CHEMICAL EVOLUTION OF STAR-FORMING REGIONS

Recent advances in the understanding of the chemical processes that oc cur during all stages of the formation of stars, from the collapse of molecular clouds to the assemblage of icy planetesimals in protoplanet ary accretion disks, are reviewed. Observational studies of the circum stellar material within 100-10,000 AU of the young star with (sub)mill imeter single-dish telescopes, millimeter interferometers, and ground- based as well as space-borne infrared observatories have only become p ossible within the past few years. Results are compared with detailed chemical models that emphasize the coupling of gas-phase and grain-sur face chemistry. Molecules that are particularly sensitive to different routes of formation and that may be useful in distinguishing between a variety of environments and histories are outlined. In the cold, low -density prestellar cores, radicals and long unsaturated carbon chains are enhanced. During the cold collapse phase, most species freeze out onto the grains in the high-density inner region. Once young stars ig nite, their surroundings are heated through radiation and/or shocks, w hereupon new chemical characteristics appear. Evaporation of ices driv es, a ''hot core'' chemistry rich in organic molecules, whereas shocks propagating through the dense envelope release both refractory and vo latile grain material, resulting in prominent SiO, OH, and H2O emissio n. The role of future instrumentation in further developing these chem ical and temporal diagnostics is discussed.