1999A&A...341..216D

We present new circular spectropolarimetric observations of the young O-type star θ¹ Orionis C obtained with the MuSiCoS echelle spectropolarimeter and the 2 m Telescope Bernard Lyot, in an attempt to detect the surface magnetic field structure invoked to explain the periodic variability of θ¹ Ori C at X-ray, UV and optical wavelengths. We obtain null detections with 250 G 1σ error bars for the disc-averaged line-of-sight (i.e. longitudinal) component of the surface magnetic field vector, from which we conclude that the polar strength of the dipole field structure is lower than 1.6 to 2.0kG (with a confidence level of 87%), depending on the exact orientation of the magnetic and rotation axes with respect to the line of sight. We report as well the unexpected discovery of strong, time variable continuum circular polarisation in the spectrum of θ¹ Ori C, as well as depolarisation structures associated with nebular emission lines, indicating that this continuum polarisation (the nature of which is as yet unclear) is produced within the immediate circumstellar environment.