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A Mapping Survey of Dense Clumps Associated with Embedded Clusters : Evolutionary Stages of Cluster-Forming Clumps

We have carried out a survey of the dense clumps associated with 14 embedded clusters in the C^18O (J=1-0) line emission with the Nobeyama 45m telescope in order to understand the formation and evolution of stellar clusters in dense clumps of molecular clouds. We have selected these clusters at distances from 0.3 to 2.1kpc and have mapped about 6' X 6' to 10' X 10'regions (corresponding to 3.8pc X 3.8pc at 2.1kpc) for all the clumps with 22" resolution (corresponding to Jeans length at 2.1kpc). We have obtained dense clumps with radii of 0.40-1.6pc, masses of 150-4600M_sun, and velocity widths in FWHM of 1.4-3.3kms^-1. Most of the clumps are found to be approximately in virial equilibrium, which implies that C^18O gas represents parental dense clumps for cluster formation. From the spatial relation between the distributions of clumps and clusters, we classified C^18O clumps into three types (Type A, B, and C). The C^18O clumps as classified into Type A have emission distributions with a single peak at the stellar clusters and higher brightness contrast than that of other target sources. Type B clumps have double or triple peaks which are associated with the cluster and moderately high brightness contrast structure. Type C clumps have also multiple peaks although they are not associated with the cluster and low brightness contrast structure. We suggest that our classification represents an evolutionary trend of cluster-forming dense clumps because dense gas in molecular clouds is expected to be converted into stellar constituents, or to be dispersed by stellar activities. Moreover, although there is a scatter, we found a tendency that the SFEs of the dense clumps increase from Type A to Type C, which also supports our scenario.