A magnetic massive star was produced by a stellar merger

Shedding light on why some massive stars have magnetic fields even though these stars’ interiors layers don’t undergo convection, researchers report observational evidence that magnetic fields form in some such stars through stellar mergers. The magnetic fields of low-mass stars, like the Sun, are produced by a dynamo generated in the convective layers of the star’s interior. Massive stars – those 8 or more solar masses at formation – do not have the convective interiors required to sustain magnetic fields in this way. However, roughly 7% of massive stars have been observed to have magnetic fields, the origin of which is poorly understood. Several mechanisms have been proposed. One possibility is that the magnetic fields could arise through mixing of stellar material, such as during a stellar merger. Here, Abigail Frost and colleagues present multi-epoch interferometric and spectroscopic observations of HD 148937, a binary system consisting of two massive stars surrounded by a bipolar nebula. Frost et al. monitored the binary system for 9 years, allowing them to determine its orbit and the properties of the constituent stars. They find that only one of the two stars is magnetic and that it appears to be younger than its companion. Using these data and theoretical models, the authors characterize the system’s evolution and conclude that the HD 148937 originally contained at least three stars; a stellar merger between two of the stars, which likely occurred only a few thousand years ago, produced the magnetic field in the merged star, and made it appear younger than its current binary companion. The same merger might also have produced the bipolar nebula surrounding the system. According to Frost et al., their inferred history of this system provides observational support for the proposal that stellar mergers produce magnetism in at least some massive stars.