How a special type of neutron star, a magnatar (might) form

https://www.scientificamerican.com/article/this-bizarre-star-could-become-one-of-the-strongest-magnets-in-the-universe/?/

In the vastness of the known universe, few things are more wondrous than a magnetar. These stars are deceptively pint-sized; they squeeze multiple suns’ worth of mass into an orb no bigger than a city. And they boast mind-bogglingly powerful magnetic fields that are trillions of times stronger than the one that encompasses our planet. A magnetar’s magnetic field is so strong, in fact, that it can crack open the star’s surface to release powerful bursts of energy that may be visible across billions of light-years. Despite these amazing properties, astronomers aren’t quite sure how magnetars form, with a myriad of possibilities on the table. “We have too many ideas, and we’re not sure which ones are right,” says Christopher White of the Flatiron Institute in New York City. Now researchers may have pinned down one possible pathway to a magnetar by finding an unusually massive and magnetic star that might be on the cusp of forming one of these enigmatic objects.

Tomer Shenar of the University of Amsterdam and his colleagues studied a pair of stars about 3,000 light-years from Earth that are collectively called HD 45166. One member of the pair had previously been identified as a Wolf-Rayet star—a very rare, hot and massive star in the final stages of its life. Such stars have exhausted their hydrogen fuel and instead burn helium, which makes them shine brighter and raises intense stellar winds that can blow off their outer layers. Studying the star in more detail, Shenar’s team discovered this was a particularly unusual Wolf-Rayet star with a magnetic field of 43,000 gauss. (Earth’s field, for comparison, is a paltry half-gauss, and our sun’s is just a single gauss.) This makes the star, whose mass is twice that of our sun, the most magnetic massive star ever discovered. “We have never detected magnetic fields in these types of stars,” Shenar says. “It turned out to have an extremely powerful magnetic field, and it is a prime candidate for becoming a magnetar.” The research was published today in Science.

Using the Canada-France-Hawaii Telescope on Mauna Kea in Hawaii—along with data from Brazil’s National Laboratory for Astrophysics, La Silla Observatory in Chile and the Roque de los Muchachos Observatory in Spain’s Canary Islands—Shenar’s team studied the star via a process called Zeeman-Doppler imaging, which can tease out details of a stellar magnetic field from subtle changes the magnetism imparts to the polarization of a star’s light. The researchers then modeled the Wolf-Rayet star’s history to better understand how its remarkable magnetic field might have formed and found that the star was likely the result of two helium-rich stars merging together. “We think it was quite a complicated merger,” Shenar says—one that possibly involved a helium-rich lower mass star spiraling into the puffy stellar atmosphere of an accompanying red supergiant. The rapid rotation of the two progenitors in the merging process would have spun up the postmerger star’s magnetic field, “amplifying it to a high degree,” says Lidia Oskinova of the University of Potsdam in Germany, who is a co-author of the new paper. “This is a new type of object,” she says.

Magnetars—only about 30 of which are known in our galaxy—are a type of neutron star, a remnant core that is left behind after a massive star ends its life. Neutron stars are the closing phase of stellar evolution, the “last stop” that dying massive stars can reach if they aren’t sufficiently hefty to collapse further to form a black hole. Many are born via a supernova—such neutron stars are created when a star’s explosive death leaves behind a dense, compressed core that is barely 10 miles across. That extreme compression—and an associated boost to the core’s rotation that leaves it spinning around several dozens of times per second—can in principle supercharge any preexisting magnetic field to reach the levels measured for magnetars: some 100 trillion gauss.