The magnetic field and wind confinement of<i>θ</i>¹Orionis C

Abstract

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 u 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 428 ^68 with respect to the rotation axis (assumed to be inclined at 458 to the line of sight). We find, in particular, that the positive magnetic pole comes closest to the observer when the variable Ha 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.