Astronomers using the European Southern Observatory’s (ESO) Very Large Telescope, along with other telescopes on the ground and in space, have discovered a new type of exotic binary star. The system AR Scorpii (also known as AR Sco), lying 380 light-years from Earth in the constellation Scorpius, contains a rapidly spinning white dwarf star, the size of Earth but containing 200,000 times more mass. The system also is home to its companion, a cool red dwarf one third the mass of the Sun. The two dwarfs orbit one another every 3.6 hours.
White dwarfs form late in the life cycles of stars. 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. A single spoonful of the matter making up a white dwarf would weigh about as much as an elephant here on Earth.
In a unique twist, this binary star system is exhibiting some brutal behavior. 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 and has never been detected before from a white dwarf system.
Tom Marsh of the University of Warwick’s Astrophysics Group and lead researcher for the project study published in the July 28 edition of the journal Nature commented, “AR Scorpii was discovered over 40 years ago, but its true nature was not suspected until we started observing it in 2015. We realized we were seeing something extraordinary within minutes of starting the observations.”
While 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 the major mystery. There is no clear answer as to whether it is associated with the white dwarf itself, 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 behavior has been observed before, but from neutron stars — some of the densest celestial objects known in the Universe — rather than white dwarfs.
Boris Gänsicke, co-author of the study, also at the University of Warwick, offered, "We've known pulsing neutron stars for nearly fifty years, and some theories predicted white dwarfs could show similar behavior. 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."