The magnetic field and wind confinement of θ1 Orionis C

We report the detection, through spectropolarimetric observations, of a strong dipolar magnetic field of presumably fossil origin at the surface of the very young O star theta (1) Ori C. The Stokes V signatures we detect are variable with time, the variations being consistent with rotational modulation. A detailed modelling of our observations indicates that this dipole field has an intensity of 1.1+/-0.1 kG and is inclined at 42degrees+/-6degrees with respect to the rotation axis (assumed to be inclined at 45degrees to the line of sight). We find, in particular, that the positive magnetic pole comes closest to the observer when the variable Halpha emission component observed on this star reaches maximum strength. This discovery represents the first definite detection of a magnetic field in an O star, as well as the first detection of a fossil field in a very young star. We also investigate in this paper the magnetic confinement of the radiatively driven wind of theta (1) Ori C in the context of the magnetically confined wind-shock model of Babel & Montmerle. In the case of theta (1) Ori C, this model predicts the formation of a large magnetosphere (extending as far as 2-3R *), consisting of a very hot post-shock region (with temperatures in excess of 10 MK and densities of about 10(10) -10(11) cm(-3) ) generated by the strong collision of the wind streams from both stellar magnetic hemispheres, as well as a dense cooling disc forming in the magnetospheric equator. We find that this model includes most of the physics required to obtain a satisfactory level of agreement with the extensive data sets available for theta (1) Ori C in the literature (and, in particular, with the recent X-ray data and the phase-resolved spectroscopic observations of ultraviolet and optical wind lines) provided that the mass-loss rate of theta (1) Ori C is at least 5 times smaller than that predicted by radiatively driven wind models. We finally show how new observations with the XMM or Chandra spacecraft could help us constrain this model much more tightly and thus obtain a clear picture of how magnetic fields can influence the winds of hot stars.