The magnetic field and confined wind of the O star theta^1 Orionis C

Aims. In this paper we confirm the presence of a globally-ordered, k G-strength magnetic field in the photosphere of the young O st ar θ 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 ofθ 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 variatio ns of lines formed in both the wind and photosphere using dynamic spectra. Results. Based on spectrum synthesis fitting of the LSD profiles, we det ermine that the polar strength of the magnetic dipole component is 1150 < Bd < 1800 G and that the magnetic obliquity is 27 ◦ < β < 68 ◦ , assuming i = 45± 20 ◦ . The best-fit values for i = 45 ◦ are Bd = 1300± 150 (1σ) G andβ = 50 ◦ ± 6 ◦ (1σ). Our data confirm the previous detection of a magnetic field i n this star, and furthermore demonstrate the sinusoidal variability of the longitudina l field and accurately determine the phases and intensities o f the magnetic extrema. The analysis of “photospheric” and “wind” line profile varia tions supports previous reports of the optical spectroscop ic characteristics, and provides evidence for infall of material within the magnetic equatorial plane.