Wooden et al.: Composition and Evolution of Interstellar Clouds 33 33 Composition and Evolution of Interstellar Clouds D. H. Wooden NASA Ames Research Center S. B. Charnley NASA Ames Research Center P. Ehrenfreund Leiden Observatory 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 chem- istry where neutral-neutral reactions and ion-molecule reactions increase the complexity of mole- cules 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 incorpor- ated 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. 1. GALACTIC INTERSTELLAR MEDIUM 1.1. Overview: Cycle of Matter from Stars through the Interstellar Medium to the Solar System As with our Sun, new stars form in the dense cores of quiescent cold molecular clouds (Boss, 2004) from interstel- lar materials. To a minor extent, pre-main-sequence stars re- plenish the interstellar medium (ISM) with material through their bipolar outflows and jets. The ISM primarily becomes enriched with nucleosynthesized “metals”, i.e., elements heavier than H and He, through supernovae (SNe) explo- sions of high-mass stars (SNe type II) and of primary stars in a low-mass binary systems (SNe type I). Other sources of enrichment include the massive winds of low-mass asymp- totic giant branch (AGB) stars and novae (e.g., Jones, 2001; Chiappini et al., 2003; Wheeler et al., 1989). Supernovae explosions (McKee and Ostriker, 1977), the UV photons from massive O and B stars (Wolfire et al., 2003), and, to a lesser extent, AGB stellar winds inject energy into the ISM. This energy is deposited in shocks and generates turbulence that acts on many different length scales to bring together, compress, and even shear apart enhancements in the inter- stellar gas density. Turbulent energy is degraded efficiently into thermal energy when turbulence is concentrated in small volumes such as in shocks and in small intermittent regions of velocity shear where viscous dissipation occurs (Vázquez- Semandeni et al., 2000). In the ISM, gas is processed rapidly through a wide range of temperatures, densities, and ioniza- tion stages, as given in Table 1, under the influence of turbu- lent and thermal processes, pressure gradients, and magnetic and gravitational forces. Interstellar clouds comprise defin- able structures in the ISM, but only represent two of five ISM components (section 1.2). Stars enrich the ISM (sec- tion 2) with the gas and dust that eventually contributes to the formation of new star systems after cooling and passing through interstellar cloud phases. Processes that contribute to increasing the complexity of solid-state and molecular materials are introduced by Irvine and Lunine (2004) and discussed here in detail; these processes primarily occur in interstellar clouds, i.e., in diffuse clouds (section 3) and mo- lecular clouds (section 4), at low temperatures (≤100 K). In interstellar clouds, molecules and solid-state materials are more protected from the destruction mechanisms — UV irra- diation, cosmic rays, fast electrons — prevalent in the highly energetic intercloud environment of the ISM of the galaxy. In the dense, hot high-mass and warm low-mass protostel- lar cores (section 5) that only comprise tiny fractions of the mass of molecular clouds, a rich gas-phase chemistry occurs that increases the complexity of the materials infalling onto