VLT/SPHERE non-detection of the proposed brown dwarf in the V471 Tauri system

Wide-field view of the sky around the unusual binary star V471 TImage and text credit: European Southern Observatory (ESO) science release. A tertiary brown dwarf companion in the V471 Tauri eclipsing binary system has been postulated since at least 2001. The star itself is centred in the above image. Detection of the brown dwarf as a residual in the fitted light curve of the eclipsing primary/secondary pair had been claimed in 2011 and indeed the presence of a fourth, planetary mass body hinted at. However the brown dwarf has not been found by a new instrument which should have been capable of easily detecting it, the SPHERE exoplanet imager at ESO/VLT. A link to the research paper itself is here (via ESO). From the science release:

The new SPHERE instrument on ESO’s Very Large Telescope has been used to search for a brown dwarf expected to be orbiting the unusual double star V471 Tauri. SPHERE has given astronomers the best look so far at the surroundings of this intriguing object and they found — nothing. The surprising absence of this confidently predicted brown dwarf means that the conventional explanation for the odd behaviour of V471 Tauri is wrong. This unexpected result is described in the first science paper based on observations from SPHERE. Some pairs of stars consist of two normal stars with slightly different masses. When the star of slightly higher mass ages and expands to become a red giant, material is transferred to other star and ends up surrounding both stars in a huge gaseous envelope. When this cloud disperses the two move closer together and form a very tight pair with one white dwarf, and one more normal star. [Such pairs are known as post common envelope binaries]. One such stellar pair is called V471 Tauri. It is a member of the Hyades star cluster in the constellation of Taurus and is estimated to be around 600 million years old and about 163 light-years from Earth. The two stars are very close and orbit each other every 12 hours. Twice per orbit one star passes in front of the other — which leads to regular changes in the brightness of the pair observed from Earth as they eclipse each other. A team of astronomers led by Adam Hardy (Universidad Valparaíso, Valparaíso, Chile) first used the ULTRACAM system on ESO’s New Technology Telescope to measure these brightness changes very precisely. The times of the eclipses were measured with an accuracy of better than two seconds — a big improvement on earlier measurements. The eclipse timings were not regular, but could be explained well by assuming that there was a brown dwarf orbiting both stars whose gravitational pull was disturbing the orbits of the stars. They also found hints that there might be a second small companion object. Up to now however, it has been impossible to actually image a faint brown dwarf so close to much brighter stars. But the power of the newly installed SPHERE instrument on ESO’s Very Large Telescope allowed the team to look for the first time exactly where the brown dwarf companion was expected to be. But they saw nothing, even though the very high quality images from SPHERE should have easily revealed it. The SPHERE images are so accurate that they would have been able to reveal a companion such as a brown dwarf that is 70 000 times fainter than the central star, and only 0.26 arcseconds away from it. The expected brown dwarf companion in this case was predicted to be much brighter.

From the paper abstract:

We report that an unprecedented contrast of [12.1 magnitudes (H band)] at a separation of 260 mas was achieved, but resulted in a non- detection. This implies that there is no brown dwarf present in the system unless it is three magnitudes fainter than predicted by evolutionary track models, and provides damaging evidence against the circumbinary interpretation of eclipse timing variations. In the case of V471 Tau, a more consistent explanation is offered with the Applegate mechanism, in which these variations are prescribed to changes in the quadrupole moment within the main-sequence star.

More (on a very different topic): The oldest stars in the universe formed 560 Myr after the Big Bang, 140 Myr later than previously thought (Scientific American). This new Planck result is important as it resolves certain difficulties which had arisen from the earlier date and also implies that the very first stars which formed (often called Population III stars) might be more easily detected in the future. This is one of the prime observational goals of the James Webb Space Telescope (JWST).


2 Responses to “VLT/SPHERE non-detection of the proposed brown dwarf in the V471 Tauri system”

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