The ISM reflects the global history of a galaxy (e.g. interaction) as well as local phenomena (e.g. star formation, radiation field). As such it links the global galactic evolution with the localized stellar evolution. An understanding of the formation and evolution of galaxies as well as the formation of stars provides insight into the physics and chemistry of the ISM. The complex structure of the ISM is a consequence of the fact that the ISM is an open system that tries to balance itself according to the physical and chemical conditions that prevail locally. Since most of the ISM is cold or, in the presence of a radiation field, warm, most of its continuum radiation and spectral line emission emerges in the MIR, FIR, and Submm. SOFIA is thus the only observatory ideally suited to study these processes.

SOFIA will allow us, to understand the chemical fractionation of matter within the galaxy. Photon dominated regions (PDRs) are the stages for the dynamical interaction of star formation and its radiation field with the ISM. The molecular clouds absorb part of the radiation field, and produce a diversity of species that only exist in a well-defined local equilibrium. The equilibrium mainly depends on the distance from the radiation source (mostly a hot star) and its radiated spectrum, and on the density, temperature, chemical composition, and geometry of the molecular cloud. The diffuse interstellar radiation field plays the dominant role in the chemistry of more isolated MCs. High resolution heterodyne spectroscopy will be used to obtain spectral maps of molecules (e.g. HD, H₂O, CO, HCN), atoms (e.g. H, C, O), ions (H⁺, C⁺, Ne⁺), and radicals (e.g. HCO⁺, CO⁺, OH⁺) in molecular cloud regions.

Massive stars dominate their environment by their strong and energetic radiation field. At later stages of their evolution, stellar matter is fed back into the interstellar region by mass loss of high mass stars (e.g. LBVs), stellar winds of evolved AGB stars, planetary nebulae, stellar jets, and violent supernova (SN) events. These stars eject processed, i.e. metal enriched, material back into space. The interaction of this material with the surrounding interstellar matter (ISM) leads to shock fronts which heat and destroy the dust and excite characteristic spectral lines. The study of the composition of the reprocessed material and its interaction with the ISM will strongly impact on our understanding of the chemistry and dynamics of the ISM as well as on the galactic chemical evolution.

The content of this page was excerpted from a PDF document "SOFIA Astronomy and Technology in the 21st Century" by Alfred Krabbe and Hans-Peter Roeser.

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