Mon. Not. R. Astron. Soc., 416, 3160-3169 (2011/October-1)
Confirmation of the magnetic oblique rotator model for the of?p star HD 191612.
WADE G.A., HOWARTH I.D., TOWNSEND R.H.D., GRUNHUT J.H., SHULTZ M., BOURET J.-C., FULLERTON A., MARCOLINO W., MARTINS F., NAZE Y., UD DOULA A., WALBORN N.R., DONATI J.-F. (The MiMeS Collaboration)
Abstract (from CDS):
This paper reports high-precision Stokes V
spectra of HD 191612 acquired using the ESPaDOnS spectropolarimeter at the Canada–France–Hawaii Telescope, in the context of the Magnetism in Massive Stars (MiMeS) Project. Using measurements of the equivalent width of the Hα line and radial velocities of various metallic lines, we have updated both the spectroscopic and orbital ephemerides of this star. We confirm the presence of a strong magnetic field in the photosphere of HD 191612, and detect its variability. We establish that the longitudinal field varies in a manner consistent with the spectroscopic period of 537.6 d, in an approximately sinusoidal fashion. The phases of minimum and maximum longitudinal field are, respectively, coincident with the phases of maximum and minimum Hα equivalent width and Hp
magnitude. This demonstrates a firm connection between the magnetic field and the processes responsible for the line and continuum variability. Interpreting the variation of the longitudinal magnetic field within the context of the dipole oblique rotator model, and adopting an inclination i
= 30° obtained assuming alignment of the orbital and rotational angular momenta, we obtain a best-fitting surface magnetic field model with obliquity β= 67°± 5° and polar strength Bd
= 2450±400 G. The inferred magnetic field strength implies an equatorial wind magnetic confinement parameter η*
≃ 50, supporting a picture in which the Hα emission and photometric variability have their origin in an oblique, rigidly rotating magnetospheric structure resulting from a magnetically channelled wind. This interpretation is supported by our successful Monte Carlo radiative transfer modelling of the photometric variation, which assumes the enhanced plasma densities in the magnetic equatorial plane above the star implied by such a picture, according to a geometry that is consistent with that derived from the magnetic field. Predictions of the continuum linear polarization resulting from Thompson scattering from the magnetospheric material indicate that the Stokes Q
variations are highly sensitive to the magnetospheric geometry, and that expected amplitudes are in the range of current instrumentation.
Based on observations obtained at the Canada–France–Hawaii Telescope (CFHT) which is operated by the National Research Council of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique of France and the University of Hawaii.
2011 The Authors Monthly Notices of the Royal Astronomical Society2011 RAS
stars: individual: HD 191612 - stars: magnetic field - stars: massive - stars: rotation
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