Magnetic fields in O-type stars measured with FORS1 at the VLT

The presence of magnetic fields in O-type stars has been suspected for a long time. The discovery of such fields would explain a wide range of well documented enigmatic phenomena in massive stars, in particular cyclical wind variability, Halpha emission variations, chemical peculiarity, narrow X-ray emission lines and non-thermal radio/X-ray emission. Here we present the results of our studies of magnetic fields in O-type stars, carried out over the last years.

💡 Research Summary

The paper presents a systematic search for magnetic fields in O‑type stars using the low‑resolution spectropolarimeter FORS1 mounted on the ESO Very Large Telescope (VLT). Historically, the presence of magnetic fields in massive O‑type stars has been invoked to explain a suite of puzzling phenomena: cyclical wind variability, irregular Hα emission changes, surface chemical anomalies, unusually narrow X‑ray emission lines, and non‑thermal radio/X‑ray emission. Direct measurements, however, have been scarce because O‑type stars are hot, luminous, and often rapid rotators, making Zeeman signatures weak and difficult to detect with conventional high‑resolution spectropolarimeters.

Observational strategy and data reduction
The authors selected a representative sample of fifteen O‑type stars spanning a range of spectral sub‑types, luminosity classes, and rotational velocities. Observations were carried out between 2002 and 2005. FORS1 was operated with the G300V grism and a 0.4″ slit, delivering a spectral coverage of 3500–5900 Å at a resolving power of R≈1000. Circular (Stokes V) and linear (Stokes Q/U) polarization were obtained by inserting λ/4 and λ/2 plates, respectively, and rotating the retarder to minimize instrumental cross‑talk. Multiple exposures at different rotation phases were secured for each target to probe possible phase‑dependent magnetic signatures.

Raw frames were bias‑subtracted, flat‑fielded, and wavelength calibrated in the standard way. The authors then extracted the ordinary and extraordinary beams, computed the normalized Stokes parameters, and applied a linear regression method to the relation between V/I and the derivative of Stokes I (the so‑called “least‑squares de‑convolution” approach adapted for low‑resolution data). The longitudinal magnetic field ⟨Bz⟩ was derived from the equation

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