The spectral variability and magnetic field characteristics of the Of?p star HD 148937
We report magnetic and spectroscopic observations and modeling of the Of?p star HD 148937 within the context of the Magnetism in Massive Stars (MiMeS) Large Program at the CanadaFrance Hawaii Telescope. Thirty two high signal to noise ratio circularly polarised (Stokes V) spectra and 13 unpolarised (Stokes I) spectra of HD 148937 were acquired in 2009 and 2010. A definite detection of a Stokes V Zeeman signature is obtained in the grand mean of all observations (in both Least Squares Deconvolved (LSD) mean profiles and individual spectral lines) The longitudinal magnetic field inferred from the St okes V LSD profiles is consistently negative, in contrast to the essentially zero field strength measured from the diagnostic null profiles. A period search of new and archival equivalent widt h measurements confirms the previously reported 7.03 d variability period. The variation of equivalent widths is not strictly periodic: we present evidence for evolution of the amount or distribution of circumstellar plasma. Interpreting the 7.03d period as the stellar rotati onal period within the context of the Oblique Rotator paradigm, we have phased the equivalent widths and longitudinal field measurements. The longitudinal field measurements show a weak s inusoidal variation of constant sign, with extrema out of phase with the Ha variation by about 0.25 cycles. From our constraint on v sin i6 45 km s 1 we infer that the rotational axis inclination i6 30 Modeling the longitudinal field phase variation directly, we obtain the m agnetic obliquityb 38 +17 28 and dipole polar intensity Bd 1020 380 +310 G. Simple modeling of the Ha equivalent width variation supports the derived geometry. The inferred magnetic config uration confirms the suggestion