Close encounter: interstellar fly-by

Rare stellar flyby event caught in action

How do we uncover the galactic past of our solar system? Its history is long gone—searching for it here is like sifting through a burned village after a fire. Instead, scientists look to other early solar systems to find the clues to our history. University of Victoria astronomer Ruobing Dong looks to the Z Canis Majoris (Z CMa) solar system—where he has made a breakthrough discovery.

Together with an international research team, Dong uncovered the first evidence of a stellar flyby disturbing the protoplanetary disk—or birthplace of planets—using the Atacama Large Millimeter/submillimeter Array (ALMA) and the Karl G. Jansky Very Large Array (JVLA). The intruder object came in close proximity to and interacted with the environment surrounding the binary protostar, causing the formation of chaotic, stretched-out streams of dust and gas in the disk.

Few convincing direct observations have ever been made, and until now, these events have remained largely theoretical. Flybys happen fast so it is difficult to catch them in action. 

“People have long speculated that lightning could cause forest fires. To prove that hypothesis, you would have to record the moment lightning strikes a tree and lights it. What we have done with our ALMA and JVLA observations is equivalent to capturing lighting striking a tree,” explains Dong.

Scientists have speculated flyby events on a few young star systems before however; those suspected “intruders” might have been bound to the primary stars. Stars are born in clusters and will experience close encounters with one another as their orbits settle. 

“In other words,” explains Dong, “we may be observing siblings within one family of stars playing with each other. Many stars are born in twins, triplets, or multiples. Interactions among siblings are different in nature and have different consequences from interactions with other star families.”

By examining the changes in Z CMa’s disk morphology—essentially flyby fingerprints—the team was able to identify the intruder, offering the most comprehensive evidence of an intruder flyby event to date.  

The meeting place of the theoretical and observational is key to this discovery. Dong and co-author Hau-Yu Baobab Liu (Institute of Astronomy and Astrophysics at Academia Sinica) were looking for evidence of gravitational instability in Z CMa system when they discovered the flyby. Meanwhile Nicolas Cuello (Université Grenoble Alpes) had just published new simulations of flybys in protoplanetary disks—which looked exactly like what Dong and Liu had observed. 

Dong happened to be giving talks at Universidad de Chile and the ALMA headquarters in 2019 when he saw Cuello. “I was like ‘Nico, I think I saw something you just simulated!’” remembers Dong, “And he was like ‘Great! Let’s compare the models with observations’. From there we finally settled on the flyby scenario. Sometimes, good things happen when people talk.”

Armed with new evidence the planet pioneers plan to investigate the frequency of flybys and how important such events are to the planet formation processes in circumstellar disks. “The two stars in the Z CMa binary systems are both experiencing accretion outbursts,” says Dong, “Previous theoretical models suggest that flyby events may trigger stellar accretion outbursts. Could this idea be further tested in the Z CMa system, as now we have a specific flyby event example?”

The team may also investigate the nature of the intruder object. It is yet to be seen at infrared wavelengths: is it an extremely young stellar object whose emission concentrate at ALMA and JVLA wavelengths or a star with an edge on disk so that it is obscured at optical and infrared wavelengths? 

The answer lies in a galaxy far, far away.