Exotic Peacocks of the Stellar World: A New Model for Colliding-Wind Binaries

A student from the University of Sydney in Australia has led a paper describing the Apep star system - dubbed as "one of the exotic peacocks of the stellar world."

In astronomy, Wolf-Rayet stars are unusually rare - a heterogeneous set of stars with broad emission lines of ionized helium, nitrogen, or carbon - in the sense that these are incredibly luminous occurrences bound to collapse in a supernova event, only leaving massive darkness, like black holes.

Harder to find, even among Wolf-Rayet stars, are those that create a glowing spiral plume. Under the right set of conditions, these sets of stars give off vast amounts of carbon dust swept by stellar winds, appearing like a pair of tails as the pair orbit each other.

The Apep triple star system, however, seems to break previously-established rules concerning these star systems. Han's work, published in the journal Monthly Notices of the Royal Astronomical Society, reports the details and offers a new model explaining this rare object's behavior.

From Apep, The Egyptian God of Chaos

"Aside from the stunning image, the most remarkable things about this star system is the way the expansion of its beautiful dust spiral left us stumped," said Yinuo Han, an honors student at the Sydney Institute for Astronomy, School of Physics at the University of Sydney.

He added that the dust in the Apep systems appear "to have a mind of its own," describing that its dust tail flows much slower than the extreme winds that were supposed to drive it.

It was first discovered by a team that included the University of Sydney Professor Peter Tuthill and Netherlands Institute for Radio Astronomy researcher Joseph Callingham, who studied the system from 2016 to 2018 and published a report soon after. It was named after Apep, the serpent deity who represents chaos and is depicted as the enemy of the sun god Ra.

Using high-resolution imaging techniques available at the European Southern Observatory's Very Large Telescope, located on Cerro Paranal in Chile's Atacama Desert, Han and his team could better observe the star system. Han likened the magnification that made his breakthrough possible to "seeing a chickpea on a table 50 kilometers away."

Creating an Updated Geometric Model

With support from the high angular resolution near-infrared imaging studies they conducted near the heart of Apep, researchers were able to generate an "improved geometric model" that is supported by gathered data on the system. This new model constrained the orbital parameters of the Wolf-Rayet binary and can also explain the anomaly in its extreme anisotropic winds.

Tuthill expressed admiration for their "relatively simple model," being able to reproduce the spiral geometry observed from Apep's dust clouds. However, they noted that the actual physics involved is not as simple. The dust spiral trailing the Wolf-Rayet binary expands four times slower than the stellar winds supposedly moving them. Researchers noted that this was not yet observed in other similar systems.

Callingham noted that Wolf-Rayets had been the subject of multiple studies as objects that are "elegantly beautiful, but potentially dangerous."