Composition and Evolution of Interstellar Clouds

In this chapter we describe how elements have been and are still being formed in the galaxy and how they are transformed into the reservoir of materials present in protostellar environments. We discuss the global cycle of matter from stars, where nucleosynthesis produces heavy elements that are ejected through explosions and winds into the interstellar medium (ISM), through the formation and evolution of interstellar cloud material. In diffuse clouds, low-energy cosmic rays impact silicate grains, amorphizing crystals, and UV photons easily penetrate, sponsoring a simple photochemistry. In dense cold molecular clouds, cosmic rays penetrate, driving a chemistry where neutral-neutral reactions and ion-molecule reactions increase the complexity of molecules in icy grain mantles. In the coldest, densest prestellar cores within molecular clouds, all available heavy elements are depleted onto grains. Dense cores collapse to form protostars and the protostars heat the surrounding infalling matter and release molecules previously frozen in ices into the gas phase, sponsoring a rich gas-phase chemistry. Some material from the cold regions and from hot or warm cores within molecular clouds probably survives to be incorporated into the protoplanetary disks as interstellar matter. For diffuse clouds, for molecular clouds, and for dense hot cores and dense warm cores, the physiochemical processes that occur within the gas and solid state materials are discussed in detail.