Astronomers using NSF’s Very Large Array (VLA) have spotted the huge flow of gas near HW2 — a massive protostar located 2,283 light-years away in the star-forming region Cepheus A — which allows the rapid growth of the protostar.
Ammonia gas infalling into an accretion disk feeding the protostar HW2. Image credit: NSF / AUI / NSF / NRAO / B. Saxton.
The large reservoir of interstellar gas needed to build up a massive star, dozens of times more massive than our Sun, piles up over wide regions of the order of a parsec (3.26 light-years).
But, it is only within circumstellar regions as large as a few hundred times an astronomical unit (AU) that gas will be ultimately collected to accrete onto a small protostar, with diameter of about a million kilometers only.
Resolving the properties of the gas flow, as it streams in the inner hundreds AU from a very young star, has long been an observational challenge, especially for the most massive stars which are found much further away from Earth than solar-type stars.
“Our observations provide direct evidence that massive stars can form through disk-mediated accretion up to tens of solar masses,” said Dr. Alberto Sanna, an astronomer at INAF and the Max-Planck-Institut für Radioastronomie.
“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.”
Cepheus A is the second nearest star-forming site where massive young stars of ten and more solar masses are born, making it an ideal laboratory for studying these challenging processes..
Dr. Sanna and his colleagues used ammonia, 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.
The VLA observations revealed a dense ring of hot ammonia gas spanning radii of 200 to 700 AU around HW2.
This structure was identified as part of an accretion disk — a key feature in star formation theories.
The astronomers 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.
The researchers 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 Professor André Oliva, an astronomer at the Université de Genève and the Space Research Center (CINESPA) at the University of Costa Rica.
Interestingly, the scientists 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.
“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,” the authors said.
“Massive stars play pivotal roles as cosmic engines, driving winds and explosions that seed galaxies with heavy elements.”
Their paper will be published in the journal Astronomy & Astrophysics.
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A. Sanna et al. 2025. Gas infall via accretion disk feeding Cepheus A HW2. A&A, in press; doi: 10.1051/0004-6361/202450330
