The SPHERE instrument attached to the VLT
The unusual binary star V471 Tauri in the constellation of Taurus
Wide-field view of the sky around the unusual binary star V471 Tauri
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New SPHERE instrument shows its power
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 [1].
One such stellar pair is called V471 Tauri [2]. 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 [3].
“There are many papers suggesting the existence of such
circumbinary objects, but the results here provide damaging evidence
against this hypothesis,” remarks Adam Hardy.
If there is no orbiting object then what is causing the odd changes
to the orbit of the binary? Several theories have been proposed, and,
while some of these have already been ruled out, it is possible that the
effects are caused by magnetic field variations in the larger of the
two stars [4], somewhat similar to the smaller changes seen in the 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. 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,” concludes Adam Hardy.
Notes
[1] Such pairs are known as post-common-envelope binaries.
[2] This name means that the object is
the 471st variable star (or as closer analysis shows, pair of stars) to
be identified in the constellation of Taurus.
[3] 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.
[4] This effect is called the
Applegate mechanism and results in regular changes in the shape of the
star, which can lead to changes in the apparent brightness of the double
star seen from Earth.
More Information
This research was presented in a paper entitled “The First Science
Results from SPHERE: Disproving the Predicted Brown Dwarf around V471
Tau” by A. Hardy et al., to appear in the Astrophysical Journal Letters on 18 February 2015.
The team is composed of A. Hardy (Universidad Valparaíso, Valparaíso,
Chile; Millennium Nucleus "Protoplanetary Disks in ALMA Early Science",
part of the Millennium Science Initiative Program, Universidad
Valparaíso), M.R. Schreiber (Universidad Valparaíso), S.G. Parsons
(Universidad Valparaíso), C. Caceres (Universidad Valparaíso), G.
Retamales (Universidad Valparaíso), Z. Wahhaj (ESO, Santiago, Chile), D.
Mawet (ESO, Santiago, Chile), H. Canovas (Universidad Valparaíso), L.
Cieza (Universidad Diego Portales, Santiago, Chile; Universidad
Valparaíso), T.R. Marsh (University of Warwick, Coventry, United
Kingdom), M.C.P. Bours (University of Warwick), V.S. Dhillon (University
of Sheffield, Sheffield, United Kingdom) and A. Bayo (Universidad
Valparaíso).
ESO is the foremost intergovernmental astronomy organisation in
Europe and the world’s most productive ground-based astronomical
observatory by far. It is supported by 16 countries: Austria, Belgium,
Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy,
the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the
United Kingdom, along with the host state of Chile. ESO carries out an
ambitious programme focused on the design, construction and operation of
powerful ground-based observing facilities enabling astronomers to make
important scientific discoveries. ESO also plays a leading role in
promoting and organising cooperation in astronomical research. ESO
operates three unique world-class observing sites in Chile: La Silla,
Paranal and Chajnantor. At Paranal, ESO operates the Very Large
Telescope, the world’s most advanced visible-light astronomical
observatory and two survey telescopes. VISTA works in the infrared and
is the world’s largest survey telescope and the VLT Survey Telescope is
the largest telescope designed to exclusively survey the skies in
visible light. ESO is a major partner in ALMA, the largest astronomical
project in existence. And on Cerro Armazones, close to Paranal, ESO is
building the 39-metre European Extremely Large Telescope, the E-ELT,
which will become “the world’s biggest eye on the sky”.
Links
Contacts
Adam Hardy
Universidad Valparaíso
Valparaíso, Chile
Tel: +56 32 2508457
Email: adam.hardy@postgrado.uv.cl
Matthias Schreiber
Universidad de Valparaíso
Valparaíso, Chile
Tel: +56 32 2399279
Email: matthias@dfa.uv.cl
Richard Hook
ESO education and Public Outreach Department
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591
Email: rhook@eso.org
Source: ESO
