Two's company: Scientists identify new class of star remnants - Astronomy & Space - Astronomy

Credit: Institute of Science and Technology Austria

In about 5 to 8 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.

Hydrogen emission spectra alternating between two peaks over the remnant’s six-minute spin period indicate a half-ring of material circling the star. Credit: Aayush Desai/Andrei Cristea/ISTA

"We initially thought it was a binary system," says Andrei Cristea, a Ph.D. 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.

For the surrounding material to be trapped in a half-ring configuration, the object must have a strong and asymmetric magnetic field, the ISTA scientists argue. Credit: Russell C. J. Kightley

"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 Ph.D. 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!"

New class of star remnants. Researchers at the Institute of Science and Technology Austria (ISTA) find two isolated, ultra-massive, X-ray emitting, highly magnetic, and rapidly rotating white dwarfs. Left to right: Ph.D. student Andrei Cristea, Assistant Professor Ilaria Caiazzo, and Ph.D. student Aayush Desai. ISTA. Credit: ISTA

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. 

Provided by Institute of Science and Technology Austria 

by Institute of Science and Technology Austria

edited by Gaby Clark, reviewed by Robert Egan 

Source: Two's company: Scientists identify new class of star remnants

Ultrathin membranes could transform hydrocarbon processing by slashing energy use - Engineering - Energy & Green Tech

Credit: Pixabay/CC0 Public Domain

A team of international researchers has developed a new class of ultrathin polymer membranes that can rapidly and selectively separate complex hydrocarbon mixtures, potentially transforming how crude oil is refined and refinery streams are processed, significantly reducing the energy required for one of the world's most energy-intensive industrial processes.

The study, "Ultrathin polymer membranes with locked intrinsic microporosity for hydrocarbon fractionation," has created a new way to form the separating layers in polymer membranes for molecular separations. The breakthrough derives from the way the cross-linking agent for the polymer film is added to the polymer during membrane fabrication.

The work is published in the journal Science.

It results in a scalable membrane technology capable of separating complex organic mixtures into valuable fractions with unprecedented efficiency. The membranes combine extremely high molecular selectivity with fast liquid transport—a combination that has long eluded scientists and engineers working in this field.

Re-thinking a century-old process

Conventional crude oil refining relies on thermal distillation, a process that consumes vast amounts of energy and accounts for around 1% of global energy use. Although membrane technologies have long promised a far more energy-efficient alternative, their industrial uptake has been limited by fundamental materials challenges.

"Membranes can, in principle, do the same job as distillation or evaporation, using far less energy," explains lead researcher Andrew Livingston, professor of chemical engineering and vice president of research and innovation at Queen Mary University of London, and CEO of Exactmer.

"The problem has been finding materials that are both fast and selective when exposed to real hydrocarbon mixtures."

Locking pores at the nanoscale

The breakthrough reported in this study lies in a new way of manufacturing polymer membranes so that their nanoscale pores are "locked" in place during formation.

The researchers focused on polymers of intrinsic microporosity, materials known for their sponge-like structure containing sub-nanometer pores. While these pores are ideal for separating molecules by size and type, the polymers normally swell when exposed to hydrocarbons, causing the pores to expand and lose selectivity.

To overcome this, the team developed an in-situ cross-linking approach that stabilizes the polymer structure while the membrane is being formed. This process locks the pores in their optimal configuration, producing what the researchers call polymers of locked intrinsic microporosity (PLIMs).

"The key was stabilizing the structure before the polymer had a chance to swell," explains Dr. Zhiwei Jiang, who led the research as head of membrane research at Exactmer and who is now assistant professor at Nanyang Technological University in Singapore.

"This preserves the tiny pores that make molecular separation possible, while still allowing hydrocarbons to flow through very quickly."

To probe the molecular origins of locking, the UCL team, led by Dr. Foglia, used quasi-elastic neutron scattering at the ISIS Neutron and Muon Source, the U.K.'s national pulsed neutron facility and an unrivaled tool for studying polymer chain dynamics.

