High-Speed Propeller Star is Fastest-Spinning White Dwarf

Spinning white dwarf

Artist’s impression of LAMOST J024048.51+195226.9, the fastest-spinning confirmed white dwarf and only second ever magnetic propeller known. Material is being pulled from the companion and flung into space at high speed. A small fraction of it is accreted, gathering in bright spots that rotate in and out of view, which allowed the detection of the rotation period. Credit: University of Warwick/Mark Garlick.

A white dwarf star that completes a full rotation once every 25 seconds is the fastest-spinning confirmed white dwarf, according to a team of astronomers led by the University of Warwick.

They have established the spin period of the star for the first time, confirming it as an extremely rare example of a magnetic propeller system: the white dwarf is pulling gaseous plasma from a nearby companion star and flinging it into space at around 3000 kilometers per second (1864 miles per second).

Reported on November 22 in Monthly Notices of the Royal Astronomical Society: Letters, it is only the second magnetic propeller white dwarf to have been identified in over 70 years thanks to a combination of powerful and sensitive instruments that allowed scientists to catch a glimpse of the speeding star.

A white dwarf is a star that has burned all its fuel and shed its outer layers, undergoing a process of shrinking and cooling over millions of years. The star that the University of Warwick team observed, named LAMOST J024048.51+195226.9 (or J0240+1952 for short), is the size of Earth but is thought to be at least 200,000 times more massive. It is part of a binary star system, and its immense gravity pulls material from its larger companion star in the form of plasma.

In the past, this plasma fell onto the white dwarf’s equator at high speed, providing the energy that has given it this dizzyingly fast spin. Put into context, one rotation of Earth takes 24 hours, while the equivalent on J0240+1952 takes a mere 25 seconds. That’s almost 20% faster than the confirmed white dwarf with the most comparable spin rate, which completes a rotation in just over 29 seconds.

However, at some point in its evolutionary history, J0240+1952 developed a strong magnetic field. The magnetic field acts a protective barrier, causing most of the falling plasma to be propelled away from the white dwarf. The remainder flows toward the star’s magnetic poles, gathering in bright spots on the surface of the star; as these rotate in and out of view, they cause pulsations in the light that astronomers observe from Earth, which they then used to measure the rotation of the entire star.

Lead author Ingrid Pelisoli of the University of Warwick’s Department of Physics said, “J0240+1952 will have completed several rotations in the short amount of time that people take to read about it. It is really incredible. The rotation is so fast that the white dwarf must have an above-average mass just to stay together and not be torn apart.

“It is pulling material from its companion star due to its gravitational effect, but as that gets closer to the white dwarf, the magnetic field starts to dominate. This type of gas is highly conducting and picks up a lot of speed from this process, which propels it away from the star and out into space.”

J0240+1952 is one of only two stars with this magnetic propeller system discovered in over past 70 years. Although material being flung out of the star was first observed in 2020, astronomers had not been able to confirm the presence of a rapid spin, a main ingredient of a magnetic propeller, as the pulsations are too fast and dim for other telescopes to observe.

To visualize the star at that speed for the first time, the University of Warwick team used the highly sensitive HiPERCAM instrument, jointly operated by the University of Warwick and the University of Sheffield with funding from the European Research Council. HiPERCAM was specially mounted on the largest functioning optical telescope in the world, the 10.4-meter-diameter (34-foot-diameter) Gran Telescopio Canarias (GTC) in La Palma, Canary Islands, to capture as much light as possible.

“These kinds of studies are possible thanks to the unique combination of the fast-imaging capability of HiPERCAM, with the largest collecting area in the world provided by GTC,” said Antonio Cabrera, head of GTC Science Operations.

Co-author Professor Tom Marsh from the University of Warwick’s Department of Physics adds: “It’s only the second time that we have found one of these magnetic propeller systems, so we now know it’s not a unique occurrence. It establishes that the magnetic propeller mechanism is a generic property that operates in these binaries, if the circumstances are right.”

“The second discovery is almost as important as the first as you develop a model for the first, and with the second you can test it to see if that model works. This latest discovery has shown that the model works really well. It predicted that the star had to be spinning fast, and indeed it does.”