Massive stellar feedback influences star formation, finds study of W4 super-large HII region

A new study has unveiled new insights into the effects of massive stars on nearby molecular gas and star formation in the W4 super-large HII region.

The study, published in Astronomy & Astrophysics, was conducted by Shen Hailiang, a Ph.D. student from the Xinjiang Astronomical Observatory of the Chinese Academy of Sciences (CAS) and his collaborators.

Massive stars, through their powerful stellar winds and radiation, play a critical role in shaping the structure and evolution of surrounding molecular clouds. In particular, their feedback can either initiate or inhibit further star formation in the vicinity, particularly within rare, super-large HII regions.

One such region, W4, is a well-known cavity structure filled with ionized material and a chimney that carries hot material to the galactic disk.

In this study, Shen and his team conducted a large-scale CO (1–0) observation 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 of CAS, they analyzed the distribution of molecular gas surrounding the W4 super-large HII region.

Their findings offer valuable insights into how feedback from massive stars influences the evolution of molecular gas and clumps in the area.

According to the researchers, 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 the researchers to examine how stellar feedback can both trigger and suppress star formation.

The results revealed that the CO gas at the boundary of the W4 HII region exhibits strong radiation, with intensity sharply increasing before gradually decreasing in the direction away from the HII region. The temperature of the gas at the boundary also shows a strong 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, the researchers identified 288 clump structures within the region, classifying them into three categories based on their distribution: HDL, bubble, and quiescent clumps. The analysis showed that HDL clumps tend to have higher excitation temperatures, lower virial parameters, higher thermal velocity dispersions, and lower L/M ratios when compared to clumps in the quiescent regions.

On the other hand, bubble clumps showed the opposite trends. The mass-radius relationship and mass cumulative distribution function also clearly distinguish the three types of clumps, confirming that feedback from the W4 HII region triggers star formation activity in the W3 HDL layer while suppressing it in the bubble boundary shell.