An action-packed white dwarf binary

A dramatic pair of white dwarfs are surprising astronomers with their unruly behaviour, challenging our ideas about how material can be transferred from one object to another.

An artist’s impression of two white dwarfs. Image: David A Aguilar (CfA).

The white dwarf binary in question is called KL Draconis, in the constellation of Draco. White dwarfs are the incredibly dense remnants of the cores of Sun-like stars, in this case with masses of 0.6 and 0.1 times that of the Sun respectively, crammed into a diameter not much larger than Earth itself. The white dwarfs of KL Dra are separated by merely 200,000 kilometres, which is half the distance from Earth to the Moon, and they orbit each other every 25 minutes. Because they are so close, the gravity of the larger white dwarf is able to steal material from the less massive companion, wrapping this gas around itself before accumulating on the white dwarf’s surface.

Observations of KL Dra with the Liverpool robotic telescope on the Canary Islands, led by Dr Gavin Ramsay of Armagh Observatory, backed up by complimentary measurements from NASA’s Swift satellite, show that every two months, on a staggeringly predictable schedule, the accretion disc explodes in some kind of outburst that causes the system to shine tens of times brighter than normal. This is surprising because in normal accretion systems, where a white dwarf is accompanied by a normal Sun-like star, the white dwarf tears hydrogen gas away from the star and into an accretion disc that gradually falls into the white dwarf’s surface. When too much of this is transferred there is an explosion on the surface, rather than in the disc, which causes a nova to be seen in our skies. However, white dwarf binaries have a fundamental difference in that they are rich in helium, not hydrogen.

At top are images of KL Dra taken with the Liverpool robotic telescope, and at the bottom images taken in ultraviolet light by Swift. The left hand images show the system prior to the outburst, and on the right during the outburst. The object at the top of each picture is a background galaxy. Images: Liverpool Telescope/Gavin Ramsay; Swift Satellite–UVOT/Gavin Ramsay.

“The fact that helium is heavier [than hydrogen], and has a higher ionisation potential means that the characteristics of helium-accreting binaries may be different to hydrogen binaries,” says Ramsay. Only twenty or so white dwarf binary systems are currently known in the Galaxy, but millions are expected to exist although most will be too faint to detect.

Ramsay says that although the mechanism behind such regular outbursts is still unknown, further observations of this system and others like it may lead to an answer. To that end his team, who are publishing their results in the Monthly Notices of the Royal Astronomical Society, are now planning a survey of ‘helium-eating binaries’ to see if they also display activity like KL Dra, and to figure out why they behave so differently to normal ‘hydrogen-munching binaries’.

As a footnote to the observations, if the white dwarfs of KL Dra do ever merge, their masses are not great enough to create a supernova. A total mass of 1.4 solar masses, called the Chandraskhar Limit, is required for supernova detonation.