When Cores Collide: New Clues from Barnard 68
Barnard 68 (B68) is a small, dense cloud of gas and dust known as a Bok globule. These clouds are often studied because they represent the earliest stages of star formation. B68, in particular, has drawn attention for its isolated location and nearly spherical shape, making it an ideal subject for observing how stars begin to form. However, recent simulations proposed a more dramatic story: a smaller “bullet”-like core may be colliding with B68. In this study, Dalei Li and collaborators aimed to test this hypothesis by searching for observable signs of such an event.
Why Sulfur Monoxide?
To look for evidence of a collision, the team used the Effelsberg 100-meter radio telescope to observe emissions from sulfur monoxide (SO), a molecule that becomes more abundant in areas affected by shock waves. These emissions were mapped across B68, allowing the team to trace the gas's motion and chemical makeup. Unlike other molecules previously studied in B68, the SO emission lines were clean and simple, offering a clearer view of the underlying motions within the cloud.
Revealing the Motion of Gas
From the SO velocity maps, three distinct velocity components were detected: one associated with the main body of B68, another with the southeastern bullet, and a third transitional component. The central part of B68 showed a smooth velocity gradient that matched a solid-body rotation model—like a spinning disc. The bullet, however, showed a relative motion of 0.4 km/s toward the main cloud. This velocity is remarkably close to the 0.37 km/s predicted by previous simulations, reinforcing the idea that a collision is indeed taking place.
Chemical Clues from SO Distribution
To better understand the chemical effects of the collision, the team analyzed how SO is distributed within the cloud. As expected, the central regions of B68 showed signs of SO depletion, which is common in cold, dense environments. However, at the edge where the bullet meets the main cloud, the SO abundance was significantly higher—likely caused by shock waves generated by the impact. The team confirmed this interpretation by using a radiative transfer model (RATRAN) that successfully reproduced the observed intensity patterns when assuming a step-like increase in SO abundance outside a central depletion zone.
Putting the Puzzle Together
Altogether, the motion and chemistry of the gas support the core-core collision scenario. Not only does the bullet appear to be colliding with B68 at the expected velocity, but the resulting shock waves are enhancing the SO emission at the point of impact. This suggests the collision is already influencing the structure and dynamics of B68. The researchers also noted a previously observed elongated structure near the impact zone, possibly created by tidal forces from the collision—though it was not resolved in the current SO data due to limited resolution.
A Trigger for Star Formation?
This study provides strong observational support for the idea that collisions between dense gas cores can trigger the gravitational collapse needed to form stars. In the case of B68, what once appeared to be a calm, stable cloud may be undergoing a transformation sparked by this very kind of collision. The findings highlight how detailed molecular observations—especially of species like SO—can uncover the dynamic processes hidden within cold, dark clouds.
Concluding Thoughts
By combining velocity measurements, chemical analysis, and modeling, Li and colleagues offer a compelling case that the southeastern bullet is impacting the rotating main core of B68. Their work not only confirms earlier predictions but also advances our understanding of how star formation can be triggered by interactions between molecular cloud cores. For students exploring the building blocks of stars, B68 offers a real-world example of how complex and dynamic these early processes can be.
Source: Li
