13CO, C18O, and CS observations toward massive dense cores

We present results of (CO)-C-13 J = 1-0, (CO)-O-18 J = 1-0, and CS J = 2-1 map observations toward 10 massive star-forming regions selected from the previous CO line survey of cold IRAS sources along the Galactic plane. CS line emission was detected from 9 out of 10 sources, and (CO)-C-13 line emission was detected in all sources. (CO)-O-18 J = 1-0 line emission was detected in only three sources. Physical parameters are derived for the detected sources. The (CO)-C-13 and (CO)-O-18 data support the idea that gradual dissipation of the internal turbulence leads to formation of denser cores and subsequent star formation. There is no apparent relationship between the CS abundance and the CO temperature. The ratios of L-IR to M-LTE vary by no more than a factor of 3 for the CS cores and by nearly 1 order of magnitude for the (CO)-C-13 cores. This indicates that star formation is determined by the gas with high volume density and does not depend on the mass of the cloud. We find that the star formation rate per unit mass is nearly constant in the dense cores. Divided into two subgroups, "low" and "high," by their FIR color indexes, the low sources are in an earlier stage than the high sources, based on the high sources' association with H2O masers.