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 the chemical composition and evolution of three giant cloud cores in Orion A, M17, and Cepheus A. This study is the culmination of a chemical survey of 32 transitions of 20 different molecules and isotopic variants in these cloud cores. Using these data, combined with observationally derived physical conditions, chemical abundances were calculated for several positions in each cloud. A global analysis of the molecular abundances shows that, although abundance differences exist, the chemical composition of giant cloud cores is remarkably homogeneous. This agreement suggests that the chemical evolution of the individual giant cloud cores is not unique. The molecular abundances of giant cloud cores are also systematically lower than those observed in the more quiescent dark cloud core TMC-1. A one-dimensional chemical model is presented that examines internal chemical structure induced by a radiation field enhanced by a factor of 103-105 above the normal interstellar radiation field. This model integrates the abundances of the various species as a function of depth, producing column densities that can be compared with observations. The one-dimensional model is unable to reproduce the abundances of many molecules for any single time. Two assumptions 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 weight placed on the photon-dominated region within smaller clumps, clumps have a detrimental effect on reproducing the abundances of other species. Models with a range of C/O ratios are also compared with the measured abundances. Good agreement between this model and the observations at two positions with disparate physical properties is found for early times (t ~ 105 yr) and for C/O increased to ~0.8. We suggest that one possible interpretation of these results is that the cores are dynamically evolving objects. Either giant cloud cores are intrinsically young objects or the dense material is effectively young by virtue of a complex interchange of material between the clumps and the interclump medium. We suggest that the CS/SO ratio can be used to probe the evolutionary state of and the initial C/O ratio in dense molecular clouds.