First massive binary star with magnetic fields found

Scientists have discovered the first massive binary star, epsilon Lupi, in which both stars have magnetic fields. A binary star is a star system consisting of two or more stars, orbiting around their common centre of mass. For the past few years, the BinaMIcS (Binarity and Magnetic Interactions in various classes of Stars) collaboration, formed to study the magnetic properties of close binaries, has been trying to find such an object. They have now discovered one using the Canada-France-Hawaii Telescope.    

"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 these stars have magnetic  fields," said Matt Shultz, a PhD candidate at the Queen's University in Canada.  In cool stars, such as the Sun, magnetic fields are generated by a convection in the outer portion of the star. However, there is no convection in the outer layers of massive star, so there is no support for a magnetic dynamo.     

Nevertheless, approximately 10 per cent of massive stars have strong magnetic fields. Two explanations have been proposed for the origin of  massive star magnetic fields, both variants on the idea of a  so-called "fossil" magnetic field, which is generated at some point in the star's past and then locked in to the star's outer portion.     

The first hypothesis is that the magnetic field is generated while the star is being formed; the second is that the magnetic field originates in dynamos driven by the violent mixing of stellar plasma when the two stars in a close binary merge. "This discovery doesn't change the basic statistics that  the BinaMIcS collaboration has assembled and we still don't know why there are so few magnetic, massive stars in close  binaries," said Shultz.     

The research shows the strengths of the magnetic fields are similar in the two stars, however, their magnetic axes are anti-aligned, with the south pole of one star pointing in approximately the same direction as the north pole of the other. "We're not sure why that is yet, but it probably points to something significant about how the stars are interacting with one another. We'll need to collect more data," Shult added.     

The research was published in the journal Monthly Notices of the Royal Astronomical Society.