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We attempt to characterize the observed variability of the magnetic helium-strong star σ Ori E in terms of a recently developed rigidly rotating magnetosphere model. This model predicts the accumulation of circumstellar plasma in two co-rotating clouds, situated in magnetohydrostatic equilibrium at the intersection between magnetic and rotational equators. We find that the model can reproduce well the periodic modulations observed in the star's light curve, Hα emission-line profile, and longitudinal field strength, confirming that it furnishes an essentially correct, quantitative description of the star's magnetically controlled circumstellar environment.

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Abstract. The rotating magnetic B stars are a class of variables consisting of He-strong and some β Cep stars which have oblique dipolar magnetic fields. Such stars develop co-rotating, torus-shaped clouds by channeling wind particles from their magnetic poles to circumstellar regions centered around the plane of their magnetic equators. The rotation of the cloud-star complex permits the study of absorptions from the cloud as it occults the star. In this paper we describe a quantitative analysis of archival IUE data to map the properties of these clouds over four stars (HD 184927, σ Ori E, β Cep, and HR 6684). By computing spectral synthesis models for these stars, we find that only β Cep has a solar-like metallicity. Our analysis also shows that the metal composition across the surfaces of all these stars is at least approximately homogeneous. Using the Hubeny code CIRCUS, we demonstrate that the periodic variations of broad-band ultraviolet continuum fluxes can be explained fully by the absorptions of the co-rotating cloud. We show next that among selected lines, those arising from low-excitation states are selectively affected by cloud absorption and turbulence. Our analysis also quantifies the cloud temperatures and column densities required to match the absorptions of a number of weak to moderate strength resonance lines. These temperatures increase with the ionization potential of the parent ions of these various lines, a result which is consistent with radiative equilibrium models in which temperature increases with proximity to the star's surface. Although these attributes appear stable from one epoch to another, dynamic processes are nonetheless at work. Both the strengths and widths of resonance lines at occultation phases indicate the presence of a turbulence in the cloud which increases inwards. The spectroscopic hallmark of this stellar class is the presence of strong C IV and N V resonance line absorptions at occultation phases and of redshifted emissions of these lines at magnetic poleon phases. The emissions have characteristics which seem most compatible with their generation by high-energy shocks at the wind-cloud interface, as predicted recently by Babel (1998).

Abstract. We report the detection of an X-ray flare on the Bp star σ Ori E with the ROSAT high resolution imager (HRI). The flare is shown to have likely occurred on the early-type star, rather than on an hypothesized late-type companion. We derive flare parameters such as total energy release, coarse estimates of size and density, and also present arguments for a magnetic origin of the flare. We place our observations in the context of a magnetic character of Bp-type stars and speculate on a common physical basis and connection between Bp and Be stars.

We study the possibility of the helium decoupling in the stellar wind of σ Ori E. To obtain reliable wind parameters for this star we first calculate NLTE wind model and derive wind mass-loss rate and terminal velocity. Using corresponding force multipliers we study the possibility of helium decoupling. We find that helium decoupling is not possible for realistic values of helium charge (calculated from NLTE wind models). Helium decoupling only seems possible for a very low helium charge. The reason for this behavior is the strong coupling between helium and hydrogen. We also find that frictional heating becomes important in the outer parts of the wind of σ Ori E due to the collisions between some heavier elements and the passive components -hydrogen and helium. For a metallicity ten times lower than the solar one, both hydrogen and helium decouple from the metals and may fall back onto the stellar surface. However, this does not explain the observed chemical peculiarity since both these components decouple together from the absorbing ions. Although we do not include the effects of the magnetic field into our models, we argue that the presence of a magnetic field will likely not significantly modify the derived results because in such case model equations describe the motion parallel to the magnetic field.