Illustration by  Mark A. Garlick
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For the first time, a small star called a white dwarf has been spotted tearing a nearby space rock into bits. Within about a million years, there will be nothing left of the rock but metallic dust on the white dwarf's surface.

Illustration by  Mark A. Garlick

Zombie Star Caught Feasting On Asteroids

Astronomers suspected that white dwarf stars dine on huge chunks of rock—new observations provide the smoking gun.

A hot, dense white dwarf star, no bigger than Earth but with as much mass as the sun, has been caught in the act of ripping an asteroid-like object to shreds. Soon it will begin swallowing up the pieces.

“Lots of people suspected this kind of thing must be happening,” says Andrew Vanderburg of the Harvard-Smithsonian Center for Astrophysics, “but this is the smoking gun.” Vanderburg is lead author of a paper describing the discovery in the latest issue of Nature.

The white dwarf, known as WD 1145+017, lies about 570 light-years away in the direction of the constellation Virgo. The first hint that the white dwarf might be behaving badly came from the Kepler space telescope, which searches for extrasolar planets by looking for the light from a star to dim as planets transit, or pass in front.

“We weren’t really looking for white dwarfs in the Kepler observations,” says Vanderburg. That’s because a white dwarf is a cosmic zombie, the corpse left behind after an aging, sun-like star swells into a red giant, sloughs off its outer layers, and collapses into a tiny, intensely hot ember. The Kepler mission focuses mostly on stars that are haven’t gone through this catastrophic process, since life-friendly planets are most likely to be found orbiting them.

While WD 1145+017 was not a Kepler target, it was in the telescope’s field of view, and it was immediately clear to Vanderburg and his planet-hunting colleagues that something was transiting. But the astronomers needed to use several small Earth-based telescopes to figure out what those objects were.

Observations revealed that the object or objects—there may be several—are whipping around the white dwarf at a blistering pace of once every 4.5 to 4.9 hours. That means they’re much closer to the star than the moon is to Earth.

They’re also tiny. A white dwarf is so small that a nearby planet passing in front would blot it out completely, or nearly so. In this case, however, the light dimmed by 40 percent at most. Even stranger, the dimming was inconsistent—greater during some transits and less during others. Sometimes the star didn’t dim when the astronomers expected it to. And moreover, after the star dimmed, it would brighten gradually. If the transit were caused by a planet, the brightening should be abrupt.

The Odd Nature of White Dwarfs

All of this might have been tough to unravel, except that astronomers have been noticing some peculiar things about white dwarfs for about a decade. Some of these stars, for instance, show evidence of relatively heavy elements in their atmospheres, including magnesium, silicon, and aluminum. “These should disappear on relatively short timescales,” says Boris Gänsicke, an astronomer at the University of Warwick in England who studies heavy-element pollution in white dwarfs but was not involved in this research. That these elements haven’t disappeared yet means they’ve been replenished relatively recently.

Another peculiarity is that many white dwarfs appear to be surrounded by swirling disks of dust, which presumably supply the heavy elements that fall into the stars’ atmospheres. In our own solar system, dust is continuously created when asteroids smash into each other. The observations by Vanderburg and his colleagues, however, suggest that something different is happening with WD 1145+017. The dust in that system, the astronomers believe, is coming from one or more planetesimals, or asteroid-like objects, as they are ripped apart by the intense gravitational field of the white dwarf, streaming a tail of dust behind them as they disintegrate. It’s the relatively large tail, not the tiny planetesimal, that’s causing the star’s light to dim.

That would explain why the stars brighten gradually during a transit. “The tail gets less opaque as it gets farther from the planetesimal,” says Vanderburg. It explains why the transits sometimes happen and sometimes don’t. “The dust can come and go,” he says. And it explains how those heavy elements, which are plentiful in rocky planetesimals, get into the atmosphere of a white dwarf.

The discovery is a coup for Vanderburg and his collaborators, but it also signals a potentially powerful technique theorists might one day use to study exoplanets themselves. “Future observations of evaporating planets and metal-polluted white dwarfs might even allow scientists to distinguish between material that originated in a planet’s core as opposed to its mantle,” astronomer Francesca Faedi, also at the University of Warwick, writes in a commentary accompanying the Nature study.

There could be other interpretations of these observations, but Gänsicke is convinced that the one from Vanderburg’s group is plausible. “I can’t think of an alternative explanation,” he says. “The exciting thing is that, yes, the story we’ve been developing over the past decade works the way we think it works.”

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