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A Mapping Survey of Dense Clumps Associated with Embedded Clusters II : Can Clump-Clump Collisions Induce Stellar Clusters?

We report the H13CO+(1-0) survey observations toward embedded clusters obtained using the Nobeyama 45m telescope, which were performed to follow up our previous study in the C18O survey with a dense gas tracer. Our aim is to address the evolution of cluster-forming clumps. We observed the same 14 clusters in C18O, which are located at distances from 0.3-2.1kpc with 27" resolution in H13CO+. We detected the 13 clumps in H13CO+ line emission and obtained the physical parameters of the clumps with radii of 0.24-0.75pc, masses of 100-1400Msun, and velocity widths in FWHM of 1.5-4.0kms^-1. The mean density is 3.9x10^4cm^-3 and the equivalent Jeans length is 0.13pc at 20K. We classified the H13CO+ clumps into three types, Type A, B, and C according to the relative locations of the H13CO+ clumps and the clusters. Our classification represents an evolutionary trend of cluster-forming clumps because dense clumps are expected to be converted into stellar constituents, or dispersed by stellar activities. We found a similar but clearer trend than our previous results for derived star formation efficiencies to increase from Type A to C in the H13CO+ data, and for the dense gas regions within the clumps traced by H13CO+ to be sensitive to the physical evolution of clump-cluster systems. In addition, we found that four out of 13 H13CO+ clumps which we named DVSOs(Distinct Velocity Structure Objects) have distinct velocity gradients at the central parts of them. Assuming that the velocity gradients represent the rigid-like rotation of the clumps, we calculated the virial parameter of the H13CO+ clumps by taking into account the contribution of rotation, and found that the DVSOs tend to be gravitationally unbound. In order to explain the above physical properties for DVSOs, we propose a clump-clump collision model as a possible mechanism for triggering formation of clusters.