Exotic White Dwarf Brutalizes its Red Dwarf Partner

Comprised of a tiny white dwarf and red dwarf that orbit one another every 3.6 hours, the AR Scorpii system was misidentified in the 1970s as a single variable star that fluctuated in brightness. But in 2015, amateur astronomers stumbled upon the star and made a note of its strange behavior. In followup observations, culminating in observing time with the Hubble Space Telescope, AR Scorpii's binary nature was revealed.

Binary stars are common in our galaxy, but this particular system has an exotic side that that is causing some confusion.

Every 1 minute and 58 seconds, the white dwarf blasts its red dwarf binary partner with an incredibly powerful beam of radiation. This pulse of radiation causes the whole system to brighten and dim like clockwork and includes radiation over a broad range of frequencies, including radio waves. And herein lies the puzzle.

White dwarfs are small husks of stars that died long ago. After sun-like stars run out of hydrogen fuel, they puff up into erupting red giants, eventually shedding their hot plasma, creating beautiful planetary nebulae. When our sun eventually dies in about 5 billion years, all that will be left behind is a nebula and tiny white dwarf in the core. These dense objects are around the size of Earth but contain the mass of 200,000 Earths. They are also highly magnetized, compressing the entire magnetic field of a large main sequence star into the volume of a small planet.

So the powerful beam of radiation sweeps through space like a lighthouse as the white dwarf spins -- in a similar way that spinning neutron stars create pulsars. This beam is generated by the white dwarf's intense magnetic field accelerating electrons to relativistic speeds. But astronomers aren't sure where these electrons are coming from as it's not clear whether they are supplied by the white dwarf or red dwarf.

"We've known about pulsing neutron stars for nearly fifty years, and some theories predicted white dwarfs could show similar behavior," said Boris Gänsicke of the University of Warwick. "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."

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