Close collision stirs spirals in accretion disk

(ORDO NEWS) — Dr. Lu Xun, Associate Scientist at the Shanghai Astronomical Observatory (SHAO) of the Chinese Academy of Sciences, along with colleagues from Yunnan University, the Harvard-Smithsonian Center for Astrophysics, and the Max Planck Institute, used high-resolution observational data from the Atacama Large Millimeter/Submillimeter Array ( ALMA) to detect a massive protostellar disk at the Galactic Center and determine how its spiral arms formed.

The team’s research showed that this disk was perturbed by a close collision with a nearby object, leading to the formation of spiral arms. This discovery shows that the formation of massive stars may be similar to the formation of stars of smaller mass through accretion disks and flybys.

During star formation, accretion disks form around newborns. They, also known as “protostellar disks”, are an important component of their formation. Accretion disks are constantly supplying gas to protostars from the environment. In this sense, they are stellar cradles where they are born and grow.

However, for massive protostars, especially early O-type stars with masses greater than 30 solar masses, the role of accretion disks in their formation is still not clear.

At about 26,000 light-years from Earth, the Galactic Center is a unique and important star-forming environment.

In addition to the supermassive black hole Sgr A*, the Galactic Center contains a huge reservoir of dense molecular gas, mostly in the form of molecular hydrogen (H2), which is the raw material for star formation. The gas begins to form stars as soon as the gravitational collapse begins.

However, the environment at the Galactic Center is unique: strong turbulence and strong magnetic fields, as well as tidal forces from Sgr A*, all significantly influence star formation in this region.

Since the distance between the Galactic Center and the Earth is vast and there are complex foreground pollutions, direct observations of the star-forming regions around the Galactic Center have been difficult.

The research team led by Dr. Lu used ALMA’s long baseline to achieve a resolution of 40 milliarcseconds. To get an idea of ​​how subtle this resolution is, an observer in Shanghai can easily spot a soccer ball in Beijing.

Through these highly sensitive ALMA observations, researchers have discovered an accretion disk at the Galactic Center.

The disk is about 4,000 astronomical units in diameter and surrounds an emerging early O-type star with about 32 times the mass of the Sun. This system is one of the most massive protostars with accretion disks and represents the first direct image of a protostellar disk in the Galactic center.

This discovery suggests that massive early O-type stars are undergoing a formation stage involving accretion disks, and this conclusion holds true for the unique environment of the Galactic Center.

What is even more interesting is that two spiral arms are clearly visible in the disk. Such arms are often found in spiral galaxies but rarely seen in protostellar disks.

Typically, spiral arms arise in accretion disks as a result of fragmentation caused by gravitational instability. However, the disk found in this study is hot and turbulent, allowing it to balance its own gravity.

Trying to explain this phenomenon, the researchers proposed an alternative explanation – the spirals were caused by an external disturbance.

This finding clearly shows that accretion disks in the early evolutionary stages of star formation are subject to frequent dynamic processes such as flybites, and these processes can significantly influence the formation of stars and planets.

Interestingly, flibits could have originated in our solar system as well: A binary star system known as Scholz’s star flew past the solar system about 70,000 years ago, likely penetrating the Oort cloud and sending comets into the inner solar system.

The current study suggests that for more massive stars, especially in the high stellar density environment around the Galactic Center, such flybys should also be frequent.

“The formation of this massive protostar is similar to the formation of its lower-mass siblings, such as the Sun, with accretion disks and flybys.

Although stellar masses are different, some of the physical mechanisms of star formation may be the same. This provides important clues to the mystery of massive star formation,” Dr. Lu said.

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