Maps of Massive Clumps in the Early Stage of Cluster Formation: Two
  Modes of Cluster Formation, Coeval or Non-Coeval?
  We present maps of 7 young massive molecular clumps within 5 target regions
in the C18O (J=1-0) line emission, using the Nobeyama 45m telescope. These
clumps, which are not associated with clusters, lie at distances between 0.7 to
2.1 kpc. We find C18O clumps with radii of 0.5-1.7 pc, masses of 470-4200 Msun,
and velocity widths of 1.4-3.3 km/s. All of the clumps are massive and
approximately in virial equilibrium, suggesting they will potentially form
clusters. Three of our target regions are associated with H II regions ("CWHRs"
from Clump with H II Regions), and the other two are without H II regions
(CWOHRs). The C18O clumps can be classified into two morphological types: CWHRs
shape a filamentary or shell-like structure, CWOHRs are spherical. The two
CWOHRs have systematic velocity gradients. Using the publicly released WISE
database, Class I and Class II protostellar candidates were identified within
the C18O clumps. The fraction of the Class I candidates among all YSO
candidates (Class I+Class II) is > 50 % in CWHRs, and < 50 % in CWOHRs. We
conclude that effects from the H II regions can be seen in (1) spatial
distributions of the clumps: filamentary or shell-like structure running along
the H II regions, (2) velocity structures of the clumps: large velocity
dispersion along shells, and (3) small age spreads of YSOs. The small spread in
age of the YSOs show that the presence of H II regions tend to trigger coeval
cluster formation.
