Astronomers find progenitor of magnetic monster

New observations by a team of astronomers, including NSF's NOIRLab's André-Nicolas Chené, may shed important light on the origin of these magnetic powerhouses. Using various telescopes around the globe, including the Canada-France-Hawai'i Telescope (CFHT) on Maunakea [1], the researchers have identified a new type of astronomical object -- a massive magnetic helium star (an unusual variant of a Wolf-Rayet star), which may be the precursor of a magnetar.

"For the first time, a strong magnetic field was discovered in a massive helium star," said Chené. "Our study suggests that this helium star will end its life as a magnetar."

Despite having been observed for more than a century by astronomers, little was known about the true nature of this star, known as HD 45166, beyond the fact that it is rich in helium, somewhat more massive than our Sun, and part of a binary system.

"This star became a bit of an obsession of mine," said Tomer Shenar, an astronomer at the University of Amsterdam and lead author of a study published in the journal Science. Having studied similar helium-rich stars before, Shenar was intrigued by the unusual characteristics of HD 45166, which has some of the characteristics of a Wolf-Rayet star, but with a unique spectral signature. He suspected that magnetic fields could explain these perplexing characteristics. "I remember having a Eureka moment while reading the literature: 'What if the star is magnetic?'," he said.

Shenar, Chené, and their collaborators set out to test this hypothesis by taking new spectroscopic observations of this star system with the CFHT. These observations revealed that this star has a phenomenally powerful magnetic field, about 43,000 gauss [2], the most powerful magnetic field ever found in a massive star. By also studying its interactions with its companion star, the team were able to make precise estimates of its mass and age.

The researchers speculate that, unlike other helium stars that eventually evolve from a red supergiant, this particular star was likely created by the merger of a pair of intermediate-mass stars.

"This is a very specific scenario, and it raises the question of how many magnetars come from similar systems and how many come from other types of systems," said Chené.

In a few million years, HD 45166, which is located 3000 light-years away in the constellation Monoceros (the Unicorn), will explode as a very bright, but not particularly energetic, supernova. During this explosion, its core will contract, trapping and concentrating the star's already daunting magnetic field lines. The result will be a neutron star with a magnetic field of around 100 trillion gauss -- the most powerful type of magnet in the Universe.

"We thought that the most likely magnetar candidates would come from the most massive of stars," said Chené. "What this research shows us is that stars that are much less massive can still become a magnetar, if the conditions are just right."

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[1] The team also relied on key archive data taken with the Fiber-fed Extended Range Optical Spectrograph (FEROS) at ESO's La Silla Observatory in Chile.

[2] Gauss is a unit of measurement of magnetic induction, also known as magnetic flux density (essentially, a measure of magnetic strength). The Sun's typical polar magnetic field is 1-2 gauss, while sunspots can achieve a magnetic field strength of around 3000 gauss.