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Researchers Observe Super-Heated Magnetosphere Around White Dwarf

An international group of astronomers has found out a very hot magnetosphere around a white dwarf, a remnant of a star similar to the Sun, thereby solving a decades-old mystery.

This is an artist’s impression of the hot white dwarf GALEXJ014636.8+323615 (white) and its ultra-hot circumstellar magnetosphere (purple) trapped with the magnetic field (green). (Image credit: N. Reindl)

The study was headed by Dr Nicole Reindl, Research Fellow of the Royal Commission 1851, based at the University of Leicester, and has been published in the Monthly Notices of the Royal Astronomical Society journal on November 7th, 2018.

White dwarfs are the end stage in the lives of stars such as the Sun. Close to the final stages of their lives, such stars eject their outer atmospheres, where a compact, hot, and dense core that cools over billions of years is left behind. In general, the temperature on their surfaces is about 100,000 °C (in contrast, the temperature of the surface of Sun is 5500 °C).

However, certain white dwarfs pose challenges to researchers since they exhibit evidence for highly ionized metals. In astronomy, any element heavier than helium are regarded as “metals”, and here, high ionization means that all but one of the outer electrons often found in their atoms have been taken away. This process requires a temperature of 1 million degrees Celsius, much higher than the surface of even the hottest white dwarf stars.

Reindl’s group used the 3.5-m Calar Alto telescope in Spain to find out and observe a white dwarf in the direction of the constellation of Triangulum, cataloged as GALEXJ014636.8+323615, located 1200 light-years from the Sun. The signatures of highly ionized metals were unraveled through the analysis of the light from the white dwarf with a method called spectroscopy, which involves dispersing light into its constituent colors. Interestingly these differed over a period of six hours—the same time taken by the white dwarf to rotate.

Reindl and her colleagues came to a conclusion that material flowing from the surface of the star is trapped by the magnetic field surrounding it, or the magnetosphere. The material is drastically heated by the shocks within the magnetosphere, stripping nearly all the electrons from the metal atoms.

It’s like a doughnut made up of ultra-hot material that surrounds the already very hot star. The axis of the magnetic field of the white dwarf is tilted from its rotational axis. This means that the amount of shock-heated material we see varies as the star rotates.

Dr Nicole Reindl, Research Fellow of the Royal Commission 1851, University of Leicester.

After decades of finding more and more of these obscure stars without having a clue where these highly ionised metals come from,” she continues, “our shock-heated magnetosphere model finally explains their origin.”

Although magnetospheres can be found around other types of stars, this is the first time one such magnetosphere has been observed around a white dwarf. The discovery could have far-reaching effects. “We simply didn’t take this into account,” admitted Reindl. “Ignoring their magnetospheres could mean measurements of other basic properties of white dwarfs are wrong, like their temperatures and masses.”

Possibilities are that a quarter of white dwarfs undergo a stage of trapping and super-heating material. Currently, Reindl and her colleagues are planning to model them in detail and to extend their study by analyzing more of these intriguing objects.

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