A close encounter more than 10,000 years ago sparked spirals near the galactic center

Cambridge, MA – An international research team from China, USA and Germany, including Qizhou Zhang at the Center for Astrophysics | Harvard & Smithsonian, used the ALMA observatory and discovered an accretion disk with two spiral arms surrounding a young star near the center of the Milky Way galaxy.

The disc may have been disrupted by a close encounter with an object being flown over, leading to the formation of the spiral arms, the team wrote in a recent paper by natural astronomy.

The accretion disks around young stars, or protostars – also called “protostellar disks” – are essential components of star formation because they continuously supply gas to growing stars from the surrounding environment. In this sense, they are stellar cradles where stars are born and grow. The accretion disks surrounding low-mass solar-like protostars have been extensively studied over the past decades, leading to a multitude of observational and theoretical achievements.

For massive protostars like the one seen in the study, especially early O-type ones over 30 solar masses, it remains unclear if and how accretion disks play a role in their formation. These massive stars are much more luminous than the Sun, with intrinsic luminosities that can reach several hundred thousand times the solar value, which strongly impact the environment of the entire galaxy. Therefore, understanding the formation of massive stars is of great importance.

The central region of the Milky Way, known as the Galactic Center, lies about 26,000 light-years from Earth and is a unique and important star-forming environment. The supermassive black hole Sagittarius A*, located at the very center of the Milky Way, is the best-known object in the region.

Additionally, there is 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 will begin to form stars once gravitational collapse is initiated. However, direct observations of star forming regions around the galactic center are difficult, given the considerable distance and foreground gas contamination between the galactic center and Earth. Very high resolution, combined with high sensitivity, is needed to resolve star formation details in this region.

For this study, the research team used ALMA’s long-range observations to achieve a resolution of 40 milliarcseconds. Using these high-resolution, high-sensitivity ALMA observations, the team discovered the accretion disk.

The disk is about 4,000 astronomical units in diameter and surrounds a protostar (specifically an O-type star) of 32 solar masses.

“This system is among the most massive protostars with accretion disks and represents the first direct imaging of a protostellar accretion disk in the galactic center,” Zhang said. “The discovery suggests that the formation of the first massive O-type stars goes through a phase involving accretion disks.”

What may be more interesting, the research team adds, is that the disc clearly displays two spiral arms. Such spiral arms resemble those found in spiral galaxies but are rarely seen in protostellar disks. Spiral arms could emerge in accretion disks due to fragmentation induced by gravitational instabilities. However, the disc discovered in this study is hot and turbulent, thus able to balance its gravity.

The team detected an object about three solar masses about 8,000 astronomical units from the disk. Through a combined analysis of analytical solutions and numerical simulations, they replicate a scenario where an object flew past the disk more than 10,000 years ago and disrupted the disk, causing spiral arms to form.

“The numerical simulation corresponds perfectly to ALMA’s observations. We conclude that the spiral arms in the disk are relics from the flyby of the intrusive object,” says Xing Lu, the lead author and associate researcher at the Shanghai Astronomical Observatory of the Chinese Academy of Sciences.

This discovery demonstrates that accretion disks in the early evolutionary stages of star formation are subject to frequent dynamical processes such as flybys, which are believed to significantly influence star and planet formation. Interestingly, flybys have also occurred in our solar system, according to the team. A binary star system known as Scholz’s Star flew past the solar system around 70,000 years ago, likely penetrating through the Oort cloud and sending comets into the inner solar system.

This study suggests that for more massive stars, especially in the high-density stellar environment around the galactic center, such flybys should also be frequent.

“Star formation is expected to be a dynamic process, with many mysteries still unsolved,” says Lu. “With more high-resolution ALMA observations to come, we hope to unravel these mysteries of star formation.”

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