Magnetic survey of emission line B‐type stars with FORS 1 at the VLT

Abstract

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 υ 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 ∼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, μ Cen, o Aqr, and Tuc. The strongest longitudinal magnetic field, Bz = 117±38 G, was detected in the Be star HD 62367. Among the Be stars studied with time series, one Be star, λ Eri, displays cyclic variability of the magnetic field with a period of 21.12 min. The binary star υ 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±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.

Magnetic field measurements and their uncertainties: the FORS1 legacy

“…For instance, McSwain (2008) observed the normal B stars NGC 3766 MG 111 and NGC 3766 MG 176, and the Be star NGC 3766 MG 200, and reported no field detection. Using the same data, Hubrig et al (2009b) reported new field detections for all three stars. iii) Inconsistencies between the analysis of the same FORS dataset performed by the same authors but at different epochs.…”

“…Between them, these surveys have included more than 40 Be and possible Be stars, of which 10 stars were reported to have weak but significant magnetic fields. The reports by Hubrig et al (2007b) and Hubrig et al (2009b) suggest that weak (∼100 G) fields may be ubiquitous in Be stars, a result that would have important consequences for modelling the Be phenomenon.…”

“…In Table 5 we followed the classication by McSwain (2008). Furthermore, there are differences among these publications as to which stars show field detections; McSwain (2008) did not find a significant field in the normal B stars NGC 3766 MG 111 or NGC 3766 MG 176, nor in the Be star NGC 3766 MG 200, all of which are reported, using the same data, as new detections by Hubrig et al (2009b).…”

Magnetic fields in classical Be stars: results of our long-term program with FORS1 at the VLT

“…Among this sample the detections of a magnetic field were achieved in 8 stars, most of them just occasionally on single nights (Hubrig et al 2007;2009a;2009b). The magnetic fields of Be stars appear to be very weak, generally of the order of 100G and less.…”

Magnetic fields in early-type stars

“…Claims that a significant fraction of classical Be stars may indeed be magnetic have been put forward based on low-resolution spectropolarimetric studies using FORS (Hubrig, North & Schöller 2007;Hubrig et al 2009;Yudin et al 2011). Although these results were later shown to be spurious (Bagnulo et al 2012), they nonetheless motivated further study.…”