Discovery of a magnetic field in the O9 sub-giant star HD 57682 by the MiMeS Collaboration

We report the detection of a strong, organised magnetic field in the O9IV star HD 57682, using spectropolarimetric observations obtained with ESPaDOnS at the 3.6-m Canada-France-Hawaii Telescope within the context of the Magnetism in Massive Stars (MiMeS) Large Program. From the fitting of our spectra using NLTE model atmospheres we determined that HD 57682 is a $17_{-9}^{+19}$ M${}_{\odot }$ star with a radius of $7.0_{-1.8}^{+2.4}$ R${}_{\odot }$, and a relatively low mass-loss rate of $1.4_{-0.95}^{+3.1}\times {10}^{-9}$ M${}_{\odot }$ yr${}^{-1}$. The photospheric absorption lines are narrow, and we use the Fourier transform technique to infer $v\sin i=15\pm 3$ km s${}^{-1}$. This $v\sin i$ implies a maximum rotational period of 31.5 d, a value qualitatively consistent with the observed variability of the optical absorption and emission lines, as well as the Stokes $V$ profiles and longitudinal field. Using a Bayesian analysis of the velocity-resolved Stokes $V$ profiles to infer the magnetic field characteristics, we tentatively derive a dipole field strength of $1680_{-356}^{+134}$ G. The derived field strength and wind characteristics imply a wind that is strongly confined by the magnetic field.