Astronomers uncover how massive stars form by tracking interstellar ammonia.
Using the U.S. National Science Foundation National Radio Astronomy Observatory’s (NSF NRAO) Very Large Array (NSF VLA), astronomers have, for the first time, captured the massive flow of gas surrounding a forming high-mass star—an essential process that drives its rapid growth.
By studying HW2, a young protostar in the star-forming region of Cepheus A approximately 2300 light years away, the team successfully detailed the structure and motion of the accretion disk supplying material to the growing star. The Institute of Space Sciences (ICE-CSIC) contributed to the study, which was published in Astronomy & Astrophysics and led by researchers from the Italian National Institute for Astrophysics (INAF) and the Max-Planck-Institut für Radioastronomie.
A closer look at stellar mass accumulation
This discovery addresses a fundamental question in astrophysics: how do massive stars, which ultimately explode as supernovae, build up such large masses? Since Cepheus A is the second-nearest region of massive star formation to Earth, it offers a rare opportunity to observe these extreme processes in action.
To investigate how material behaves around the forming star, the researchers tracked ammonia (NH3)—a molecule common in both interstellar clouds and industrial use on Earth—using it as a tracer to follow gas movements. Their observations revealed a hot, dense ring of ammonia gas stretching between 200 and 700 astronomical units (AU) from the star, forming part of an accretion disk, a structure central to many models of star formation.
Measuring extreme gas infall
They found that gas in the disk is simultaneously rotating and collapsing inward toward HW2. The rate at which material is falling onto the star was calculated at two thousandths of a solar mass per year, among the highest ever measured for a massive protostar. This confirms that accretion disks are capable of maintaining such intense flows of gas even when the star has already reached 16 times the mass of the 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 researchers also analyzed their data alongside advanced computer simulations of how massive stars form. “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,” explained Prof. André Oliva, who carried out the simulation work.
Their observations also revealed irregularities in the disk’s shape and motion, pointing to the influence of external gas flows, known as “streamers,” that appear to be supplying material to one side of the disk. Similar features have been detected in other stellar nurseries and are thought to be essential in sustaining the growth of massive stars through continued disk feeding.
A case that ends decades of debate
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, researcher at ICE-CSIC and affiliated with the Institute of Space Studies of Catalonia (IEEC), co-author of the study, who conducted some pivotal observations of HW2 in the late ‘90.
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.
Cutting-edge technology reveals hidden features
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.
Reference: “Gas infall via accretion disk feeding Cepheus A HW2” by A. Sanna, A. Oliva, L. Moscadelli, C. Carrasco-González, A. Giannetti, G. Sabatini, M. Beltrán, C. Brogan, T. Hunter, J. M. Torrelles, A. Rodríguez-Kamenetzky, A. Caratti o Garatti and R. Kuiper, 21 May 2025, Astronomy & Astrophysics.
DOI: 10.1051/0004-6361/202450330
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