Discovery of a magnetic field in the B pulsating system HD 1976

The presence of a magnetic field can have a strong impact on the evolution of a binary star. However, only a dozen of magnetic OB binaries are known as of today and available to study this effect, including very few magnetic pulsating spectroscopic OB binaries. We aim at checking for the presence of a magnetic field in the B5IV hierarchical triple system HD 1976 with spectropolarimetric data obtained with Narval at the Bernard Lyot Telescope (TBL). We use orbital parameters of HD 1976 available in the literature to disentangle the Narval intensity spectra. We compute Stokes V profiles with the Least Square Deconvolution (LSD) technique to search for magnetic signatures. We then derive an estimate of the longitudinal magnetic field strength for each observation and for various line lists. Our disentangling of the intensity spectra shows that HD 1976 is a double-lined spectroscopic (SB2) binary, with the lines of the secondary component about twice broader than the ones of the primary component. We do not identify the third component. Moreover, we find that clear magnetic signatures are present in the spectropolarimetric measurements of HD 1976 and seem to be associated with the primary component. We conclude that HD 1976 is a magnetic slowly-pulsating double-lined spectroscopic binary star, with an undetected third component. It is the second such example known (with HD 25558).Comment: Accepted in A&A Letter to the Editor, 4 pages, 2 figures, 2 table

The Incidence of Magnetic Fields in Massive Stars: An Overview of the MiMeS Survey Component

With only a handful of known magnetic massive stars, there is a troubling deficit in the scope of our knowledge of the influence of magnetic fields on stellar evolution, and almost no empirical basis for understanding how fields modify mass loss and rotation in massive stars. Most remarkably, there is still no solid consensus regarding the origin physics of these fields - whether they are fossil remnants, or produced by contemporaneous dynamos, or some combination of these mechanisms. This article will present an overview of the Survey Component of the MiMeS Large Programs, the primary goal of which is to search for Zeeman signatures in the circular polarimetry of massive stars (stars with spectral types B3 and hotter) that were previously unknown to host any magnetic field. To date, the MiMeS collaboration has collected more than 550 high-resolution spectropolarimetric observations with ESPaDOnS and Narval of nearly 170 different stars, from which we have discovered 14 new magnetic stars.Comment: 7 pages (+1 for questions), 3 figures, to appear in proceedings of Stellar polarimetry: From birth to deat

The magnetic field of zeta Orionis A

Zeta Ori A is a hot star claimed to host a weak magnetic field, but no clear magnetic detection was obtained so far. In addition, it was recently shown to be a binary system composed of a O9.5I supergiant and a B1IV star. We aim at verifying the presence of a magnetic field in zeta Ori A, identifying to which of the two binary components it belongs (or whether both stars are magnetic), and characterizing the field.Very high signal-to-noise spectropolarimetric data were obtained with Narval at the Bernard Lyot Telescope (TBL) in France. Archival HEROS, FEROS and UVES spectroscopic data were also used. The data were first disentangled to separate the two components. We then analyzed them with the Least-Squares Deconvolution (LSD) technique to extract the magnetic information. We confirm that zeta Ori A is magnetic. We find that the supergiant component zeta Ori Aa is the magnetic component: Zeeman signatures are observed and rotational modulation of the longitudinal magnetic field is clearly detected with a period of 6.829 d. This is the only magnetic O supergiant known as of today. With an oblique dipole field model of the Stokes V profiles, we show that the polar field strength is ~ 140 G. Because the magnetic field is weak and the stellar wind is strong, zeta Ori Aa does not host a centrifugally supported magnetosphere. It may host a dynamical magnetosphere. Its companion zeta Ori Ab does not show any magnetic signature, with an upper limit on the undetected field of $\sim$ 300 G

Observational effects of magnetism in O stars: surface nitrogen abundances

We investigate the surface nitrogen content of the six magnetic O stars known to date as well as of the early B-type star tau Sco. We compare these abundances to predictions of evolutionary models to isolate the effects of magnetic field on the transport of elements in stellar interiors. We conduct a quantitative spectroscopic analysis of the sample stars with state-of-the-art atmosphere models. We rely on high signal-to-noise ratio, high resolution optical spectra obtained with ESPADONS at CFHT and NARVAL at TBL. Atmosphere models and synthetic spectra are computed with the code CMFGEN. Values of N/H together with their uncertainties are determined and compared to predictions of evolutionary models. We find that the magnetic stars can be divided into two groups: one with stars displaying no N enrichment (one object); and one with stars most likely showing extra N enrichment (5 objects). For one star (Theta1 Ori C) no robust conclusion can be drawn due to its young age. The star with no N enrichment is the one with the weakest magnetic field, possibly of dynamo origin. It might be a star having experienced strong magnetic braking under the condition of solid body rotation, but its rotational velocity is still relatively large. The five stars with high N content were probably slow rotators on the zero age main sequence, but they have surface N/H typical of normal O stars, indicating that the presence of a (probably fossil) magnetic field leads to extra enrichment. These stars may have a strong differential rotation inducing shear mixing. Our results should be viewed as a basis on which new theoretical simulations can rely to better understand the effect of magnetism on the evolution of massive stars.Comment: 14 pages, 6 figures. Accepted by A&