Sunday, November 28

White dwarf breaks rotation record – Earth-sized remnant of stars is only the second known “propeller star” –

Remnants of stars as propellers: Astronomers have discovered the fastest rotating white dwarf. The roughly earth-sized remnant of the stars only needs 25 seconds to rotate – that’s 20 percent faster than the previous record holder. Even more unusual, however: the white dwarf sucks material from its stellar neighbor, but cannot hold it and instead hurls it at high speed into nearby space. This makes it only the second known “propeller star”, as the team reports.

White dwarfs are ultra-dense remnants of stars that remain after the collapse of a sun-like star and the ejection of its envelopes. Most of them are rather small and keep cooling down over time. However, there are also some white dwarfs that show unusual traits. These include particularly heavy representatives with strong magnetic fields, particularly fast rotating white dwarfs and a handful of star remnants in binary star systems that emit rapid, strong flashes of rays – possibly generated by the interactions of their magnetic field with the material sucked in by the partner.

A flicker from the binary star system

Astronomers working with Ingrid Pelisoli from the University of Warwick have now discovered a new record holder among the fast-moving white dwarfs. The focus of their study was the system LAMOST J024048.51 + 195226.9, J0240 + 1952 for short, which was only discovered in 2020. It consists of a red dwarf and a roughly Earth-sized white dwarf that is at least 200,000 times heavier than Earth. This remnant of the stars was noticed because it also generates periodic radiation pulses in almost all wave ranges.

What is behind the strange flickering of the white dwarf, Pelisoli and her colleagues have now examined more closely with the help of the HiPERCAM instrument at the Gran Telescopio Canarias in La Palma. The combination of the ten-meter mirror telescope with the instrument designed for high temporal resolution enabled the team to record precise light curves of the system and to analyze them.

Fastest rotation known so far

The result: Part of the flickering is due to the extremely fast rotation of the white dwarf. As the astronomers determined, the remnant of the star revolves around itself in just 24.93 seconds – an absolute record. “So far, only three other systems are known whose rotation periods are less than 40 seconds,” explains the team. J0240 + 1952, however, rotates another 20 percent faster than the fastest among them to date.

“The rotation of this white dwarf is so fast that it has to have an above-average mass in order not to be torn apart by it,” says Pelisoli. “In the short time it takes to read this, it has already made several revolutions – that’s really unbelievable.”

White dwarf is a propeller star

A second peculiarity of this white dwarf is even more exciting: it could only be the second known “propeller star”. Astronomers gave this nickname to a white dwarf in the AE Aquarii binary star system, around 300 light-years away, because it is so strongly magnetized and rotates so quickly that it cannot hold the material extracted from its partner star: it hurls it out into space at an enormous speed and creates it thereby additional bursts of radiation.

“The material is sucked in, but when it gets closer to the white dwarf, its magnetic fields begin to dominate,” explains Pelisoli. Combined with the high speed of rotation, this ensures that a large part of the material is thrown away at more than 3,000 kilometers per second – like mud from a rapidly rotating propeller.

No more isolated cases

“For more than seven decades, AE Aquarii was alone among the thousands of known star systems with these extraordinary properties,” write the astronomers. “But that could have changed now.” Because the high speed of rotation and the light curves of the flickering of J0240 + 1952 suggest that this white dwarf could also be such a “propeller system”. For the first time, AE Aquarii is no longer alone.

“It is only the second time that we have found one of these magnetic propeller systems. So we now know that AE Aquarii was not an isolated incident, ”says co-author Tom Marsh of the University of Warwick. “Instead, it establishes that this propellor mechanism is a peculiarity that can occur in such dual systems under certain conditions.”

This is also important because you can now check the model that you developed on the basis of the first case directly on the second case study. “But our current discovery shows that the model works well,” Marsh said. (Notices of the Royal Astronomical Society: Letters, 2021; doi: 10.1093 / mnrasl / slab116)

Quelle: University of Warwick

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