Magnetars, the universe's most potent magnets, are ultra-dense deceased stars dotting our galaxy, possessing immense magnetic fields. Their formation is a mystery to astronomers. However, with the aid of numerous global telescopes, including those from the European Southern Observatory (ESO), researchers have discovered a star predicted to become a magnetar. This discovery introduces a new celestial object, the massive magnetic helium stars, that elucidates the birth of magnetars.
Artist’s impression of HD 45166, the star that might become a magnetar. Credit: ESO
The star HD 45166, despite being observed for over a century, remained a puzzle not easily solved by traditional models. Its known characteristics include being one half of a binary star system [1], rich in helium, and several times the mass of our Sun.
"My fascination with this star was quite intense," admits Tomer Shenar, the primary author of a study on HD 45166 published in Science and an astronomer at the University of Amsterdam, Netherlands. Alongside co-author and ESO astronomer Julia Bodensteiner, based in Germany, they affectionately dubbed HD 45166 the 'zombie star'.
Having previously studied helium-rich stars, Shenar hypothesized that magnetic fields may be the missing piece. Magnetic fields have known impacts on star behavior and could explain why HD 45166, located approximately 3000 light-years away in the Monoceros constellation, defies conventional models. Shenar, now at the Centre for Astrobiology in Madrid, Spain, had an epiphany, "Could the star be magnetic?"
Shenar and his team launched an investigation using various global facilities. In February 2022, they utilized an instrument on the Canada-France-Hawaii Telescope that measures magnetic fields. They also depended on archival data from the Fiber-fed Extended Range Optical Spectrograph (FEROS) at ESO's La Silla Observatory in Chile.
After conducting the observations, Shenar sought the expertise of Gregg Wade, a specialist in star magnetic fields at the Royal Military College of Canada. Wade confirmed Shenar's suspicion, "Indeed, my friend, it's magnetic."
The team discovered that HD 45166 boasts a stupendously strong magnetic field of 43,000 gauss, making it the most magnetic massive star currently known. "The entire helium star surface matches the magnetic strength of the most powerful man-made magnets," says co-author Pablo Marchant from KU Leuven's Institute of Astronomy in Belgium.
This discovery signifies the first-ever identified massive magnetic helium star. Shenar finds it thrilling to discover a new type of celestial object, especially one that has been hiding in plain sight.
Additionally, it offers insights into the origins of magnetars, compact expired stars with magnetic fields a billion times stronger than HD 45166. The team's calculations suggest that HD 45166 will evolve into a magnetar. As it succumbs to its own gravity, its magnetic field will intensify, ultimately transforming into a compact core with a magnetic field around 100 trillion gauss, the universe's most potent magnet.
The team also discovered that HD 45166 has a smaller mass than previously believed, around double the mass of the Sun, and its binary star orbits at a significantly greater distance. Moreover, their research suggests that HD 45166 resulted from the merger of two smaller helium-rich stars. "Our findings completely redefine our understanding of HD 45166," concludes Bodensteiner.
This research was presented in a paper to appear in Science (doi: science.org/doi/10.1126/science.ade3293).
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