The heavier elements in atmospheres of white dwarf stars result from planets being torn apart and swallowed, with the debris mixed into the stars' outer edges, according to a theory published in the Monthly Notices of the Royal Astronomical Society.
To astronomers any element heavier than helium in the spectrum of a star is called a metal, even though some of those most commonly found there are not metals, chemically speaking.
White dwarfs are exceptionally dense stars, packing masses close to (and sometimes exceeding) the Sun into a volume the size of the Earth. Most are thought to be largely made up of carbon and oxygen, since they are formed from stars that lack the mass to fuse these into heavier elements. However, what we are actually able to study is an atmosphere made up largely of either hydrogen or helium. While whichever element is dominant makes up 99.9% of the atmosphere, traces of metals are sometimes found.
"The precise origin of the metals has remained a mystery and extreme differences in their abundance between stars could not be explained," says Professor Martin Barstow, a Pro-Vice-Chancellor at the University of Leicester. "It was believed that this material was 'levitated' by the intense radiation from deeper layers in the star."
The metals were particularly mysterious because the atmospheres do no mix hydrogen and helium – either we see almost entirely one or the other. It is thought that this is because the intense gravity of such a dense star separates the elements by weight. Hydrogen-dominated atmospheres are thought to shroud helium layers underneath. If even helium will not mix with hydrogen under such conditions it is puzzling to find heavier elements doing so.
However, when Barstow's daughter Jo was on a summer work placement at Leicester as a break from her doctorate at Oxford she combined with her father to construct an alternative theory. Using the Far Ultraviolet Spectroscopic Explorer they investigated the ratio of some of the heavier elements seen in the outer reaches of 89 white dwarfs.
"We found that in stars with polluted atmospheres the ratio of silicon to carbon matched that seen in rocky material, much higher than found in stars or interstellar gas,” Barstow the older explains. This suggests that the material comes from rocky planets or asteroids. "The new work indicates that around one-third of all hot white dwarfs are contaminated in this way, with the debris most likely in the form of rocky minor planet analogues.”
Further evidence for the theory comes from the observation that the contaminated stars are also often those with rings of dusty debris or gas, indicative of planets having been pulled apart. On the other hand, the relationship between temperature and metal presence, while present, was not nearly as strong as one would expect if the material was being sourced internally.
In cooler white dwarfs, with temperatures below 20,000°C the metals would sink through the atmosphere in a few days, suggesting they are constantly being replenished. However, where temperatures are higher radiation pressure could counteract gravity to keep them where we can detect their presence for much longer periods.
The finding is further evidence of how common planets are, something we are increasingly learning from other sources. Since white dwarfs are the end stage of the evolution of stars like the Sun, this is encouraging news for planet hunters more interested in stars like our own than red dwarfs. On the other hand, it is dismal tidings for those giving thought to the long term future of the planet – even if it survives being incorporated into the sun during its red giant phase our home could end up as a streak of carbon, silicon and iron contaminating the outer atmosphere of a white dwarf.
On a more immediate level, analysis of the stellar atmospheres could tell us something about the make-up of rocky planets around other stars.