As many as half of all stars in our galaxy come in pairs called binary systems – meaning they are composed of multiple stars orbiting a common center. In a new study published in the Monthly Notices of the Royal Astronomical Society, Matt Shultz – a Canadian Ph.D. student at Queen’s University in Ontario, Canada – announces the discovery of a unique stellar object: Two massive stars with magnetic fields within a binary system.
The system, Epsilon Lupi, resides in the constellation Lupus, approximately 500 light-years from Earth. As part of the Binarity and Magnetic Interactions in various classes of Stars (BinaMIcs) consortium – led by Dr. Evelyne Alecian of France’s University of Grenoble – Shultz studies the magnetic properties of close binary stars.
Astronomers have known for many years that Epsilon Lupi, the fourth brightest star in the southern constellation of Lupus, is actually a pair of blue stars roughly 6,000 times more luminous than our Sun. Each star in the pair has a mass of approximately 7-8 solar masses. By observing the star system with the Canada-France-Hawaii telescope (CFHT) on Mauna Kea in Hawaii, Shultz was able to discover that both stars in the system have their own magnetic field. The origin of these fields around massive stars is a mystery Shultz hopes this discovery will help solve.
Shultz comments in a statement, “The origin of magnetism amongst massive stars is something of a mystery and this discovery may help to shed some light on the question of why any of these stars have magnetic fields.”
In cooler stars, like the Sun, we know that magnetic fields are generated as a result of strong convection processes in the star’s outer layer. This is not the case with stars like Epsilon Lupi as hotter, more massive stars lack a similar convection process in the outer stellar envelope. Despite the absence of convection, up to 10% of massive stars in our galaxy have a strong magnetic field. There are two possible explanations for this, both involving the notion of a “fossil” magnetic field – which is a field generated some time during the star’s life and locked into the stellar surface.
Such a magnetic field could be generated while the star is forming. Another possibility is that when two stars merge to form a binary system, the violent exchange of material could generate a magnetic field.
“This discovery allows us to rule out the binary merger scenario,” Shultz said. The data collected indicates the newly discovered magnetic fields are similar in strength, but their magnetic axes are not aligned – meaning the magnetic south pole of one star lines up with the magnetic north pole of the other star.
“We're not sure why yet, but it probably points to something significant about how the stars are interacting with one another,” Shultz added.
Due to their close proximity, the stellar duo’s respective magnetospheres – an area surrounding each star that is influenced by its magnetic field – could be interacting. This interaction could potentially slow down the stars’ rotation like a giant cosmic brake, ultimately pulling the stars towards each other.
Since binary systems are common throughout the galaxy, they are invaluable tools for astronomers. Understanding how magnetic fields are generated around massive stars will provide keen insight into how stars evolve.