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Register a new userSpectral Variations of Of?p Oblique Magnetic Rotator Candidates in the Magellanic Clouds
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Authors
Nolan R. Walborn
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Optical spectroscopic monitoring has been conducted of two O stars in the Small and one in the Large Magellanic Cloud, the spectral characteristics of which place them in the Of?p category, which has been established in the Galaxy to consist of oblique magnetic rotators. All of these Magellanic stars show systematic spectral variations typical of the Of?p class, further strengthening their magnetic candidacy to the point of virtual certainty. The spectral variations are related to photometric variations derived from OGLE data by Naze et al. (2015) in a parallel study, which yields rotational periods for two of them. Now circular spectropolarimetry is required to measure their fields, and ultraviolet spectroscopy to further characterize their low-metallicity, magnetically confined winds, in support of hydrodynamical analyses.
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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 Halpha 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. 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 we obtain a best-fit surface magnetic field model with obliquity beta=67pm 5 deg and polar strength Bd=2450pm 400 G . The inferred magnetic field strength implies an equatorial wind magnetic confinement parameter eta*~50, supporting a picture in which the Halpha emission and photometric variability have their origin in an oblique, rigidly rotating magnetospheric structure resulting from a magnetically channeled wind. This interpretation is supported by our successful Monte Carlo radiative transfer modeling of the photometric variation, which assumes the enhanced plasma densities in the magnetic equatorial plane above the star implied by such a picture. Predictions of the continuum linear polarisation resulting from Thompson scattering from the magnetospheric material indicate that the Stokes Q and U variations are highly sensitive to the magnetospheric geometry, and that expected amplitudes are in the range of current instrumentation. (abridged)
All known Galactic Of?p stars have been shown to host strong, organized, magnetic fields. Recently, five Of?p stars have been discovered in the Magellanic Clouds. They posses photometric citep{Naze} and spectroscopic citep{Walborn} variability compatible with the Oblique Rotator Model (ORM). However, their magnetic fields have yet to be directly detected. We have developed an algorithm allowing for the synthesis of photometric observables based on the Analytic Dynamical Magnetosphere (ADM) model of citet{Owocki}. We apply our model to OGLE photometry in order to constrain their magnetic geometries and surface dipole strengths. We predict that the field strengths for some of these candidate extra-Galactic magnetic stars may be within the detection limits of the FORS2 instrument.
Massive O-type stars play a dominant role in our Universe, but many of their properties remain poorly constrained. In the last decade magnetic fields have been detected in all Galactic members of the distinctive Of?p class, opening the door to a better knowledge of all O-type stars. With the aim of extending the study of magnetic massive stars to nearby galaxies, to better understand the role of metallicity in the formation of their magnetic fields and magnetospheres, and to broaden our knowledge of the role of magnetic fields in massive star evolution, we have carried out spectropolarimetry of five extra-Galactic Of?p stars, as well as a couple of dozen neighbouring stars. We have been able to measure magnetic fields with typical error bars from 0.2 to 1.0 kG, depending on the apparent magnitude and on weather conditions. No magnetic field has been firmly detected in any of our measurements, but we have been able to estimate upper limits to the field values of our target stars. One of our targets, 2dFS 936, exhibited an unexpected strengthening of emission lines. We confirm the unusual behaviour of BI 57, which exhibits a 787 d period with two photometric peaks and one spectroscopic maximum. The observed strengthening of the emission lines of 2dFS 936, and the lack of detection of a strong magnetic field in a star with such strong emission lines is at odd with expectations. Together with the unusual periodic behaviour of BI 57, it represents a challenge for the current models of Of?p stars. The limited precision that we obtained in our field measurements (in most cases as a consequence of poor weather) has led to field-strength upper limits that are substantially larger than those typically measured in Galactic magnetic O stars. Further higher precision observations and monitoring are clearly required.
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