Magnetic survey of emission line B-type stars with FORS1 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 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 8m 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, =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 =-102+-10G at MJD54333.018 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. Comment: 9 pages, 8 figures, 3 tables, accepted for publication in Astronomische Nachrichten

Full text

Page 1

arXiv:0907.0882v1 [astro-ph.SR] 5 Jul 2009
Astron. Nachr. / AN 330, No.7, 708–716 (2009) / DOI 10.1002/asna.200911236
Magnetic survey of emission line B-type stars with FORS1 at the VLT⋆
S. Hubrig1,⋆⋆, M. Sch¨ oller2, I. Savanov3, R.V. Yudin4,5, M.A. Pogodin4,5, St.ˇStefl6, Th. Rivinius6, and
M. Cur´ e7
1Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
2European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
3Institute of Astronomy of the Russian Academy of Sciences, Pyatnitskaya St. 48, 119017, Moscow, Russia
4Pulkovo Observatory, Saint-Petersburg, 196140, Russia
5Isaac Newton Institute of Chile, Saint-Petersburg Branch, Russia
6European Southern Observatory, Casilla 19001, Santiago 19, Chile
7Departamento de F´ ısica y Astronom´ ıa, Facultad de Ciencias, Universidad de Valpara´ ıso, Chile
Received 2009 May 25, accepted 2009 Jun 30
Published online 2009 Jul 20
Key words
polarization – stars: early-type – stars: magnetic fields
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 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 8m
Kueyen telescope. We detect weak photospheric magnetic fields in four field Be stars, HD62367, µCen, oAqr, and ǫTuc.
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, λEri, displays cyclic variability of the magnetic field with a period of 21.12min.
The binary star υSgr, 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 MJD54333.018 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.
c ? 2009 WILEY-VCH Verlag GmbH&Co.KGaA, Weinheim
1 Introduction
The majority of emission line B-type stars constitute so-
called Be stars, which are defined as rapidly rotating main
sequence stars showing normal B-type spectra with super-
posed Balmer line emission. In addition, these stars are
characterized by episodic dissipation and formation of a
new circumstellar (CS) disk-like environment, non-radial
pulsations, and photometricandspectroscopicvariability.A
number of physical processes in classical Be stars (e.g., an-
gular momentum transfer to a CS disk, channeling stellar
wind matter, accumulationof material in an equatorial disk,
etc.) are more easily explainable if magnetic fields are in-
voked (e.g. Brown et al. 2004). Cassinelli et al. (2002) sug-
gested a Magnetically Torqued Disk model, in which a suf-
ficiently strong magnetic field (of the order of 300G) chan-
nels a flow of wind material towards the equatorial plane to
form a disk. Maheswaran (2003, 2005) developed the Mag-
⋆Based on observations obtained at the European Southern Observa-
tory, Paranal, Chile (ESO programmes 077.D-0406(A), 279.D-5042(A),
380.D-0480(A), 080.D-0383(A)).
⋆⋆Corresponding author: shubrig@aip.de
netic Rotator Wind Disk model, in which Keplerian disks
may be formed by magnetic fields of the order of a few tens
of Gauss. Very recently, Maheswaran & Cassinelli (2009)
obtained solutions for the structure and evolution of a pro-
todisk region, i.e. the disk region that is initially formed
when wind material is channeled by dipole-type magnetic
fieldstowardstheequatorialplane,showingthatmagnetoro-
tational instability may assist in the formation of a quasi-
steady disk. Accordingto their calculations, magnetic fields
of the order of a few tens of Gauss will be able to channel
wind flow into a protodisk region.
Due to the high rotation of Be stars and the presence of
strong Balmer emission lines, magnetic field measurements
are difficult and rare. The only reported magnetic field de-
tection using high-resolution spectropolarimetry of ω Ori
(80±40G; Neiner et al. 2003) was not confirmed by recent
observations with ESPaDOnS (Grunhut et al., in prepara-
tion). A longitudinal magnetic field at a level larger than
3σ has previously been diagnosed in low-resolution spec-
tropolarimetric FORS1 observations of the three Be stars
HD56014 (=EWCMa; Hubrig et al. 2007), HD148184
(=χOph; Hubrig et al. 2007), and in HD208057 (=16Peg;
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Astron. Nachr. / AN (2009)709
Hubrig et al. 2006a). HD208057 has v sini=104kms−1
and was classified as a Be star by Merrill & Burwell (1943)
due to the detection of double emission in Hα. Recently,
Henrichs et al. (2009)confirmed the presence of a magnetic
field with new measurements using the spectropolarime-
ters Narval at the T´ elescope Bernard Lyot, France, and ES-
PaDOnS at the Canada France Hawaii Telescope, during
2007. However, the presence of emission in Hα was not
detected by these observations. Thus, the question whether
this star is a classical Be star remains open.
