2010ApJ...719.1813H

We report the H¹³CO⁺ (J = 1-0) survey observations toward embedded clusters obtained using the Nobeyama 45 m 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 to 2.1 kpc with a 27'' resolution (corresponding to the Jeans length for most of our targets) in H¹³CO⁺. We detected the 13 clumps in H¹³CO⁺ line emission and obtained the physical parameters of the clumps with radii of 0.24-0.75 pc, masses of 100-1400 M_☉, and velocity widths in FWHM of 1.5-4.0 km/s. The mean density is ∼3.9x10⁴/cm3 and the equivalent Jeans length is ∼0.13 pc at 20 K. We classified the H¹³CO⁺ clumps into three types, type A, type B, and type C according to the relative locations of the H¹³CO⁺ clumps and the clusters (see our previous study). 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 the clump-cluster systems. In addition, we found that 4 out of 13 H¹³CO⁺ clumps, which we named "Distinct Velocity Structure Objects" (DVSOs), have distinct velocity gradients at their central parts, i.e., at the location of the embedded clusters. 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 the rotation and found that the DVSOs tend to be gravitationally unbound. In order to explain the above physical properties for DVSOs in a consistent way, we propose a clump-clump collision model as a possible mechanism for triggering the formation of clusters.