The spectral variability and magnetic field characteristics of the Of?p star HD 148937

G.A. Wade, J. Grunhut, G. Gräfener, I.D. Howarth, F. Martins, V. Petit, J.S. Vink, S. Bagnulo, C.P. Folsom, Y. Nazé, N.R. Walborn, R.H.D. Townsend, C.J. Evans and the MiMeS Collaboration


Figure 2. Left panel: Fit to the optical spectrum #1206203. The black solid line is the observed spectrum, the dashed red line the best fit model (Teff = 40000 K, log g = 4.0, log L/L = 5.8, Ṁ = 10−7 M/yr, v = 2600 km s−1 ). The Balmer lines are not reproduced by the TLUSTY models. (see Sect. 3). Right panel: Fit of the UV-optical-IR SED. The red solid line is the best fit model (log L/L = 5.8 and E(B − V) = 0.67). The black solid line is the FUSE-IUE spectrum. The black points correspond to UBVJHK photometry.

Abstract

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 Canada-France-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 Stokes 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 width 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 rotational 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 sinusoidal variation of constant sign, with extrema out of phase with the Hα variation by about 0.25 cycles. From our constraint on v sin i ≤45 km s−1, we infer that the rotational axis inclination i ≤30°. Modeling the longitudinal field phase variation directly, we obtain the magnetic obliquity β = 38 +17°-28 and dipole polar intensity βd = 1020-380+310 G. Simple modeling of the Hα equivalent width variation supports the derived geometry. The inferred magnetic configuration confirms the suggestion of Nazé et al (2010), who proposed that the weaker variability of HD 148937 as compared to other members of this class is a consequence of the stellar geometry. Based on the derived magnetic properties and published wind characteristics, we find a wind magnetic confinement parameter η* ≈ 20 and rotation parameter W = 0.12, supporting a picture in which the Hα emission and other line variability have their origin in an oblique, rigidly rotating magnetospheric structure resulting from a magnetically channeled wind.

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Last Revised: 2011 September 23rd