Additionalspectropolarimetric
needed to firmly establish the presence of weak magnetic
fields in Be stars. Due to the sparseness of the available
magnetic field measurements, it is currently not possible
to test models, which describe the role of weak magnetic
fields in launching and stabilizing circumstellar disks in Be
stars. To obtain constraints on the origin of magnetic fields
in early B-type stars, it is especially important to study
the incidence of magnetic fields in members of clusters
of different ages. Here we report the results of our four
observing runs with the multi-mode instrument FORS1
installed at the 8m Kueyen telescope at the VLT carried
out in the last few years with the goal to prove the presence
of magnetic fields in a sample of 16 Be stars, the binary
emission-line B-type star star υ Sgr, and fourteen members
of the young open cluster NGC3766 in the Carina spiral
arm. The investigation of this cluster, which has a high
content of Be stars, normal early B-type stars, and He
peculiar stars, allows us to draw some first conclusions of
incidence of magnetic fields in different groups of early
B-type stars.
measurementsare
2 Observations and data reduction
The observations reported here have been carried out from
2006 to 2008 in service and visitor mode at the VLT with
the FOcal Reducer low dispersion Spectrograph (FORS1).
FORS1 is a multi-mode instrument equipped with polari-
sation analyzing optics comprising super-achromatic half-
wave and quarter-wave phase retarder plates, and a Wol-
laston prism with a beam divergence of 22′′in standard
resolution mode. The HD numbers, visual magnitudes and
spectral types of the studied emission line field B-type stars
based on the SIMBAD database are listed in Table 1.
Time-resolvedseries of ten Be stars have been observed
in 2006 in service mode with the GRISM 600B at the res-
olution R=2000 in the wavelength range 3480–5890˚ A to
cover all hydrogen Balmer lines from Hβ to the Balmer
jump. We used the Tektronix chip and a non-standard read-
out mode, A,1×1,low, allowing us to obtain a signal-to-
noise ratio of a few hundred for measurements of the cir-
cular polarisation. Further, the readout time was reduced to
about 40s by windowing the CCD.
Since we do not know the magnetic field topology on
the surface of Be stars, it is reasonable to study the po-
larisation induced in the spectral lines by the Zeeman ef-
Table 1
paper.
Emission line field B-type stars discussed in this
HD
Number
Other
Name
V
Spectral
Type
33328
41335
56014
58011
58715
58978
60855
62367
88661
91465
105435
120324
127793
148184
158427
181615
209409
224686
λEri
V696Mon
27CMa
NNCMa
β CMi
FYCMa
V378Pup
BD−04 2062
QYCar
pCar
δ Cen
µCen
η Cen
χOph
αAra
υ Sgr
oAqr
ǫTuc
4.2
5.3
4.7
7.2
2.9
5.6
5.7
7.1
5.8
3.4
2.6
3.5
2.3
4.4
2.8
4.6
4.7
4.6
B2IVne
B2Vne
B3IIIe
B1/B2Ib/IInne
B8Ve
B1II
B2/B3V
B9
B2IVnpe
B4Vne
B2IVne
B2Vnpe
B1.5Vne
B2Vne
B2Vne
B8pI
B7IVe
B9IV
fect across the stellar surface through longer time series of
exposures with short integration time and with low detec-
tion limit. It is well known that many Be stars exhibit X-ray
flares and rapid variability of absorption features (dimples).
These features are believed to be produced from an abla-
tion of photospheric material caused by a nearby flare (e.g.
Smith et al. 1993).Hence,it is quite possiblethat measuring
the magnetic field on the stellar surface every few minutes,
we will be able to study a local transient magnetic field. In
this respect we note that the Magnetic Rotator Wind-Disk
model (Maheswaran 2003) does not require large-scale or-
ganized magnetic fields, axially symmetric fields, or uni-
formly strong fields across the entire stellar surface. This
modelalsoqualitativelyappliesto stars withmagneticfields
consisting of flux loops that emerge from lower latitudes
and thread the disk around the Be star.
