An international team of astronomers has discovered just the second and third examples of a rare class of star system composed of two core stars orbiting each other and surrounded by a spectacular disk of gas and dust.

“If there were a planet in one of these systems,” says Michael Poon, one of the study authors, “it would be like the planet Tatooine from Star Wars.”

“You would see two suns in the sky orbiting each other. In addition,” he adds, “there’s a disk around the stars. Picture Saturn’s rings but much, much larger – with the stars in the middle.”

Due to the fact that these disks eventually give rise to families of planets similar to our solar system, they are known as protoplanetary disks. The recently found systems are unusual since their disks are at an angle to their central stars’ orbits.

“The discovery of objects like these,” according to J.J. Zanazzi, a postdoctoral scholar at the faculty’s Canadian Institute for Theoretical Astrophysics, “is important for our understanding of planet formation. Planets are born from them so the existence of disks around binary stars shows it’s likely we will find more planets orbiting binaries.”

“They’ll also help us understand whether life can exist on a planet that orbits a binary star at an angle because of how that orientation affects temperature and other conditions.”

A report detailing the discovery of Bernhard-1 and Bernhard-2 was published in Astrophysical Journal Letters.

Bernhard-1 and Bernhard-2 are so far away that we can’t see the two stars at their centers separately (such pairs of stars are known as binary stars). Instead, we merely detect a single light source and gauge the binary’s overall brightness.

The new objects were discovered by studying the intricate and unique fluctuations in brightness induced by their peculiar geometry. The new systems’ light curves are consistent with the light curve of the first such system ever identified, an object known as Kearns Herbst 15D (KH 15D).

The light curves of Bernhard-1 and Bernhard-2 both dip to a small portion of their maximum brightness on average every 112 and 62 days, respectively. These dips indicate that, as seen from Earth, one of the stars in each pair is migrating behind the disk. The system’s brightness returns to normal when the star reemerges.

Additionally, the co-authors discovered that both objects’ brightness varied over substantially longer periods of time when they compared recent observations with archival data dating back decades. This long-term pattern indicated that the disk and stars were at an angle to one another, according to the author’s earlier analysis of KH 15D and the work of other experts.

Like most of the planets and moons in our solar system, binary stars and their protoplanetary disks condense from the same massive, spinning cloud of material, thus the disk often lies in the same plane as the orbits of the stars. Think about two figure skaters who are swirling around each other while holding hands and skating on the same ice surface as other skaters.

But KH 15D, Bernhard-1, and Bernhard-2 are unusual because their circumbinary disks are at an angle to the planes of the orbiting stars. This inclination causes the disks to wobble like a spinning top as they travel between us and the stars, a phenomenon known as precession, which in turn reduces the brightness of the center stars. That cycle of fading for KH 15D could last anywhere between 60 and 6,000 years.

The signature light curve of objects like KH 15D is made up of two kinds of changes in brightness.

An amateur astronomer named Klaus Bernhard, who is also a member of the Bundesdeutsche Arbeitsgemeinschaft für Veränderliche Sterne, discovered Bernhard-1 and Bernhard-2 by examining data from the Zwicky Transient Facility. Every two days, the ZTF’s sensor scans the whole northern sky, collecting information on innumerable objects over extended periods of time.

Bernhard found candidates that resembled KH 15D by searching through the data. He then told other teammates about his results, and after more research, they discovered Bernhard-1 and Bernhard-2.

Now that scientists have found two more of these rare objects in space, they are hopeful that more will be found.

“Just this month, Gaia released its most recent data,” adds Zanazzi of the space mission that has been observing a billion stars in the Milky Way Galaxy since its launch in 2013. “And now that we have this model for these objects, we’re hopeful we can use it to find more objects to add to the list.”

“We’re also hopeful more observers will look at Bernhard-1 and Bernhard–2 for longer periods,” adds Poon. “We’re lucky that KH 15D has been observed at a special time where its orientation causes the light of the central stars to dim. We’re confident that Bernhard-1 and Bernhard-2 also exist in this favourable orientation, so having more observations will increase our understanding of these rare objects.”

Image Credit: Poon, Zhu, Zanazzi, U of T; Sahl Rowther, et al, Warwick University

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