Discovery of Infalling Motion with Rotation of the Cluster-Forming Clump
  S235AB and Its Implication to the Clump Structures
  We report the discovery of infalling motion with rotation of S235AB the
massive cluster-forming clump (~10^3 Mo) in the S235 region. Our C18O
observations with the 45m telescope at the Nobeyama Radio Observatory have
revealed the elliptical shape of the clump. Position-velocity (PV) diagram
taken along its major axis exhibits two well-defined peaks symmetrically
located with respect to the clump center, which is similar to that found for a
dynamically infalling envelope with rotation around a single protostar modeled
by N. Ohashi and his collaborators, indicating that the cluster-forming clump
is also collapsing by the self-gravity toward the clump center. With analogue
to Ohashi'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
structures of the clump such as the relation among the global mass infall rate
(~10^-3 Mo/yr), formation of a compact core (with a mass and size of ~4 Mo and
< 0.1 pc) at the center, and a massive star (~11 Mo) forming in the core.
