New Insights into the Formation of Giant Stars

Astronomers utilizing the very large array (VLA) from NSF have made a significant discovery of a massive gas flow near HW2. Cephaus A enables rapid protostar growth.

Extensive reservoirs of interstellar gas are essential for forming giant stars, several times the size of our Sun, accumulating over a vast scale of approximately Parsec (3.26 light years).

Ultimately, gas collects in a local area several hundred times larger than the Astronomical Unit (AU) to attach to small protostars nearly one million kilometers wide.

The flow, originating from very young stars to hundreds of AU away, has long presented observational challenges, particularly for the largest stars distant from solar-type stars.

“Our observations present direct evidence that giant stars can form with masses reaching dozens of solar masses through disks,” stated Dr. Alberto Sanna, an astronomer from INAF and the Max-Planck-Institut für Radioastronomie.

“The exceptional wireless sensitivity of the VLA enabled us to discern features on a scale as small as 100 AU, giving us unprecedented insights into this formation process.”

Cephaus A represents the second closest star-forming region where large young stars of over 10 solar masses have been observed, providing an ideal setting for investigating these complex processes.

Dr. Sanna and colleagues employed ammonia, a common molecule in interstellar gas clouds, widely used on Earth as a tracer mapping gas dynamics around stars.

VLA observations revealed a dense ring of high-temperature ammonia gas with a radius of 200-700 AU surrounding HW2.

This structure was recognized as a component of the accretion disk, a crucial feature in star formation theory.

Astronomers found that the gas in this disk flowed inward and rotated around a young star.

Remarkably, the mass accreting onto HW2 was measured at one-two-thousandth of a solar mass annually.

These findings confirm that accretion disks can sustain such extreme mass transfer rates, even while the central star’s mass reaches 16 times that of the Sun.

The researchers also compared their findings with advanced simulations of large-scale star formation.

“The results align closely with theoretical predictions, suggesting that ammonia gas near HW2 nearly collapses at free-fall speeds and rotates at sub-critical speeds.

Interestingly, the asymmetry of the disk structure and turbulent flow indicate the presence of an external gas stream, known as a streamer, potentially supplying new material to one side of the disk.

Such streamers have been detected in other star-forming regions and may be significant in refreshing accretion disks around giant stars.

This discovery resolves decades of debate about whether HW2 and protostars can similarly form accretion disks capable of sustaining rapid growth.

It also reinforces the concept that similar physical mechanisms drive star formation across various mass categories.

“This research enhances our comprehension of how large stars are formed and influences broader inquiries into the evolution and chemical enrichment of galaxies in the universe,” the author remarked.

“Massive stars function as essential cosmic engines, generating winds and explosions that distribute heavy elements throughout the galaxy.”

Their paper will be published in the journal Astronomy and Astrophysics.

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A. Sanna et al. 2025. Gas infall through accretion disk feeding Cephaus A HW2. A&A in press; doi: 10.1051/0004-6361/202450330