1994A&A...288..601Z

Eleven dense molecular clouds associated with Sharpless H II regions have been observed in the J=2-1 CS and C³⁴S lines with the 13.7-m radio telescope in Metsaehovi. In addition, we have observed a few clouds in the J=1-0 HCN line. Almost all of the clouds represent sites of massive star formation and have been studied earlier in some other molecular lines. We have compared the spatial structure of the sources as seen in the different lines and discuss the possible reasons for the similarities and differences between them. The CS emitting regions have been modelled using LVG and Monte Carlo radiative transfer codes. We have found that the CS column densities derived in the framework of the uniform cloud models are too low, in some cases probably by an order of magnitude. The most plausible physical explanations involve small scale clumpiness in the clouds and/or the presence of low-density molecular envelopes which shield partly the emission from the core in the CS main isotope line. However the HCN data do not support the hypothesis of a scattering envelope. The masses derived from the CS data are close to the virial masses for most sources. The corresponding average densities along the line of sight are rather low, n∼10³-10⁴ cm^–3. The densities found from the comparison with the CS J=7-6 data are 2-3 orders of magnitude higher. This implies strong density inhomogeneities in the cores. There are size-linewidth and size-density correlations similar to those found earlier for CO clouds and dense cores in dark and Sharpless clouds.