Confirmation of the magnetic oblique rotator model for the Of?p star HD 191612★

Abstract

This paper reports high-precision Stokes V spectra of HD 191612 acquired\nusing the ESPaDOnS spectropolarimeter at the Canada-France-Hawaii Telescope, in\nthe context of the Magnetism in Massive stars (MiMeS) Project. Using\nmeasurements of the equivalent width of the Halpha line and radial velocities\nof various metallic lines, we have updated both the spectroscopic and orbital\nephemerides of this star. We confirm the presence of a strong magnetic field in\nthe photosphere of HD 191612, and detect its variability. We establish that the\nlongitudinal field varies in a manner consistent with the spectroscopic period\nof 537.6 d, in an approximately sinusoidal fashion. This demonstrates a firm\nconnection between the magnetic field and the processes responsible for the\nline and continuum variability. Interpreting the variation of the longitudinal\nmagnetic field within the context of the dipole oblique rotator model we obtain\na best-fit surface magnetic field model with obliquity beta=67\\pm 5 deg and\npolar strength Bd=2450\\pm 400 G . The inferred magnetic field strength implies\nan equatorial wind magnetic confinement parameter eta*~50, supporting a picture\nin which the Halpha emission and photometric variability have their origin in\nan oblique, rigidly rotating magnetospheric structure resulting from a\nmagnetically channeled wind. This interpretation is supported by our successful\nMonte Carlo radiative transfer modeling of the photometric variation, which\nassumes the enhanced plasma densities in the magnetic equatorial plane above\nthe star implied by such a picture. Predictions of the continuum linear\npolarisation resulting from Thompson scattering from the magnetospheric\nmaterial indicate that the Stokes Q and U variations are highly sensitive to\nthe magnetospheric geometry, and that expected amplitudes are in the range of\ncurrent instrumentation. (abridged)\n