Recently, the scientific research team of the Shanghai Observatory of the Chinese Academy of Sciences, in cooperation with Yunnan University, the Harvard Smithsonian Center for Astrophysics in the United States, and the Max Planck Institute in Germany, used the high-resolution observation data of the Atacama millimeter / submillimeter wave array telescope (ALMA) to discover a massive newborn Star accretion disk that was swept by nearby objects at a close distance in the direction of the center of the Milky Way galaxy, thus generating a spiral arm structure.
This new discovery proves that the formation process of massive stars is similar to that of low mass stars, both of which undergo accretion disks and flyovers. This achievement was published in the international academic journal Nature astronomy on May 30.
In the process of star formation, accretion disks are formed around the newborn stars. This accretion disk, also known as the "protostellar disk", is a key link in the process of star formation. Newborn stars continue to gather gas from their environment through accretion disks and grow up gradually. Therefore, the accretion disk can be said to be the cradle of star birth and growth. Astronomers have studied the accretion disks of small mass stars similar to the sun for decades, and there are abundant observational and theoretical results. However, it is not clear whether accretion disks also exist in the formation of stars with greater mass, especially early type O stars with more than 30 times the mass of the sun. These massive early stars are much brighter than the sun, and their luminosity can reach hundreds of thousands of times that of the sun, which can severely affect the environment of the whole galaxy. Therefore, it is of great significance to understand the formation process of these massive stars.
The center of the Milky way is about 26000 light-years away from us. It is a unique and important star forming region. There is a supermassive black hole SGR a*, and there are also tens of millions of solar mass star forming raw materials - dense hydrogen molecular gas. Once these gases collapse under the action of self gravity, they will begin to form stars. However, there are very special environments in the center of the Milky Way galaxy, such as strong turbulence and strong magnetic field, which will severely affect star formation activities. Therefore, the star formation process in the central region of the Milky way may be different from the familiar star formation process around the solar system. However, because the central region of the Milky way is too far away from the earth, and there is a complex foreground gas barrier 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 galaxy. Therefore, astronomers must choose a telescope with high resolution and sensitivity to observe and study the details of star formation.
The scientific research team used the Alma interferometer array located in Chile, South America, to carry out long baseline observations in the central region of the Milky Way galaxy, with a resolution of about 40 milliarcseconds. The observation accuracy at this resolution is just like that we can stand in Shanghai and clearly see a football in Beijing. With such high-resolution and sensitive observations, researchers found an accretion disk with a diameter of about 4000 astronomical units rotating around an early type O star 32 times the mass of the sun near the center of the Milky Way galaxy. This is one of the most massive protostars with accretion disks found so far, and it is the first time for astronomers to directly image the protostar disk in the center of the Milky Way galaxy. This finding shows that accretion disks are indeed involved in the formation of massive early type O stars, and this conclusion is still valid in the special environment such as the center of the Milky Way galaxy.
More surprisingly, the accretion disk has a pair of obvious spiral arms. This kind of spiral arm structure is common in galactic disks, but rare in protostellar disks. It is generally believed that this kind of spiral arm structure is caused by the gravitational instability of the accretion disk itself, resulting in fragmentation. However, this study found that the gas temperature in the accretion disk of this massive early type O star is high and the turbulence is strong, which is enough to maintain the stability of the accretion disk itself. Therefore, the researchers believe that there is another possible explanation, that is, the rotating arm is generated by external disturbance. At a distance of several thousand astronomical units around the accretion disk, researchers just found an object three times the mass of the sun, which may be the source of external disturbance. In order to test this conjecture, they first checked dozens of possible historical trajectories of this celestial body by using analytical calculation, and found that only under one trajectory can it disturb the accretion disk. Then the researchers tracked this trajectory on the high-performance supercomputer platform of the Shanghai Observatory using numerical simulation, and reproduced the complete process of the celestial body passing over the accretion disk more than 10000 years ago and stirring out the spiral arm structure in the accretion disk. The results of analytical calculation and numerical simulation are completely consistent with the observation results. Therefore, the spiral arm in this accretion disk is likely to be a relic left by the visiting process of surrounding celestial bodies.
This discovery fully shows that in the early stage of star formation, the evolution of accretion disk will be frequently affected by dynamic processes such as flyby, which will significantly affect the formation of stars and planets. The results of this study show that for more massive stars, especially in the high stellar density environment such as the center of the Milky way, this flyby should be extremely frequent.