Exceptional performance in crude oil and refinery streams

When tested with synthetic crude oil, PLIM membranes showed up to 10-fold higher permeance than existing state-of-the-art membranes while maintaining high selectivity. The membranes were able to discriminate effectively between hydrocarbon molecules that differ only slightly in size.

In tests using real Arabian Extra Light crude oil, the membranes:

• Removed 99.8% of hydrocarbons heavier than 15 carbon atoms
• Reduced sulfur-containing compounds by 93%, a critical step in protecting downstream catalysts and equipment

The membranes also performed particularly well with refinery streams such as virgin naphtha. In these tests, they efficiently separated light hydrocarbons (C4–C6), suitable for fuel upgrading, from heavier naphtha fractions used to produce plastics and chemicals—all at permeances comparable to commercial desalination membranes.

Designed for scale-up

Crucially, the researchers demonstrated that the membranes can be manufactured at scale. Using roll-to-roll processing, they produced sheets more than a meter wide and integrated them into standard spiral-wound membrane modules commonly used in industry.

"These membranes aren't just laboratory curiosities," said Dr. Adam Oxley, first author of the research paper and now deputy vice president of membranes at Exactmer. "They can be produced using established manufacturing techniques and fitted into existing industrial module designs. At Exactmer, we are building these new techniques into membranes used for high-value separations in organic solvents."

Long-term testing showed stable performance over 30 days of continuous operation, indicating strong potential for real industrial deployment.

A more sustainable pathway for refining

While the global energy system is transitioning toward lower-carbon alternatives, demand remains for fuels, chemicals, solvents and materials derived from hydrocarbons. Improving the efficiency of existing separation processes is therefore essential to reducing emissions during the transition period.

By enabling membrane-based separations that are both fast and selective, the PLIM technology could allow industries from oil refining to pharmaceuticals to:

• Cut energy consumption dramatically
• Reduce carbon emissions
• Operate with smaller, more flexible processing units
• Integrate selective desulfurization earlier in the refining process

The researchers note that the same pore-locking concept could be extended to other liquid separation challenges, including chemical manufacturing, solvent recovery and emerging bio-based feedstocks.

Looking ahead

The team is now exploring greener solvents for membrane manufacture and investigating how PLIM membranes could be deployed in targeted hybrid processes alongside existing refinery infrastructure and the manufacture of high-value pharmaceuticals in organic solvents.

"This work shows that membrane-based molecular separation in organic liquids is no longer just a theoretical possibility," said Livingston. "With the right materials design, it can be fast, selective, scalable—and ready for industry."

Dr. Zachary P. Smith, associate professor of chemical engineering, Massachusetts Institute of Technology (MIT), said, "As all chemists know, 'like dissolves like.' So how can you separate hydrocarbon liquids using a hydrocarbon polymer without the polymer itself dissolving while in use? Livingston and his team have developed an approach to 'lock' their polymers in place, making them stable under aggressive conditions.

"More than that, they have shown that this approach works with some of the newest and most innovative emerging polymers in membrane science, helping to push the field into untapped areas of application."

Ryan P. Lively, professor in the School of Chemical & Biomolecular Engineering at the Georgia Institute of Technology, added,

"One of the key technological barriers facing membrane deployment in crude oil refining [is/was] the very low productivity of the membrane units. The membranes from Livingston's research are more than 100 times more productive than the first-generation membrane materials—the fact that this was achieved along with improved separation efficiency is a remarkable achievement.

"The composition of the membrane selective layer is interesting. The polymer backbones used had been considered previously, and cross-linked polymers had been considered previously, but the special combination that the team discovered really hit a sweet spot in terms of membrane performance.

"Being able to go from a small postage-stamp test to a full-size membrane module in such a short time indicates that the prospects for membrane-based oil refining are bright. Indeed, this article and others in the academic literature continue to indicate that there are real economic and environmental benefits to moving forward with membranes for oil refining at larger and larger scales." 

Provided by Queen Mary, University of London 

by Queen Mary, University of London

edited by Sadie Harley, reviewed by Robert Egan 

Source: Ultrathin membranes could transform hydrocarbon processing by slashing energy use