of supernovae shocks on the evolution of dust.

Our list of sources (Table 1) includes two fields specifically dedicated to characterizing the accumulated effects of supernovae shocks on the dust size distribution in the WISM. One field will cover dust in the warm gas, at both the local and intermediate velocities, seen in the direction of the halo star HD 93521 (Spitzer & Fitzpatrick 1993), and a second will look at the low density ionized gas heated by the nearby star Spica (Reynolds 1985). For both regions depletion measurements derived from absorption spectroscopy show that significant dust erosion has occurred. Three other fields will probe the effects of shocks on the dust as a function of shock speed and gas density, these include: two intermediate velocity clouds of different densities, as inferred from the ratio of atomic to molecular gas, which show morphological signs of interaction with the local ISM, and a field on the Cygnus Loop where dust is being processed in a fast shock (v~200 km/s) propagating through the diffuse ISM. For this last source second look spectroscopic observations will be used to estimate the contribution of atomic lines to the emission observed in the broad bands.

$\bgroup\color{bluesedi}$\triangleright$\egroup$ Dust evolution in the CISM from cirrus to molecular clouds

The evolution of dust in the CISM, which is thought to result from accretion, coagulation and shattering processes, depends on the density structure, the amplitude of turbulent motions and the variations in the radiation field related to extinction. The accretion of molecules onto grain surfaces is known to occur in opaque molecular clouds. It can also occur in the more diffuse and transparent media provided that the impinging atoms can chemically bind to the grain surfaces. Both the processes of coagulation and shattering are influenced by turbulence. For coagulation, the turbulence modifies and accelerates its effect on the dust size distribution and the outcome depends on the relative velocities of the grains as a function of size. Through the effects of intermittency (a term which refers to the existence of local velocity gradients and vorticity which are significantly larger than the average), small grains may acquire sufficiently large relative velocities to make their mutual coagulation the dominant process in the early evolution of the dust size distribution. This result is opposite to the classical picture, where the large grains move faster than the smaller ones resulting in a gradual removal of all small grains during coagulation (Fig. 2 and Völk et al. 1980). Turbulence could also be energetic enough to drive grain shattering in collisions, an efficient mechanism for the formation of small particles.This possibility have recently received strong support from mid-IR ISOCAM images of an atomic cirrus cloud which show that the small grains emission coincide spatially with a velocity component of the H I emission which exhibits a large vorticity (Fig. 3). The angular resolution provided by SIRTF is critical to identify intermediate steps in dust evolution. The well-known line width-size scaling law established with CO and H I observations characterizes turbulent mixing in the ISM. It relates in a statistical sense physical sizes to time-scales. For nearby interstellar matter, the SIRTF angular resolution from a few to 38 arcsec translates into physical scales of ~0.01 pc. Turbulent mixing on these scales is ~10E5 years. This is commensurate with the time-scales of coagulation and accretion for densities of 10E4 H/cm^3.

Figure 3.
Comparison of HII, PAH mid-infrared (5-8.5 microns ISOCAM filter) emission and IRAS 100 micron data of the Ursa Major cirrus cloud (Miville-Deschenes et al. 2000) showing that the small grain emission coincides spatially with a velocity component of the HI emission corresponding to a large vorticity.

The existence of SED variations on angular scales smaller than the IRAS resolution is illustrated by ISOPHOT observations of a cirrus knot in Fig. 4. These observations show that the spectrum observed at the highest angular resolution is intermediate between the spectra of the mean high latitude dust and cold dust (shown in Fig. 1). We may thus be witnessing an intermediate step in the evolution of dust. The SEDI database will provide much broader statistics on SED variations as a function of angular scales to find many more examples of on-going dust processing in transition regions. The identification of such intermediate steps in the processing of dust is essential in order to follow the effects of turbulence on dust and, in particular, on the coagulation of grains.

The source list in Table 1 covers the full diversity of cloud parameters from transparent cirrus to dark clouds, encompassing the full range of interstellar densities (HI/H2 transition), column densities (extinction), masses (from gravitationally bound to unbound more transient strcutures) and turbulent energies. They sample the CISM through its evolution from transparent cirrus to giant molecular clouds.

Figure 4.
The SED of a cirrus knot taken from the IRAS Point Source Catalog: ISOPHOT observations (filled circles), IRAS ISSA maps (filled triangles), and IRAS ADDSCAN (highest resolution achievable with IRAS, open circles). The effect of observing at different spatial resolutions is seen at 60 micron: the highest spatial resolution (ISOPHOT) yields the largest emission whereas the 5' resolution ISSA maps gives the lowest emission and the intermediate emission is given by the ADDSCAN data.

$\bgroup\color{bluesedi}$\triangleright$\egroup$ Photo-Dissociation Regions

PDRs and H II regions are ideally suited targets to study the role of photons in the evolution of dust. IRAS and ISO observations have shown that in PDRs, including the surfaces of molecular clouds heated by the general Galactic interstellar radiation field, the abundance of PAHs and very small grains are often observed to be enhanced with respect to the mean values in the diffuse ISM (Bernard et al. 1993). ISOCAM observations of PDRs have spatially resolved changes in the PAH band intensity ratios and the band-to-continuum contrast. These observations, which indicate an evolution of the cold dust in the dense molecular gas as it cycles back into the diffuse atomic gas, leave many questions open. For example: What process regenerates small particles in PDRs? Is it the disaggregation of coagulated particles? If so, is it driven by grain-grain collisions or photons? Alternatively, do the observations point to a decoupling between the gas and dust arising from the anisotropic interaction of photons with grains (Weingartner & Draine 1999)?

For the PDRs excited by ionizing stars, these observations will elucidate the evolution of dust within H II regions. In intense radiation fields close to hot stars the IRAS images and ISO spectra show that PAHs are destroyed while the mid-IR emission of VSGs is enhanced (Cox & Roelfsema 1999). The proposed SEDI observations will address these questions by spatially resolving variations in the size distribution and large grain emission properties across PDRs. The PDRs and H II regions in our list of sources (Table 1) sample a broad range of excitation conditions and density. For some sources the exciting star is a run-away which provides a mean to test evolutionary time-scales.

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