A close encounter more than 10,000 years ago stirred up spirals in an accretion disk

Dr. Xing Lu, an associate researcher from Shanghai Astronomical Observatory, Chinese Academy of Sciences, together with his collaborators from Yunnan University, Harvard-Smithsonian Center for Astrophysics, and Max Planck Institute, have used high resolution observational data from the Atacama Large Millimeter/submillimeter Array (ALMA) to discover a massive protostellar disk in the Galactic Center. This disk has been perturbed by close encounter with a nearby object, leading to the formation of spiral arms. This finding demonstrates that the formation of massive stars could be similar to that of lower-mass stars, through accretion disks and flybys. The result is published in Nature Astronomy on May 30th.

During the formation of stars, accretion disks will arise around new-born stars. These accretion disks, also known as ‘protostellar disks’, are an essential component in star formation. Accretion disks 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, and 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, the environment in the Galactic Center is unique, with strong turbulence, strong magnetic fields, and tidal forces from Sgr A*. Therefore, star formation in this region would be substantially affected, and may exhibit differences than the well-studied star forming regions in the solar neighborhood. Yet, the distance between the Galactic Center and us is huge, and there are complicated foreground contaminations. As a result, direct observations of star forming regions around the Galactic Center have been challenging. Astronomers have to choose telescopes that can provide very high resolutions and high sensitivities, so as to resolve details of star formation in the Galactic Center.

The research team led by Dr. Lu have used the long baseline observations of the Atacama Large Millimeter/submillimeter Array (ALMA) to achieve a resolution of 40 milliarcseconds. At such a resolution, one can observe in Shanghai and easily spot a football hidden in Beijing. With these high resolution, high sensitivity ALMA observations, the researchers discovered an accretion disk in the Galactic Center. The disk has a diameter of about 4000 astronomical units, and is surrounding a forming early O-type star that is about 32 times the mass of the Sun. This system is among the most massive protostars with accretion disks, and represents the first direct imaging of a protostellar disk in the Galactic Center. The 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 unique environment of the Galactic Center. 

What is more interesting is that the disk clearly displays two spiral arms. Such spiral arms are often found in spiral galaxies, but are rarely seen in protostellar disks. Usually, spiral arms could emerge in accretion disks thanks to the fragmentation induced by gravitational instabilities. However, the disk discovered in this research is hot and turbulent, thus is able to balance its own gravity. As such, the researchers proposed an alternative explanation: the spirals are induced by external perturbations. The researchers happen to detect an object of about 3 solar masses at several thousand astronomical units away from the disk, which could be the source of the external perturbations. To verify this proposition, the researchers examined several dozens of possible orbits of this object using analytical calculations, and found that only one of these orbits is able to perturb the disk. Then, they carried out a numerical simulation on the high-performance supercomputing platform of Shanghai Astronomical Observatory, to trace the trajectory of the intruding object, and successfully reproduced the entire history of the object flying by the disk more than 10,000 years ago and stirring up spirals in the disk (Figure 1). It is worth noting that such simulations are time consuming, taking up to one week to complete. However, the researchers had found the only suitable orbit through analytical solutions, and therefore they did not have to attempt different physical conditions back and forth, but simply ran one simulation and got the solution in one shot, which is an efficient approach. In conclusion, the nice match among analytical calculations, the numerical simulation, and the ALMA observations, provides robust evidence that the spiral arms in the disk are relics of the flyby of the intruding object.

This finding clearly demonstrates that accretion disks at 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. As such, one cannot regard accretion disks as isolated systems when studying their evolution, but should take dynamic interactions into account. It is interesting to note that flybys may have taken place in our own solar system too: a binary stellar system named the 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. The current 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 this massive protostar is similar to it lower-mass cousins like the Sun, with accretion disks and flyby events involved. Although stellar masses are different, certain physical mechanisms in star formation could be the same. This provides important clues to solving the mystery of massive star formation.” says Dr. Lu, “We have submitted new requests to observe with ALMA to further improve the resolution by a factor of three, which will push ALMA to its very limit, in order to recognize finer details in this unique accretion disk.”
Figure 1: The three plots starting from bottom left are snapshots from the numerical simulation, depicting the system right at the flyby event, 4000 years after, and 8000 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 in the model 12,000 years after the flyby event.

Xing Lu, Shanghai Astronomical Observatory, CAS

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