Magnetic field measurements and wind-line variability of OB-type stars^*

¹ SRON, Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands e-mail: rschnerr@science.uva.nl;r.schnerr@sron.nl
² Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
³ GEPI, UMR 8111 du CNRS, Observatoire de Paris-Meudon, 5 place Jules Janssen, 92195 Meudon Cedex, France
⁴ European Space Astronomy Centre (ESAC), Research & Scientific Support Department of ESA, Villafranca del Castillo, PO Box 50727, 28080 Madrid, Spain
⁵ European Southern Observatory, Karl Schwarzschildstrasse 2, 85748 Garching bei München, Germany
⁶ Leiden Observatory, University of Leiden, PO Box 9513, 2300 RA, Leiden, The Netherlands
⁷ Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK
⁸ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 27 April 2007
Accepted: 17 February 2008

Abstract

Context. The first magnetic fields in O- and B-type stars that do not belong to the Bp-star class, have been discovered. The cyclic UV wind-line variability, which has been observed in a significant fraction of early-type stars, is likely to be related to such magnetic fields.

Aims. We attempt to improve our understanding of massive-star magnetic fields, and observe twenty-five carefully-selected, OB-type stars.

Methods. Of these stars we obtain 136 magnetic field strength measurements. We present the UV wind-line variability of all selected targets and summarise spectropolarimetric observations acquired using the MUSICOS spectropolarimeter, mounted at the TBL, Pic du Midi, between December 1998 and November 2004. From the average Stokes I and V line profiles, derived using the LSD method, we measure the magnetic field strengths, radial velocities, and first moment of the line profiles.

Results. No significant magnetic field is detected in any OB-type star that we observed. Typical 1σ errors are between 15 and 200 G. A possible magnetic-field detection for the O9V star 10 Lac remains uncertain, because the field measurements depend critically on the fringe-effect correction in the Stokes V spectra. We find excess emission in UV-wind lines, centred about the rest wavelength, to be a new indirect indicator of the presence of a magnetic field in early B-type stars. The most promising candidates to host magnetic fields are the B-type stars δ Cet and 6 Cep, and a number of O stars.

Conclusions. Although some O and B stars have strong dipolar field, which cause periodic variability in the UV wind-lines, such strong fields are not widespread. If the variability observed in the UV wind-lines of OB stars is generally caused by surface magnetic fields, these fields are either weak (few hundred G) or localised.