Foreachstarweperformedtime-resolvedmagneticfield
measurementsoveronehour(correspondingto a time series
of 20to 30measurementsperstar). In this way we get infor-
mation on the behaviour of the localized transient magnetic
fieldoveratleast apartofthestellar surface.Asimilarstudy
was conductedfor the Be star λEri in the past by Mathys &
Smith (2000) with CASPEC at the ESO 3.6m telescope,
where some constraints on a possible presence of a mag-
netic field were discussed.
The mean longitudinal magnetic field ?Bz? is the aver-
age over the stellar hemisphere visible at the time of ob-
servation of the component of the magnetic field parallel
to the line of sight, weighted by the local emergent spectral
line intensity.The detectionofa weakmagneticfield ?Bz?=
136±16G in one of the stars observed with time-series,
χOph, was already reported in our previous work on Be
stars (Hubrig et al. 2007). After re-inspection of the previ-
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710S. Hubrig et al.: Magnetic survey of emission line B-type stars
ous measurements of χOph we noted that the detection of
this field involved a Zeeman feature in the Hβ line filled
with strong emission. Excluding this line from the mea-
surements we obtain a much weaker magnetic field ?Bz?=
17±18G. At the low FORS1 resolution of R=2000 it is
difficult to decide whether the observed Zeeman feature is
indeed related to the presence of a magnetic field or not and
its study at a higher resolution will be worthwhile. This star
is not further considered in this work.
Our measurements revealed that one Be star, λEri
(=HD33328), displays a cyclic variability of the magnetic
field. While a few other Be stars in our sample appeared
variable too, only for λEri did the amplitude peak stand out
nicely. Understandably, a confirmation of such a behaviour
would immediately stimulate a deeper theoretical investiga-
tion, and test and further constrain the recently developed
magnetic field models for Be stars. For this reason, we car-
ried out follow-up observations of λEri in 2007 November
during two halves of consecutive nights. For these observa-
tions we used GRISM 1200B and an 0.′′4 slit (R=4000) to
observe the spectral range 3730–4970˚ A, which includes all
Balmer lines fromHβ to H12.This time, however,no cyclic
variability of the magnetic field in λEri was detected. We
discuss the possible explanation for the absence of cyclic
variability in the Sect. 3.1. Three more Be stars, 27CMa,
oAqr, and ǫTuc were observed during this run of two half
nights.
Hubrig et al. (2007) reported on the presence of a weak
magnetic field ?Bz?=38±10G in the binary emission-line
B-type star υ Sgr (=HD181615). Additional three observa-
tions of this binary system were obtained in service mode
during 2007 August and September with GRISM 1200B
and an 0.′′4 slit
The most recent observations presented here have been
carried out in a visitor run on 2008 March 23 and 24
in the framework of the study of 15 B-type members of
the open cluster NGC3766. The results of this study have
in part been reported by McSwain (2008). Unfortunately,
the polarimetric spectra of the member star NGC3766-031
were strongly contaminated by a close companion and have
not been further considered in our study. During this run,
apart from the Be star members of this cluster, we also
observed the field Be stars V696Mon, 27CMa, NNCMa,
β CMi, FYCMa, V378Pup, and HD62367.These observa-
tions have been carried out with the GRISM 600B and a slit
width of 0.′′4.
All observations obtained in 2007 and 2008 have been
carried out with a new mosaic detector with blue optimised
E2V chips, which was implemented in FORS1 in 2007. It
has a pixel size of 15µm (compared to 24µm for the previ-
ous Tektronix chip) and higher efficiency in the wavelength
range below 6000˚ A. With the new mosaic detector and the
grism 600B we were able to cover a much larger spectral
range, from 3250 to 6215˚ A, and from 3680 to 5130˚ A us-
ing grism 1200B. To achieve the highest possible signal-to-
noise (S/N) ratio – as is required for accurate measurements
of stellar magnetic fields – the non-standard, 200kHz, low,
1×1, readout mode was used, which makes it possible to
achieve a S/N ratio of more than 1000 with only one sin-
gle exposure. For each star observed in 2007–2008 we usu-
ally took three to five continuous series of two exposures at
the position angles of the retarder waveplate +45 and −45.
