A Star Passed too Close and Tore Out a Chunk of a Protoplanetary Disk

When it comes to observing protoplanetary disks, the Atacama Large Millimetre/sub-millimetre Array (ALMA) is probably the champion. ALMA was the first telescope to peer inside the almost inscrutable protoplanetary disks surrounding young stars and watch planets forming. ALMA advanced our understanding of the planet-forming process, though our knowledge of the entire process is still in its infancy.

According to new observations, it looks like chaos and disorder are part of the process. Astronomers using ALMA have watched as a star got too close to one of these planet-forming disks, tearing a chunk away and distorting the disk’s shape.

What effect will it have on planetary formation?

A team of researchers led by Ruobing Dong, an astronomer at the University of Victoria in Canada, made the discovery. Their paper is titled “A likely flyby of binary protostar Z CMa caught in action.” They published their paper in the journal Nature Astronomy.

Astronomers know that when a star forms, the left-over material resides in a disk around the star called a protoplanetary disk. Planets form from that material. We know that they form via accretion, but there’s a lot of questions around how exactly that happens.

ALMA was the first to observe this process in any detail. In 2014 astronomers using ALMA imaged the disk around the young star HL-Tauri in greater detail than ever before. The image showed gaps in the disk, which scientists interpreted as “lanes” where planets are forming, sweeping up gas and dust and leaving a gap in the process.

ALMA still spends some of its time peering into protoplanetary disks. In this new study, a team of researchers used ALMA and the Very Large Array (VLA) to examine the disk around the Z Canis Majoris (Z CMa) star system. They spotted the aftermath of a stellar interloper’s path through the star’s disk. The interloper wasn’t part of Z CMa, and its path through the system left an imprint. It warped Z CMa’s disk structure and created a stretched-out gas and dust stream.

Z CMa is a pre-main-sequence group of stars about 3750 light-years away in the constellation Canis Major. Astronomers thought that Z CMa was a binary pair, but this study identified two additional probable stars in the system. The stars in Z CMa are young, only about 300,000 years old. At that age, planets haven’t formed yet. The stars are still in their mass-accretion phase themselves.

Astronomers know more about these stellar fly-bys from simulations than we do from observations. Researchers have only observed a few of them because they’re difficult to see. According to lead author Dong, it’s like trying to watch lightning strike a tree.

“Observational evidence of flyby events is difficult to obtain because these events happen fast, and it is difficult to capture them in action. What we have done with our ALMA Band 6 and VLA observations is equivalent to capturing lightning striking a tree,” said Ruobing Dong, an astronomer at the University of Victoria in Canada and the principal investigator on the new study. “This discovery shows that close encounters between young stars harbouring disks do happen in real life, and they are not just theoretical situations seen in computer simulations. 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.” 

This isn’t the first time astronomers have observed the stream and wondered about it. “The origin of the streamer has been a long-standing puzzle,” the authors point out in their paper. Previous hypotheses for the streamer say it could be the wall of a cavity carved out by molecular outflow, a spiral arm created by gravitational instability, or the remnant of a recently-captured low-mass cloudlet.

But the identification of a point source at the end of the streamer is a new development.

A “point source” is astronomy jargon for a potential star or other object. In this case, the researchers found a point source (C) at about 4700 astronomical units from Z CMa. The source is at the end of the stream of gas and dust, which is about 2000 AU long. They interpreted the point source as the interloper and the stream as the effect. The team says that point source C hasn’t been observed in optical or infrared yet. Their initial observations show that it’s likely a one million-year-old planetary-mass object or a solar-mass young stellar object (YSO) shrouded by gas and dust.

In the Z CMa system, the flyby disrupted the disks of two stars, both of which showed “…accretion outbursts, which may be facilitated by perturbations to the host disk by flybys,” they write in their paper.

Solitary stars like our Sun are not the norm. Most stars are in binary pairs or even in triplets or quadruplets. In those systems, disk disruptions aren’t rare. But it’s frequently the companion stars that disrupt the disks, not interlopers. This makes Z CMa stand out.

