A (CO)-O-17 survey toward ultracompact HII regions

We have observed 16 regions of massive star formation in the J = 1-0, 2-1 r otational transitions of (CO)-O-17 and the J = 2-1 transition of (CO)-O-18 With the IRAM 30 m telescope. We have also observed the J = 2-1, 3-2 rotati onal transitions of (CO)-O-17 with the KOSMA 3 m telescope. The (CO)-O-17-e mitting clumps typically have sizes of 1 pc and average densities and tempe ratures of 10(5) cm(-3) and 25 K, respectively. Clump masses from 10(3) to 4 x 10(4) 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 valu es obtained using measurements of the millimeter/submillimeter dust continu um emission and find that the dust continuum masses tend to be higher by ab out 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 ra tio of "O-star mass" to gas mass is roughly 1% in most cases.