Stars can generate powerful shock waves when gas and dust from the star collide with material in the surrounding space. Astronomers have now captured a stunning example around the dead star RXJ0528+2838 using the European Southern Observatory’s Very Large Telescope (ESO’s VLT).
This discovery is puzzling because, based on known mechanisms, this small stellar remnant should not be able to produce the visible structure around itself. This unexpected discovery challenges current ideas about how dead stars exchange matter and energy with their surroundings.
“We found something that had never been seen before and, more importantly, was completely unexpected,” says Simon Scaringi, associate professor at Durham University in the UK and co-lead author of the published study. nature astronomy.
“Our observations reveal a powerful outflow that, according to our current understanding, should not be there,” says Christian Ilkiewicz, a postdoctoral researcher at the Nicolaus Copernicus Astronomical Center in Warsaw, Poland, and co-leader of the study.
Astronomers use the term ‘outflow’ to describe material expelled from an object into space.
A shock wave around a dead star
RXJ0528+2838 is about 730 light years from Earth. Like the Sun and other stars, it revolves around the center of the galaxy. As it moves through space, it encounters gas between the stars, generating a type of shock wave known as a bow shock.
Noel Castro Segura, a research fellow at the University of Warwick in the UK and a collaborator on the study, described it as “a curved arc of material, similar to a wave forming on the front of a ship.”
Bow shocks typically occur when material flowing away from a star collides with its surroundings. However, in the case of RXJ0528+2838, astronomers cannot identify any known process that completely matches what they observed.
a white dwarf without a disk
RXJ0528+2838 is a white dwarf, the leftover core of a dying low-mass star, and is orbited by a Sun-like companion.
In such binary systems, material may be pulled away from the companion and transferred onto the white dwarf. That material often forms a disk around the dead star. The disk nourishes the white dwarf, while some matter may also be thrown back into space in powerful outflows.
RXJ0528+2838 is different. Astronomers see no evidence of such a disk, leaving them without a common explanation for the outflow and the surrounding nebula.
Scaringi says, “It’s surprising that a supposedly quiet, discreet system could drive such a spectacular nebula; it was one of those rare ‘wow’ moments.”
VLT observations confirm source
This unusual structure was first seen in images taken with the Isaac Newton Telescope in Spain. Its strange appearance prompted researchers to examine it more closely using the MUSE instrument on ESO’s VLT.
Ilkiewicz explains, “Observations with the ESO MUSE instrument allowed us to map the bow shock in detail and analyze its structure. This was important to confirm that the structure did indeed originate from the binary system, and not from an unrelated nebula or interstellar cloud.”
The size and shape of the bow shock suggest that RXJ0528+2838 has been producing a powerful outflow for at least 1000 years.
This creates another problem. Scientists don’t yet know how a dead star with no disk can sustain such outflows for so long, although the system’s magnetic field could provide an important clue.
A magnetic field can hold some part of the north
RXJ0528+2838 is known to have a strong magnetic field, which is also confirmed by MUSE observations.
Rather than allowing material from the companion star to settle into a disk, the magnetic field appears to direct that material directly onto the white dwarf.
“Our discovery shows that even without a disk, 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,” explains Ilkiewicz.
Researchers suspect that the magnetic field may be linked to a hidden source of energy, which Scaringi describes as a ‘mystery engine’. But the explanation is incomplete.
According to observations, the white dwarf’s current magnetic field can withstand the bow shock for only a few hundred years. This structure appears to have existed for at least 1000 years, meaning that magnetic fields alone cannot yet explain the entire phenomenon.
Search for “Mystery Engine”
Astronomers will need to examine many more binary systems to determine how these powerful outflows could form without disks.
ESO’s upcoming Extremely Large Telescope (ELT) could play an important role by allowing scientists to study both known systems and very faint examples in greater detail.
Scaringi hopes the telescope will “map more of these systems as well as weaker systems and explore similar systems in detail, ultimately helping to understand the mysterious energy source that remains unexplained.”
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