For more than a century, astronomers have been puzzled by the mysterious nature of HD 45166, a dying Wolf-Rayet star located about 3,000 light-years from Earth. This star, like other stars in this class, is helium-rich and has a companion. However, it exhibits peculiar behavior that distinguishes it from all known Wolf-Rayet stars, including an unexpectedly high rate of mass loss and an unusual pattern of winds.
A team of astronomers led by Tomer Shenar of the University of Amsterdam has made a groundbreaking discovery, revealing the true nature of this celestial enigma. The study revealed that HD 45166 has an incredibly powerful magnetic field, making it the first Wolf-Rayet magnetic star ever identified. This discovery not only resolves previous mysteries associated with the binary system, but also provides valuable insights into the transformation of massive stars into the most magnetic objects in the Universe. The study was published in the journal Science
Wolf-Rayet stars are rare and often considered some of the brightest stars in the Milky Way. These stars are nearing the end of their lifetimes because their cores have run out of atomic fuel for fusion. They are highly heated, bright, low in hydrogen but rich in carbon and nitrogen. Losing mass rapidly through stellar winds, they eventually become supernovae in a relatively short cosmic time span. The ejection of external matter causes their core to collapse into a neutron star, an incredibly dense object only 20 kilometers in size and 2.4 times the mass of the Sun.
Magnetars, on the other hand, are a type of neutron star with mind-bogglingly powerful magnetic fields – about 1000 times stronger than those of ordinary neutron stars, and quadrillions of times more powerful than the Earth’s magnetic field. The origin of such strong magnetic fields remains unknown, but some models suggest that stars may possess magnetic fields even before supernova and core collapse. However, no star at the end of its life that possesses a magnetic field of sufficient strength has yet been discovered.
This is where the interest in HD 45166 comes in. Previous measurements of this binary system have shown that the Wolf-Rayet star is helium-rich, has a mass about four times that of the Sun, and orbits a B-type star in a narrow orbit of 1.6 days duration. However, these properties contradicted our ideas about the evolution of binary systems and the mechanisms of stellar wind launching.
Driven by curiosity, Chenard and his collaborators embarked on a new set of observations to find evidence for a magnetic field in HD 45166. Their efforts paid off when they not only discovered a magnetic field, but also obtained a new set of measurements that completely changed our understanding of the binary system.
First, HD 45166’s magnetic field strength was found to be 43,000 gauss – the strongest magnetic field ever recorded in a massive star. Second, the star was found to have a much smaller mass than previously thought, only twice the mass of the Sun. Finally, it was found that the orbital period of the binary system is much longer, about 8,200 days, which contradicts the previously discovered periodicity of 1.6 days, which was explained by internal oscillations of the B-type companion star.
This groundbreaking discovery not only sheds light on the true nature of HD 45166, but also deepens our understanding of the mechanisms of magnetar formation. It opens new avenues for research and exploration of the fascinating world of massive stars and their transformations into highly magnetic objects.