Strongly magnetic, rapidly rotating B-type stars with relatively weak winds form centrifugal magnetospheres (CMs), as the stellar wind becomes magnetically confined above the Kepler co-rotation radius. Approximating the magnetic field as a dipole tilted by an angle β with respect to the rotation axis, the CM plasma is concentrated in clouds at and above the Kepler radius along the intersection of the rotational and magnetic equatorial planes. Stellar rotation can bring such clouds in front of the stellar disk, leading to absorption of order 0.1 magnitude ($\sim 10 {{\ \rm per\ cent}}$ of continuum flux). However some stars with prominent CMs, such as σ Ori E, show an emission bump in addition to absorption dips, which has been so far unexplained. We show that emission can occur from electron scattering toward the observer when CM clouds are projected off the stellar limb. Using the Rigidly Rotating Magnetosphere model, modified with a centrifugal breakout density scaling, we present a model grid of photometric light curves spanning parameter space in observer inclination angle i, magnetic obliquity angle β, critical rotation fraction W, and optical depth at the Kepler radius τK. We show that τK of order unity can produce emission bumps of the magnitude ∼0.05 seen in σ Ori E. We discuss the implications for modeling the light curves of CM stars, as well as future work for applying the radiative transfer model developed here to 3D MHD simulations of CMs.
Early-type B stars with strong magnetic fields and rapid rotation form centrifugal magnetospheres (CMs), as the relatively weak stellar wind becomes magnetically confined and centrifugally supported above the Kepler co-rotation radius. CM plasma is concentrated at and above the Kepler co-rotation radius at the intersection between the rotation and magnetic field axis. Stellar rotation can cause these clouds of material to intersect the viewer’s line-of-sight, leading to photometric eclipses. However, for stars with strong (∼10 kG) magnetic fields and rapid rotation, CMs can become optically thick enough for emission to occur via electron scattering. Using high-precision space photometry from a sample of stars with strong Hα emission, we apply simulated light curves from the Rigidly Rotating Magnetosphere model to directly infer magnetic and rotational properties of these stars. By comparing the values inferred from photometric modelling to those independently determined by spectropolarimetry, we find that magnetic obliquity angle β, viewer inclination i and critical rotation fraction W can be approximately recovered for 3 of the 4 stars studied here. However, there are large discrepancies between the optical depth at the Kepler radius τK expected from magnetometry, and the values required to match the observations. We show that τK of order unity is needed to reasonably match the light curve morphology of our sample stars.
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