Binary T Tauri Stars Found Responsible for Carving Giant Bubble in Barnard 18 Molecular Cloud

Astronomers have found evidence of two baby stars forming within a dense cloud of gas and dust 450 light-years away. They identified a unique bubble in the center of a stellar nursery called Barnard 18 in the Taurus molecular cloud complex, which was probably created by the two emerging stars. This is only the second time astronomers have discovered such a bubble linked to a growing star's ejection of material, or "outflow." The discovery could provide insight into how stars shape their surroundings as they develop.

Star formation is a complex and chaotic process. It begins with a compact, cold cloud of dust and gasses including hydrogen. The cloud eventually collapses under its gravity, causing a clump to form and pull in more material from its surroundings. When the clump reaches a certain mass, the pressure and heat from within ignite the hydrogen and create a new star, as reported by Science Alert.

Molecular Wavelengths Differences

However, as the baby star grows and gains mass, it pushes against its surroundings. Some of the material is expelled along the star's magnetic field lines and escapes into space as jets. Additionally, the young star creates winds that clear out a large space within the cloud where it was born. These outflows are known as feedback, and they're believed to have a significant impact on the growth of protostars and the evolution of the interstellar medium - the gas and dust that floats in the gaps between stars.

Observing the inner workings of a star as it forms within a molecular cloud can be challenging due to the cloud's high density. Shorter wavelengths of light cannot pass through the cloud, but longer wavelengths can. Barnard 18 is a dark nebula, meaning it doesn't emit or reflect light and appears as a dark spot in optical observations. To gain insight into the cloud, a group of astronomers from the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) in China used radio wavelengths.

They analyzed the carbon monoxide signal, which can be used to map out the structures within a gas cloud, using two different radio telescopes. As a result, they discovered a bubble-like structure within the Barnard 18 molecular cloud. The additional examination uncovered even more information. According to Yan Duan, an astronomer from NAOC and the lead author of the team's paper, they discovered an outflow located in the center of the molecular bubble after analyzing data from the Five College Radio Astronomy Observatory (FCRAO) survey of the Taurus molecular cloud.

(Photo : (Duan et al., ApJ, 2023))
A three-color map of the bubble's structure. The blue corresponds to hydrogen, the red to carbon monoxide, and the green 250 μm observations.

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Binary T Tauri Stars

Barnard 18 also contains a previously identified object of interest, called HH 319, which is a Herbig-Haro object. These objects are formed when protostellar jets shoot out from the source stars at high speeds, hitting the molecular cloud and causing it to emit light. HH 319 is situated in the center of the outflow found by Li and his team, providing evidence for the source of the bubble.

However, there were multiple potential origins as stars can move, no star was visible at the center of the bubble, and many young stars were present in the area. After examining their positions, the scientists concluded that the bubble was likely created by a binary pair of T Tauri stars. These stars are less than a million years old, have not yet started hydrogen fusion, and are still gaining mass. The team determined that the binary is the most probable source for the current position of the bubble.

The team's research suggests that the bubble in Barnard 18 was created approximately 70,000 years ago by the activity of the binary T Tauri stars. This, they believe, showcases the significant impact that T Tauri stars can have on their surroundings. Further observations will be required to verify these results. The findings were published in The Astrophysical Journal.

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