Studies of the interstellar medium (ISM) address the diffuse gas and dust between stars, and their role in the life cycles of stars and of galaxies. Derived from the infall of gas from intergalactic space, the ISM is the substrate from which stars condense and into which stellar winds and supernovae expand. All aspects of galactic evolution -- the way galaxies appear from afar, the rate at which they form stars of various types, and the manner in which they intensify their internal magnetic fields -- are manifestations of how the ISM reacts to internal forces (e.g., supernovae) and to external forces (e.g., collisions between galaxies). Despite fifty years of progress, many open questions remain: for instance, how do newborn stars affect the gas clouds from which they formed? What is the mechanism by which dust grains align with the magnetic field? How can this effect be used to interpret observations of dense, cold molecular clouds? What are the basic dynamics and structure of these objects, from which stars form? In 2002, CITAzens have been addressing these questions with techniques ranging from fundamental physics, to analytical and numerical modeling of important phenomena, to the detailed investigation of individual regions, as outlined below.
This last year M.-A. Miville-Desch\^enes participated to several studies on dust evolution in the ISM using Infrared Space Observatory (ISO) and Spitzer data. It includes work done in collaboration with L. Pagani (Paris) where they discovered in L134N one of the coldest core found in molecular clouds (dust temperature ~ 7.6K). This work brings very interesting insight on the dust evolution scenario (formation of fractal aggregates) and really challenges dust theory. M.-A. Miville-Desch\^enes also used dust models and radiative transfer code to characterize dust properties in diffuse interstellar clouds using infrared observations. In particular he showed that the smallest dust grains (PAH-like) coagulate on bigger grains very rapidly in the gas condensation process. This has an impact on our understanding of the thermodynamical and chemical scenario of the formation of long-lived molecular structures. In collaboration with F. Boulanger (IAS, Orsay), M.-A. Miville-Desch\^enes also worked on the reprocessing and analysis of all the spectro-imagery (CVF) observations obtained with ISOCAM. This analysis allowed them to obtain the very general mid-infrared spectrum of the diffuse Galactic ISM dust emission.
The InfraRed Astronomical Satelite (IRAS) made a survey of the whole sky at 12, 25, 60 and 100 microns. This data set is very useful to study interstellar dust emission but it suffers from several instrumental problems (striping, zero level, responsivity variations) that can limit its use. In collaboration with G. Lagache (IAS, France), M.-A. Miville-Desch\^enes made a complete reprocessing of the IRAS data using the McKenzie cluster of CITA, improving significantly the quality of the data that can now be used as a template to estimate the dust emission contribution to the bands of the Planck satelite in the submm/mm.
A first result with these newly reprocessed IRAS data has already been obtained by combining 25 micron IRAS data and new 24 micron Sptizer data of a diffuse cloud. M.-A. Miville-Desch\^enes and J. Ingalls (IPAC, Pasadena) made a power spectrum analysis of these two data sets and showed for the first time a break in the power spectrum slope of interstellar emission. This break is interpreted as a geometrical effect (2D-3D transition) where the high angular resolution of the Spitzer data allows us to trace the structure of the skin of the cloud observed.
M.-A. Miville-Desch\^enes, G. Joncas (Laval University), F. Boulanger (IAS, France) and E. Falgarone (ENS, France), used 21 cm interferometric observations to determine the power spectrum of the 3D density and velocity field in a local HI cloud (the Ursa Major cirrus), over 3 orders of magnitude in scale. This work shows that interstellar turbulence has a similar power spectrum than incompressible turbulence (Kolmogorov). This result puts important constraints on the impact of compressibility and magnetic field in MHD turbulent fluids.
M.-A. Miville-Desch\^enes worked on the difficult problem of separating Galactic, extra-galactic and cosmological signals in diffuse regions of the sky (a problem known as ``component separation''). He is involved in several projects devoted to the observation of very low column density HI clouds, to characterize their properties in the far-infrared and in the sub-millimeter and to evaluate the possibility of using 21 cm observations to remove foreground emission. These observations are done with the DRAO (Penticton, Canada) and GBT (Virginia, USA) radio-telescopes, and with Spitzer. This work is done in collaboration with P. Martin (CITA), J. Lockman (NRAO) and the IAS team (France). In collaboration with J.-P. Bernard (CESR, Toulouse), M.-A. Miville-Desch\^enes also developed an all-sky model that allows to predict the emission from every foreground emissions in the Planck bands.
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