THE CHEMICAL-COMPOSITION AND EVOLUTION OF GIANT MOLECULAR CLOUD CORES- A COMPARISON OF OBSERVATION AND THEORY

We present the results of an observational and theoretical study of th e chemical composition and evolution of three giant cloud cores in Ori on A, M17, and Cepheus A. This study is the culmination of a chemical survey of 32 transitions of 20 different molecules and isotopic varian ts in these cloud cores. Using these data, combined with observational ly derived physical conditions, chemical abundances were calculated fo r several positions in each cloud. A global analysis of the molecular abundances shows that, although abundance differences exist, the chemi cal composition of giant cloud cores is remarkably homogeneous. This a greement suggests that the chemical evolution of the individual giant cloud cores is not unique. The molecular abundances of giant cloud cor es are also systematically lower than those observed in the more quies cent dark cloud core TMC-1. A one-dimensional chemical model is presen ted that examines internal chemical structure induced by a radiation f ield enhanced by a factor of 10(3)-10(5) above the normal interstellar radiation held. This model integrates the abundances of the various s pecies as a function of depth, producing column densities that can be compared with observations. The one-dimensional model is unable to rep roduce the abundances of many molecules for any single time. Two assum ptions have been investigated to improve the agreement between theory and observations. These are adding clumps and raising the initial C/O ratio. We find that the inclusion of clumps in the chemical model can reproduce the abundance of C and C+. However, because of the greater w eight placed on the photon-dominated region within smaller clumps, clu mps have a detrimental effect on reproducing the abundances of other s pecies. Models with a range of C/O ratios are also compared with the m easured abundances. Good agreement between this model and the observat ions at two positions with disparate physical properties is found for early times (t similar to 10(5) yr) and for C/O increased to similar t o 0.8. We suggest that one possible interpretation of these results is that the cores are dynamically evolving objects. Either giant cloud c ores are intrinsically young objects or the dense material is effectiv ely young by virtue of a complex interchange of material between the c lumps and the interclump medium. We suggest that the CS/SO ratio can b e used to probe the evolutionary state of and the initial C/O ratio in dense molecular clouds.