A magnetic massive star has experienced a stellar merger
Smoker, J. V.; Shenar, T.; Sana, H.; Barbá, R. H.; Mahy, L.; Fabry, M.; Marchant, P.; Bowman, D. M.; Frost, A. J.; Mérand, A.; Morrell, N. I.; Wade, G.; Schneider, F. R. N.; Le Bouquin, J. -B.; Farhang, A.; Barron, J.
Belgium, Chile, Canada, France, Germany, Israel, United Kingdom, Iran
Abstract
Massive stars (those ≥8 solar masses at formation) have radiative envelopes that cannot sustain a dynamo, the mechanism that produces magnetic fields in lower-mass stars. Despite this, approximately 7% of massive stars have observed magnetic fields, the origin of which is debated. We used multi-epoch interferometric and spectroscopic observations to characterize HD 148937, a binary system of two massive stars. We found that only one star is magnetic and that it appears younger than its companion. The system properties and a surrounding bipolar nebula can be reproduced with a model in which two stars merged (in a previous triple system) to produce the magnetic massive star. Our results provide observational evidence that magnetic fields form in at least some massive stars through stellar mergers.