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

TL;DR

This study maps massive molecular clumps in early cluster formation stages, revealing two formation modes—coeval and non-coeval—based on morphological, kinematic, and star formation activity differences influenced by H II regions.

Contribution

It provides detailed C18O maps of young massive clumps, classifies their structures, and links H II region influence to cluster formation modes, highlighting the role of environment in star formation.

Findings

C18O clumps are massive, virialized, and likely to form clusters.

Two morphological types: filamentary/shell-like with H II regions, spherical without.

H II regions influence clump structure, velocity dispersion, and star formation age spread.

Abstract

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.