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Using the Atacama Large Millimeter/submillimeter Array (ALMA), astronomers have discovered a massive accretion disk with two spiral arms around a 32-solar-mass protostar in the Galactic center.

A schematic view of the history of the accretion disk and the flyby object: 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. Image credit: Lu et al., doi: 10.1038/s41550-022-01681-4.

Protostellar disks — accretion disks around newborn stars — 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, 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.

In the new study, Harvard & Smithsonian’s Center for Astrophysics astronomer Qizhou Zhang and colleagues used ALMA to image an accretion disk of a few solar masses surrounding a massive protostar in the Sagittarius C molecular cloud in the Galactic center.

The disk has a diameter of about 4,000 astronomical units (AU) 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,” Dr. Zhang said.

“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 gravity.

The astronomers detected an object of about three solar masses at about 8,000 AU 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,” said Dr. Xing Lu, an astronomer at the Shanghai Astronomical Observatory of the Chinese Academy of Sciences.

“We conclude that the spiral arms in the disk are relics of the flyby of the intruding object.”

The team’s work was published in the journal Nature Astronomy.


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