Discovery of a magnetic field in the rapidly rotating O-type secondary of the colliding-wind binary HD 47129 (Plaskett's star)

We report the detection of a strong, organized magnetic field in the secondary component of the massive O8III/I+O7.5V/III double-lined spectroscopic binary system HD 47129 (Plas- kett's star), in the context of the Magnetism in Massive Star s (MiMeS) survey. Eight inde- pendent Stokes V observations were acquired using the ESPaDOnS spectropolarimeter at the Canada-France-Hawaii Telescope and the Narval spectropolarimeter at the Telescope Bernard Lyot. Using Least-Squares Deconvolution we obtain definite detections of signal in Stokes V in 3 observations. No significant signal is detected in the di agnostic null (N) spectra. The Zeeman signatures are broad and track the radial velocity of the secondary component; we therefore conclude that the rapidly-rotating secondary co mponent is the magnetized star. Cor- recting the polarized spectra for the line and continuum of the (sharp-lined) primary, we mea- sured the longitudinal magnetic field from each observation . The longitudinal field of the secondary is variable and exhibits extreme values of−810± 150 G and +680± 190 G, im- plying a minimum surface dipole polar strength of 2850± 500 G. In contrast, we derive an upper limit (3σ) to the primary's surface magnetic field of 230 G. The combina tion of a strong magnetic field and rapid rotation leads us to conclude that th e secondary hosts a centrifugal magnetosphere fed through a magnetically confined wind. We r evisit the properties of the op- tical line profiles and X-ray emission - previously interpre ted as a consequence of colliding stellar winds - in this context. We conclude that HD 47129 represents a heretofore unique stellar system - a close, massive binary with a rapidly rotat ing, magnetized component - that will be a rich target for further study.