More details on the observing technique with FORS1 can
be found elsewhere (e.g., Hubrig 2004a, 2004b, 2008).
The mean longitudinalmagnetic field is diagnosed from
the slope of a linear regression,
geffe
4πmec2λ21
I
where V is the Stokes parameter which measures the circu-
lar polarisation,I is the Stokes parameterobservedin unpo-
larized light, geffis the effective Land´ e factor, e is the elec-
troncharge,λ is the wavelength,methe electronmass, c the
speed of light, dI/dλ is the derivativeof Stokes I, and ?Bz?
is the mean longitudinal magnetic field. V0/I0is a constant
term taking into account the remaining instrumental polar-
ization. Our experience from the study of a large sample of
magnetic and non-magnetic Ap and Bp stars (Hubrig et al.
2006b)revealedthat this regressiontechniqueis veryrobust
and that detections at a significance level larger than 3σ re-
sult only for stars possessing magnetic fields.
Longitudinal magnetic fields were measured in two
ways: using only the absorption hydrogen Balmer lines or
using the whole spectrum including all available absorption
lines. Lines showingevidencefor emission were not used in
the determination of the magnetic field strength. The feasi-
bility of longitudinal magnetic field measurements in mas-
sive stars using FORS1 in spectropolarimetric mode was
demonstrated by recent studies of early B-type stars (e.g.,
Hubrig et al. 2006a; Hubrig et al. 2008; Hubrig et al. 2009).
V/I = −
dI
dλ?Bz? + V0/I0,
(1)
3 Results
The results of our magnetic field measurements are sum-
marised in Table 2. In the first three columns we give the
name of the targets, indicate whether the star was observed
in a longer time series, and the modified Julian date of
the middle of the exposures. The mean longitudinal mag-
netic field ?Bz?allmeasured using all absorption lines is
presented in Col. 4. The mean longitudinal magnetic field
?Bz?hydrusing all hydrogen lines in absorption is listed in
Col. 5. All quoted errors are 1σ uncertainties. In Col. 6 we
identify new detections by ND and confirmed detections
by CD. We note that all claimed detections have a signif-
icance of at least 3σ, determinedfrom the formal uncertain-
ties we derive. These measurements are indicated in bold
face. While data for stars with extended time series were
also analysedusingonlythe pairwisesettings ofthe retarder
waveplate angle of +45◦and −45◦(see Sect. 3.1), the val-
ues of the measured magnetic field presented here were de-
termined using all of the up to 30 exposures.
We detect weak photospheric magnetic fields in four
field Be stars, HD62367, µCen, oAqr, and ǫTuc. The
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Astron. Nachr. / AN (2009)711
Table 2
uncertainties.
Longitudinal magnetic fields of emission line B-type stars measured with FORS1. All quoted errors are 1σ
NameTime
Series
MJD
?Bz?all
[G]
?Bz?hydr
[G]
Comment
λEriY
Y
Y
53955.400
54432.116
54433.114
54549.982
54432.163
54433.134
54549.996
54549.082
54549.060
54548.995
54549.048
54549.095
53889.996
53890.332
53869.232
53869.295
53862.329
53869.353
54333.018
54343.098
54361.071
53955.185
54432.026
54433.007
53869.405
54432.026
54550.261
54550.066
54549.020
54550.016
54549.117
54550.327
54550.186
54549.151
54550.375
54550.261
54550.066
54549.020
54550.016
54550.016
−39±33
22±11
−4±11
30±40
21±20
−28±14
60±38
17±43
−73±34
121±58
106±46
99±32
65±53
73±96
48±45
−80±24
−14±28
−5±39
−78±8
−73±9
19±13
85±28
−48±22
−28±22
9±24
74±24
−106±55
−123±40
−134±42
−99±48
−79±31
294±53
1522±34
−11±29
128±40
−55±38
2±36
112±34
43±29
89±28
−13±42
11±15
−6±12
44±51
32±25
−38±17
49±43
30±55
−90±41
70±70
62±52
117±38
57±88
108±102
85±66
98±33
−5±32
10±45
−102±10
−98±10
−3±11
98±31
−46±31
−13±28
−1±26
61±28
−75±58
−194±62
−129±58
−115±67
−89±34
310±65
1559±38
−3±40
115±38
−67±52
−13±45
89±38
62±39
120±34
V696Mon
27CMa
NNCMa
β CMi
FYCMa
V378Pup
HD62367
QYCar
pCar
δ Cen
µCen
η Cen
αAra
υ Sgr
ND
Y
Y
Y
Y
Y
Y
ND
CD
CD
oAqrYND
ǫTucY
ND
NGC3766-025
NGC3766-045
NGC3766-047
NGC3766-073
NGC3766-083
NGC3766-094
NGC3766-170
NGC3766-196
NGC3766-200
NGC3766-041
NGC3766-055
NGC3766-111
NGC3766-161
NGC3766-176
CD
CD
CD
CD
ND
ND
ND
largest longitudinal magnetic field, ?Bz?=117±38G, was
detected using hydrogen lines in the Be star HD62367.
