A Statistical Study of Massive Cluster-Forming Clumps
  We report results of the observations of 15 regions in several molecular
lines for a statistical study of massive cluster-forming clumps. We identified
24 clumps based on the C18O (J=1-0) data obtained by the NRO 45 m telescope,
and found that 16 of them are associated with young clusters. The clumps
associated with clusters have a typical mass, radius, and molecular density of
~1 X 10^3 Mo, ~0.5 pc, ~1 X 10^5 cm^-3, respectively. We categorized the clumps
and clusters into four types according to the spatial coincidence of gas and
star density, and discussed their evolutions: Clumps without clusters (Type 1),
clumps showing good correlations with clusters (Type 2), clumps showing poor
correlations with clusters (Type 3), and clusters with no associated clumps
(Type 4). Analyses of the velocity structures and the chemical compositions
imply that the clump + cluster systems should evolve from Type 1 to Type 4. We
found that some of the Type 2 clumps are infalling on the clump-scale to form
clusters at the clump center, which should commonly occur in the beginning of
cluster formation. Interestingly, all of the identified Type 1 clumps are
likely to be older than the Type 2 clumps in terms of chemical compositions,
suggesting that they have been gravitationally stable for a long time possibly
being supported by the strong magnetic field of > 1 mG.Type 1 clumps younger
than the observed Type 2 clumps should be very rare to find because of their
short lifetime.
