This image from the Digitized Sky Survey (DSS) shows the region of the sky around the dead star RXJ0528+2838, which is located at the very centre of the image. Credit: ESO/ Digitized Sky Survey 2. Acknowledgement: D De Martin.

ESO astronomers have been surprised by the observation of a mysterious shock wave around a dead star.

Gas and dust flowing from stars can, under the right conditions, clash with a star’s surroundings and create a shock wave.

Now, astronomers using the European Southern Observatory’s Very Large Telescope (ESO VLT) have imaged a beautiful shock wave around a dead star — a discovery that has left them puzzled.

According to all known mechanisms, the small, dead star RXJ0528+2838 should not have such a structure around it.

This discovery, as enigmatic as it is stunning, challenges our understanding of how dead stars interact with their surroundings.

Co-author of the study, Simone Scaringi, associate professor at Durham University, UK, said: “We found something never seen before and, more importantly, entirely unexpected.”

Study co-lead Krystian Ilkiewicz, a postdoctoral researcher at the Nicolaus Copernicus Astronomical Center in Warsaw, Poland, added: “Our observations reveal a powerful outflow that, according to our current understanding, shouldn’t be there.”

‘Outflow’ is the term used by astronomers to describe the material that is ejected from celestial objects. 

The central square image, taken with the MUSE instrument on ESO’s Very Large Telescope, shows shock waves around the dead star RXJ0528+2838. When a star moves through space it can push away nearby material creating a so-called bow shock, which in this image is glowing in red, green and blue. The colours represent hydrogen, nitrogen and oxygen, respectively. These shocks are usually produced by a strong outflow expelled from the star. However, in the case of RXJ0528+2838 – a white dwarf with a Sun-like companion – astronomers discovered that the shock wave can’t be explained by any known mechanism. Some hidden energy source, perhaps magnetic fields, could be the answer to this mystery. Credit: ESO/ K Ilkiewicz and S Scaringi et al.

Bow shock

The star RXJ0528+2838 is located 730 light-years away and, like the Sun and other stars, it rotates around our galaxy’s centre.

As it moves, it interacts with the gas that permeates the space between stars, creating a type of shock wave called a bow shock.

Collaborator Noel Castro Segura, research fellow at the University of Warwick, explained a bow shock as: “a curved arc of material, similar to the wave that builds up in front of a ship.” 

These bow shocks are usually created by material outflowing from the central star, but in the case of RXJ0528+2838, none of the known mechanisms can fully explain the observations. 

RXJ0528+2838 is a white dwarf – the leftover core of a dying low-mass star – and has a Sun-like companion orbiting it.

In such binary systems, the material from the companion star is transferred to the white dwarf, often forming a disc around it.

While the disc fuels the dead star, some of the material also gets ejected into space, creating powerful outflows.

But RXJ0528+2838 shows no signs of a disc, making the origin of the outflow and resulting nebula around the star a mystery. 

Scaringi said: “The surprise that a supposedly quiet, discless system could drive such a spectacular nebula was one of those rare ‘wow’ moments.” 

Crucial

The team first spotted a strange nebulosity around RXJ0528+2838 on images from the Isaac Newton Telescope in Spain.

Noticing its unusual shape, they observed it in more detail with the MUSE instrument on ESO’s VLT. 

Ilkiewicz explained: “Observations with the ESO MUSE instrument allowed us to map the bow shock in detail and analyse its composition.

“This was crucial to confirm that the structure really originates from the binary system and not from an unrelated nebula or interstellar cloud.”

The shape and size of the bow shock imply that the white dwarf has been expelling a powerful outflow for at least 1,000 years.

Strong magnetic field

Scientists don’t know exactly how a dead star without a disc can power such a long-lasting outflow – but they do have a guess. 

This white dwarf is known to host a strong magnetic field, which has been confirmed by the MUSE data.

This field channels the material stolen from the companion star directly onto the white dwarf, without forming a disc around it.

Ilkiewicz said: “Our finding shows that even without a disc, these systems can drive powerful outflows, revealing a mechanism we do not yet understand.

“This discovery challenges the standard picture of how matter moves and interacts in these extreme binary systems.”

The results hint at a hidden energy source, likely the strong magnetic field, but this ‘mystery engine’, as Scaringi puts it, still needs to be investigated.

The data show that the current magnetic field is only strong enough to power a bow shock lasting for a few hundred years, so it only partly explains what the astronomers are seeing. 

To better understand the nature of such discless outflows, many more binary systems need to be studied. Scaringi said that ESO’s upcoming Extremely Large Telescope (ELT) will help astronomers to: “map more of these systems as well as fainter ones and detect similar systems in detail, ultimately helping in understanding the mysterious energy source that remains unexplained.”

The study is published in Nature Astronomy.