Among the Be stars studied with time series, the Be star
λEri displays cyclic variability of the magnetic field with a
period of 21.12min.
The binary star υ Sgr, in the initial rapid phase of mass
exchange between the two components with strong emis-
sion lines in the visible spectrum (Koubsk´ y et al. 2006), is
a magnetic variable star, probably on a timescale of a few
months. The maximum longitudinal magnetic field ?Bz?=
−110±10G at MJD54333.018was measured using hydro-
gen lines.
The cluster NGC3766 seems to be extremely interest-
ing, where we find evidence for the presence of a mag-
netic field in seven early B-type cluster members out of
the fourteen members observed. The strongest magnetic
field ?Bz?=1559±38G was measured in the He-weak star
NGC3766-170, followed by the second strongest mag-
netic field ?Bz?=310±65G measured in the He-strong star
NGC3766-094. Among the cluster member Be stars, the
strongest magnetic field ?Bz?=−134±42G was measured
in NGC3766-47. Surprisingly, magnetic fields of a similar
order were also discovered in the normal early B-type stars
NGC3766-111 and NGC3766-176.
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712S. Hubrig et al.: Magnetic survey of emission line B-type stars
0.00.5 1.01.52.0
Phase
-500
0
500
Magnetic Field (Gauss)
0100
Frequency (1/day)
200300400
0
50
100
150
200
Amplitude (Gauss)
Fig.1
magnetic field measurements of λEri using hydrogen lines
in 2006 August.
Phase diagram and amplitude spectrum for the
In the following subsections we describe the time-
resolved observations of nine Be stars (Sect. 3.1), discuss
the results of the measurements of other stars with magnetic
field detections at 3σ level (Sect. 3.2), and in Sect. 3.3 we
present the magnetic field measurements of members of the
young open cluster NGC3766.
3.1
covery of magnetic field cyclic variability in λEri
Time-resolved magnetic field measurements: dis-
The nine Be stars with time-resolved magnetic field mea-
surements are very bright objects. The corresponding inte-
gration time for a single measurement of the magnetic field
with the Kueyen 8-m telescope and FORS1 accounts for
only a few seconds. Taking into account time for overheads
(retarder waveplate setting plus readout time), we were able
to obtain during one hour 21 consecutive measurements for
the faintest Be star in our sample (QYCar) and 30 mea-
surements for the brightest Be star in our sample (ηCen).
The periodicity of the time-resolved magnetic field mea-
surements was analysed by application of Breger’s code
(1990). In the obtained amplitude spectra a 2.4σ peak cor-
respondingto a period of 21.12min was detected in the data
set of measurements carried out using hydrogen lines in the
star λEri in 2006 August (see lower panel in Fig. 1) . This
peak appears with a 2.2σ in the data set of measurements
carriedoutusingthewholespectrum(lowerpanelinFig. 2).
The correspondingdata sets for measurements using hydro-
gen lines and the whole spectrum are presented in Table 3.
To confirm the detected variability, we repeated our mea-
surements of λEri about 16 months later on two consecu-
0.0 0.51.01.52.0
Phase
-500
0
500
Magnetic Field (Gauss)
0100
Frequency (1/day)
200300400
0
50
100
150
Amplitude (Gauss)
Fig.2
magnetic field measurementsof λEri using all lines in 2006
August.
