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Babel, Jaques and Montmerle, Thierry Abstract We present here the hypothesis of a dipolar magnetic field embedded in the radiation-driven wind of &theta1 Orionis C and analyze the X-ray data in the framework of the magnetically confined wind shock (MCWS) model (Babel & Montmerle 1997). We find that both the ROSAT HRI count rate and the periodic variability of &theta1 Orionis C are quantitatively reproduced by the model, provided that the field has an intensity at the stellar surface of B* 3 300 G. The variability is caused in large part by the circumstellar cooling disk predicted by the MCWS model. These results give strong support to a magnetic origin for the variability of &theta 1 Orionis C and render this star the best candidate for a high- mass analog to magnetic Bp stars. &theta1 Ori C is the principle ionization source of the Orion Nebula. It is a young O7 star. It is also believed to be one of the few candidates to be a high-mass analog to chemically-peculiar magnetic Bp stars. Typically, high-mass stars do not exhibit large magnetic fields, but &theta1 Ori C does. The major effect of the presence of a magnetic field is to channel the wind towards the magnetic equator. A front will come from each hemisphere, causing a strong shock. The exact configuration of the magnetic field will affect how this shock plays out. In this paper, the authors assume two different types of magnetic field. In one, the "closed magnetosphere," the approximation is made that the wind shock does not distort the magnetic field lines. In the "open magnetosphere" case, the magnetic field lines only define input conditions for the wind shock. The authors do take into account radiative cooling and forces that act upon the material post-shock. The authors found that the main characteristic feature of such a star would be a high-density cooling disk around the magentic equator. They hypothesize that there would be a large accumulation of energy density at that location. Then, even small perturbations could case the material in the disk to either fall back into the star or eject out away from the star. Another feature found was a significant increase in column-density across the disk, which would of course be expected to block some of the light from the star itself. In the Magnetically Channeled Wind Shock (MCWS) model, a number of things could cause the observed variation of the source. These include the afforementioned absorption due to the cold disk, eclipses by the star, and eclipses by the cold disk. This variablity is found to depend greatly on wavelength, which could lead one to guess that the real factor is absorption due to the cold disk if forced to choose between those three options. It was found that the absorbing disk plays an important role; simulations lacking the disk showed much lower variability than those with the disk. The authors conclude by claiming strong support for the magnetic origin or the variability in &theta1 Ori C. |