Dr. Xing Lu, an associate researcher at the Chinese Academy of Sciences’ Shanghai Astronomical Observatory (SHAO), worked with collaborators from Yunnan University, the Harvard-Smithsonian Center for Astrophysics, and the Max Planck Institute. They discovered a massive protostellar disk in the Galactic Center and determined how its spiral arms were formed using high-resolution observational data from the Atacama Large Millimeter/submillimeter Array (ALMA).
According to the research, this disk was agitated by a brief encounter with a neighboring object, causing the spiral arms to develop. This discovery suggests that big stars can originate in the same way as lower-mass stars, via accretion disks and flybys.
On May 30th, 2022, the findings were reported in Nature Astronomy.
Accretion disks emerge around nascent stars during star formation. The accretion disks, also known as “protostellar disks,” are crucial in the birth of stars. Gas from the surroundings is continually fed into protostars by accretion disks. They are stellar cradles in this sense, where stars are born and raised.
However, the function of accretion disks in the development of huge protostars, particularly early O-type ones with masses more than 30 solar masses, remains unclear.
The Galactic Center, located roughly 26,000 light-years from Earth, is a unique and crucial star-forming environment. Aside from Sgr A*, the Galactic Center has a tremendous reservoir of dense molecular gas, primarily in the form of molecular hydrogen (H2), which is the raw material for star formation.
Once gravitational collapse begins, the gas begins to produce stars.
The environment at the Galactic Center, on the other hand, is unique, with tremendous turbulence and magnetic fields, as well as tidal forces from Sgr A*, all of which have a significant impact on star formation.
Direct studies of star-forming areas near the Galactic Center have proven difficult due to the large distance between the Galactic Center and Earth and the presence of intricate foreground contaminations.
Using ALMA’s extended baseline observations, the research team lead by Dr. Lu was able to attain a resolution of 40 milliarcseconds. To understand how precise that resolution is, imagine being able to identify a football in Beijing from Shanghai.
The researchers detected an accretion disk in the Galactic Center using these high-resolution, high-sensitivity ALMA images. A developing, early O-type star with a mass around 32 times that of the Sun is surrounded by a disk with a diameter of roughly 4,000 astronomical units.
This system is one of the most massive protostars with accretion disks in the Galactic Center, and it marks the first direct imaging of a protostellar disk in the galaxy.
The result shows that large early-O type stars go through a development phase that involves accretion disks, and this conclusion holds true in the Galactic Center’s peculiar environment.
What is even more intriguing is that the disk has two spiral arms prominently displayed. In spiral galaxies, such arms are common, but in protostellar disks, they are uncommon. Spiral arms appear in accretion disks as a result of fragmentation caused by gravitational instability.
The disk revealed in this study, on the other hand, is heated and chaotic, allowing it to balance its own gravity.
The researchers suggested an alternative explanation for this phenomenon, claiming that the spirals were caused by external disruption. After discovering an object of around three solar masses several thousand astronomical units out from the disk—possibly the cause of the external perturbation—the researchers offered this interpretation.
To back up their claim, the researchers computed a number of different orbits for this item. Only one of these orbits was determined to be capable of perturbing the disk to the observed amount.
They then used the Shanghai Astronomical Observatory’s high-performance supercomputing facility to run a numerical simulation to track the invading object’s path. The scientists were able to recreate the complete history of the item passing by the disk over 10,000 years ago when it would have caused spirals to form in the disk.
The nice match among analytical calculations, the numerical simulation, and the ALMA observations provides robust evidence that the spiral arms in the disk are relics of the flyby of the intruding object.
Dr. Xing Lu, Associate Researcher, Shanghai Astronomical Observatory, Chinese Academy of Sciences
The discovery shows that accretion disks in the early phases of star formation are prone to frequent dynamic events such as flybys, which can have a significant impact on the development of stars and planets.
Flybys may well have occurred in the solar system as well. Around 70,000 years ago, a binary star system known as Scholz’s Star passed by the solar system, perhaps piercing the Oort cloud and delivering comets to the inner solar system.
According to the current study, such flybys should be common for more massive stars, particularly in the high-stellar-density environment at the Galactic Center.
Dr. Lu added, “The formation of this massive protostar is similar to its lower-mass cousins like the Sun, with accretion disks and flyby events involved. Although stellar masses are different, certain physical mechanisms in star formation could be the same. This provides important clues to solving the mystery of massive star formation.”
Dr Lu. X, et al. (2022) A massive Keplerian protostellar disk with flyby-induced spirals in the Central Molecular Zone. Nature Astronomy doi:10.1038/s41550-022-01681-4.