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27 July 2016
Artist’s impression of the exotic binary star system AR Scorpii
Astronomers using ESO’s Very Large Telescope, along with other telescopes on the ground and in space, have discovered a new type of exotic binary star. In the system AR Scorpii a rapidly spinning white dwarf star powers electrons up to almost the speed of light. These high energy particles release blasts of radiation that lash the companion red dwarf star, and cause the entire system to pulse dramatically every 1.97 minutes with radiation ranging from the ultraviolet to radio. The research will be published in the journal Nature on 28 July 2016.
In May 2015, a group of amateur astronomers from Germany, Belgium and the UK came across a star system that was exhibiting behaviour unlike anything they had ever encountered. Follow-up observations led by the University of Warwick and using a multitude of telescopes on the ground and in space , have now revealed the true nature of this previously misidentified system.
The star system AR Scorpii, or AR Sco for short, lies in the constellation of Scorpius, 380 light-years from Earth. It comprises a rapidly spinning white dwarf , the size of Earth but containing 200 000 times more mass, and a cool red dwarf companion one third the mass of the Sun , orbiting one another every 3.6 hours in a cosmic dance as regular as clockwork.
AR Scorpii in the constellation of Scorpius
In a unique twist, this binary star system is exhibiting some brutal behaviour. Highly magnetic and spinning rapidly, AR Sco’s white dwarf accelerates electrons up to almost the speed of light. As these high energy particles whip through space, they release radiation in a lighthouse-like beam which lashes across the face of the cool red dwarf star, causing the entire system to brighten and fade dramatically every 1.97 minutes. These powerful pulses include radiation at radio frequencies, which has never been detected before from a white dwarf system.
Lead researcher Tom Marsh of the University of Warwick’s Astrophysics Group commented: “AR Scorpii was discovered over 40 years ago, but its true nature was not suspected until we started observing it in 2015. We realised we were seeing something extraordinary within minutes of starting the observations.”
The observed properties of AR Sco are unique. They are also mysterious. The radiation across a broad range of frequencies is indicative of emission from electrons accelerated in magnetic fields, which can be explained by AR Sco’s spinning white dwarf. The source of the electrons themselves, however, is a major mystery — it is not clear whether it is associated with the white dwarf itself, or its cooler companion.
Wide-field view of the sky around the exotic binary star system AR Scorpii
AR Scorpii was first observed in the early 1970s and regular fluctuations in brightness every 3.6 hours led it to be incorrectly classified as a lone variable star . The true source of AR Scorpii’s varying luminosity was revealed thanks to the combined efforts of amateur and professional astronomers. Similar pulsing behaviour has been observed before, but from neutron stars — some of the densest celestial objects known in the Universe — rather than white dwarfs.
Artist’s impression video of the exotic binary star system AR Scorpii
Boris Gänsicke, co-author of the new study, also at the University of Warwick, concludes: "We've known pulsing neutron stars for nearly fifty years, and some theories predicted white dwarfs could show similar behaviour. It's very exciting that we have discovered such a system, and it has been a fantastic example of amateur astronomers and academics working together."
Zooming in on the exotic binary star AR Scorpii
 The observations underlying this research were carried out on: ESO’s Very Large Telescope (VLT) located at Cerro Paranal, Chile; the William Herschel and Isaac Newton Telescopes of the Isaac Newton Group of telescopes sited on the Spanish island of La Palma in the Canaries; the Australia Telescope Compact Array at the Paul Wild Observatory, Narrabri, Australia; the NASA/ESA Hubble Space Telescope; and NASA's Swift satellite.
 White dwarfs form late in the life cycles of stars with masses up to about eight times that of our Sun. After hydrogen fusion in a star’s core is exhausted, the internal changes are reflected in a dramatic expansion into a red giant, followed by a contraction accompanied by the star’s outer layers being blown off in great clouds of dust and gas. Left behind is a white dwarf, Earth-sized but 200 000 times more dense. A single spoonful of the matter making up a white dwarf would weigh about as much as an elephant here on Earth.
