2016ApJ...832..205S

We report the discovery of infalling motion with the rotation of S235AB, a massive cluster-forming clump (∼1×10³ M_☉) in the S235 region. Our C¹⁸O observations with the 45 m telescope at the Nobeyama Radio Observatory have revealed an elliptical shape of the clump. A position-velocity diagram taken along its major axis exhibits two well-defined peaks symmetrically located, with respect to the clump center. This is similar to that found for a dynamically infalling envelope with rotation around a single protostar, modeled by N. Ohashi et al., indicating that the cluster-forming clump is also collapsing by the self-gravity toward the clump center. With analogue to Ohashi et al.'s model, we made a simple model of an infalling, rotating clump to fit the observed data. Based on the inferred model parameters, as well as results of earlier observations and simulations in the literature, we discuss the structures of the clump such as the relation among the global mass infall rate (∼1×10^–3 M_☉ yr^–1), formation of a compact core (with a mass and size of ∼4 M_☉ and \lesssim  0.1 pc) at the center, and a massive star (∼11 M_☉) forming in the core.