A groundbreaking study recently published in Astronomy & Astrophysics has provided new insights into the dynamic interplay between massive stars, molecular gas, and star formation in one of the most well-known super-large HII regions in our galaxy: the W4 super-large HII region. The study, led by Shen Hailiang, a Ph.D. student from the Xinjiang Astronomical Observatory of the Chinese Academy of Sciences (CAS), sheds light on how stellar feedback from massive stars can both foster and hinder the formation of new stars, particularly in environments like W4, where intense stellar winds and radiation dominate.
The Role of Massive Stars in Star Formation
Massive stars, defined as those with several times the mass of the Sun, are known for their powerful stellar winds and intense radiation. These energetic outputs have a profound impact on their surrounding environment, particularly on molecular clouds, which are the birthplaces of new stars. While these massive stars can help ignite the formation of new stars by compressing the surrounding gas, their feedback can also suppress further star formation by dispersing the gas or heating it to the point where it cannot collapse into new stars.
The study focuses on a particularly interesting example of this complex interaction—the W4 super-large HII region. Located in the Perseus Arm of the Milky Way, W4 is a vast region of space filled with ionized hydrogen gas, or HII gas, which has been energized by radiation from nearby massive stars. Within this region, researchers observed a distinctive structure: a large cavity filled with ionized gas, as well as a chimney that transports hot gas from the region up toward the galactic disk.
The W4 HII region provides a unique environment in which to study how stellar feedback can influence the surrounding molecular gas. The feedback from the massive stars in the region, through their stellar winds and radiation, shapes the distribution of molecular gas and plays a crucial role in regulating star formation.
Observing the Molecular Gas and Clump Distribution
In order to investigate the effects of this stellar feedback, Shen and his collaborators conducted an extensive CO (1–0) observation of the W4 super-large HII region and its neighboring W3 giant molecular cloud, using data from the Purple Mountain Observatory’s 13.7-meter millimeter-wave telescope. The team focused on a range of carbon monoxide (CO) isotopologues, including 12CO, 13CO, and C18O, to study the distribution of molecular gas in this region.
The team discovered that the W3/4 molecular cloud, which encompasses both the W3 giant molecular cloud and the W4 HII region, can be divided into three distinct regions based on the properties of the gas present:
- The High-Density Layer (HDL region) – This region is directly influenced by the feedback from massive stars. The feedback compresses the gas, creating a dense, star-forming region.
- The Diffuse “Bubble Region” – Situated between the HDL region and the W4 HII region, this region contains low-density gas that has been shaped by the radiation and stellar winds but is not dense enough to form stars easily.
- The “Spontaneous Star Formation Region” – Located far from the intense feedback of the HII region, this area shows little evidence of being affected by the stellar winds and radiation from the massive stars, and star formation can proceed in a more typical fashion.
This unique molecular structure provided the team with an opportunity to explore how the feedback from massive stars can have different effects depending on the density and location of the gas.
Key Findings and Stellar Feedback Mechanisms
The study’s most important results stem from detailed analysis of the molecular gas at the boundary of the W4 HII region. The team found that CO gas in this boundary region exhibited a strong correlation with the radiation from the HII region. Specifically, the CO intensity sharply increased at the boundary of the HII region and gradually decreased with distance from it. Similarly, the gas temperature at the boundary exhibited a direct relationship with 8μm radiation—a wavelength associated with the infrared emission from heated dust and molecular gas.
This observation provides strong evidence for two key mechanisms at play in this region: sweeping of gas and radiation heating. As the W4 HII region expands, the intense radiation from the massive stars sweeps up nearby gas, heating it and pushing it outward. Additionally, the study also revealed signs of ionized flow erosion, in which the fast-moving gas is eroded by the radiation and stellar winds, further shaping the distribution of gas in the region.
Clumps and Star Formation in W4
The study also identified 288 clump structures within the W4 region, which were classified into three categories: HDL clumps, bubble clumps, and quiescent clumps. These clumps represent regions of gas that have the potential to form new stars under the right conditions.
- HDL Clumps: These clumps, located in the high-density layer influenced by stellar feedback, tend to exhibit higher excitation temperatures, lower virial parameters (which suggest gravitational stability), higher thermal velocity dispersions, and lower L/M ratios (indicating higher efficiency for star formation).
- Bubble Clumps: Located in the diffuse bubble region, these clumps show the opposite trends. Their properties suggest that they are less likely to undergo star formation compared to HDL clumps due to the low density and disrupted conditions caused by the stellar feedback.
- Quiescent Clumps: These clumps, located in the regions farthest from the feedback influence, show properties that are more typical of isolated star-forming regions, with higher stability for star formation.
These clumps were further analyzed in terms of their mass-radius relationship and mass cumulative distribution function, which allowed the researchers to differentiate between the three types of clumps. This classification further confirmed that the feedback from the W4 HII region triggers star formation in the HDL clumps while suppressing it in the bubble clumps at the boundary.
Conclusion: The Role of Stellar Feedback in Star Formation
Shen Hailiang’s study represents a significant contribution to our understanding of how feedback from massive stars influences molecular gas and star formation, particularly in rare and complex environments like the W4 super-large HII region. The team’s findings demonstrate that the feedback from these stars can both trigger and suppress star formation, depending on the region’s density and proximity to the HII region.
In regions like the HDL clumps, where gas is dense and compressed by stellar winds and radiation, the feedback appears to trigger new star formation. On the other hand, in the diffuse bubble region, where gas is less dense and more scattered, the feedback tends to inhibit star formation by dispersing the gas and preventing it from collapsing into new stars.
These findings offer important insights into the complex processes that govern star formation in regions influenced by massive stars. As future observations continue to refine our understanding of these processes, the work by Shen and his team lays the groundwork for further studies into the delicate balance between star formation and stellar feedback in the most extreme environments of our galaxy.
Reference: Hailiang Shen et al, Triggered and dispersed under feedback of super HII region W4, Astronomy & Astrophysics (2024). DOI: 10.1051/0004-6361/202450914