SYDNEY: A cannibalistic binary star system - predicted by theory but never seen - has finally been found, according to a study published today in U.S. journal Science.
"It was beginning to look as if these systems either didn't exist, or were too difficult to find using current telescopes," said lead author Stuart Littlefair, from the University of Sheffield, in England. "Our result shows, finally, that these systems do exist."
Binary stars are systems where two stars orbit around each other. A 25 year-old theoretical prediction said that 70 per cent of one class of binary star systems should contain a brown dwarf, a dim type of star too small to sustain nuclear fusion.
But according to the study, none of the approximately 1,600 such systems, called accreting binaries, found so far by astronomers had been conclusively shown to contain this smallest of star types.
"Finding the brown dwarf was important because their existence had been predicted for so long but, try as we might, we couldn't find any brown dwarfs in accreting binaries," said Littlefair.
The system containing the brown dwarf is called a 'cataclysmic variable'. Cataclysmic variables are made up of a white dwarf and a companion star that orbit around each other in less than one day,
White dwarves are the body left over when a low or medium mass star, such as our Sun, runs out of fuel, dies and collapses. It becomes extremely dense - a mass the size of the Sun contained in a volume about equal to that of Earth.
In a cataclysmic variable system, the orbiting companion star is so close to the white dwarf that it is distorted by the white dwarf's gravity. Material from the companion star is pulled in a spiral towards the white dwarf, heating up enough to glow and form a bright 'accretion disc'.
As the companion star loses mass to the white dwarf, it shrinks and dims, becoming a brown dwarf. Brown dwarfs are bodies with too little mass left to sustain hydrogen nuclear fusion, and have a mass between that of giant planets and the smallest stars.
As the brown dwarf loses mass, its gravity weakens and it expands, causing the length of its orbit to increase.
By this theory, cataclysmic variable systems with a long orbit should have a brown dwarf as a companion star, and it's predicted that 70 per cent of cataclysmic variables should have evolved this feature.
But finding the dim brown dwarf has been difficult. "The brown dwarf is too faint to actually see directly," said Littlefair, because the dim star is lost against the relatively bright white dwarf and accretion disc.
Images from the Sloan Digital Sky Survey (SDSS) were used as they could detect much fainter objects that in previous surveys, according to the study. Additionally the researchers took telescope images of the binary system, called SDSS 1035, as the white dwarf passed behind the brown dwarf.
While it was still difficult to see the brown dwarf directly, its presence was determined by measuring the mass and radius implied from the change the light eminating from the white dwarf, said the researchers.
"Without using this technique we would never have known SDSS 1035 contains a brown dwarf," said Littlefair. "[This research] shows that we can actually find and study these objects, which will hopefully allow us to learn a lot about brown dwarfs in future."
