The distribution of CS and NH_3_ in star-forming regions.

The molecules CS and NH_3_ are expected on theoretical grounds to trace high density interstellar gas in a similar fashion. However, observations show that CS is often widespread while NH_3_ appears concentrated in dense cores. This result is interpreted here in terms of a clumpy model of molecular clouds, and a differential rate for chemistry of the two species. These CS cores are unresolved and numerous; most of them probably disperse rather than collapse to form a star.