First ALMA animation of encircling young twin stars

Artist’s impression of the young XZ Tau binary system. The two young stars in the system each have a protoplanetary disc around them that is tilted relative to each other. The two young stars orbit in a different plane from that of either of the discs. (Credit: ALMA (ESO / NAOJ / NRAO))

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The researchers analyzed accumulated data from the Atacama Large Millimeter / submillimeter Array (ALMA) and plotted the motion of a young XZ Tauri twin star system over three years. This very first “ALMA animation” of twin stars sheds new light on the origins of binary stars and the planets that will form around them.

“This achievement was made possible by the high resolution and rich archival data of ALMA,” says Takanori Ichikawa, first author of the research paper and former graduate student at Kagoshima University, Japan. “This research used three years of observational data. The results show the feasibility of a new research method using radio astronomical animations instead of conventional images. Hope this method will help to clarify various astronomical phenomena in the future.

The Sun is a single star, but the Universe is full of binary stars, which are two stars orbiting each other. During its youth, each young star in a binary system is surrounded by a protoplanetary disk made up of molecular gas and dust. This disc is known to be the site of the formation of the planets. Many planets associated with binary stars have in fact been detected, but how disks form in binary star systems and how planets form in these systems remains a mystery.

“In order to study the formation of planets in binary star systems, it is important to accurately determine the orbital motion of the two stars and the inclination of individual protoplanetary disks,” explains Shigehisa Takakuwa, professor at the University of Kagoshima.

The researchers suggested two training mechanisms for binary systems; one is the bursting of a single large gaseous disc and the other is the fragmentation of the largest molecular cloud due to violent turbulence. In the first case, astronomers assume that the orbit of binary stars and individual disks should be on the same plane. On the other hand, in the latter case, the orbital plane of the binary stars and the plane of the disks should be different. This is a major problem that will affect the final orbits of the planets in binary systems.

The research team searched the ALMA data archives and obtained data from the young XZ Tau system taken in 2015, 2016 and 2017. They carefully analyzed the data and for the first time performed an animation of the movement. orbital of binary stars, showing that XZ Tau B has moved 3.4 astronomical units (3.4 times the radius of Earth’s orbit) around XZ Tau A during those three years.

The team discovered the three-dimensional structure of the orbit. In addition, by analyzing the Doppler effect and the distribution of the disk’s radio waves around each star in the XZ Tau system, they found that these disks are significantly misaligned with respect to each other, and also not in the same plane as the binary orbit.

Previous observations with ALMA had found examples of young binary stars with protoplanetary disks tilted relative to each other. However, this is the first time that the orbital motion of a binary system has been clarified, showing that the tilt is different from that of circumstellar disks. These results support the idea that the XZ Tau system was formed via molecular cloud fragmentation.

“This is a great example of using ALMA’s rich archives,” says Takakuwa. “The archives pave the way for young researchers to immediately start conducting cutting-edge research. This is true for this study. “I am very honored to contribute to such interesting research as an undergraduate student,” said Miyu Kido of Kagoshima University, the second author of the research paper. “I hope to use this experience in my own future research.”

These observational results have been published in Takanori Ichikawa et al. “Misaligned circumstellar discs and orbital motion of the young binary XZ Tau” in the Astrophysical Journal on September 23, 2021.

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