White dwarf observed 'cannibalising' rocky object

NASA's Kepler 2 spacecraft may have found the "smoking gun" showing for the first time how white dwarfs can vaporise rocky remnants that survive within the dead star's planetary system.

Artist's conception depicting a small, planetary body being vaporised as it orbits its white dwarf host.
CfA/Mark A. Garlick

A white dwarf star has been observed ‘cannibalising’ a small rocky object that is spiralling around it.

The discovery validates a previous theory that white dwarfs strip material off remnant planets that survive within their planetary system.

White dwarfs form when sun-like stars age and grow into red giants, before losing about half their mass and shrinking to around one hundredth of their original size. The dead stellar remnants are known as white dwarfs.

NASA’s Kepler 2 spacecraft (K2) observed a cosmic object formed from dust and rock orbiting white dwarf WD 1145+017. The object, thought to be the size of a large asteroid, is the first planetary object to be confirmed transiting a white dwarf.

“We are for the first time witnessing a miniature ‘planet’ ripped apart by intense gravity, being vaporised by starlight and raining rocky material onto its star,” says Andrew Vanderburg, graduate student from the Harvard-Smithsonian Center for Astrophysics and lead author of the paper.

K2 focussed on a patch of sky in Virgo during its first observing session from 30 May 2014 to 21 August 2014 and was able to measure the variable brightness of the white dwarf. Using the transit method, which astronomers use to discover planetary bodies outside our Solar System through the dimming of the host star’s light as they pass in front of it, the small planetary body was discovered.

The object orbits the dead star about once every 4.5 hours, spiralling close to the white dwarf’s extreme heat and gravitational force.

But the team noticed that the transit in this case did not have the usual symmetrical U-shaped pattern, rather an asymmetric, elongated slope pattern that would usually indicate the presence of a comet-like tail. The team were able to use this information to suggest a ring of dusty debris around the white dwarf and the signs of a small planet being stripped of its material.

“The eureka moment of discovery came on the last night of observation with a sudden realisation of what was going around the white dwarf. The shape and changing depth of the transit were undeniable signatures,” says Vanderburg.

The team also found signs of heavier elements polluting the atmosphere of WD 1145+017. This is unusual because white dwarfs are supposed to have chemically pure surfaces. The presence of polluting, heavier elements like calcium, silicon, magnesium and iron implies the presence of an asteroid or a small planet being torn apart by the white dwarf's gravity and littering the dead star’s surface with its stripped material.“For the last decade we’ve suspected that white dwarf stars were feeding on the remains of rocky objects, and this result may be the smoking gun we’re looking for,” says Fergal Mullally, staff scientist of K2 at SETI and NASA’s Ames Research Center in Moffett Field, California. “However, there's still a lot more work to be done figuring out the history of this system.”


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