Hau-Yu Baobab Liu is an astronomer at the Institute of Astronomy and Astrophysics at Academia Sinica in Taiwan and a co-author on the paper. In a press release, Liu said, “Most often, stars do not form in isolation. The twins, or even triplets or quadruplets, born together may be gravitationally attracted and, as a result, closely approach each other. During these moments, some material on the stars’ protoplanetary disks may be stripped off to form extended gas streams that provide clues to astronomers about the history of past stellar encounters.”

Our Solar System has experienced its own disruptive events. There’ve been no stellar fly-bys that we know of, but there’ve been other events that have shaped the planets. Jupiter’s migration changed the course of events in our system, and something knocked Uranus onto its side. A collision between the young Earth and a protoplanet probably created the Moon. So we know that disruptive events shape solar systems. But these events in our Solar System took place later when the planets were already formed.

What does a stellar flyby mean for planetary formation at such an early stage as in Z CMa? What are the long-term effects?

That’s open to speculation at this point. In an e-mail exchange with Universe Today, lead author Dong talked about some possibilities. He prefaced his remarks by saying that we don’t know much about the effects yet.

“In general, we could think of a flyby as a “shake up” of the system as it perturbs the material distribution in the disk, and there may be both pros and cons on planet development,” Dong said.

Dong made an analogy to building something out of wooden blocks. If the block structure is bumped and knocked over, that’s bad. But what if we’re not building something out of wooden blocks, and instead, we’re making a stir-fry? In that case, it could result in a better stir-fry.

“So I guess in a protoplanetary disk, a flyby event may cause material to mix, i.e., pieces of material could run into each other after a flyby, and they wouldn’t have a chance to meet if the disk had not been perturbed. This could be good in forming objects or growing them to larger sizes.”

That’s only one potential outcome, Dong explained. But if the young system was already forming planetesimals, it could destroy the nascent planets. “On the other hand, if loosely bound pebbles (or larger objects) have already formed, a flyby event may cause them to collide with each other, and those collisions may destroy already formed objects, in which case perturbations are not so good,” he said.

How could these flybys affect the star itself? In the Z CMa system, the interloper caused accretion outbursts. Could the outbursts affect the solar system’s frost line? Would the frost line be pushed further out?

“Another independent effect is that flyby events may cause the central star to have outbursts, thus for a period of time, the star becomes a lot brighter. In those events, the frost lines would be pushed further from the star. Ices may melt or evaporate, and droplets and molecules may interact with each other in the air. This could cause a series of ripple events both in chemistry and in physics, so complicated that I’m not sure if we could simply classify them as “good” or “bad” for planet formation,” Dong said.

Dong said that studying the evolution and growth of young star systems throughout the galaxy helps scientists better understand our own Solar System’s origin. “Studying these types of events gives a window into the past, including what might have happened in the early development of our own Solar System, critical evidence of which is long since gone. Watching these events take place in a newly forming star system provides us with the information needed to say, ‘Ah-ha! This is what may have happened to our own Solar System long ago.’ Right now, VLA and ALMA have given us the first evidence to solve this mystery, and the next generations of these technologies will open windows on the Universe that we have yet only dreamed of.”

There’s no question that a flyby like this one would affect the system’s future. But what exactly those effects are is an open question and one that Dong would like to address in future work.

“All in all, I suppose what we can say is that we expect flyby events to have some profound impacts on planet development, and the exact effect in a specific system may depend on the stage of planet formation in that system,” Dong concluded.

More:

• Press Release: ALMA Catches “Intruder” Redhanded in Rarely Detected Stellar Flyby Event
• Research Paper: A likely flyby of binary protostar Z CMa caught in action
• Universe Today: Here are 20 Protoplanetary Disks, With Newly Forming Planets Carving Out Gaps in the Gas and Dust