Magnetic survey of emission line B-type stars with FORS1 at the VLT

We report the results of our search for magnetic fields in a sample of 16 field Be stars, the binary emission-line B-type star nu Sgr, and in a sample of fourteen members of the open young cluster NGC 3766 in the Carina spiral arm. The sample of cluster members includes Be stars, normal B-type stars and He-strong/He-weak stars. Nine Be stars have been studied with magnetic field time series obtained over similar to 1 hour to get an insight into the temporal behaviour and the correlation of magnetic field properties with dynamical phenomena taking place in Be star atmospheres. The spectropolarimetric data were obtained at the European Southern Observatory with the multi-mode instrument FORS 1 installed at the 8 m Kueyen telescope. We detect weak photospheric magnetic fields in four field Be stars, HD 62367, mu Cen, o Aqr, and epsilon Tuc. The strongest longitudinal magnetic field, < B(z)> = 117 +/- 38 G, was detected in the Be star HD 62367. Among the Be stars studied with time series, one Be star, lambda Eri, displays cyclic variability of the magnetic field with a period of 21.12 min. The binary star nu Sgr, in the initial rapid phase of mass exchange between the two components with strong emission lines in the visible spectrum, is a magnetic variable star, probably on a timescale of a few months. The maximum longitudinal magnetic field < B(z)> = -102 +/- 10 G at MJD 54333.018 was measured using hydrogen lines. The cluster NGC 3766 seems to be extremely interesting, where we find evidence for the presence of a magnetic field in seven early B-type stars out of the observed fourteen cluster members. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim