3 ISM structure and evolutionThe IRAS images played an important role in establishing our present perception of the ISM by revealing its intricate morphology, similar to that of cirrus clouds in the Earth's atmosphere. They guided many follow-up molecular and H I observations probing the kinematics and density structure of nearby cirrus and molecular clouds. Similarly, the combination of the SIRTF, H I and CO observations put together in the SEDI Legacy Program will form the basis for many extensive investigations of the ISM by the astronomical community.
3.1 ISM StructureAmong the various observational means of imaging the structure of interstellar matter, observations of the IR dust emission remain unique for tracing interstellar matter over a wide range of conditions and, in particular, across the key HI to H2 chemical transition where neither HI nor molecular lines are good tracers (Boulanger et al. 1998). Three of the SIRTF bands, namely the 8 micron IRAC band and the 70 and 160 micron MIPS bands, are particularly suited to trace interstellar matter due to their sensitivity to column densities as low as 10E20 H/cm^2. These bands and their color combinations will trace the spatial distribution of specific ISM components. For example, the combination of the 70 and 160 micron MIPS maps allow us to trace UV-shielded dense gas, including dense cores, with an efficiency much greater than that achievable with present millimeter and submillimeter telescopes. The 8 micron emission will trace diffuse matter distributed over regions with small extinction (A<1), with an angular resolution of 5 arcsec. The diffuse ISM has never been observed with such an angular resolution and over such a large dynamic range of size-scales (from the SIRTF angular resolution to the large scale probed by DIRBE). The SEDI images will constitute a unique dataset on the structure of the diffuse ISM which can be used to search for signatures of extreme physical conditions. Spatial differences in gas velocity and in dust emission are observed to follow non-Gaussian statistics (Abergel et al. 1996, Lis et al. 1996). These non-Gaussian tails are associated with localized but scattered regions in the maps. The study of these specific regions, which contribute to the non-Gaussian statistics of the velocity field and/or the dust distribution, is of general importance for ISM studies because the physical and chemical processes triggered in these regions may have lasting and observable consequences for the gas and dust properties. The SEDI maps will be used to identify and statistically characterize the regions, which contribute to the non-Gaussian tails in the brightness images, as a function of angular scale and environmental parameters (e.g., molecular/atomic gas, mean gas column density). Once identified these regions can be further studied using H I and molecular observations as probes of the density, chemistry and velocity structure of the CISM gas. 3.2 Physical processes involving dustSeveral processes couple dust to the chemical and thermodynamical evolution of the gas. First, in the space penetrated by stellar radiation, photo-electrons from small dust grains are the main heating source for the gas. Second, small dust particles, if charged, may in some environments be the main coupling agent between interstellar matter and the magnetic field. Third, chemical reactions on dust surfaces, e.g. H2 formation, are an essential part of interstellar chemistry. Fourth, by attenuating UV starlight dust contributes to the protection of molecules from photo-dissociation (molecular lines are the main cooling agents for dense gas). In all these processes, small dust particles play a key role. By providing a new view of their evolution throughout the ISM, the SEDI observations will contribute to a better understanding of their role in the physics and chemistry of the ISM. For example, the key assumption made in current models of the two-phase ISM is that small dust particles must be present in the WISM in order to account for its heating (Wolfire et al. 1995). This can be tested by the SEDI observations described in section 2.2. It will also be possible to quantify the photoelectric heating yield by correlating the SEDI observations of PDRs with measurements of gas cooling lines which can be observed with SIRTF ([O I] 63 micron, H2 rotational lines).
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