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The five-minute
white dwarf waltz

Posted: 10 March 2010

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Utilising the resolving power of the ten-metre Keck telescope in Hawaii, astronomers from the University of Warwick and Radboud University in the Netherlands have confirmed the existence of a double white dwarf system where the two stars orbit one another every 5.4 minutes.

The two white dwarfs are in a death grip and are destined to merge. Image: GSFC/D.Berry.

This dizzying orbital rate is incrementally decreasing as the two white dwarfs – the left over cores of Sun-like stars that have expended their nuclear fuel and ceased to be – are spiralling gradually into each other. Already, a bridge of gas has extended from one to another, transferring gas. We often see this accretion occurring in systems involving one white dwarf and a normal Sun-like companion, but double white dwarfs are much rarer.

“We now have close to two dozen systems where mass exchange occurs between the two stars, with periods ranging from the record 5.4 minutes up to about one hour for the wider systems,“ Warwick’s Dr Danny Steeghs tells Astronomy Now. The star system, known as HM Cancri, was discovered eleven years ago by the Rosat satellite, which detected an X-ray pulse every 5.4 minutes. What was causing the pulse was at the time unclear – we now know it is the accretion of the gas onto one of the white dwarfs – and nobody was willing to believe that it could be an extremely tight pair of white dwarfs (they are less than 100,000 kilometres apart) without concrete proof.

Movie showing the waltzing white dwarfs, and their eventual dramatic merging event. Credit: GSFC/D.Berry.

Steeghs, along with fellow Warwick colleague Professor Tom Marsh and Professor Paul Groot and Dr Gijs Nelemans of Radboud University, now have that proof. They measured the radial velocity ‘wobble’ of HM Cancri, in much the same way that astronomers discover exoplanets through the Doppler Effect as their host stars wobble towards and away from us.

Aside from the ‘gosh-wow’ factor of the incredibly rapid rotation rate and proximity of the two white dwarfs, the discovery has wider implications for Type Ia supernovae and the expansion of the Universe. In February, a pair of astronomers at the Max Planck Institute in Germany announced that they had used the Chandra X-ray Observatory to determine that the majority of Type Ia supernovae are caused by two white dwarfs merging (see our report here). Although the white dwarfs of HM Cancri do not possess enough mass between them to explode as a supernova when they merge, the study of their orbits and behaviour will help astronomers better understand white dwarf mergers. Because Type Ia supernovae are crucial ‘standard candles’ for measuring astronomical distances, understanding their behaviour is important for measuring the expansion of the Universe and the strength of the mysterious dark energy, which is causing the expansion to accelerate.

“One reason [to investigate binary white dwarfs] is to establish whether one can produce sufficient Type Ia explosions via this route,” says Steeghs. “Other reasons include the expected gravitational wave signals from the large population of double white dwarfs that are expected to be present within the Milky Way Galaxy.” Steeghs estimates that there could be a few hundred million double white dwarf systems in the Milky Way alone. HM Cancri is the most extreme binary star system yet found, but the chances are that it is not alone.