The Strange Situation Of The Lost Dwarf

The new SPHERE instrument on ESO's Very Large Telescope has been utilized to hunt for a brown dwarf estimated to be orbiting the unusual double star V471 Tauri. SPHERE has given astrophysicists the best look so far at the environment of this fascinating object and they discovered—nothing. The astonishing absence of this confidently foretold brown dwarf means that the conventional clarification for the odd behaviour of V471 Tauri is incorrect. 

Some pairs of stars comprise of two normal stars with somewhat different masses. When the star of somewhat higher mass ages and expands to turn into a red giant, material is transported to other star and ends up neighbouring both stars in a huge gaseous envelope. When this cloud scatters the two move near to each other and form a very tight pair with one white dwarf , and one more normal star.

One such stellar couple is called V471 Tauri. It is a fellow of the Hyades star cluster in the constellation of Taurus and is expected to be about 600 million years old and about 163 light-years from Earth. The two stars are very close and circle each other every 12 hours. Twice per orbit one star passes in front of the other—which leads to frequent changes in the brightness of the pair witnessed from Earth as they eclipse each other.

A team of astrophysicists headed by Adam Hardy (Universidad Valparaíso, Valparaíso, Chile) first utilized the ULTRACAM system on ESO's New Technology Telescope to calculate these brightness changes very accurately. The times of the eclipses were measured with a precision of better than two seconds—a big development on earlier calculations.

The eclipse timings were not consistent, but could be clarified well by supposing that there was a brown dwarf circling both stars whose gravitational pull was troubling the orbits of the stars. They also discovered clues that there might be a second small companion object.

Up to now though, it has been difficult to in fact image a faint brown dwarf so close to much brighter stars. But the power of the recently installed SPHERE tool on ESO's Very Large Telescope permitted the team to search for the first time precisely where the brown dwarf companion was projected to be. But they saw nothing, even though the very high quality pictures from SPHERE should have easily discovered it.

"There are many papers proposing the presence of such circumbinary objects, but the outcomes here provide destructive proof against this hypothesis," said Adam Hardy.

If there is no circling object then what is producing the odd changes to the orbit of the binary? Several theories have been suggested, and, while some of these have at present been ruled out, it is potential that the effects are produced by magnetic field variations in the larger of the two stars, to some extent similar to the smaller changes observed in the Sun.

"A study such as this has been essential for many years, but has only become probable with the arrival of powerful new instruments such as SPHERE. This is how science works: observations with new technology can either confirm, or as in this case disprove, earlier concepts. This is an outstanding way to start the observational life of this amazing instrument," closes Adam Hardy.

This study was issued in a paper named "The First Science Results from SPHERE: Disproving the Predicted Brown Dwarf around V471 Tau" by A. Hardy et al., to publish in the Astrophysical Journal Letters on 18 February 2015.



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