CS and HNCO Surveys of Massive Dense Cores

In recent years we have carried out extensive surveys of dense high-mass star-forming cores in CS and HNCO lines with the SEST 15m and OSO 20-m telescopes. From the CS maps and optically thin C34S emission we derive the basic physical parameters of the cores (size, LTE mass, mean density, virial mass) and obtain their statistical distributions. We analyse the dependence of these parameters on galactocentric distance. In particular, a galactic gradient in the mean density of the cores has been found. The CS and C34S probably indicate a small scale clumpiness in the cores. The HNCO results show the presence of very dense hot gas in the objects. HNCO abundances are found to be enhanced in high velodity gas. HNCO integrated line intensities correlate well with those of thermal SiO emission. This indicates a spatial coexistence of the two species and may hint at a common production mechanism, presumably based on shock chemistry.