Accretion disks, also called ‘protostellar disks’, are important components of star formation. These disks continuously feed gas from the environment to protostars. In that sense, they are stellar cradles where stars are born and raised.
Recently, a team of astronomers reported the discovery of a huge protostellar disk in the The center of the Milky Way† The team also determined how the spiral arms were formed.
The disk is about 4,000 astronomical units in diameter and surrounds a forming early 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 consists of Dr. LU Xing, an associate researcher of the Shanghai Astronomical Observatory (SHAO) of the Chinese Academy of Sciences, and collaborators of Yunnan University, the Harvard-Smithsonian Center for Astrophysics and the Max Planck Institute.
According to the research, this disk was agitated by a close encounter with a neighboring object, causing the spiral arms develop. This discovery suggests that large stars may form similarly to lower-mass stars via accretion disks and flybys.
Using observations from ALMA, the research team achieved a resolution of 40 milliarcseconds. With these high-resolution, high-sensitivity ALMA observations, the team discovered an accretion disk in the Galactic Center†
The discovery suggests that early O-type massive stars are undergoing a formation phase with accretion disks, and this conclusion is valid for the unique environment of the Galactic Center.
The fascinating fact is that the disc’s two spiral arms are visible. Such weapons are usually found in spiral galaxies but rarely in protostellar disks. Spiral arms in accretion disks are generally formed by fragmentation caused by gravitational instability.
However, this newly discovered protostellar disk is hot and turbulent, allowing it to balance its gravity.
In an effort to explain this phenomenon, the researchers proposed an alternative explanation: that the spirals were caused by external disturbance. The researchers proposed this explanation after detecting an object about three solar masses — possibly the source of the external disturbance — several thousand astronomical units away from the disk.
The team verified this theorem by calculating several dozen possible orbits of this object. They found that only one of these orbits could disrupt the disk to the observed level.
They then ran a numerical simulation on the powerful supercomputing platform of the Shanghai Astronomical Observatory to track the trajectory of the invading object. They successfully reproduced the entire history of the object flying past the disk more than 10,000 years ago when it would have caused spirals in the disk.
dr. LU said† “The nice match between analytical calculations, the numerical simulation and the ALMA observations provide robust evidence that the spiral arms in the disk are remnants of the flyby of the invading object.”
“This finding demonstrates that accretion disks in early evolutionary stages of star formation are subject to frequent dynamic processes such as flybys, and these processes can have a significant impact on the formation of stars and planets.”
“The formation of this massive protostar is similar to its lower-mass cousins, such as the Sun, with accretion disks and flyby events. While stellar masses are different, certain physical mechanisms in star formation may be the same. This provides important clues as to how the star is formed.” mystery of the formation of massive stars.”
- Lu, X., Li, GX., Zhang, Q. et al. A massive Keplerian protostellar disk with flyby-induced spirals in the Central Molecular Zone. Nat Astron (2022). DOI: 10.1038/s41550-022-01681-4
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