A close encounter more than 10,000 years ago caused spirals in the accretion disk

Dr. Lu Xing, a research associate at the Shanghai Astronomical Observatory (SHAO) of the Chinese Academy of Sciences, along with collaborators from Yunnan University, the Harvard-Smithsonian Center for Astrophysics and the Max Planck Institute , used high-resolution observations given from the Atacama Large Millimeter/submillimeter Array (ALMA) to discover a massive protostellar disk in the galactic center and determine how its spiral arms formed.

The group’s research shows that this disc was disrupted by a close encounter with a nearby object, leading to the formation of the spiral arms. This discovery demonstrates that the formation of massive stars can be similar to that of lower mass stars, through accretion disks and flybys.

The results were published in natural astronomy May 30.

During star formation, accretion disks appear around nascent stars. These accretion disks, also called “protostellar disks”, are an essential part of star formation. Accretion disks continuously supply the protostars with gas from the environment. In this sense, they are stellar cradles where stars are born and grow.

For massive protostars, especially early O-type ones over 30 solar masses, however, the role of accretion disks in their formation is unclear.

At a distance of approximately 26,000 light-years from Earth, the Galactic Center is a unique and important star-forming environment. Besides 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 once gravitational collapse begins.

However, the environment of the galactic center is unique, with strong turbulence and magnetic fields as well as tidal forces of Sgr A*, all of which significantly affect star formation in this region.

Since the distance between the galactic center and the Earth is huge and there are complicated foreground contaminations, direct observations of star forming regions around the galactic center have been difficult.

The research team led by Dr. Lu used ALMA’s long baseline observations to achieve a resolution of 40 milliarcseconds. To get a sense of the fineness of this resolution, it would allow an observer in Shanghai to easily spot a soccer ball in Beijing.

Using these high-resolution, high-sensitivity ALMA observations, the researchers discovered an accretion disk in the galactic center. The disk is about 4,000 astronomical units in diameter and surrounds a forming O-type star with a mass about 32 times that of the sun. This system is among the most massive protostars with accretion disks and represents the first direct imagery of a protostellar disk in the galactic center.

The finding suggests that massive early O-type stars go through a phase of formation involving accretion disks, and this finding holds true for the unique environment of the Galactic Center.

What is more interesting is that the disc clearly displays two spiral arms. Such arms are often found in spiral galaxies but are rarely seen in protostellar disks. In general, spiral arms emerge in accretion disks due to fragmentation induced by gravitational instability. However, the disc discovered in this research is hot and turbulent, which allows it to balance its own gravity.

In trying to explain this phenomenon, the researchers offered another explanation, namely that the spirals were induced by an external disturbance. The researchers offered this explanation after detecting an object about three solar masses – possibly the source of the external disturbance – several thousand astronomical units from the disk.

To verify this proposition, the researchers calculated several dozen possible orbits of this object. They found that just one of these orbits could perturb the disk at the observed level. They then carried out a numerical simulation on the high-performance supercomputing platform of the Shanghai Astronomical Observatory to trace the trajectory of the intruding object. Scientists managed to reproduce the whole story of the flying object near the disk more than 10,000 years ago, when it would have waved spirals in the disk.

“The good match between the analytical calculations, the numerical simulation and the ALMA observations provides strong evidence that the spiral arms in the disc are relics from the intrusive object’s flyby,” Dr Lu said.

This finding clearly demonstrates that accretion disks in the early evolutionary stages of star formation are subject to frequent dynamical processes such as flybys and that these processes can significantly influence star and planet formation.

Interestingly, flybys may also have taken place in our own solar system: a binary star system known as Scholz’s Star flew through the solar system around 70,000 years ago, likely penetrating through the cloud of Oort and sending comets into the inner solar system.

The current study suggests that for more massive stars, especially in the high-density stellar environment around the galactic center, such flybys should also be frequent. “The formation of this massive protostar is similar to that of its lower-mass cousins ​​like the sun, with accretion disks and flyby events involved. Although the stellar masses are different, certain physical mechanisms in the formation stars could be the same. This provides important clues to solve the mystery of massive star formation,” said Dr. Lu.