2000ApJ...536..393H

We have observed 16 regions of massive star formation in the J=1-0, 2-1 rotational transitions of C¹⁷O and the J=2-1 transition of C¹⁸O with the IRAM 30 m telescope. We have also observed the J=2-1, 3-2 rotational transitions of C¹⁷O with the KOSMA 3 m telescope. The C¹⁷O-emitting clumps typically have sizes of 1 pc and average densities and temperatures of 10⁵ cm^–3 and 25 K, respectively. Clump masses from 10³ to 4x10⁴ M_☉ are found, and most of the clumps are virialized in the sense that their gravitational energy is of the same order as bulk kinetic energy inferred from the line width. We also compare the derived masses with values obtained using measurements of the millimeter/submillimeter dust continuum emission and find that the dust continuum masses tend to be higher by about a factor of 2. We estimate the masses of early-type stars (more massive than 15 M_☉) embedded in the sources of our sample and conclude that the ratio of ``O-star mass'' to gas mass is roughly 1% in most cases.