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

S. Hubrig$^1$, M. Sch"oller$^2$, I. Savanov$^3$, R.V. Yudin$^4$, M.A. Pogodin$^4$,
St. {v S}tefl$^5$, Th. Rivinius$^5$, and M. Cur'e$^6

1 - AIP, Potsdam, Germany; 2 - ESO, Germany; 3 - Russian Academy of Sciences, Moscow, Russia;
4 - Pulkovo Observatory, Russia; 5 - ESO, Chile; 6 - Universidad de Valpara'iso, Chile

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 upsilon Sgr,
and in a sample of fourteen members of the open young cluster NGC3766
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
~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 FORS1 installed at the 8,m
Kueyen telescope.
We detect weak photospheric magnetic fields in
four field Be stars, HD62367, mu Cen, o Aqr, and epsilon Tuc.
The strongest longitudinal magnetic field,
< Bz >=117+-38G, was detected
in the Be star HD62367. Among the Be stars studied with time series,
one Be star, lambda Eri,
displays cyclic variability of the magnetic field with a period of 21.12min.
The binary star upsilon 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 < Bz >=-102+-10G at
MJD 54333.018 was measured using hydrogen lines.
The cluster NGC3766 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.

Reference: Astronomische Nachrichten 330, 708-716
Status: Manuscript has been accepted