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Extrasolar asteroids pollute white dwarf stars
Posted: 30 March 2012

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Unexpected elements have been found in the atmospheres of white dwarfs which suggest that the stars may have been eating broken-up asteroids.

Most of the exoplanets that we know of today have been discovered around main sequence stars which are in the prime of their lives. However in billions of years time, many of these stars will have gone through a swollen red giant phase and then contracted to become a stellar remnant known as a white dwarf.

An impression of an asteroid breaking up into dust around a white dwarf. Image: NASA/JPL-Caltech

White dwarfs are composed of either hydrogen or helium, and should be completely free of any heavier elements, which astronomers call “metals.” However, this is not always the case.

Using spectra obtained from several large telescopes, Boris Gaensicke from the University of Warwick found that many white dwarfs have metals in their atmospheres. "We found oxygen, magnesium, silicon, and iron, which are elements common in terrestrial bodies," Gaensicke told delegates of the National Astronomy Meeting.

It is also known from infrared measurements that a small percentage of white dwarfs have circumstellar discs. Circumstellar discs are more often seen around young stars, as it is a natural by-product of star formation. However, the discs surround the white dwarfs are thought to have a different origin.

If an asteroid is perturbed by an unseen planet, it could be sent on a collision course with the white dwarf. The intense tidal forces of the white dwarf would then rip the asteroid apart before it actually impacting the star; thus creating a disc of dust. This dust is then accreted onto the white dwarf, polluting it with heavy metals, and this material can accrete onto white dwarfs at a phenomenal rate of one million kilograms every second.

It is possible to measure the minimum mass of the accreted material, and Gaensicke said that this varies between the mass of Ceres and the mass of Pluto. If more material than this were to be accreted onto a white dwarf, such as the equivalent of an Earth-mass planet, then the spectrum would become so polluted with metals that it would be almost unrecognisable as a white dwarf.

Thus by measuring the abundance of the white dwarf atmospheres, it is possible to measure the abundance of rocky objects around white dwarfs.