2012A&A...537A.148N

Following the indirect detection of a magnetic field in the β Cephei star V2052 Oph by Neiner and collaborators in 2003 with the Musicos spectropolarimeter, we remeasured the magnetic field of this star to attempt to directly confirm the detection of a magnetic field and study its configuration in greater detail. We used the Narval spectropolarimeter installed at TBL (Pic du Midi, France), which is about 20 times more efficient than the Musicos spectropolarimeter. We applied the least-squares deconvolution (LSD) technique to various groups of lines to measure the circular polarisation of the light coming from V2052 Oph. We synthesized the measured Stokes V profiles with a centred and off-centred dipole model. For the first time, we clearly detect the Zeeman signature in the Stokes V profiles of V2052 Oph and thus directly prove the presence of a magnetic field in this star. The modulation with the rotation period is also confirmed and reflects an oblique dipole field. Thanks to the small error bars on the measurements, we are able to study the behaviour of different groups of lines and the centring of the dipole in the star. We find that the dipole is most likely off-centred along the magnetic axis and that He spots are present at the surface next to the magnetic axis. We conclude that V2052 Oph is a magnetic He-strong β Cep star, with a dipole field, probably off-centred, with B_pol∼400G and He patches close to the magnetic poles.