Phase diagram and amplitude spectrum for the
tive nights on 2007 November 27 and 28 over few hours on
each night to sample two different phases of its rotation pe-
riod. However, this time our observations did not reveal any
significant periodicity in any of the data sets. Previous stud-
ies of λEri indicate that spectral line profiles exhibit short-
time periodic variability due to non-radial pulsations with a
period of 0.7days (e.g., Kambe et al. 1993; Rivinius et al.
2003). In addition to these line profile variations, the He I
λ6678 line is reported to show dimples with a duration of
2–4hours (Smith 1994). Smith suggests that such changes
in the line profileare consistentwith the stellar rotationrate,
as if caused by a rooted active spot on the surface. The ob-
served rapid optical line variability develops over tens of
minutes or less, implying that violent high-energy events
occur close to the surface of this star (Smith et al. 1997).
Based on multiwavelength observations in optical, X-rays
and FUV, Smith et al. (1997) proposed that the observed
violent processes on the surface of this star show great sim-
ilarity with magnetic flaring. Our observations indicate that
a strong field could possibly exist locally, but with a topol-
ogy such that its net effect can appear only sporadically in
disk-integrated variations.
Trying to understand why we failed to detect any peri-
odicity in our measurements in the follow-up observations,
we studied the line variations in the polarimetricspectra ob-
tained with FORS1 on all three observing nights. Interest-
ingly, while spectral lines in Stokes I spectra obtained in
2006 August appear fairly symmetrical, the spectral lines
showratherstrongvariabilityinthetwotimeseriesobtained
in 2007 November.In Fig. 3 we present for each time series
all spectrain the spectral region4500to 4730˚ A overlapped.
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Astron. Nachr. / AN (2009)713
Table 3
2006 August.
Magnetic field time series for λEri obtained in
MJD
?Bz?hydr
[G]
?Bz?all
[G]
53955.37939
53955.38134
53955.38328
53955.38523
53955.38718
53955.38913
53955.39108
53955.39303
53955.39498
53955.39693
53955.39888
53955.40084
53955.40279
53955.40475
53955.40671
53955.40867
53955.41062
53955.41259
53955.41455
53955.41652
53955.41848
53955.42045
−160±195
−83±197
69±186
−349±187
465±201
158±190
52±207
−263±204
−83±184
−218±219
−142±206
148±203
230±205
248±204
5±210
−267±210
−201±205
−103±211
−26±205
211±213
−96±217
77±219
−140±160
−45±162
−15±153
−239±154
271±165
206±156
−211±170
−117±168
−96±151
−281±180
−126±169
90±166
35±168
−6±167
−29±172
−269±172
−52±168
−43±172
137±167
147±174
−34±177
−36±178
To show the line profile variations in more detail, we plot-
ted in Fig. 4 five spectra for each time series correspond-
ing to equidistant time intervals over the full time used to
obtain each data set. The spectra obtained in 2006 August
are presented in the bottom of the plot, and the spectra ob-
tained on 2007 November 27 and 28 are presented in the
middle and at the top of the plot, respectively. Asymmetric
line profiles are well visible in spectral lines Al III λ4530,
Si III λ4553, O II λ4675, and He I λ4713 in the time se-
ries obtained in 2007 November. Since the topology of the
magnetic field is not known, it is difficult to estimate the
impact of non-radial pulsations causing strong line asym-
metries on our measurements. It is quite possible that lines
of differentelements behavedifferentlywith respect to their
pulsation amplitudes and shapes of the line profiles. We be-
lieve that high resolution spectropolarimeters will be more
suitable for field measurements in pulsating stars, since at
higher resolution the Zeeman features in individual lines
can be studied separately. Our time-resolved magnetic field
measurements of the remaining Be stars indicate that four
other Be stars may display a magnetic cyclic variability on
the time scales of minutes or tens of minutes. The stars
QYCar, δ Cen, αAra, and ǫTuc show weak signals in the
Fourier transforms of our data sets, corresponding to pe-
riods of 21.86min, 27.74min, 9.37min, and 4.27min, re-
spectively. These stars are good candidates for future time-
resolved magnetic field observations with high-resolution
spectropolarimeters.
4500 4550 4600 4650 4700
Wavelength [A]
0.95
1.00
1.05
1.10
1.15
Normalized Flux
He I
O IIO IIO IIO IIO II
Si IIISi IIISi IIIAl IIIAl III
Nov. 28 2007
Nov. 27 2007
Aug. 08 2006
Fig.3
tainedon 2006August8 (bottom),2007November27(mid-
dle), and 2007 November 28 (top).
