Radiation transport and non--LTE analysis of interstellar molecular lines. II. Carbon monosulfide

We present numerical solutions to the coupled equations of radiative transfer and statistical equilibrium in the rotational transitions of carbon monosulfide (CS), similar to those in our earlier paper on carbon monoxide. Static, spherical, and microturbulent models are constructed over a wide range in density, abundance, and temperature, and the results are compared with observations of massive clouds near H II regions and of dark dust clouds and globules. The optical depths found for CS are quite small in all cases, typically 9 or approx. =2, and the J=1→0 line is found to be a weak maser in the rarer isotopic species in some denser clouds. No single value of the abundance fits all massive clouds, but typical central densities and relative abundances are 5 x 104< or =n/sub H/2< or =2 x 105, 2 x 10-10 < or =n/sub CS//n/sub H/2< or =2 x 10-9. Mean densities are a factor 5--10 lower because strong density gradients are required to reproduce the small spatial extent of the observed CS emission. The observations are dominated by instrumental effects, and it is difficult to determine whether source-to-source abundance variations actually exist. In the four globules which show emission in the two lowest transitions, n/sub CS//n/sub H/2approx. =5--10 x 10-10 and n/sub H/2approx. =5--20 x 103 cm-3. The observational results for L134 and the Taurus dark cloud are greatly discrepant and cannot be modeled for any reasonable choice of parameters