Accretion disks, also known as “protostellar disks”, are key components of star formation. These discs continuously supply the protostars with gas from the environment. In this sense, they are stellar cradles where stars are born and grow.
Recently, a team of astronomers reported the discovery of a massive protostellar disk in the Center of the Milky Way. The team also determined how his spiral arms were formed.
The disc is about 4,000 astronomical units in diameter and surrounds a forming O-type star with a mass about 32 times that of the Sun.
The discovery was made using high-resolution observational data from the Atacama Large Millimeter/submillimeter Array (ALMA). The team includes Dr LU Xing, a research associate at the Shanghai Astronomical Observatory (SHAO) of the Chinese Academy of Sciences, and collaborators from Yunnan University, the Harvard-Smithsonian Center for Astrophysics and the Max Planck Institute.
According to research, this disk was shaken by a close encounter with a nearby object, causing the spiral arm develop. This finding suggests that large stars may originate similarly to lower mass stars via accretion disks and flybys.
Using ALMA observations, the research team achieved a resolution of 40 milliarcseconds. Using these high-resolution, high-sensitivity ALMA observations, the team discovered an accretion disk in the Galactic Center.
The finding suggests that massive early O-type stars go through a phase of formation involving accretion disks, and this finding holds true for the unique environment of the Galactic Center.
The fascinating fact is that the two spiral arms of the disc are visible. These weapons are usually found in spiral galaxies but rarely in protostellar disks. Spiral arms in accretion disks are usually formed due to fragmentation induced by gravitational instability.
However, this newly discovered protostellar disk is hot and turbulent, allowing it to balance its gravity.
In trying to explain this phenomenon, the researchers offered another explanation, namely that the spirals were induced by an external disturbance. The researchers offered this explanation after detecting an object about three solar masses – possibly the source of the external disturbance – several thousand astronomical units from the disk.
The team verified this proposition by calculating several dozen possible orbits of this object. They found that just one of these orbits could perturb the disk at the observed level.
They then carried out a numerical simulation on the high-performance supercomputing platform of the Shanghai Astronomical Observatory to trace the trajectory of the intruding object. They managed to reproduce the whole story of the object flying near the disc more than 10,000 years ago when it would have swirled spirals in the disc.
Dr. Lu said, “The good match between the analytical calculations, the numerical simulation and the ALMA observations provides strong evidence that the spiral arms in the disc are relics from the intrusive object’s flyby.”
“This finding demonstrates that accretion disks in the early evolutionary stages of star formation are subject to frequent dynamical processes such as flybys, and these processes can significantly influence the star formation and the planets.
“The formation of this massive protostar is similar to that of its lower-mass cousins like the Sun, with accretion disks and flyby events involved. Although the stellar masses are different, certain physical mechanisms in the formation stars could be the same. This provides important clues to solve the mystery of massive star formation.
Journal reference:
- Lu, X., Li, GX., Zhang, Q. et al. A massive Keplerian protostellar disk with flyover-induced spirals in the central molecular zone. Nat Astron (2022). DO I: 10.1038/s41550-022-01681-4
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