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Analysis of spectral observations of interstellar dark clouds requires the conversion of the observed quantities of column density and spectral line velocity profile into physical quantities such as density, temperature, abundance, and velocity field. A Monte Carlo radiative transfer code has been developed to model molecular emission from interstellar dark clouds. This model is based on code from Bernes (1979, AA, 73, 67) and has been revised to include emission from molecules other that CO. This revised code allows modeling of density, molecular abundance, kinetic temperature, and velocity field as a function of radius in a spherical cloud for emission from linear molecules with calculated collisional cross sections such as CO, CS, HCN, HC$_{3}$N, N$_{2}$H$^{+}$, and OCS.
Modeled spectra of CS (J=2-1) emission are compared with observations of the dark cloud L134N (Swade 1989, ApJS, 71, 219). In this case observed spectra can be closely simulated by a cloud model with an exponentially decreasing density distribution (n$_{max}$ = 3x10$^{4}$ cm$^{-3}$), constant kinetic temperature (12 K), constant CS fractional abundance (3x10$^{-9}$), and a static velocity field.
In addition, theoretical line profiles for CS are generated using a constant kinetic temperature while varying the density distribution, molecular abundance gradient, and velocity field within a spherical dark cloud. These simulations show that self-absorption occurs in low level CS transitions under most conditions where there is significant central density and molecular abundance regardless of the density distribution. Variations in the velocity field are used to generate emission line profile asymmetries.
