An encounter ten thousand years ago stirred up a vortex in the accretion disk!

During the star formation process, an accretion disk is formed around the newborn star. This accretion disk, also known as a "protostellar disk," is a key link in the star formation process. Newborn stars continue to gather gas from the environment through the accretion disk and gradually grow. Therefore, the accretion disk can be said to be the cradle of the birth and growth of stars. Astronomers have been studying the accretion disks of low-mass stars similar to the sun for decades, and both observational and theoretical results are relatively rich. However, for more massive stars, especially early O-type stars with more than 30 times the mass of the sun, it is still unclear whether accretion disks also exist during their formation. These more massive early stars are much brighter than the sun, with a luminosity of up to hundreds of thousands of times that of the sun, and can dramatically affect the environment of the entire galaxy. Therefore, it is of great significance to understand the formation process of these massive stars.

Recently, Associate Researcher Lyu Xing of the Shanghai Astronomical Observatory of the Chinese Academy of Sciences, in collaboration with Yunnan University, the Harvard-Smithsonian Center for Astrophysics in the United States, and the Max Planck Institute in Germany, used high-resolution observation data from the Atacama Millimeter/Submillimeter Array (ALMA) to discover a massive newborn star accretion disk in the direction of the center of the Milky Way that was passed closely by surrounding celestial bodies, thus producing a spiral arm structure. This new discovery proves that the formation process of massive stars is similar to that of low-mass stars, and both will experience processes such as accretion disks and flybys. The results were published in Nature Astronomy on May 30.

Superaccretion disk near the center of the Milky Way

The center of the Milky Way is about 26,000 light-years away from us and is a unique and important star-forming region. There is a supermassive black hole Sgr A* here, as well as dense hydrogen molecular gas, the raw material for star formation with tens of millions of solar masses. Once these gases collapse under the action of self-gravity, they will begin to form stars. However, the center of the Milky Way has a very special environment, such as strong turbulence, strong magnetic fields and tidal effects of Sgr A*, which can drastically affect star formation activities. Therefore, the star formation process in the center of the Milky Way may be different from the star-forming process we are familiar with around the solar system.

However, because the center of the Milky Way is too far away from the Earth, and there is a complex foreground gas obstruction between the center of the Milky Way and the solar system, these factors make it very difficult for astronomers to directly observe the star-forming region in the center of the Milky Way. Therefore, astronomers must choose telescopes with extremely high resolution and sensitivity to observe and study the details of star formation.

The research team led by Lv Xing used the ALMA interferometer array in Chile, South America, to conduct long-baseline observations of the central region of the Milky Way, with a resolution of about 40 milliarcseconds. The observation accuracy at such a resolution is like being able to clearly see a football in Beijing while standing in Shanghai.

With such high-resolution and high-sensitivity observations, researchers discovered an accretion disk with a diameter of about 4,000 AU in the area near the center of the Milky Way, which is rotating around an early O-type star with a mass of 32 times that of the sun. This is one of the most massive protostars with accretion disks discovered so far, and it is also the first time that astronomers have directly imaged the protostar disk at the center of the Milky Way.

This discovery shows that accretion disks are indeed involved in the formation of massive early O-type stars, and this conclusion still holds true in special environments such as the center of the Milky Way.

External disturbances affect the evolution of accretion disks

What is different is that the accretion disk seen by scientists such as Lü Xing has a pair of obvious spiral arm structures. This spiral arm structure is common in galactic disks, but relatively rare in protostellar disks. It is generally believed that this spiral arm structure is caused by the gravitational instability of the accretion disk itself, which leads to its fragmentation. However, this study found that the gas temperature and turbulence in the accretion disk of this massive early O-type star are high, enough to maintain the stability of the accretion disk itself. Therefore, the researchers believe that there is another possible explanation, that the spiral arms are caused by external disturbances. A few thousand astronomical units away from this accretion disk, there happens to be an object with a mass of three times that of the sun, which may be the source of the external disturbance.

To verify this hypothesis, the researchers first used analytical calculations to check dozens of possible historical trajectories of this celestial body, and found that it could only disturb the accretion disk under one trajectory. Subsequently, the researchers used numerical simulations on the high-performance supercomputer platform of the Shanghai Astronomical Observatory to track this trajectory, reproducing the complete process of this celestial body passing over the accretion disk more than 10,000 years ago and stirring up a spiral arm structure in the accretion disk. It is worth mentioning that this type of numerical simulation takes a long time and takes about a week to run. Because the researchers used analytical calculations to find the only suitable trajectory in advance, there is no need to repeatedly try different physical conditions, but to hit the bull's eye in one run, saving a lot of time. In the end, the results of both analytical calculations and numerical simulations completely corresponded to the observations. Therefore, the spiral arms in this accretion disk are likely to be relics left behind by the visit of surrounding celestial bodies.

This discovery fully demonstrates that in the early stages of star formation, the evolution of accretion disks will be frequently affected by dynamic processes such as flybys, which will significantly affect the formation of stars and planets. Therefore, when studying the evolution of accretion disks, they cannot be viewed as isolated systems, but these dynamic effects should be carefully considered.

Interestingly, there is evidence that about 70,000 years ago, a binary star system called Schulz's Star flew close to the solar system, which may have disturbed the Oort Cloud and sent a batch of comets into the inner solar system. The results of this study suggest that for more massive stars, especially in high-star density environments such as the center of the Milky Way, such flybys should be extremely frequent.

"The formation process of this massive star is somewhat similar to that of low-mass stars like the sun, both of which involve accretion disks and flybys. Although the masses are different, some physical mechanisms in the star formation process are unified. This provides important clues to solve the mystery of massive star formation." Lv Xing said, "We have submitted a new ALMA observation application, hoping to increase the resolution by another 3 times and push the telescope to the limit, in order to see the details hidden in this accretion disk."