University of Warwick astronomers have helped discover a beautiful yet mysterious shock wave around a dead star - a completely unexpected discovery.
Gas and dust flowing from stars can clash with a star's surroundings and create a shock wave. Now, a collaboration of 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.
The star (called RXJ0528+2838), reported in Nature Astronomy, is a white dwarf in a binary system with a Sun-like star orbiting it, and is located 730 light-years away. Typically, the material from a companion star transfers to the white dwarf, which forms a disc around the dwarf and causes some of the material also getting ejected into space as powerful outflows.
Sometimes, these outflows of gas and dust can clash with their surroundings to create shock waves, yet this dead white dwarf shows no signs of a disc of material, making the origin of its outflows and surrounding shock wave a mystery.
Noel Castro Segura, Research Fellow at the University of Warwick, co-author of the study said: "When stellar outflows ram into interstellar gas under the right conditions, they produce bow shocks - curved, comet-like arcs of shocked material, similar to the wave that builds up in front of a ship. Bow shocks are powerful tools, because their size and shape let us directly gauge how much energy is carried by the outflow."
"However, here we present the discovery of an enigmatic bow shock around the magnetic system RXJ0528+2838. Against all expectations, this disc-less accreting white dwarf shows a prominent bow shock that cannot be explained by any of the known mechanisms."
The size of shock wave from the white dwarf is roughly 3800 times the distance between Earth and the Sun. To try to explain how a disc-less white dwarf could produce a bow shock like this, the team used the spectrographic MUSE instrument on the ESO's VLT.
The shape and size of the bow shock suggests that the white dwarf must have been expelling powerful outflows without a disc for at least 1,000 years. The results also showed that the dead star has a strong magnetic field, which suggests it could have been channelling material accreted from its companion star directly into the white dwarf, without forming a disc around it.
Yet 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.
"This star is known as a polar white dwarf, which unlike other accreting white dwarfs, do not collect a disc of material around them from their companion. The surprise that a supposedly quiet, discless system could drive such a spectacular nebula was one of those rare 'wow' moments," adds Simone Scaringi, Associate Professor at Durham University and co-lead author.
"To try and understand this, we really need to try and find more examples elsewhere in the galaxy. In this case, this white dwarf is quite close to Earth and therefore we can see it well. The hunt is on now to try and discover more examples of this, to help develop our understanding and offer a physics-based solution to the mystery."
Dr Noel Castro Segura concludes: "This finding adds a new piece to the puzzle that had not been considered until now, and points to an additional, powerful energy-loss channel - likely linked to the strong magnetic field - that may help resolve long-standing discrepancies between observations and theoretical predictions for the evolution of binary stars in our Galaxy."
