A new type of star offers clues to the origin of magnetars

2023-08-17 20:00:10

Throughout our galaxy you can find very high-density dead stars with ultra-strong magnetic fields: the magnetares, the most powerful magnets in the cosmos. It is a type of neutron star, which is formed when massive stars collapse and explode in the form of a supernova. However, the origin of the magnetar type is unclear.

Now, using multiple telescopes, a team of researchers has discovered a living star that is likely to become a magnetar. This finding, published in Sciencemarks the discovery of a new type of astronomical object: massive magnetic helium starsin addition to shedding light on the mysterious origin of magnetars.

The first massive magnetic helium star has been discovered: it is rich in helium and is part of the binary HD 45166

Despite having been observed for more than 100 years, conventional models could not explain the enigmatic nature of one of the two stars that make up the galaxy. binary HD 45166, specifically the one that is rich in helium and a few times more massive than our Sun. Little else was known about it, the protagonist of the new study that belongs to a group of hot and evolved stars called Wolf-Rayet.

“This star became almost an obsession”, affirms Tomer Shenarlead author of a study and astronomer at the University of Amsterdam (Netherlands).

“Tomer and I refer to HD 45166 (in this context it is the helium-rich star, not the two in the binary) as the zombie star, It’s not just because it’s so unique, but also because I jokingly said it turns Tomer into a zombie,” says co-author and astronomer Julia Bodensteiner of the European Southern Observatory (ESO).

Having studied similar helium-rich stars before, Shenar thought that the magnetic fields they could solve the case. In fact, these fields are known to influence the behavior of stars and could explain why traditional models were unable to describe the behavior of HD 45166, which is located about 3000 light years away, in the constellation Monoceros.

Investigation after the eureka moment

“I remember having a eureka moment while reading the literature about it: What if the star is magnetic?”, says Shenar, who currently works at the Center for Astrobiology (CAB) in Madrid.

Shenar and his team set out to study the star using multiple facilities spread across the globe. The main observations were made in February 2022 using an instrument installed in the Canada-France-Hawaii Telescope that can detect and measure magnetic fields.

The researchers also relied on key archival data taken with the instrument. WILD (Fiber-fed Extended Range Optical Spectrograph), at ESO’s La Silla Observatory in Chile.

After the observations were made, Shenar asked the co-author Gregg Wade, an expert in star magnetic fields from the Royal Military College of Canada, to examine the data. Wade’s response confirmed Shenar’s hunch: “Well, my friend, whatever it is, it’s definitely magnetic.”.

The star has an incredibly strong magnetic field of 43,000 gauss, making it the most magnetic massive star found so far.

Shenar’s team had discovered that the star has an incredibly strong magnetic field of 43,000 gaussmaking HD 45166 the most magnetic massive star found to date.

That value represents the strongest magnetic field ever detected in a star that exceeds the so-called Chandrasekhar mass limitabove which stars can collapse into neutron stars, like magnetars.

“The entire surface of this helium star has a magnetic field about 100,000 times stronger than Earth’s.”, explains the co-author, Paul Marchantastronomer at the KU Leuven Institute of Astronomy (Belgium).

This observation marks the discovery of the first massive magnetic helium star. “It is exciting to discover a new type of astronomical object”, says Shenar, “especially when it’s been hiding in plain sight the whole time.”

It’s exciting to discover a new kind of astronomical object, especially when it’s been hiding in plain sight all along.”

Tomer Shenar (University of Amsterdam/CAB)

In addition, it provides clues to the origin of magnetarscompact dead stars associated with magnetic fields at least a billion times stronger than that of HD 45166.

Birth of the magnetar after the death of the star

In fact, 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 eventually the star will become a very compact core with a magnetic field of around 100 trillion gauss, the most powerful type of magnet in the universe.

It was formed by the merger of two smaller helium-rich stars.

Shenar and his team also discovered that HD 45166 has a smaller mass than previously calculated, 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 formed through the merger of two smaller, helium-rich stars. “Our findings completely reshape our understanding of HD 45166,” concludes Bodensteiner.

Reference:

Tomer Shenar et al. “A massive helium star with a sufficiently strong magnetic field to form a magnetar”. Science2023