European Southern Observatory (ESO) has been using the new SPHERE instrument on the Very Large Telescope (VLT) to look for a brown dwarf they assumed is orbiting the double star V471 Tauri (in the constellation Taurus), according to a press release. SPHERE has operated beautifully and provided the astronomers with - nada. Nothing. Bupkis.
SPHERE has given astronomers "the best look so far," but the predicted dwarf is missing, which means the strange behavior of V471 Tauri has not been explained. The surprise findings are published in the first science paper revealing SPHERE's observations published in Astrophysical Journal Letters.
Some star pairings do not have identical stars. One star might have a higher mass and upon expanding to become a red giant, some of its material is transferred to the star with lower mass. The material ends up a gaseous envelope surrounding both stars. When the cloud dissipates, the stars get closer and form a tight couple consisting of one white dwarf and one normal star. Pairs like that are known as post-common-envelope binaries.
This is the case of V471 Tauri, named so because it is the 471st star - rather, pair of stars - to be identified in the constellation Taurus. V471 Tauri, estimated to be 600 million years old, is a member of the Hyades star cluster. It is located about 163 light-years away from Earth.
The sister stars orbit each other every 12 hours. Twice during that period, one star passes in front of the other, causing regular shifts in brightness as viewed from the Earth as they eclipse each other.
The ULTRACAM system on ESO's New Technology Telescope was first used by a team of astronomers lead by Adam Hardy, from the Universidad Valparaíso in Valparaíso, Chile. With the new system, the eclipses were measured with an improved two second accuracy.
The irregular timing of the eclipses was explained by a predicted brown dwarf whose gravitational pull was messing with the orbiting stars' timing. Astronomers even expected a second, smaller companion of the phantom brown dwarf.
Until the SPHERE instrument, faint brown dwarfs were difficult - impossible - to see, so the team was excited to test out SPHERE's powers. Alas, the brown dwarf failed to be. According to the press release, "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."
"There are many papers suggesting the existence of such circumbinary objects, but the results here provide damaging evidence against this hypothesis," said Hardy, according to the press release.
Other theories have been proposed. One possible theory is that magnetic field variations in the larger of the two stars causes changes similar to those seen in our sun.
"A study such as this has been necessary for many years, but has only become possible with the advent of powerful new instruments such as SPHERE," said Hardy, according to the press release. "This is how science works: observations with new technology can either confirm, or as in this case disprove, earlier ideas. This is an excellent way to start the observational life of this amazing instrument."
