Stars can produce powerful shock waves when gas and dust streaming away from them collide with material in the surrounding space. Astronomers have now captured a striking example around the dead star RXJ0528+2838 using the European Southern Observatory’s Very Large Telescope (ESO’s VLT).
The discovery is puzzling because, based on known mechanisms, this small stellar remnant should not be capable of producing the structure seen around it. The unexpected finding challenges current ideas about how dead stars exchange matter and energy 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 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.
Astronomers use the term ‘outflow’ to describe material expelled from an object in space.
A Shock Wave Around a Dead Star
RXJ0528+2838 lies about 730 light-years from Earth. Like the Sun and other stars, it travels around the center of the Milky Way. As it moves through space, it encounters gas between the stars, producing a type of shock wave known as a bow shock.
Noel Castro Segura, research fellow at the University of Warwick in the UK and collaborator in this study, describes it as “a curved arc of material, similar to the wave that builds up in front of a ship.”
Bow shocks are typically produced when material flowing away from a star slams into its surroundings. In the case of RXJ0528+2838, however, astronomers cannot identify any known process that fully accounts for what they observed.
A White Dwarf With No Disk
RXJ0528+2838 is a white dwarf, the leftover core of a dying low-mass star, and it is orbited by a Sun-like companion.
In binary systems like this one, material can be pulled away from the companion and transferred onto the white dwarf. That material often forms a disc around the dead star. The disc feeds the white dwarf, while some matter can also be thrown back into space in powerful outflows.
RXJ0528+2838 is different. Astronomers see no evidence of such a disc, leaving them without the usual explanation for the outflow and the surrounding nebula.
“The surprise that a supposedly quiet, discless system could drive such a spectacular nebula was one of those rare ‘wow’ moments,” says Scaringi.
VLT Observations Confirm the Source
The unusual structure was first noticed in images taken with the Isaac Newton Telescope in Spain. Its strange appearance prompted the researchers to investigate it more closely using 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 bow shock’s size and shape suggest that RXJ0528+2838 has been producing a powerful outflow for at least 1000 years.
That creates another problem. Scientists do not yet know how a dead star without a disc could sustain such an outflow for so long, although the system’s magnetic field may provide an important clue.
A Magnetic Field May Hold Part of the Answer
RXJ0528+2838 is known to possess a strong magnetic field, something the MUSE observations also confirmed.
Instead of allowing material from the companion star to settle into a disc, the magnetic field appears to guide that matter directly onto the white dwarf.
“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 researchers suspect that the magnetic field could be connected to a hidden source of energy, which Scaringi describes as a ‘mystery engine.’ But the explanation remains incomplete.
According to the observations, the white dwarf’s present-day magnetic field could sustain a bow shock for only a few hundred years. The structure appears to have existed for at least 1000 years, meaning the magnetic field alone cannot yet explain the full phenomenon.
Searching for the “Mystery Engine”
Astronomers will need to investigate many more binary systems to determine how these powerful outflows can form without discs.
ESO’s upcoming Extremely Large Telescope (ELT) could play an important role by allowing scientists to study both known systems and much fainter examples in greater detail.
Scaringi expects the telescope “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.”
