2009ApJ...705..468H

We have carried out a survey of the dense clumps associated with 14 embedded clusters in the C¹⁸O (J = 1-0) line emission with the Nobeyama 45 m 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 2.1 kpc and have mapped about 6'x6'-10'x10' regions (corresponding to 3.8 pcx3.8 pc at 2.1 kpc) for all the clumps with 22'' resolution (corresponding to Jeans length at 2.1 kpc). We have obtained dense clumps with radii of 0.40-1.6 pc, masses of 150-4600 M_☉, and velocity widths in FWHM of 1.4-3.3 km/s. Most of the clumps are found to be approximately in virial equilibrium, which implies that C¹⁸O gas represents parental dense clumps for cluster formation. From the spatial relation between the distributions of clumps and clusters, we classified C¹⁸O clumps into three types (Type A, B, and C). Type A clumps 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 also have 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 dispersed by stellar activities. Moreover, although there is a scatter, we found a tendency that the star formation efficiencies of the dense clumps increase from Type A to Type C, which also supports our scenario.