White dwarf lashes red dwarf with mystery ray
Astronomers using the NASA/ESA Hubble Space 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 is powering 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.
, 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 same size as Earth but containing 200,000 times more mass, and a cool red dwarf companion one third the mass of the Sun . They are orbiting one another every 3.6 hours in a cosmic dance as regular as clockwork.
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.
|This wide-field image from the Digitized Sky Survey 2 shows the rich starfields surrounding the exotic|
binary star system AR Scorpii [Credit: Digitized Sky Survey 2/Davide De Martin]
The observed properties of AR Sco are unique. And 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, or its cooler companion.
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.
Boris Gansicke, co-author of the new study, also at the University of Warwick, concludes: "We've known about 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."
 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.
 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 since regular fluctuations in observed brightness occur as the two stars orbit each other and one blocks some of the light from the other.
Source: ESA/Hubble Information Centre [July 27, 2016]