When massive stars form, their strong stellar winds and radiation significantly impact the structure, properties, and evolution of nearby molecular clouds. This influence can either trigger or suppress further star formation, especially in rare super-large HII regions. One such region, W4, is a well-known cavity structure with a chimney that carries hot material to the galactic disk and is filled with ionized material.

SHEN Hailiang, a PhD student from the Star Formation and Evolution Group at the Xinjiang Astronomical Observatory, Chinese Academy of Sciences, together with his collaborators, conducted a large-scale CO (1-0) observations of the W4 super-large HII region and the neighboring W3 giant molecular cloud. Using 12CO/13CO/C18O data collected from the 13.7-meter millimeter-wave telescope at the Purple Mountain Observatory, the researchers analyzed the large-scale distribution of molecular gas around the W4 super-large HII region. The research offers valuable insights into how feedback from massive stars influences the evolution of molecular gas and clumps in the area.

This result was published in the international astronomical journal Astronomy & Astrophysics.  

The W3/4 molecular cloud is divided into three distinct regions based on its gas distribution: the high-density layer (HDL region), shaped by feedback and containing dense gas; the diffuse "bubble region", influenced by feedback but with low-density gas; and the "spontaneous star formation region", located far from the feedback area .  This unique structure allowed researchers to simultaneously investigate how stellar feedback can both trigger and suppress star formation.

The research results indicate that the CO gas at the boundary of the W4 HII region exhibits strong radiation, with its intensity increasing sharply before gradually decreasing in the direction away from the HII region. The gas temperature at the boundary also shows a good correlation with the 8μm radiation, both exhibiting higher values. These observations provide clear evidence for the expansion sweeping and radiation heating at the boundary of the HII region, as well as ionized flow erosion.

In addition, 288 clump structures were identified in the region, and based on their distribution characteristics, they were classified into "HDL," "bubble," and "quiescent" clumps. Analysis showed that compared to the quiescent region clumps, HDL clumps generally have higher excitation temperatures, lower virial parameters, higher thermal velocity dispersions, and lower L/M ratios. In contrast, bubble clumps show the opposite properties. The mass-radius relationship and mass cumulative distribution function also clearly distinguish the three types of clumps. These results confirm that the feedback from the W4 HII region triggers star formation activity in the W3 HDL layer while suppresses it in the bubble boundary shell.

This research was supported by the National Key R&D Program and the National Natural Science Foundation of China.