A team of astronomers resolve the mystery of massive star formation with the Very Large Array telescope

Using the U.S. National Science Foundation National Radio Astronomy Observatory‘s (NSF NRAO) Very Large Array (VLA), astronomers have revealed for the first time the huge flow of gas near a massive star in the making, which allows its rapid growth. By observing the young star HW2 in the Cepheus A region, located 2300 light years from Earth, researchers have resolved the structure and dynamics of an accretion disk feeding material to this massive star.

The Institute of Space Studies of Catalonia (IEEC — Institut d’Estudis Espacials de Catalunya), trough researchers at the Institute of Space Sciences (ICE-CSIC), is involved in the study published today in Astronomy & Astrophysics, led by the Italian National Institute for Astrophysics (INAF) and the Max-Planck-Institut für Radioastronomie.

This finding sheds light on a central question in astrophysics: how do massive stars, which often end their lives as supernovae, accumulate their immense mass? Cepheus A is the second closest site of massive star formation to Earth, making it an ideal laboratory for studying these challenging processes.

The research team used ammonia (NH3), a molecule commonly found in interstellar gas clouds and widely used industrially on Earth as a tracer to map the gas dynamics around the star. Observations revealed a dense ring of hot ammonia gas spanning radii of 200 to 700 astronomical units (AU) around HW2. This structure was identified as part of an accretion disk—a key feature in star formation theories.

The study found that gas within this disk is both collapsing inward and rotating around the young star. Remarkably, the infall rate of material onto HW2 was measured at two thousandths of a solar mass per year—one of the highest rates ever observed for a forming massive star. These findings confirm that accretion disks can sustain such extreme mass transfer rates even when the central star has already grown to 16 times the mass of our Sun.

“Our observations provide direct evidence that massive stars can form through disk-mediated accretion up to tens of solar masses,” said Dr. Alberto Sanna, lead author of the study. “The NSF VLA’s unparalleled radio sensitivity allowed us to resolve features on scales on the order of 100 AU only, offering unprecedented insights into this process,” he added.

The team also compared their observations with state-of-the-art simulations of massive star formation. “The results aligned closely with theoretical predictions, showing that ammonia gas near HW2 is collapsing almost at free-fall speeds while rotating at sub-Keplerian velocities—a balance dictated by gravity and centrifugal forces,” said Prof. André Oliva, who performed the detailed simulations.

The study uncovered asymmetries in the disk’s structure and turbulence, suggesting that external streams of gas—known as “streamers”—may be delivering fresh material to one side of the disk. Such streamers have been observed in other star-forming regions and may play a crucial role in replenishing accretion disks around massive stars.

This discovery resolves decades of debate over whether HW2, and protostars alike, can form accretion disks able to sustain their rapid growth. It also reinforces the idea that similar physical mechanisms govern star formation across a wide range of stellar masses.

“HW2 has been known since more than 40 years by now and still inspires new generations of astronomers,” said José María Torrelles, IEEC researcher at the ICE-CSIC and co-author of the study, who conducted some pivotal observations of HW2 in the late ‘90s.

In the early 2000s, thanks to powerful instruments such as the NRAO’s VLA, the Submillimeter Array (SMA) of the Smithsonian Astrophysical Observatory (SAO) and the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA), Torrelles and other collaborators presented evidence that HW2 had precisely an accretion disk around it with an associated jet. The study unequivocally proves there is indeed an accretion disk with a unique pattern of rotation and gas falling towards the protostar and puts an end to a scientific discussion that remained for 25 years.

The findings were made possible by high-sensitivity NSF VLA observations conducted at centimeter wavelengths in 2019. The researchers targeted specific ammonia transitions that are excited at temperatures above 100 Kelvin, enabling them to trace dense and warm gas near HW2.

“These results highlight the power of radio interferometry to probe the hidden processes behind the formation of the most influential object in our galaxy,” said Dr Todd Hunter of the NRAO,” and, in ten years, the next upgraded version of the VLA will make it possible to study circum-stellar ammonia at scales of our solar system,” he added.

This work not only advances our understanding of how massive stars form but also has implications for broader questions about galaxy evolution and chemical enrichment in the universe. Massive stars play pivotal roles as cosmic engines, driving winds and explosions that seed galaxies with heavy elements.

Press release prepared in collaboration with the Institute of Space Sciences.