The magnetic field and confined wind of the O star θ1 Orionis C

Aims. In this paper we confirm the presence of a globally-ordered, kG-strength magnetic field in the photosphere of the young O star theta(1) Orionis C, and examine the properties of its optical line profile variations. Methods. A new series of high-resolution MuSiCoS Stokes V and I spectra has been acquired which samples approximately uniformly the rotational cycle of theta(1) Orionis C. Using the Least-Squares Deconvolution ( LSD) multiline technique, we have succeeded in detecting variable Stokes V Zeeman signatures associated with the LSD mean line profile. These signatures have been modeled to determine the magnetic field geometry. We have furthermore examined the profile variations of lines formed in both the wind and photosphere using dynamic spectra. Results. Based on spectrum synthesis fitting of the LSD profiles, we determine that the polar strength of the magnetic dipole component is 1150 less than or similar to B-d less than or similar to 1800 G and that the magnetic obliquity is 27 degrees less than or similar to beta less than or similar to 68 degrees, assuming i = 45 +/- 20 degrees. The best-fit values for i = 45 degrees are B-d = 1300 +/- 150 G (1 sigma) and beta = 50 degrees +/- 6 degrees (1 sigma). Our data confirm the previous detection of a magnetic field in this star, and furthermore demonstrate the sinusoidal variability of the longitudinal field and accurately determine the phases and intensities of the magnetic extrema. The analysis of "photospheric" and "wind" line profile variations supports previous reports of the optical spectroscopic characteristics, and provides evidence for infall of material within the magnetic equatorial plane.