The mystery of the missing brown dwarf in the V471 Tauri system

The sky around the double star V471 Tauri, visible in low luminosity in the middle of the image (Image ESO/Digitized Sky Survey 2)
The sky around the double star V471 Tauri, visible in low luminosity in the middle of the image (Image ESO/Digitized Sky Survey 2)

SPHERE (Spectro-Polarimetric High-contrast Exoplanet REsearch) is an instrument built especially to take direct pictures of exoplanets but the first scientific article based on its observations was about the binary system V471 Tauri. A group of astronomers led by Adam Hardy carried out this research, published in the journal “Astrophysical Journal Letters”, explaining why it led to a surprise.

The astronomers expected that there was a brown dwarf orbiting this double star but SPHERE didn’t find anything. It’s a surprising result because the presence of a brown dwarf was by far the most plausible explanation for the strange behavior of V471 Tauri.

V471 Tauri is a double star about 163 light years from Earth. This pair is formed by two stars that are very close, so much that they orbit each other in just 12 hours. One of the two stars is a white dwarf while the other is still in its normal life stage.

This type of pair is created when the most massive star ages, becomes a red giant and part of its gas is transferred to the other star. This phenomenon ends up creating an envelope of gas that surrounds both stars. Over time, this gas disperses while the two stars get closer and start orbiting each other in very short time.

The team led by Adam Hardy also used the ULTRACAM system on ESO’s NTT (New Technology Telescope) telescope to measure very accurately the changes in brightness of the binary system V471 Tauri. The astronomers expected the variations to be very regular, instead they weren’t. The most likely explanation was that there was a brown dwarf orbiting the two stars slightly altering their orbits.

Brown dwarfs are objects on the boundary between planets and stars and their brightness is so limited that they were discovered only in 1995. Thanks to the new generation astronomical instruments, we now know that they’re very numerous and perhaps as many as normal stars.

SPHERE is a great instrument installed on ESO’s Very Large Telescope (VLT) at the Paranal Observatory in Chile and was used to search for the brown dwarf in the V471 Tauri system. Until now it was impossible to detect the extremely faint light of a brown dwarf close to stars that are normal and therefore much more luminous. SPHERE is the perfect instrument for this kind of observations but in this case it found nothing.

At this point, the mystery remains of the anomalies in the orbits of the two stars of V471 Tauri. It may be due to the Applegate mechanism, whose effects are observed to a lesser extent also on the Sun. In simple words, changes in the magnetic field of the bigger star can result in changes in its form and consequently in its apparent brightness. In this case, the observations from the Earth would be altered by these changes.

Adam Hardy pointed out that this study was possible thanks to an instrument such as SPHERE. It allowed to disprove his team’s theory and science works that way too. Now they can proceed with further studies that will improve our knowledge and that also thanks to that extraordinary instrument.

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