Stars are similar in size to small “childhood experiences” For the first time, astronomers have achieved direct imaging of the protostar disk at the center of the Milky Way

During star formation, an “accretion disk” is created around the newborn star. Accretion disks, also known as “protostellar disks”, are a key link in the process of star formation, and studying accretion disks is of great significance to understanding the formation process of these massive stars.

Recently, Lü Xing, an associate researcher at the Shanghai Astronomical Observatory of the Chinese Academy of Sciences, cooperated with scholars from Yunnan University, the Harvard-Smithsonian Center for Astrophysics in the United States, and the Max Planck Institute in Germany to use the high-resolution observation data of the Atacama Millimeter/Submillimeter Array Telescope (ALMA) to discover a massive neonatal star accretion disk that was skimmed by surrounding celestial bodies at close range and produced a spiral arm structure in the direction of the center of the Milky Way. The discovery proves that massive and low-mass stars form a similar process, both undergoing processes such as accretion disks and flybys. On May 30, the study was published in Nature-Astronomy.

Numerical simulation of the flyby process .png

Numerical simulation of the flyby process Courtesy of the interviewee

Who moved the “Stellar Baby Dinner Plate”

“The accretion disk is like a ‘stellar baby’s dinner plate’, which continuously transmits the nutrients needed for development to the stellar baby.” Lu Xing, the paper’s first author, told China Science Daily, “Newborn stars can grow up gradually by continuously accumulating gas from the environment through accretion disks.” ”

The Sun is an ordinary star, and astronomers have accumulated decades of research on the formation of Sun-like stars and have a good understanding of their accretion disks.

But there are some “big” stars in the Milky Way that weigh dozens of times more than the Sun. In particular, early O-type stars with more than 30 times the mass of the Sun can change the physical and chemical properties of the interstellar medium in the Milky Way through feedback effects such as intense radiation, stellar winds and supernova explosions, thus affecting the evolution of galaxies. Therefore, it is important to understand the formation process of these massive stars.

How did these “big” stars “eat” at the beginning of their birth? Are there also accretion discs as their plates? These questions have been plaguing astronomers.

Discover the “large plate”

Compared to the secluded galactic edge of the solar system (which is at the edge of the Milky Way), the center of the Milky Way resembles a noisy and bizarre downtown area. There is not only the supermassive black hole Sgr A*, but also the dense hydrogen molecular gas, which provides abundant raw materials for massive star formation. But strong tidal action from Sgr A* and surrounding star clusters has made it difficult for the gas to settle down and to breed new stars.

However, previously, astronomers in the center of the Milky Way accidentally found a large group of stellar babies after eating and drinking “full burps” – dozens of hidden outflows. This means that in the lively environment at the center of the Milky Way, baby stars can still thrive.

“The process of star formation in the central region of the Milky Way may be different from the star-making process around the solar system that we are familiar with.” “However, because the central region of the Milky Way is too far from Earth, and there is a complex foreground gas occlusion between the center of the Milky Way and the solar system, it is very difficult to observe the star formation region at the center of the Milky Way.” ”

To understand the process of star formation at the center of the Milky Way, it is necessary to choose a telescope with extremely high resolution and sensitivity. To this end, the scientific research team led by Lü Xing used the ALMA interference array in Chile, South America, to conduct a long baseline observation of the central region of the Milky Way galaxy, with a resolution of 40 milliarcoseconds.

“The accuracy of observation at this resolution is like standing in Shanghai and can clearly see a football in Beijing.” Lu Xing explained.

Sure enough, with high-resolution, high-sensitivity observations, the researchers saw a large accretion disk with a radius of 2,000 times the distance from Earth to the Sun. By studying how quickly the accretion disk rotates, the team extrapolated the mass of the star at the center of the disk and found that it was a “hunk” with 32 times the mass of the sun.

This is one of the most massive nascent stars with accretion disks found so far, and it is the first direct imaging of the protostellar disk in the central region of the Milky Way. The researchers found that this large star, like ordinary stars like the Sun, “eats” through an accretion disk.

Restoring the “encounter” of 10,000 years ago

Even more unexpectedly, the researchers found that the accretion disk had a pair of distinct spiral arm structures.

“This spiral arm structure is common in galactic disks, but rare in protostellar disks.” Li Guangxing, one of the corresponding authors of the paper and an associate professor at Yunnan University, explains, “It is generally believed that this spiral arm structure is caused by the gravitational instability of the accretion disk itself, resulting in fragmentation. However, we found that the accretion disk of this massive early O-type star has a higher gas temperature and strong turbulence, which is enough to maintain the stability of the accretion disk itself. Thus, one possible explanation is that the spiral arm is generated by external perturbations. ”

In further observations, the researchers found a celestial body with 3 times the mass of the Sun at a distance of thousands of astronomical units around the accretion disk, which may be the source of external perturbations.

To test this conjecture, the researchers examined dozens of possible historical trajectories of the object using analytical calculations and found that it could disturb the accretion disk only in one trajectory. They then tracked the trajectory using numerical simulations on the shanghai observatory’s high-performance supercomputer platform, recreating the object’s complete process of skimming the accretion disk more than 10,000 years ago and stirring up the spiral arm structure in the accretion disk.

In the end, both the analytical calculations and numerical simulation results correspond exactly to the observations. Therefore, it can be determined that the spiral arm in this accretion disk is likely to be a remnant left by the surrounding celestial body visitation process.

The findings confirm that, similar to small-mass stars such as the Sun, large star babies also use accretion disks as dinner plates, and that eating may be disturbed by external celestial bodies, causing the plates to deform and disrupt the eating rhythm.

“That is to say, the early formation process of large and small stars has undergone the same accretion disk, fly-by perturbation, etc., with similar ‘childhood experiences’.” Lü Xing explains, “Although there is a difference in mass, some physical mechanisms in the process of star formation are unified. ”

This provides important clues to solving the mystery of massive star formation, and the article’s reviewers believe that “it is fascinating to see flybys at such an early stage of evolution … Such interactions are of great significance for the study of accretion disks and planet formation. ”

At present, Lü Xing and others have submitted a new application for ALMA observation, “we hope to increase the resolution by another 3 times, push it to the limit of the ALMA telescope, and see more details hidden in this accretion disk.” (Source: China Science Daily, Zhang Shuanghu, Huang Xin)

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