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.
An international research team from China, the U.S., and Germany has
used high-resolution observational data from ALMA and discovered a
massive accretion disk with two spiral arms surrounding a 32 solar mass
protostar in the Galactic Center. This disk could be perturbed
by a close encounter with a flyby object, thus leading to the formation
of the spiral arms. This finding demonstrates that the formation
of massive stars may be similar to that of lower-mass stars through
accretion disks and flybys.
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, 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 undoubtedly be the supermassive black hole
Sgr A*. Besides that, there is a massive reservoir of dense molecular
gas, mainly 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 considerable distance and 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 used the long-baseline observations of ALMA to
achieve a resolution of 40 milliarcseconds. Wecan easily spot a baseball
hidden in Osaka from Tokyo at such a resolution. With these
high-resolution,high-sensitivityALMA 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. 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 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 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 also happened in our Solar System. 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 also be frequent. "The formation of stars should be a dynamical process, with many mysteries still unresolved," said Xing Lu. "With more upcoming high-resolution ALMA observations, we expect to disentangle these mysteries in star formation."
Scientific Paper
Additional Informaton
These research results were published by X. Lu et al. as "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 original image release was published by the National Astronomical Observatory of Japan (NAOJ)
an ALMA partner on behalf of East Asia.
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