 This red dwarf is an M type star. M type stars are the most common class in the Harvard classification system, which uses single letters to group stars according their spectral characteristics. The famously awkward to remember sequence of classes runs: OBAFGKM, and is often remembered using the mnemonic Oh Be A Fine Girl/Guy, Kiss Me.
 A variable star is one whose brightness fluctuates as seen from Earth. The fluctuations may be due to the intrinsic properties of the star itself changing. For instance some stars noticeably expand and contract. It could also be due to another object regularly eclipsing the star. AR Scorpii was mistaken for a single variable star as the orbiting of two stars also results in regular fluctuations in observed brightness.
This research was presented in a paper entitled “A radio pulsing white dwarf binary star”, by T. Marsh et al., to appear in the journal Nature on 28 July 2016.
The team is composed of T.R. Marsh (University of Warwick, Coventry, UK), B.T. Gänsicke (University of Warwick, Coventry, UK), S. Hümmerich (Bundesdeutsche Arbeitsgemeinschaft für Veränderliche Sterne e.V., Germany; American Association of Variable Star Observers (AAVSO), USA) , F.-J. Hambsch (Bundesdeutsche Arbeitsgemeinschaft für Veränderliche Sterne e.V., Germany; American Association of Variable Star Observers (AAVSO), USA; Vereniging Voor Sterrenkunde (VVS), Belgium), K. Bernhard (Bundesdeutsche Arbeitsgemeinschaft für Veränderliche Sterne e.V., Germany; American Association of Variable Star Observers (AAVSO),USA), C.Lloyd (University of Sussex, UK), E. Breedt (University of Warwick, Coventry, UK), E.R. Stanway (University of Warwick, Coventry, UK), D.T. Steeghs (University of Warwick, Coventry, UK), S.G. Parsons (Universidad de Valparaiso, Chile), O. Toloza (University of Warwick, Coventry, UK), M.R. Schreiber (Universidad de Valparaiso, Chile), P.G. Jonker (Netherlands Institute for Space Research, The Netherlands; Radboud University Nijmegen, The Netherlands), J. van Roestel (Radboud University Nijmegen, The Netherlands), T. Kupfer (California Institute of Technology, USA), A.F. Pala (University of Warwick, Coventry, UK) , V.S. Dhillon (University of Sheffield, UK; Instituto de Astrofisica de Canarias, Spain; Universidad de La Laguna, Spain), L.K. Hardy (University of Warwick, Coventry, UK; University of Sheffield, UK), S.P. Littlefair (University of Sheffield, UK), A. Aungwerojwit (Naresuan University, Thailand), S. Arjyotha (Chiang Rai Rajabhat University, Thailand), D. Koester (University of Kiel, Germany), J.J. Bochinski (The Open University, UK), C.A. Haswell (The Open University, UK), P. Frank (Bundesdeutsche Arbeitsgemeinschaft für Veränderliche Sterne e.V., Germany) and P.J. Wheatley (University of Warwick, Coventry, UK).
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”.
ESO’s Very Large Telescope (VLT): http://www.eso.org/paranal/
NASA/ESA Hubble Space Telescope: https://www.spacetelescope.org/
Australia Telescope Compact Array: https://www.narrabri.atnf.csiro.au/
NASA's Swift satellite: http://swift.gsfc.nasa.gov/
Isaac Newton Group of telescopes: http://www.ing.iac.es/
Research paper: http://www.eso.org/public/archives/releases/sciencepapers/eso1627/eso1627a.pdf
Photos of the VLT: http://www.eso.org/public/images/archive/category/paranal/
Images of Hubble: http://www.spacetelescope.org/images/archive/category/spacecraft/
Link to science paper: http://www.spacetelescope.org/static/archives/releases/science_papers/heic1616/heic1616a.pdf
Images, Text, Credits: ESO/M. Garlick/University of Warwick/IAU and Sky & Telescope/Digitized Sky Survey 2. Acknowledgement: Davide De Martin/ESA/Hubble/Mathias Jäger/Department of Physics, University of Warwick/Tom Marsh/Boris Gänsicke/Videos: ESO/L. Calçada/University of Warwick/Digitized Sky Survey 2/N, Risinger (skysurvey.org).
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