September 30, 2023

Astronomers discover possible progenitor of a magnetar

Magnetars – neutron stars with an extremely strong magnetic field – are among the strongest magnets in the universe, but until now it was not clear how they are formed. Astronomers have now detected a possible magnetic precursor for the first time. About 3,000 light-years away, it is a massive star that has lost its hydrogen envelope and has a helium core. Spectroscopic measurements now show that this helium star has a magnetic field of about 43,000 gauss. This makes the star, dubbed HD 45166, the largest massive magnetar ever discovered and could represent a whole new class of star. At the same time, this makes a helium star a suitable precursor to a magnetar.

When a massive star explodes in a supernova, the leftover remnant is a black hole or neutron star – a very dense and compact celestial body. However, in some cases, magnetars, neutron stars with an unusually strong magnetic field, form. It’s only 20 to 30 kilometers in diameter, but it’s extremely dense and heavy, like one or two suns. In addition, magnetars rotate very quickly and have the strongest magnetic fields known in the universe. Its magnetic field strength can reach more than a million teslas. But how these more extreme magnets form in the universe remains unclear. According to one theory, magnetars inherit the magnetic field from their massive ancestors. The magnetic field is concentrated and strengthened by the collapse of the stellar core into a neutron star. 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 cycles.

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Another candidate for magnetic progenitors is massive helium stars. These subtypes of Wolf-Rayet stars form when a massive, extremely hot, and luminous star gradually loses all of its hydrogen shell due to strong stellar winds. An interaction with a partner star or the merger of two white dwarfs can also result in a helium star. The result is an exposed star core that contains little hydrogen but plenty of helium. If such a helium star has a magnetic field, it can also be strengthened when it collapses into a neutron star, turning the rest into a magnetar. But evidence for this scenario has also been missing so far: “Although strongly magnetized low-mass helium stars have already been observed, massive stars beyond the Chandrasekhar limit have not been observed,” explain Tomer Shenar of the University of Amsterdam and colleagues. The Chandrasekhar limit is the mass limit at which a stellar remnant becomes unstable and explodes in a supernova.

A helium star with a strong magnetic field

Now astronomers have closely examined one of these helium stars. About 3,000 light-years away, the hot, helium-rich star HD 45166 was discovered some time ago. However, the only things known so far about its properties are that it has a distantly orbiting companion star that 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. That’s why Schnarr and his team targeted HD 45166 using a special spectrometer on the Canada-France-Hawaii Telescope (CFHT) on Mauna Kea. In addition, they evaluated spectral data from other telescopes.

Analysis of the observational data revealed several revealing anomalies about the helium star: Most of its light was circularly polarized, and the oxygen lines in the spectrum characteristically split into two components, Schnarr and his colleagues found. The so-called Zeeman division indicates that there must be a magnetic field on the surface of this star – the greater the division of the spectral line, the stronger the magnetic field. From this data, astronomers were able to determine that the helium star HD 45166 should have a magnetic field with a flux density of 43,000 gauss. This makes it the most massive magnetic star ever observed. “The entire surface of a helium star is magnetic, as is the strongest man-made magnet,” explains co-author Pablo Marchant of KU Leuven in Belgium. At the same time, HD 45166 is the first massive magnetar helium. He belongs to a whole new class of superstars. “It’s exciting to discover a new breed of astronomical object, especially when it’s been hidden all along,” Schnarr says.

(Video: ESO)

magnet predecessor

So this helium star can 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. Since the magnetic flux is preserved, but the size of the star is greatly reduced, the magnetic field on the surface of the resulting neutron star becomes correspondingly stronger. “Based on a measurement of 43,000 gauss and a neutron star radius of about twelve kilometers, we calculate the expected magnetic field of the neutron star at about 110 trillion gauss,” the astronomers wrote. “This is within the range of magnetars.” This star could solve the mystery of how magnetars form.

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At the same time, a complementary simulation by the team also confirms a previous thesis about these magnetic strains. Because, as Schnarr and his colleagues found, HD 45166 most likely formed from the merger of the helium-rich cores of two intermediate-mass progenitor stars. These circled so closely together that they first split and then expelled their shells as they fused. Only then did the massive helium magnetar form.

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