Overlapped spectra of λEri for the data sets ob-
Apartfromthe studyofperiodicityin thetimeseries,we
used all magnetic field measurements obtained in the time
series together to determine their average magnetic field.
We obtainedfield detectionsat the 3σ level for two Be stars,
µCen and oAqr. The detected fields are very weak, of the
order of one hundred Gauss. In Fig. 5 we present Stokes I
and Stokes V spectra of oAqr in the spectral region includ-
ing the Hδ and Hγ lines. Noticeable Zeeman features are
well visible at the positions of both hydrogen lines. Two
more observations of this star were obtained during our vis-
itor run in 2007 November. The measured magnetic field
showed negativepolarity at a significance level of only 2.2σ
on November 27 and of 1.3σ on November 28.
3.2
for the presence of a weak magnetic field
Other emission line B-type stars showing evidence
HD62367: This Be star has only been marginally studied
in the past, mainly due to its rather faint magnitude
V =7.1. It is also one of the faintest Be stars in our sam-
ple(seeTable1).AccordingtoYudin(2001)thespectral
type of the star is B8e and the v sini=114kms−1. This
star was observed only once and exhibits the strongest
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714S. Hubrig et al.: Magnetic survey of emission line B-type stars
4500455046004650 4700
Wavelength [A]
1.0
1.2
1.4
1.6
Normalized Flux
Fig.4
nights: on 2006 August 8 (bottom), 2007 November 27
(middle), and 2007 November 28 (top).
Spectrum variability of λEri on three different

0.4
0.6
0.8
1.0
1.2
4100 41504200 4250 4300 4350
Wavelength [A]
-0.10
-0.05
0.00
0.05
0.10
Stokes V/I [%]
Normalized Flux
˚
Hδ
Hγ
Fig.5
gion including the Hδ and Hγ lines.
Stokes I and Stokes V spectra of oAqr in the re-
magnetic field among the Be stars in our sample. Using
hydrogen Balmer absorption lines we obtained ?Bz?=
117±38G.
ǫTuc = HD224686: This Be star was classified as B8V
with Teff=13000K and logg =3.90 by Levenhagen
& Leister (2006), who also determined v sini=300
kms−1. A weak magnetic field at a 3σ significance
level, ?Bz?=74±24G, was detected during our observ-
ing run in 2007 November.
4460 44704480 4490
˚
4500
Wavelength [A]
0.0
0.5
1.0
Stokes V/I [%]
MJD 53519.91
MJD 54333.02
MJD 54343.10
MJD 54361.08
Fig.6
star υSgr over two years in the vicinity of Mg II λ4481.
Observed Stokes V spectra of the emission line
υ Sgr = HD181615: The emission-line star υ Sgr is a very
unusual object, frequently classified as a Be star due to
the presence of strong emission lines in the visible spec-
trum. It seems to be a magnetic variable star, probably
on a few months timescale with a maximum longitudi-
nal magnetic field ?Bz?=−102±10G measured in hy-
drogenlinesonMJD54333.018.InFig.6wepresentthe
Stokes V spectra in the vicinity of Mg II λ4481 taken
on four different dates over two years. The evolution-
ary status for this star is not obvious due to the fact that
it is a single-line spectroscopic binary system currently
observed in the initial rapid phase of mass exchange be-
tween the two components (Koubsk´ y et al. 2006). The
star dominating the optical and UV line spectra is less
massive and has a spectral type B8pI, while the second,
almost invisible component is more massive by a factor
of 1.57 and has a spectral type O9V. The optically visi-
ble star is hydrogen poor and the observed spectrum is
extremely line rich (see Fig. 7). Hubrig et al. (2007) re-
ported the detection of distinctive Zeeman signatures in
the Ca II H&K lines, which are probably formed in the
circumstellar disk around this star. Future monitoringof
the magnetic field of υ Sgr over a few months with a
high resolution spectropolarimeter would be of extreme
interest to understand the role of the magnetic field in
the evolutionary process of mass exchange in a binary
system.
