A schematic view of the history of the accretion disk and the flyby object (a – c). The three plots starting from the bottom left are snapshots from the numerical simulation, capturing the system right at the flyby event, 4,000 years after, and 8,000 years after, respectively. The top right image is from the ALMA observations, showing the disk with spirals and the two objects around it, corresponding to the system at 12,000 years after the flyby event (d).
Image credit: Lu et al.
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Accretion disks around protostars, also known as ‘protostellar disks,’ are essential components in star formation because they continuously feed gas into protostars from the environment. In this sense, they are stellar cradles where stars are born and raised. Accretion disks surrounding solar-like low-mass protostars have been extensively studied in the last few decades, leading to a wealth of observational and theoretical achievements. For massive protostars, especially early O-type ones of more than 30 solar masses, however, it is still unclear whether and how accretion disks play a role in their formation. These massive stars are far more luminous than the Sun, with intrinsic luminosities up to several hundreds of thousands of times the solar value, which strongly impact the environment of the entire Galaxy. Therefore, understanding the formation of massive stars is of great importance.

At a distance of about 26,000 light-years away from us, the Galactic Center is a unique and important star-forming environment. The most well-known object here would certainly be the supermassive black hole Sgr A*. Other than that, 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 start to form stars once gravitational collapse is initiated. However, direct observations of star-forming regions around the Galactic Center are challenging, given the large distance as well as the contamination from foreground gas between the Galactic Center and us. A very high resolution, combined with high sensitivity, is necessary to resolve details of star formation in this region.

The research team has used the long-baseline observations of ALMA to achieve a resolution of 40 milliarcseconds. At such a resolution, we can easily spot a baseball hidden in Osaka from Tokyo. With these high-resolution, high-sensitivity ALMA observations, the team has discovered an accretion disk around the Galactic Center. The disk has a diameter of about 4,000 astronomical units and is surrounding a forming early O-type star of 32 solar mass. “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,” said Qizhou Zhang, a co-author and an astrophysicist at the Center for Astrophysics, Harvard & Smithsonian. This discovery suggests that the formation of massive early O-type stars does go through a phase with accretion disks involved, and such a conclusion is valid for the Galactic Center.

What is more interesting is that the disk 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 disk discovered in this study is hot and turbulent, thus able to balance its own gravity. The team detected an object of about three solar masses at about 8,000 astronomical units away from the disk. Through a combined analysis of analytic solutions and numerical simulations, they reproduce a scenario where an object flew by the disk more than 10,000 years ago and perturbed the disk, leading to the formation of spiral arms. “The numerical simulation matches perfectly with the ALMA observations. We conclude that the spiral arms in the disk are relics of the flyby of the intruding object,” said Xing Lu, the lead author and an associate researcher at the Shanghai Astronomical Observatory of the Chinese Academy of Sciences.

This finding clearly demonstrates that accretion disks at the early evolutionary stages of star formation are subject to frequent dynamic processes such as flybys, which would substantially influence the formation of stars and planets. It is interesting to note that flybys have happened in our own solar system too. A binary stellar system known as Scholz’s Star flew by the solar system about 70,000 years ago, probably penetrating through the Oort cloud and sending comets to the inner solar system. This study suggests that for more massive stars, especially in the high stellar density environment around the Galactic Center, such flybys should be frequent as well. “The formation of stars should be a dynamical process, with many mysteries still being unresolved,” said Xing Lu. “With more upcoming high-resolution ALMA observations, we expect to disentangle these mysteries in star formation.”

Additional information 

These research results were published as X. Lu et al, “A massive Keplerian protostellar disk with flyby-induced spirals in the Central Molecular Zone,” in Nature Astronomy (DOI:10.1038/s41550-022-01681-4).

This research was supported by the initial funding of scientific research for high-level talents at Shanghai Astronomical Observatory, JSPS KAKENHI Grant Number JP20K14528, and the National Natural Science Foundation of China grants W820301904 and 12033005.

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Organisation for Astronomical Research in the Southern Hemisphere (ESO), the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the Ministry of Science and Technology (MOST) and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI).

ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.