Unveiling the birth secrets of massive stars

Using the US National Radio Astronomy Observatory (NRAO) and Very Large Array (VLA), astronomers, among whom a researcher form UNIGE, 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 Cepheus A, located 2300 light years from Earth, researchers have resolved the structure and dynamics of an accretion disk feeding material to this massive star. 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? The study is to be published in Astronomy & Astrophysics.

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.”

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 during his UNIGE affiliation.

Interestingly, 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.

Known since the 90s, HW2 had to wait 40 years before its mysteries were unravelled. 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 objects in our Galaxy. The next upgraded version of the VLA, planned for ten years from now, should make it possible to study circum-stellar ammonia at scales of the Solar system.

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," said Prof. André Oliva, "driving winds and explosions that seed galaxies with heavy elements."

Link to the article on arXiv

News