3.3 Members of the open cluster NGC3766
The results of the study of fifteen early B-type members of
the open cluster NGC 3766 have in part been reported by
McSwain (2008), who announcedtwo definite detections in
He peculiar stars, NGC3766-094 and NGC3766-170, one
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Astron. Nachr. / AN (2009)715
3700 40004300
Wavelength [A]
4600 49005200
0.4
0.6
0.8
1.0
Normalized Flux
Fig.7
Normalized FORS1 Stokes I spectrum of υSgr.
marginal detection in one Be star, NGC3766-047, and one
marginal detection in a Be star candidate, NGC3766-045.
A careful treatment of the spectropolarimetric data allowed
us to detect weak magnetic fields in three additional mem-
bers of this cluster: in the Be star NGC3766-200 and in the
two normalB-type stars NGC3766-111andNGC3766-176.
Theclustermemberswithdetectedmagneticfieldsarehigh-
lighted in bold face in Table 2. As was already mentioned
in Sect. 2, the polarimetric spectra of NGC3766-031 were
contaminated by a close companion and have not been con-
sidered in our study. As an example, in Fig. 8 we present
the observed Stokes I and Stokes V profiles of the He pe-
culiar member of this cluster and of another cluster mem-
ber, which was classified as a potential Be star by Shob-
brook (1985) with longitudinal magnetic fields of ?Bz?=
+1559±38G and ?Bz?=−194±62G, respectively.
Themagneticfields have beendetectedin stars with Teff
in the range from 15500K to 21435K and loggpolarfrom
4.61 to 3.51 (McSwain 2008), indicating that the presence
of a magnetic field is not directly related to the stellar evo-
lutionary phase on the main sequence.
4 Discussion
Our search for magnetic fields in Be stars revealed that
while their magnetic fields are rather weak, fields of the or-
der of 100G and less are not rare. Weak magnetic fields
are considered to provide a mechanism for launching and
stabilizing circumstellar disks in Be stars (e.g. Brown et al.
2008). Since a large fraction of stars in our sample was ob-
served only once, a non-detection of their magnetic field
may be explained by temporal variability of their magnetic
fields. A cyclical variability with a period of 21.12min was
detected in one data set of time series in λEri, but could not
be confirmed in the two follow-up time series. The cluster

0.4
0.4
0.6
0.8
1.0
38003850 3900 3950
Wavelength [A]
-0.4
-0.2
0.0
0.2
Stokes V/I [%]
Normalized Flux
˚

0.4
0.6
0.8
1.0
3800 3850 3900 3950
Wavelength [A]
-0.3
-0.2
-0.1
-0.0
0.1
0.2
Stokes V/I [%]
Normalized Flux
˚
Fig.8
blue spectralregionaroundhighnumberBalmer lines of the
He-peculiarmemberNGC3766-170oftheyoungopenclus-
ter NGC3766 with the magnetic field ?Bz?=1559±38G,
measured on hydrogen lines. Lower panel: Stokes I and
Stokes V spectra around high number Balmer lines for
the candidate Be star NGC3766-45, with a magnetic field
?Bz?=−194±62G measured on hydrogen lines.
Upper panel:Stokes I and Stokes V spectra in the
NGC 3766seems tobe extremelyinteresting,wherewe find
clear evidence for the presence of a magnetic field in seven
early B-type cluster members out of fourteen members.
Since magnetic fields can potentially have a strong im-
pact on the physics and evolution of B-type stars, it is criti-
cal to answer the principal question of the possible origin of
such magnetic fields. One important step would be to con-
duct observations of members of open clusters and associ-
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716S. Hubrig et al.: Magnetic survey of emission line B-type stars
ations at different age. To date, we studied the presence of
magnetic fields only in members of a young open cluster in
the Carina spiral arm known for its high content of early-B
type stars, NGC3766, with very surprising results. Along
with strong magnetic fields detected in He-peculiar stars,
weak magnetic fields have been detected in a few normal
B type stars and in a few Be stars. We note that the inabil-
ity to detect magnetic fields in Be stars and normal B-type
stars in the past is probably related to the weakness of these
fields. Future observations will be worthwhile to determine
the structure of these fields using high signal-to-noise spec-
tropolarimetric time series.
Acknowledgements. We are grateful to the referee Dr. J. Madej
for useful comments. MAP and RVY acknowledge the sup-
port obtained by RFBR grant No07-02-00535a and Sci.Schole
No6110.2008.2, and MC acknowledges the support obtained by
DIUV grant 08/2007. This research has made use of the SIMBAD
database, operated at CDS, Strasbourg, France.
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