More than half of all stars are born as members of multiple star systems, but the process of multiple star formation is not well understood. Therefore, unraveling the mystery of the multi-star formation mechanism is very important to establish a comprehensive star formation theory. To date, several scenarios for multiple star formation have been proposed, and discussions on formation scenarios have yet to converge.
Artistic impression of three protostars IRAS 04239+2436. Source: ALMA (ESO/NAOJ/NRAO)
To understand the formation process of multiple stars, it is necessary to use high-resolution and high-sensitivity instruments like ALMA to directly observe the moment when multiple protostars (stars in the process of formation) are born. In addition, recent observations of protostars often report gas structures called "streamlines," which are flows of gas toward the protostar.
Observing these streamlines is important because they show how the protostar absorbed gas and grew, but it is currently unclear how these streamlines formed. Because gas flows around protostars in multi-star systems are expected to have complex structures, detailed observations using ALMA's high resolution are a powerful tool for studying the origins of gas flows.
Gas distribution around the ternary protostar IRAS04239+2436, (left) SO emission observed by ALMA, (right) numerical simulation reproduction by supercomputer ATERUI. Protostars A and B, shown in blue on the left, represent radio waves from the dust surrounding the protostar. In Protostar A, two unresolved protostars are thought to exist. In the image on the right, the positions of the three protostars are indicated by blue crosses. Source: ALMA (ESO/NAOJ/NRAO), J.-E. Leeetal. Leeetal.
Detailed observations and discoveries
The team used ALMA to observe radio waves emitted by sulfur monoxide (SO) molecules around the young multi-star system IRAS04239+2436. IRAS04239+2436 is a "ternary protostar system", a system composed of three protostars, about 460 light-years away from us. The research team expects to detect SO molecules in the region where the shock wave occurs, and to see violent gas movements around the protostar. As a result of the observation, they detected SO molecules around three times the protostar, and found that the distribution of SO molecules formed a large spiral arm extending up to 400 astronomical units. In addition, they successfully obtained the velocity of gas containing SO molecules based on the frequency shift of radio waves caused by the Doppler effect.
Based on the analysis of gas motion, they found that the spiral arms traced by SO molecules were indeed streamlines flowing to the triple protostar. "The most profound feature of our ALMA images is the detection of large, well-defined multi-arm structures in the SO radiation," said Li, explaining the significance of the discovery. "My first impression was that these structures were dancing together, rotating around the central protostar system, but later we discovered that the spiral arms are channels of material that feed small stars."
Supercomputer "ATERUI" simulates multiple star formation. The film shows that multiple protostars are born in filamentous turbulent gas clouds, which excite spiral arms and disturb the surrounding gas as they move. Source: Tomaki Matsumoto, Takaaki Takeda, 4D2U project, National Astronomical Observatory of Japan
Meaning and comparative analysis
To further study the gas movement, the research team compared the gas velocity derived from this observation with the velocity derived from numerical simulations. These simulations were performed using the astronomy-dedicated supercomputers "ATERUI" and "ATERUIII" at the National Astronomical Observatory of Japan's Center for Computational Astrophysics. In the simulation, three protostars formed in the gas cloud, and the disturbed gas around the three protostars excited shock waves in the form of spiral arms.
"We found that the spiral arms exhibit gas flows toward the three protostars; they are the streamlines that feed the gas to the protostars," said Matsumoto, who led the study's numerical simulations. "The simulated gas velocities are in good agreement with the observations, showing that numerical simulations can indeed explain the origin of the streamlines."
Hybrid scheme for multi-star formation
By comparing observational data and numerical simulation results, the team studied how this triple protostar was born. So far, there are two options for the formation of multiple stars. The first is the "turbulent fragmentation scenario", in which a turbulent gas cloud fragments into gas condensates, and each condensate evolves into a protostar. The second is the "disk fragmentation scenario", in which the gas disk around a protostar fragments to form a new protostar, thereby producing multiple stars.
The tripling of protostars observed here can be explained by a hybrid scenario in which the star formation process begins with a turbulent proto-gas cloud, similar to the turbulent fragmentation scenario, and then new protostar seeds are produced in the disk, similar to the disk fragmentation scenario, and the surrounding gas turbulence causes the spiral arms to extend widely. The observations are very similar to the simulations, suggesting that the observed triplet protostar is the first object confirmed to have formed multiple stars through a mixing scheme.
"This is the first time that the origins of protostars and meteors have been comprehensively elucidated simultaneously. The combination of ALMA observations and simulations is a powerful tool for revealing the secrets of star formation," Matsumoto said.
Implications for planet formation and future research
Li believes the study also sheds light on the difficulties of planet formation in multi-star systems. She said: "Planets are born in the disk of gas and dust that formed around the protostar. In this three-protostar system, the protostar is located in a small region, the disk around the protostar is small, and the protostar orbiting the protostar peels the disk away from other protostars. Planets are formed in a long-term calm environment. Therefore, IRAS04239+2436 is unlikely to be an environment conducive to planet formation."
Matsumoto discusses the implications of this research for our understanding of multiple star formation. "The actual observation of a forming multi-star system through a hybrid scheme will go a long way toward resolving the debate about multi-star formation schemes. Furthermore, this study not only confirms the existence of recently noted streamlines, but also explains how they form, marking a major advance."
Jeong-EunLee and others introduced this research in the paper "Triple spiral arms of triple spiral arms of triple protostar system imaged by molecular lines" published in the Astrophysical Journal.