Astronomers from our Physics department have discovered a striking and unexplained shock wave surrounding a dead star.
The never-before-seen phenomenon has been discovered around a white dwarf, named RXJ0528+2838, the remnant of a low-mass star, around 730 light years from Earth.
Mystery source of shock wave
RXJ0528+2838 is being orbited by a companion star. Usually, material from the companion transfers to the white dwarf, fuelling the dead star and creating powerful outflows.
Under the right conditions, these outflows of gas and dust can clash with their surroundings to create a shock wave, or 'nebula'.
However, RXJ0528+2838 shows no signs of a disc, making the power source of its shock wave a mystery.
Despite this, the white dwarf is surrounded by a huge bow-shaped shock wave extending roughly 3,800 times the distance between Earth and the Sun.
Challenging understandings of star evolution
The discovery, published today in Nature Astronomy, challenges current understanding of how dead stars interact with their surroundings and how they evolve.
This unexplained 'energy leak' may also affect our understanding of how binary stars (two starts gravitationally bound to each other, orbiting a common centre of mass) evolve over cosmic time.
RXJ0528+2838 is a polar white dwarf, which unlike other accreting white dwarfs, do not collect a disc of material around them from their companion.
With no disc, experts would not expect this type of star to have any bow shock wave, or nebular, around it.
The object was first noticed by a final year Physics student during a project searching for the remnants of stellar explosions, called nova shells.
Further research, co-led by Associate Professor Dr Simone Scaringi, showed that the distinctive tail of this structure ruled it out as being a nova shell.
To study the object in more detail the team secured time using the MUSE instrument on the European Southern Observatory's Very Large Telescope (ESO's VLT).
This allowed them to map the bow shock in detail and analyse its composition.
Unidentified energy source
The size and shape of the bow shock suggest that the white dwarf has been driven by a powerful outflow for at least 1,000 years.
Their analysis also confirmed the presence of a strong magnetic field, directing incoming material without creating a surrounding disc.
While the magnetic field could contribute to the energy driving the shock wave, current measurements cannot fully explain its scale or longevity.
The team's data shows that the current magnetic field is only marginally strong enough to power a bow shock lasting for a few hundred years at most, so this cannot explain the discovery.
The team believes an additional, as yet unidentified, energy source must be involved.
Researchers are now seeking further examples of similar systems elsewhere in the Milky Way.
Discovering more objects like RXJ0528+2838 will help determine whether this phenomenon is rare or previously overlooked.
Dead star creating a shock wave as it moves through space. Credit: ESO/K. Ilkiewicz & S. Scaringi et al
Dead star creating a shock wave as it moves through space. Credit: ESO/K. Ilkiewicz & S. Scaringi et al.
Wide-field view (DSS) of the area of the sky around the star RXJ0528+2838. Credit: ESO/Digitized Sky Survey 2. Acknowledgement: D. De Martin
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