In the vastness of the Universe, any new object with interesting properties can spur the search for similar objects, potentially establishing a new class of stars. In a paper published in Astronomy & Astrophysics and an arXiv preprint, researchers from the Institute of Science and Technology Austria (ISTA) describe two stellar remnants that share five properties, including X-ray emission, despite being isolated objects. According to the team, these two remnants are sufficient to define a new class of stars.
In about five to eight billion years, our Sun is expected to evolve into a white dwarf—an extremely dense, Earth-sized stellar remnant that has exhausted its fuel and shed its outer layer. But while our Sun is a solitary star, research over the past 15 years has demonstrated that binary or multi-star systems are far more common than astronomers once thought. When a dense and compact remnant like a white dwarf is involved in a binary system, it often 'snatches away' material from its companion star. This process, called accretion, usually emits X-rays in what is considered a 'signature' signal.
Now, scientists from the group of Ilaria Caiazzo, assistant professor at the Institute of Science and Technology Austria (ISTA) , confirm the detection of an X-ray signal in not just one, but two isolated objects called Gandalf and Moon-Sized. Highly magnetic and rapidly rotating, these two objects are called "merger remnants" as they each formed as a result of a violent cosmic collision. By emitting X-rays in the absence of a companion, they now form a new class of their own.
Gandalf—the Lord of the Half Rings?
Gandalf is not exactly a fresh discovery. Caiazzo first observed it during her postdoctoral research and classified it as an interesting object due to signals that hint at the presence of material around it.
"We initially thought it was a binary system," says Andrei Cristea, a PhD student in the Caiazzo group and first author of the paper published in Astronomy & Astrophysics, about Gandalf. "At the remnant's extremely high level of magnetism, its spin should be synchronized with its companion's orbit, similarly to Earth's rotation with the Moon's orbit," he adds. However, the fastest orbit period observed to date is 80 minutes. Gandalf, on the other hand, rotates on its axis every six minutes. According to Cristea, this is but one of its puzzling features.
"If Gandalf were involved in a binary system, it would have been highly unsynchronized, which might have made it even more puzzling than it already is. But we never found a companion. So, where does the circumstellar material come from?"
To help answer this question, the team drew on a clue from optical emission spectra, an observation technique widely used in astronomy.
"We saw hydrogen emission spectra that exhibited a double-peaked signature, similar to cat ears," says Cristea. "Usually, this signature indicates the presence of a disk of material surrounding a merger remnant. However, by examining the signal more closely, we realized that it was alternating between the two peaks over the remnant's six-minute spin period." This curious observation matched the existence of a half-ring of material circling the star. "We have never seen anything like that before in any white dwarf," he adds.
The team went on to argue that for the material surrounding the merger remnant to be trapped asymmetrically in a half-ring configuration, the object must have a strong and asymmetric magnetic field.
"To note, white dwarfs of similar age and evolutionary stage are typically nonmagnetic," says Cristea. "While highly magnetic white dwarf remnants are already an exception, Gandalf is now one of only two known merger remnants to feature asymmetric magnetization." All these puzzling reasons led Cristea to name this stellar object after the famous protagonist in J.R.R. Tolkien's novels, who likes to speak in riddles.
Moon-Sized—Gandalf's more evolved twin?
Even though the team did not find a companion for Gandalf, it might still have a 'twin' in a completely different area of the Universe.
When Caiazzo published her discovery of a white dwarf she called "Moon-Sized" in 2021 , this stellar object presented a range of unique properties. In addition to being very highly magnetic and rotating rapidly, it also packed a mass equivalent to the Sun into a size comparable to that of the Moon—or slightly larger, as the new evidence in an arXiv preprint led by Aayush Desai, another PhD student in the Caiazzo group, shows.
The ISTA astronomers found that Moon-Sized and Gandalf share five distinct characteristics. In addition to being ultra-massive, highly magnetic, and rapidly rotating, these two remnants are also companionless, and they both emit X-rays. These five common properties led the ISTA scientists to propose Gandalf and Moon-Sized as two members of a new class of remnants.
However, the two objects also differ significantly: unlike Gandalf, Moon-Sized shows no signs of material surrounding it. In addition, while Gandalf is the result of a collision that happened 60 to 70 million years ago, Moon-Sized is seven to eight times older, as its merger event took place around 500 million years ago. Another important difference is that Gandalf's X-ray emissions are 100 times brighter, suggesting that Moon-Sized might be an older, more evolved 'twin' that may be losing its source of X-rays.
What are the criteria for defining a new class of stars or remnants?
Astronomers agree that the closer an object is to us in the Universe, the more likely it is to be common. Nevertheless, any new object could spark interest in the community.
Caiazzo explains: "If we find one new object in the vastness of the Universe, what are the chances of it being the only one? Usually, one stellar object with new characteristics is more than enough for us to start looking for similar ones. But here, we actually found two objects with five overlapping features. This is plenty for a new class of star remnants!"
X-rays and the mysteries of stellar evolution
The team proposes several scenarios to explain their findings, particularly the source of the X-rays.
In the first scenario, a highly magnetized star could rotate rapidly enough to generate a powerful force that extracts material from itself. "This is my favorite scenario because it only accounts for the white dwarf itself rather than material originating from outside the star remnant," says Desai. According to the team, this so-called outflow scenario is known from highly magnetized neutron stars called pulsars, though it has never been modeled in a white dwarf remnant.
In their second scenario—this time involving an "inflow" of material— they propose that a 'leftover' trail of material originating from the merger event may not have completely accreted onto the star remnant following the blast. By orbiting around the merger remnant at high eccentricity—meaning moving away over a large orbit, far from the star, before returning closely—this trail could 'fall back' on the remnant over hundreds of millions of years.
In their third scenario, the team explores another source of "inflow" of external material.
"We know that a third of white dwarfs are 'polluted,'" says Desai. "They are so dense that we would expect external material, such as asteroids or even disrupted planetary bodies, to collapse onto them." While Gandalf shows some signs of pollution, possibly through carbon- or silicon-rich materials, the team did not detect such signals from the considerably older Moon-Sized. "This scenario seems less likely, as it does not fully explain why we see the X-rays in both objects right now," Desai explains.
Although the team has uncovered key insights about Moon-Sized and Gandalf, further research is needed to understand how these stars might influence their planetary systems.
"The two objects we identified so far have lots of similarities, but also differences," explains Desai. "Finding more such remnants will help us exclude scenarios and perhaps find other explanations altogether."
For now, the challenge remains to determine whether any of the five overlapping parameters is decisive for belonging to this new class.
