Astronomers discover possible precursor of a magnetar

Helium star with magnetic field

The massive helium star HD 45166 has an unusually strong magnetic field and therefore belongs to a completely new class of stars. © ESO/ L. Calçada

Magnetars – neutron stars with an extremely strong magnetic field – are among the strongest magnets in the universe, but how they are formed was previously unclear. Now astronomers have discovered a possible magnetar precursor for the first time. It is a massive star around 3,000 light-years away that has lost its hydrogen shell and whose core consists primarily of helium. Spectral measurements now show that this helium star has a magnetic field of around 43,000 Gauss. This makes the star, named HD 45166, the most magnetically massive star ever detected and could represent a whole new class of stars. At the same time, this makes the helium star a suitable precursor to a magnetar.

When a massive star explodes in a supernova, the relic left behind is a black hole or a neutron star - an extremely dense, compact celestial body. In some cases, however, magnetars, neutron stars with an unusually strong magnetic field, are formed. They are only 20 to 30 kilometers in size, but are extremely dense and as heavy as one or two suns. Magnetars also rotate extremely quickly and have the strongest known magnetic fields in the cosmos. Their magnetic field strength can reach more than a million Tesla. But how these most extreme magnets in the cosmos are created is still unclear. According to one theory, magnetars inherit the magnetic field from their massive predecessor stars. As the star core collapses into a neutron star, the magnetic field is concentrated and strengthened. However, stronger magnetic fields have so far only been detected in a few massive main sequence stars, but not in massive stars at the end of their life cycle.

Massive helium stars are another candidate for magnetar precursors. This subtype of Wolf-Rayet star is formed when a massive, very hot and luminous star gradually loses its entire hydrogen shell due to its strong stellar winds. The interaction with a partner star or the merger of two white dwarfs can also lead to a helium star. The result is an exposed star core that contains hardly any hydrogen but a lot of helium. If such a helium star has a magnetic field, it could also be amplified when it collapses into a neutron star, turning the remnant star into a magnetar. But there has been a lack of evidence for this scenario either: “Although strongly magnetized low-mass helium stars have already been observed, massive ones beyond the Chandrasekhar limit have not been observed,” explain Tomer Shenar from the University of Amsterdam and his colleagues. The Chandrasekhar limit is the mass limit above which a remnant star becomes unstable and explodes in a supernova.

A helium star with a strong magnetic field

The astronomers have now examined one of these helium stars in more detail. The hot, helium-rich star HD 45166, located around 3,000 light-years away, was discovered a long time ago. However, all that was known about its properties so far was that it has a distant orbiting companion star and resembles a Wolf-Rayet star. However, there were features that didn't fit the picture, including a mass of less than four solar masses and unusually high proportions of nitrogen, carbon and oxygen in the spectrum. So Shenar and his team targeted HD 45166 using a special spectropolarimeter on the Canada-France-Hawaii Telescope (CFHT) on Mauna Kea. They also evaluated spectral data from other telescopes.

Analysis of the observation data revealed several revealing anomalies in the helium star: most of its light was circularly polarized and the oxygen lines in the spectrum were characteristically split into two components, as Shenar and his colleagues found. This so-called Zeeman splitting indicates that there must be a magnetic field on the surface of this star - the more the spectral line is split, the stronger the magnetic field. From this data, the astronomers were able to determine that the helium star HD 45166 must have a magnetic field with a flux density of 43,000 Gauss. This makes it the most magnetic massive star ever observed. “The entire surface of the helium star is as magnetic as the strongest man-made magnets,” explains co-author Pablo Marchant from KU Leuven in Belgium. At the same time, HD 45166 is the first massive magnetic helium star. It therefore belongs to a whole new class of stars. “It's exciting to discover a new class of astronomical objects,” says Shenar, “especially when they've been hidden this whole time.”

(Video: ESO)

Precursor of a magnetar

This helium star could therefore meet the requirements to become a magnetar in a few million years. “With a mass of 2.03 solar masses, we expect this Wolf-Rayet-like star to collapse into a neutron star,” the team explains. Because the magnetic flux is preserved but the size of the star shrinks drastically, the magnetic field on the surface of the resulting neutron star becomes correspondingly stronger. “Based on the measured 43,000 Gauss and a radius of the neutron star of around twelve kilometers, we calculate the expected magnetic field of the neutron star to be around 110 trillion Gauss,” write the astronomers. “That is within the range for magnetars.” This star could therefore solve the mystery of the formation of magnetars.

At the same time, a supplementary simulation by the team also confirms an earlier thesis about such magnetar precursors. As Shenar and his colleagues discovered, HD 45166 most likely formed from the merger of the helium-rich cores of two intermediate precursor stars. These circled each other so closely that they first split their shells and then ejected them when they merged. Only then did the massive, magnetic helium star come into being.

Source: Tomer Shenar (University of Amsterdam) et al., Science, doi: 10.1126/science.ade3293

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