Magnetars are the strongest magnets in the universe. These ultra-dense dead stars with ultra-strong magnetic fields can be found throughout our galaxy, but astronomers don’t know exactly how they form. Now, using several telescopes around the world, including European Southern Observatory (ESO) facilities, researchers have detected a living star that is likely to become a magnetar. The discovery marks the discovery of a new type of astronomical object – giant magnetized helium stars – and sheds light on the origin of magnetars.

Despite having been observed for over 100 years, the mysterious nature of the star HD 45166 could not be easily explained by conventional models, and little was known about it beyond the fact that it is one of a pair of stars. [1]It is rich in helium and several times more massive than our Sun.

“This star has become an obsession for me,” says Tomar Schenar, an astronomer at the University of Amsterdam in the Netherlands and lead author of a study on this object published today in Science. “Tomer and I refer to HD 45166 as a ‘zombie star’,,'” says co-author and ESO astronomer Julia Bodensteiner based in Germany. “Not only because this star is so unique, but also because I joked that it turns Tomar into a zombie.”

Having previously studied similar helium-rich stars, Schenar thought magnetic fields might settle the matter. In fact, magnetic fields are known to affect the behavior of stars and may explain why traditional models fail to describe HD 45166, which is located approximately 3000 light-years away in the constellation Monoceros. “I had a eureka moment while reading the literature: ‘What if the star is magnetized?’,” says Shenar, who currently works at the Center for Astrobiology in Madrid, Spain.

Shenar and his team began studying the star using several facilities around the world. The key observations were made in February 2022 on the Canada-France-Hawaii Telescope using an instrument that can detect and measure magnetic fields. The team also relied on key collection data taken with the fibre-fed Extended Range Optical Spectrograph (FEROS) at ESO’s La Silla Observatory in Chile.

Once the observations came in, Shenar asked co-author Greg Wade, an expert on the magnetic fields of stars at the Royal Military College of Canada, to cross-check the data. Wade’s response confirmed Shenar’s guess: “Well my friend, whatever this thing is – it sure is magnetic.”

Schenar’s team had found that the star has an incredibly strong magnetic field of 43,000 gauss, making HD 45166 the most magnetically massive star ever found. [2], “The magnetic field of the entire surface of a helium star is about 100,000 times stronger than that of Earth,” explains astronomer and co-author Pablo Marchant of the KU Leuven Institute of Astronomy in Belgium. This observation marks the discovery of the first massive magnetized helium star. “Uncovering a new type of astronomical object is exciting,” says Shenar, “especially when it’s always been hiding in plain sight.”

In addition, it provides clues to the origin of magnetars, compact dead stars with magnetic fields at least a billion times stronger than HD 45166’s. The team’s calculations suggest that this star will end its life as a magnetar. As it collapses under its own gravity, its magnetic field will strengthen, and the star will eventually form a very compact core with a magnetic field of about 100 trillion gauss. [3] – The most powerful type of magnet in the universe.

Shenar and his team also found that the mass of HD 45166 is smaller than previously reported, about twice the mass of the Sun, and that its stellar pair orbits at a much greater distance than previously thought. Furthermore, their research indicates that HD 45166 may have formed from the merger of two smaller helium-rich stars. “Our findings completely reshape our understanding of HD 45166,” Bodensteiner concluded.

notes

[1] While HD 45166 is a binary system, in this text HD 45166 refers to the helium-rich star, not both stars.

[2] The magnetic field of 43,000 gauss is the strongest magnetic field ever detected in a star that exceeds the Chandrasekhar mass limit, which is the critical limit above which stars can collapse into neutron stars (magnetars are a type of neutron star).

[3] In this text, a billion refers to one followed by nine zeros and a trillion refers to one followed by 12 zeros.

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