Scientists are the first to discover the environmental characteristics of the birthplace of the star’s “multiple twins.”

AlmassOP, an international cooperation team led by Liu Tie of the Shanghai Astronomical Observatory, used the Maxwell Telescope (JCMT) led by the Center for Astronomy and Science of the Chinese Academy of Sciences and the Atacama Large Millimeter Wave/Submillimeter Wave Array (ALMA) in Chile to observe the star-forming region of Orion, and found that the formation of a small-mass multi-star system is related to the physical properties of the “cloud nucleus” of its birthplace – it is easier to form a multi-star system in a denser, more turbulent region. This is the first time astronomers have explored the formation of a multi-star system from the scale of a cloud core. This research result laid the foundation for the formation theory of the construction of multi-star systems and the subsequent search for multi-star systems buried deep in the clouds and in the early stages of star formation. The research results were published in the prestigious astronomy journal Astrophysical Journal in June 2022.

Astronomers observing star-forming regions within the Milky Way have found that mature “twins,” “triplets,” and even “quadruplets” are common among stars. In previous studies, due to telescope resolution limitations, astronomers generally focused only on the incubation process of individual “stellar embryos.” In recent years, as telescope resolution has increased, more and more “multiple embryos” have been identified— they are all in a gravitational bondage system, with each member embryo at an unequal distance between each member embryo. However, for astronomers, the origins of these binary/multi-star systems remain an unsolved mystery.

In order to explore the environment and mechanism of these “multiple” star formation, researchers in the ALMASOP science team set their sights on the birthplace of stars, the “cloud nucleus”. Cloud nuclei are the cradles of cold gas and dust in molecular clouds that form stars through gravitational collapse. Each cloud core has a different gas density, mass, size, and flow characteristics of the gas inside. However, in previous studies, astronomers have not considered these physical properties of the “cloud nucleus” of the birthplace of stars, nor have they studied how these properties affect the number of stars born within the cloud nucleus.

In response, the ALMASOP team members used the JCMT telescope to discover 49 dense and cold cloud nuclei that are forming stars in the constellation Of Orion, while using ALMA’s ultra-high resolution to reveal the internal structure of these dense cloud nuclei for the first time. The researchers found that of the 49 cloud nuclei, 13 are nurturing multi-star systems, while others are forming single stars.

JCMT observations gave the researchers the physical characteristics of these 49 cloud nuclei: their size averaged about 0.1pc (20,626 solar-terrestrial distances), and their masses were only a few solar masses. However, when the researchers compared the cloud nuclei that formed only single stars separately from the cloud nuclei that formed multiple stars, they found that the column density and number density of the hydrogen molecules in these cloud nuclei were significantly different. “We believe that the cores of clouds that form binary/multi-star systems tend to have a higher gas density than those that form single stars, and the size of the cloud nuclei does not show a significant difference.” Luo Qiuyi, the first author of the work and a doctoral student at the Shanghai Observatory, said, “This phenomenon is easy to understand, and the denser cloud core will be susceptible to further fragmentation by internal self-gravitational disturbances.” She also noted that multi-star systems are easier to spawn in denser and more turbulent cloud nuclei.

The ALMASOP team also used the Nobesan 45m (NRO-45m) telescope to observe the N2H+ molecular lines of these 49 cloud nuclei. Observations show that the line width of the N2H+ molecule is greater in cloud nuclei that only form binary/multi-star stars. Professor Tatematsu, who led the NRO-45m observation project, said the observation of N2H+ molecules at NRO-45m reflected turbulence levels within the cloud nucleus well, and the study demonstrated that binary/multi-star systems are more likely to form in chaotic cloud nuclei.

Co-author of the study, Lu Shengyuan of the Taiwan Institute of Astronomy and Astrophysics (ASIAA), said: “The observational data and analysis results conducted by JCMT have laid the cornerstone for subsequent observations and research work at ALMA, and the unprecedented high sensitivity and high resolution provided by ALMA allow us to see protostars buried deep in the cloud nucleus.” So we can do similar studies on larger samples of dense cloud nuclei in the hope of a more thorough understanding of star formation. ”

Figure: G205.46-14.56 Cloud cluster in the constellation Orion. The yellow contour line represents the dense cloud nucleus found by JCMT, and the embedded image is the 1.3 mm continuous spectral radiation observed by ALMA. These observations have given astronomers a better understanding of the formation of various star systems in dense cloud nuclei.

“We also need to consider the effects of magnetic fields in future analyses. Magnetic fields may inhibit the fragmentation of dense cloud nuclei, so we are looking forward to using JCMT and ALMA in the next phase of research in this area.” Liu Tie, co-corresponding author of the project and head of the ALMASOP project, looked forward to it. (Source: Shanghai Astronomical Observatory, Chinese Academy of Sciences)

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