Contaminated white dwarfs: Scientists solve riddle of celestial archaeology

Scientists from the University of Leicester and University of Arizona investigated hot, young, white dwarfs -- the super-dense remains of Sun-like stars that ran out of fuel and collapsed to about the size of Earth. Their research is featured in the journal Monthly Notices of the Royal Astronomical Society, published by Oxford University Press.

It has been known that many hot white dwarfs' atmospheres, essentially of pure hydrogen or pure helium, are contaminated by other elements -- like carbon, silicon and iron. What was not known, however, was the origins of these elements, known in astronomical terms as metals.

"The precise origin of the metals has remained a mystery and extreme differences in their abundance between stars could not be explained," said Professor Barstow, a Pro-Vice-Chancellor at the University of Leicester whose research was assisted by his daughter Jo, a co-author of the paper, during a summer work placement in Leicester. She has now gone on to be an astronomer working in Oxford on extra-solar planets.

"It was believed that this material was "levitated" by the intense radiation from deeper layers in the star," said Professor Barstow.

Now the researchers have discovered that many of the stars show signs of contamination by rocky material, the leftovers from a planetary system.

The researchers surveyed 89 white dwarfs, using the orbiting Far Ultraviolet Spectroscopic Explorer telescope to obtain their spectra (dispersing the light by colour) in which the "fingerprints" of carbon, silicon, phosphorus and sulphur can be seen, when these elements are present in the atmosphere.

"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.

'The new work indicates that at around a 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. This implies that a similar proportion of stars like our Sun, as well as stars that are a little more massive like Vega and Fomalhaut, build systems containing terrestrial planets. This work is a form of celestial archaeology where we are studying the 'ruins' of rocky planets and/or their building blocks, following the demise of the main star.

'The mystery of the composition of these stars is a problem we have been trying to solve for more than 20 years. It is exciting to realise that they are swallowing up the leftovers from planetary systems, perhaps like our own, with the prospect that more detailed follow-up work will be able to tell us about the composition of rocky planets orbiting other stars," said Professor Barstow.

The study also points to the ultimate fate of Earth billions of years from now -- ending up as merely contamination within the white dwarf remnant of our Sun.