Mystery shockwave around dead star stuns astronomers: 'We found something never seen before and entirely unexpected.'

Using the Very Large Telescope (VLT), astronomers have discovered an unexpected shockwave around a dead star. The team behind the shock discovery (in more ways than one) is baffled by this beautiful arc of material, which defies expectations and current theories surrounding such stellar remains. Thus, it could redefine our understanding of how dead stars interact with their surroundings.

This stellar remnant is a white dwarf, the exhausted stellar core left over when a star around the same mass as the sun runs out of the fuel for nuclear fusion and sheds its outer layers. This white dwarf is designated RXJ0528+2838 and is located 730 light-years away from Earth. It has a companion star in orbit around it, from which the dead star is stripping material with its gravity. In normal circumstances, this matter would usually form a disk around the white dwarf that gradually drops material to its surface, leaving other stellar matter to be lost to space via powerful outflows that fling it into space. However, in the case of RXJ0528+2838, there is no trace of such a disk, and that makes the presence of the shockwave entirely unexpected.

"We found something never seen before and, more importantly, entirely unexpected," team leader Simone Scaringi of Durham University in the U.K. said in a statement. "The surprise that a supposedly quiet, discless system could drive such a spectacular nebula was one of those rare 'wow' moments."

The shockwave, observed by the VLT's Multi Unit Spectroscopic Explorer (MUSE), is being created as the white dwarf moves through space, rotating around the center of the Milky Way as it does, much as the sun and our galaxy's other stars do. The dead star is pushing the interstellar gas in front of it as it travels, creating a type of shockwave called a bow shock, similar to how a wave builds up in front of the bow of a ship.

"Our observations reveal a powerful outflow that, according to our current understanding, shouldn't be there," Krystian Iłkiewicz, a postdoctoral researcher at the Nicolaus Copernicus Astronomical Center in Warsaw, Poland, said.

The team first saw hints at this unexpected structure when they examined images of RXJ0528+2838 captured by the Isaac Newton Telescope in Spain. Noting the unusual shape of this formation, they followed these initial observations up with the VLT's MUSE instrument.

"Observations with the MUSE instrument allowed us to map the bow shock in detail and analyse its composition," Iłkiewicz added. "This was crucial to confirm that the structure really originates from the binary system and not from an unrelated nebula or interstellar cloud."

Both the size and the shape of the bow shock around RXJ0528+2838 imply to the team that material has been flowing away from this white dwarf for at least 1,000 years. That is a mystery in itself, as scientists can't explain how a white dwarf with no surrounding disk of matter can maintain an outflow for such a prolonged period of time.

The team isn't completely in the dark with regards to this, though. They suspect that the powerful magnetic field of RXJ0528+2838 is channeling stellar matter stolen from the companion star directly to the white dwarf, without giving a disk a chance to form.

"Our finding shows that even without a disc, these systems can drive powerful outflows, revealing a mechanism we do not yet understand,” Iłkiewicz said. "This discovery challenges the standard picture of how matter moves and interacts in these extreme binary systems."

However, even with this magnetic field accounted for, the team thinks that the shockwave should only have been maintained for around 100 years. That means there must be another energy source or "mystery engine" in this system that has been powering this outflow and the shockwave it creates for a period ten times longer than that.

Solving this mystery could depend on studying further binary systems of this type using the forthcoming Extremely Large Telescope (ELT), currently under construction in the same location as the VLT operates, the Atacama Desert region of northern Chile.

Scaringi predicts that the 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 team's research was published on Monday (Jan. 12) in the journal Nature Astronomy.