VLA Captures Direct Evidence of Rapid Accretion in Forming Massive Star

Using the Very Large Array (VLA) of the U.S. National Science Foundation’s National Radio Astronomy Observatory (NSF NRAO), astronomers have, for the first time, directly observed a dramatic accretion process of gas surrounding a forming massive star—a key mechanism that drives the star’s rapid growth.

The study focuses on the protostar HW2, located in the Cepheus A star-forming region approximately 2,300 light-years from Earth. Conducted by an international team from the Italian National Institute for Astrophysics (INAF), the Max Planck Institute for Radio Astronomy in Germany, and Spain’s Institute of Space Sciences (ICE-CSIC), the findings were published in Astronomy & Astrophysics.

By tracing the motion of ammonia (NH₃) molecules, the team discovered a high-temperature, high-density ring of gas—part of an accretion disk—spanning 200 to 700 astronomical units (AU) from the star (1 AU ≈ 150 million kilometers). Observations revealed gas collapsing inward at a rate of two-thousandths of a solar mass per year, while retaining rotational motion—one of the highest known accretion rates for massive protostars. This confirms that accretion disks play a crucial role in the mass buildup of such stars.

Comparing the data with advanced simulations of stellar formation, the researchers found that the gas infall speeds approach free-fall conditions, with rotation slower than theoretical expectations—consistent with a model where gravity and centrifugal forces are balanced. The irregular structure of the disk further suggests that it is being fed by external gas flows. These “stream-fed” accretion mechanisms may be essential for sustaining the long-term growth of massive stars.

This study settles a decades-long debate by providing definitive evidence that disk accretion can support the formation of massive stars. Previous observations with the VLA and the Submillimeter Array (SMA) of the Smithsonian Astrophysical Observatory had hypothesized the presence of such a disk around HW2. The new data conclusively confirms this hypothesis.