Maps of Massive Clumps in the Early Stage of Cluster Formation: Two Modes of Cluster Formation, Coeval or Non-coeval?

We present maps of seven young massive molecular clumps within five target regions in C18O (J = 1-0) line emission, using the Nobeyama 45?m telescope. These clumps, which are not associated with clusters, lie at distances between 0.7 and 2.1?kpc. We find C18O clumps with radii of 0.5-1.7?pc, masses of 470-4200 M ?, and velocity widths of 1.4-3.3?km s?1. 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), while the other two are unassociated with H II regions (CWOHRs). The C18O clumps can be classified into two morphological types: CWHRs with a filamentary or shell-like structure and spherical CWOHRs. The two CWOHRs have systematic velocity gradients. Using the publicly released WISE database, Class I and Class II protostellar candidates are identified within the C18O clumps. The fraction of 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) the spatial distributions of the clumps: filamentary or shell-like structure running along the H II regions; (2) the velocity structures of the clumps: large velocity dispersion along shells; and (3) the small age spreads of YSOs. The small spreads in age of the YSOs show that the presence of H II regions tends to trigger coeval cluster formation.