The universe is a dangerous place, and sometimes protostar systems don't get to condense into stars and planets in peace. Astronomers have long theorized the process of planetary formation is sometimes disrupted when a star passes so close its gravity creates distortions. Now they have been amazed to see it in action, an event one called “equivalent to capturing lightning striking a tree.”
We know stars move around relative to each other. For example, 70,000 years ago a red dwarf came close enough to our solar system to change cometary orbits. With most stars being formed in relatively dense clusters, computer simulations suggest close encounters during the few million years it takes for star systems to form should be quite common. However, they're also so brief it takes a lot of luck to witness one.
Nevertheless, that is what Dr Ruobing Dong of the University of Victoria, Canada, and co-authors think they have done. In Nature Astronomy, they describe observations taken with the Atacama Large Millimeter/Submillimeter Array (ALMA) and the Karl G. Jansky Very Large Array (VLA) of the Z Canis Majoris (Z Cma) star system, which is a million years into becoming a binary star system.
Instead of the usual elliptical disk of gas and dust on its way to forming planets, Z CMa has long streams of material stretching out around it, which Dong and colleagues attribute to the gravitational influence of the intruder star.
“Observational evidence of flyby events is difficult to obtain because these events happen fast and it is difficult to capture them in action,” Dong said in a statement. “This discovery shows that close encounters between young stars harboring disks do happen in real life.... Prior observational studies had seen flybys, but hadn’t been able to collect the comprehensive evidence we were able to obtain of the event at Z CMa.”
It's not that unusual for protostellar disks to be disrupted, but in the vast majority of cases interactions within the system are thought responsible. Since most stars form as pairs or part of larger groups, stars can kick up pressure waves in the disk while orbiting each other. When spotting a disturbed disk, astronomers often can't tell if an intruder or the system stars are responsible.
However, “when a stellar encounter occurs, it causes changes in disk morphology – spirals, warps, shadows, etc. – that could be considered as flyby fingerprints,” Dr Nicolás Cuello of the Université Grenoble Alpes said.
In the case of Z CMa, the paper's authors didn't just consider the long streamers distinctive enough to indicate a flyby event, they used them to identify the intruder, now voyaging away from the system.
The suspected intruder is now 4,700 astronomical units (0.07 light-years) from Z CMa's stars. The stream it created in its passage extends almost half that distance.
Roughly 3,700 light-years from Earth, Z CMa is made up of a star about as massive as the Sun and one five times more massive. Notably, it is a “double burster” with both stars experiencing sudden dramatic outbursts as material from the disk falls on them. It's possible this is a coincidence, but Dong, Cuello, and co-authors are keen to investigate the possibility the intruder's influence is responsible.
For a long time it was thought that planetary systems were the exception, existing only when stars passed so close to each other one could draw out material that eventually became planets. We now know this is not true, that planets are the norm, and will form even in isolated star systems. Nevertheless, it appears likely flybys like this one, occurring at crucial times in a system's development, will alter the composition and orbits of planets that do form, and astronomers are keen to learn how.