With an unexpected discovery, the international ALMA Orion Planck Survey of Galactic Cold Clusters (ALMASOP) team recently observed a young quadruple star system in the star forming region of the constellation Orion. The discovery was made during a high-resolution study of 72 dense nuclei in the Orion Giant Molecular Clouds (GMC) with the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. These observations provide a convincing explanation for the origin and formation mechanisms of binary and multiple star systems.

The group was led by Professor Liu Ti from the Shanghai Astronomical Observatory (CAS-SHAO). Researchers from CAS-SHAO, School of Astronomy and Space Sciences (CAS-SASS), NRC Herzberg Astronomy and Astrophysics, Japan National Astronomical Observatory (NAOJ), Max Planck Institute for Astronomy (MPIA), Korea Astronomy and Space participated. Institute of Science (KASI), Academia Sinica Institute for Astronomy and Astrophysics (ASIAA), NSF NOIRLab’s Gemini International Observatory, Armagh Observatory and Planetarium, NASA’s Jet Propulsion Laboratory, and numerous universities and institutes.

ALMA observation of the G206.93-16.61E2 star forming region shows 1.3 mm emission (blue) and egress of CO molecules (orange). Image credit: SHAO/ Qiuyi Luo and others. (2023)

It is a well-known fact among astronomers and astrophysicists that about half of the stars in the Milky Way are in binary systems. Knowing how multiple star systems formed is essential to understanding galactic evolution, planet formation, and the emergence of life. The most common theory of star formation (the nebula hypothesis) states that stars form in the densest regions of molecular clouds (called “dense nuclei”). Although this theory explains the individual star systems very well, the mechanisms that lead to the formation of multi-star systems are still not fully understood.

It is currently believed that multiple star systems formed through the disintegration of cloud cores during their early evolution, but historically observations are lacking. To investigate this mystery, the ALMASOP team examined 72 young and cold cores in GMC in the constellation Orion for thermal emissions corresponding to a wavelength of 1.3 mm in the extremely high frequency (EHF) range.
By observing the dense cold core at Orion B GMC about 1,500 light-years from Earth (labeled G206.93-16.61E2), they observed a system of four stellar objects.

They consisted of two protostars and two gas concentrations that are likely to undergo gravitational collapse in the near future. They also observed that the greatest distance between four objects in the system was about 1,000 astronomical units (AU), 33 times the distance between the Sun and Neptune (30 AU). This is in contrast to the last observation of the quad system in 2015 by another international team using ALMA. In this case, the research team observed a young protostar and three gravitationally dense gas clouds that will form new stars in about 40,000 years.

In this case, however, the observed quadrilateral system had a distance of well over 1000 astronomical units (AU). Emission spectra of the dust also revealed several long strip-like structures connecting the four objects and extending outward. To determine the role these structures play, the team ran numerical simulations comparing a quadruple system similar to the one they observed. Based on the results, the team suggests that these elongated strips could be “funnels” that transport gas from the outer shell of the core to the protesters and connect the newborn stars.

Cluster G205.46-14.56 located in the Orion molecular cloud complex. Yellow contours represent dense nuclei detected by JCMT, and magnified images show sustained emission from the 1.3 mm ALMA observation. Image credit: Qiuyi Luo et al. (2022).

Said Luo Qiu-yi, Ph.D. Shao Academy of Sciences student and first author of the study:

“The extreme compactness and proximity of this system is an exciting discovery. The analysis shows that in the future this system will most likely form a gravitationally dependent four-star system. We have no explanation for how gas leaks propagate, as it may be related to gas deposition processes by system elements. It highlights the complex interactions between the members that make up a higher-order star system.”

Professor Liu added, “The simulations confirm that these strips can serve as large-scale accumulation streamers.” “Therefore, the two gas clusters in the system have the potential to form a star based on the strength of these continuous stripes. Additional streamers can also break apart and form new stars.”

Finally, the observations revealed complex gas outflows caused by stellar winds created by protostars in the system, resulting in the loss of some of the accumulated gas and dust. Similar to what is observed around active galactic nuclei (AGN), where winds produced by a supermassive black hole (SMBH) blast material from the center of the galaxy, this could influence the evolution of this system. In future observations with ALMA and other millimeter/submillimeter observatories, scientists hope to uncover more details about multi-star systems in formation. Source