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We present the results of a systematic study of the chemical composition of three giant molecular cloud cores, OMC-1, M17, and Cepheus A. The observations were obtained using the FCRAO 15 element array to map the emission of 21 molecules and isotopic variants in each source. The maps of molecular emission exhibit both striking similarities and differences between species. To ascertain if the differences in the distribution of emission are due to abundance variations, we have developed a model of the physical conditions that determine the molecular excitation.
We have utilized optically thin emission from the symmetric top molecule CH$_{3}$C$_{2}$H to determine the kinetic temperature. The velocities and line widths of the methyl acetylene emission are similar to those of the high density tracer HC$_{3}$N, confirming that the CH$_{3}$C$_{2}$H emission arises from the dense gas. Temperatures found are 20 - 45 K, considerably less than implied by CO emission. To estimate the molecular hydrogen density, four transitions of HC$_{3}$N with J$_{u}$ between 4 and 16 have been observed with similar beam sizes. Using the temperatures independently determined from methyl acetylene, the observed HC$_{3}$N transitions were fitted with an excitation model to determine the distribution of n(H$_{2}$), with resulting densities of 10$^{5}$ cm$^{-3}$ to $>$ 10$^{6}$ cm$^{-3}$.
These values of density and temperature have been used to model the molecular excitation conditions along the Orion ridge and derive accurate column densities. Correcting for changes in the physical conditions does account for some of the relative differences in emission. However, several species still exhibit abundance variations. The results of a time-dependent chemical model are compared with the observed abundances. Since the chemical reaction rates are intimately linked to the physical conditions, the derived values of density and temperature are also critical inputs to the chemical model. This research is a systematic effort to examine the extent to which gas phase chemical models can account for the observed variations and, in cases where it fails, to examine other possibilities.
