Astronomers identify a possible magnetar precursor
MAGNETARS ARE objects with magnetic fields a thousand trillion times stronger than Earth’s. They are a subclass of neutron stars, the remnants of massive stars. But while astronomers known how run-of-themill neutron stars are formed, they aren’t sure yet of the specific conditions that create a magnetar, whose magnetic field is some 100 to 1,000 times stronger than a neutron star’s.
At least part of the answer might lie in a star called HD 45166, the subject of a paper published Aug. 17 in Science. HD 45166 is a Wolf-Rayet star; these massive, helium-rich stars typically weigh 25 times the mass of the Sun and blast out powerful winds of charged particles. Because of their high mass, they rapidly fuse hydrogen into helium in their cores, living for a fraction of the Sun’s lifetime before exploding as supernovae and leaving behind neutron stars or black holes.
But HD 45166 is an oddball among Wolf-Rayets at only 2 solar masses — a real lightweight. And its magnetic field has a strength of 43 kilogauss, or 100,000 times stronger than Earth’s magnetic field. That makes it the most magnetic massive star ever found.
“We’ve never detected a magnetic field in a massive helium star that will undergo core collapse [a type of supernova],” says study leader Tomer Shenar of the University of Amsterdam. “It’s really a new type of star.” Shenar and his colleagues think HD 45166 didn’t evolve the way other Wolf-Rayets do (as part of some massive stars’ life cycles), but perhaps as the product of a merger between two other more intermediatemass helium-rich stars.
Despite the staggering strength of HD 45166’s magnetic field, it is still 10 billion times below that of a magnetar.
But in a few million years, when HD 45166 explodes as a supernova and leaves behind a neutron star, its magnetic field will be confined to a region just 12 miles (20 km) across — the size of a typical neutron star. Because magnetic flux is conserved, compressing the field will boost its strength by about 10 billion times, creating a magnetar.
“We thought that the most likely magnetar candidates would come from the most massive of stars,” said study co-author André-Nicolas Chené of the National Science Foundation’s National Optical-Infrared Astronomy Research Laboratory in Hilo, Hawaii, in a statement. “What this research shows us is that stars that are much less massive can still become a magnetar, if the conditions are just right.”