Вадим Дудченко
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A white dwarf in the binary system LAMOST J024048.51+195226.9 (J0240+1952 for short) completes a full rotation once every 24.93 seconds, making it the fastest spinning confirmed white dwarf, according to an analysis of data from the HiPERCAM instrument on the 10.4-m Gran Telescopio Canarias.

An artist’s impression of LAMOST J024048.51+195226.9, the fastest spinning confirmed white dwarf and only second ever magnetic propeller known. Image credit: Mark Garlick / University of Warwick.

The J0240+1952 binary system is located 2,016 light-years away in the constellation of Aries.

Also known as CRTS J024048.5+195227, it consists of a white dwarf and a much larger companion star.

The white dwarf is the size of the Earth but is thought to be at least 200,000 times more massive.

Its immense gravity is pulling material from the companion star in the form of plasma. In the past, this plasma was falling onto the white dwarf’s equator at high speed, providing the energy that has given it this dizzyingly fast spin.

The white dwarf has a spin period of 24.9328 seconds. That’s almost 20% faster than CTCV J2056-3014, a white dwarf with the most comparable spin rate, which completes a rotation in 29.61 seconds.

“J0240+1952 will have completed several rotations in the short amount of time that people take to read about it, it is really incredible,” said Dr. Ingrid Pelisoli, an astronomer in the Department of Physics at the University of Warwick.

“The rotation is so fast that the white dwarf must have an above average mass just to stay together and not be torn apart.”

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 will flow towards the star’s magnetic poles. It gathers in bright spots on the surface of the star and as these rotate in and out of view they cause pulsations in the light that the astronomers observe from Earth, which they then used to measure the rotation of the entire star.

“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,” Dr. Pelisoli said.

“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 seventy years.

“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,” said Professor Tom Marsh, also from the Department of Physics at the University of Warwick.

“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.”

A paper on the findings was published in the Monthly Notices of the Royal Astronomical Society: Letters.


Ingrid Pelisoli et al. 2021. Found: a rapidly spinning white dwarf in LAMOST J024048.51+195226.9. MNRASL 509 (1): L31-L36; doi: 10.1093/mnrasl/slab116


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