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’s 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 structure around it. This discovery, as enigmatic as it’s stunning, challenges our understanding of how dead stars interact with their surroundings.
“We found something never seen before and, more importantly, entirely unexpected,” says Simone Scaringi, associate professor at Durham University, UK and co-lead author of the study published today in Nature Astronomy. “Our observations reveal a powerful outflow that, according to our current understanding, shouldn’t be there,” says Krystian Ilkiewicz, a postdoctoral researcher at the Nicolaus Copernicus Astronomical Center in Warsaw, Poland and study co-lead. ‘Outflow’ is the term used by astronomers to describe the material that is ejected from celestial objects.
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, “a curved arc of material, similar to the wave that builds up in front of a ship,” explains Noel Castro Segura, research fellow at the University of Warwick in the UK and collaborator in this study. 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 left-over 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.
“The surprise that a supposedly quiet, discless system could drive such a spectacular nebula was one of those rare ‘wow’ moments,” says Scaringi.
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. “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,” Ilkiewicz explains.
The shape and size of the bow shock imply that the white dwarf has been expelling a powerful outflow for at least 1000 years. 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. “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,” Ilkiewicz explains.
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. 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,” as Scaringi foresees.