Research finds three baby stars fed by spiral arms of gas

This discovery is important for understanding the process of multiple star formation, which is still a mystery in astronomy. More than half of all stars are born in multiple star systems, but how they form is not well understood. There are several scenarios proposed for multiple star formation, but none of them can explain all the observations. 

Presence of streamers

One of the key features observed in many protostellar systems is the presence of streamers, which show how the protostars accrete gas from their surroundings. However, it is unclear how these streamers form and what role they play in multiple-star formation. By observing a triple protostar system with ALMA, the team was able to reveal the origin and structure of the streamers in unprecedented detail.

“The most striking feature of our ALMA images is the well-defined large multiple-arm structures detected in the SO emissions,” says Lee, who is the lead author of the study published in The Astrophysical Journal Letters. “My first impression was that they were dancing together, spinning around the central protostellar system, although, later, we found that the spiral arms are channels of material feeding the baby stars.” 

The team also suggests that the spiral arms may have formed due to gravitational instability in a rotating disk of gas and dust around the protostars. This instability could cause parts of the disk to collapse and form streamers that connect to the protostars.

To further investigate the gas motion, the research team compared the velocity of the gas derived from this observation with that from numerical simulations which reproduce the formation of multiple stars within the natal gas cloud. These simulations were conducted using “ATERUI” and “ATERUI II,” supercomputers dedicated to astronomy in the Center for Computational Astrophysics at the National Astronomical Observatory of Japan (NAOJ). 

In the simulation, three protostars form in the gas cloud, and the gas disturbed around the triple protostars excites shock waves in the form of spiral arms. “We found that the spiral arms exhibit gas flows toward the triple protostars; they are streamers supplying gas to the protostars,” says Matsumoto who led the numerical simulations in this research. “The velocity of the gas derived from the simulations and the observations matches well, indicating that the numerical simulation can indeed explain the origin of the streamers.”

Correlating observations and simulations

By comparing the observations with the numerical simulations, the research team investigated how this triple protostar was born. Up until now, two scenarios had been proposed for the formation of multiple stars. The first is the ‘turbulent fragmentation scenario,’ in which the turbulent gas cloud fragments into gas condensations, each of which evolves into a protostar. The second is the ‘disk fragmentation scenario,’ where the gas disk surrounding protostar fragments forms a new protostar, leading to multiple stars. 

The triple protostar observed here can be explained by a hybrid scenario in which the star formation process starts as a turbulent natal gas cloud, similar to the turbulent fragmentation scenario, and then, the seeds of new protostars are produced in the disk, like in the disk fragmentation scenario, and the surrounding gas turbulence causes the spiral arms to extend widely. 

The observational results are very similar to the simulation results, indicating that the observed triple protostars are the first objects confirmed to demonstrate the formation of multiple stars by a hybrid scenario. Matsumoto says, “This is the first time that the origin of protostars and the streamers have been simultaneously and comprehensively clarified. The combination of ALMA observations and simulations is a powerful tool for revealing the secrets of star formation.”

Lee suggests that this study also sheds light on the difficulty of planet formation in multiple star systems. She says, “Planets are born in gas and dust disks that form around protostars. In the case of this triple protostar system, the protostars are located within a small area, the disks around the protostars are small, and orbiting protostars strip the disks from other protostars. Planets form in a calm environment over a long period. Therefore, it is unlikely that IRAS 04239+2436 is a conducive environment for the formation of planets.”

Matsumoto discusses the impact of this study on our understanding of multiple star formation. “The actual observation of a multi-star system in formation through the hybrid scenario will significantly contribute to resolving debates over multiple star formation scenarios. Furthermore, this research didn’t only confirm the existence of the recently noticed streamers but also explained how they formed, marking a significant advancement.”

The findings were reported in The Astrophysical Journal Letters.

Study abstract:

Most stars form in multiple-star systems. For a better understanding of their formation processes, it is important to resolve the individual protostellar components and the surrounding envelope and disk material at the earliest possible formation epoch, because the formation history can be lost in a few orbital timescales. Here we present Atacama Large Millimeter/submillimeter Array observational results of a young multiple protostellar system, IRAS 04239+2436, where three well-developed large spiral arms were detected in the shocked SO emission. Along the most conspicuous arm, the accretion streamer was also detected in the SO2 emission. The observational results are complemented by numerical magnetohydrodynamic simulations, where those large arms only appear in magnetically weakened clouds. Numerical simulations also suggest that the large triple spiral arms are the result of gravitational interactions between compact triple protostars and the turbulent infalling envelope.