First detection of a magnetic field in the fast rotating runaway Oe star zeta Ophiuchi

0.21 MB·English

by  S. Hubrig

Astron. Nachr./AN332,No.2,147–152(2011)/DOI10.1002/asna.201111516
First detection of a magnetic field in the fast rotating runaway Oe star
ζ Ophiuchi⋆
S.Hubrig1,⋆⋆,L.M.Oskinova2,andM.Scho¨ller3
1 AstrophysikalischesInstitutPotsdam,AnderSternwarte16,14482Potsdam,Germany
2 Universita¨tPotsdam,Institutfu¨rPhysikundAstronomie,14476Potsdam,Germany
3 EuropeanSouthernObservatory,Karl-Schwarzschild-Str.2,85748Garching,Germany
1
1
0 Received2011Jan21,accepted2011Jan26
2 Publishedonline2011Feb15

n
a Keywords stars:mass-loss–stars:early-type–stars:magneticfield–stars:kinematicsanddynamics–X-rays:stars
J –stars:individual:ζOphiuchi

8
ThestarζOphiuchiisoneofthebrightestmassivestarsinthenorthernhemisphereandwasintensivelystudiedinvarious
2
wavelengthdomains.Thecurrentlyavailableobservationalmaterialsuggeststhatcertainobservedphenomenaarerelated

to the presence of a magnetic field. We acquired spectropolarimetric observations of ζOph with FORS1 mounted on
]
R the8-mKueyentelescopeoftheVLTtoinvestigateifamagneticfieldisindeedpresentinthisstar.Usingallavailable
S absorptionlines,wedetect ameanlongitudinal magneticfieldhBziall = 141±45G,confirmingthemagnetic nature
ofthisstar.WereviewtheX-raypropertiesofζOphwiththeaimtounderstandwhethertheX-rayemissionofζOphis
.
h dominatedbymagneticorbywindinstabilityprocesses.
p
- (cid:13)c 2011WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
o
r
t
s 1 Introduction tometricobservations,althoughBalona&Kambe(1999)fa-
a
[ voredaperiodintheregionof1cycle/day.
During the last years a gradually increasing number of O,
1 ζOph is also well-known for its variability in the X-
earlyB-type,andWRstarshavebeeninvestigatedformag-
v
ray band. Oskinova etal. (2001) studied the ASCA obser-
neticfields,andasaresult,aboutadozenmagneticO-type
0
vations of ζOph that covered just more than the expected
0 starsarepresentlyknown(e.g.,Hubrigetal.2008;Martins
5 et al. 2010; Hubrig et al. 2011a). The recent detections of rotation period of the star. A clearly detected periodic X-
5 magnetic fields in massive stars generate a strong motiva- rayfluxvariabilitywith∼20%amplitudeintheASCApass-
. band(0.5-10keV)wasreported.Aperiodof0.77dwasde-
1 tiontostudythecorrelationsbetweenevolutionarystate,ro-
0 tationvelocity,andsurfacecomposition,andtounderstand tected and a possible connection with the recurrence time
1 (0.875d±0.167d)oftheDACsinUVspectraofthestarwas
theoriginandtheroleofmagneticfieldsinmassivestars.
1 discussed.TheDACsinthespectraofOstarsarecommonly
The star ζOphiuchi (=HD149757) of spectral type
:
v O9.5V is a well-knownrapidly rotating runaway star with explainedbylarge-scalestructuresinthestellarwind,mod-
i ulated by rotation and possibly related to a surface mag-
X extremelyinterestingcharacteristics.It undergoesepisodic
netic field (Cranmer & Owocki 1996). Waldron etal. (in
r mass loss seen as emission in Hα, and it is possible that
a it rotates with almost break-up velocity with vsini = preparation, private communication) found that SUZAKU
400kms−1 (Kambe et al. 1993). Various studies indicate data on ζOph suggest a period of ∼0.98d that is consis-
tentbutslightlylargerthantheX-rayperiodicitiesfoundin
differenttypesof spectraland photometricvariability.The
ASCAdata(Oskinovaetal.2001)andinChandraHETGS
UVresonancelinesshowmultiplediscreteabsorptioncom-
data(Waldron2005).Inaddition,theHETGSdataappearto
ponents (DAC) in the UV (e.g. Howarth et al. 1984) and
indicate an additionalcyclic period of ∼0.33d in the hard
strong line profile variations in optical spectra reconciled
X-rayband(>1.2keV).
with travelingsectorialmodesof high degree(e.g. Reid et
al. 1993). Highly precise MOST (Microvariabilityand Os-
Theresultsofourpreviousstudiesseemtoindicatethat
cillationsofStars)satellitephotometryin2004hasyielded
thepresenceofamagneticfieldismorefrequentlydetected
atleasta dozensignificantoscillationfrequenciesbetween
incandidaterunawaystarsthaninstarsbelongingtoclusters
1 and 10 cycles/day, hinting at a behaviour similar to
or associations (Hubrig et al. 2011b; Hubrig et al. 2011a).
β Cephei-typestars(Walkeretal.2005).Nounambiguous Thecurrentlybestavailableastrometric,spectroscopic,and
rotationperiodcouldbeidentifiedinspectroscopicandpho-
photometricdatawereusedtocalculatethekinematicalsta-
⋆ Basedonobservations obtained attheEuropeanSouthern Observa- tusofmagneticO-typestarswithpreviouslyunknownspace
tory,Paranal,Chile(ESOprogramme081.C-0410(A)). velocities.TheresultssuggestthatmostofthemagneticO-
⋆⋆ Correspondingauthor:[email protected] typestarscanbeconsideredascandidaterunawaystars.
(cid:13)c 2011WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
148 S.Hubrig,L.M.Oskinova&M.Scho¨ller:MagneticfieldintherunawayOestarζOph
TheavailableobservationalmaterialsuggeststhatζOph
isamainsequencesinglestarinthefieldwithrunawaychar-
acteristics.Usually,toexplaintheoriginofmassivestarsin
thefield,twomechanismsarediscussedintheliterature.In
onescenario,closemultibodyinteractionsinadensecluster
causeoneormorestarstobescatteredoutoftheregion(e.g.
Leonard & Duncan 1990). For this mechanism, runaways
are ejected in dynamical three- or four-body interactions.
Analternativemechanisminvolvesasupernova(SN)explo-
sionwithinaclosebinary,ejectingthesecondaryduetothe
conservation of momentum (Zwicky 1957; Blaauw 1961).
Blaauw (1952) suggested the origin of ζOph in the Scor-
piusOB2associationduetoitspropermotionvector,which
pointsawayfromtheassociation.Morerecently,Hoogerw-
erfetal.(2001)suggestedthatthestargaineditspaceveloc-
ity of ∼30kms−1 in a supernovaexplosionwithin a close
binary in Upper Scorpius about 1–2Myr ago. The authors
identified PSR B1929+10 as an associated pulsar with a
characteristicageof∼3Myr,consistentwiththekinematic
ageofζOphwithintheuncertainties.Tetzlaffetal.(2010) Fig.1 (onlinecolourat: www.an-journal.org)Combined
reinvestigatedthescenarioofabinarySNinUpperScorpius IR Spitzer IRAC (3.6µm blue, 4.5µm green, 8.0µm red)
involvingζOph and PSR B1929+10and concludedthatit image of the bow shock around the runaway star ζOph.
isverylikelythatbothobjectswereejectedduringthesame Archival data have been used. Galactic coordinates are
supernova event. In their work, the considered association shown.Theimagesizeis∼36′×31′.
age range implies that the progenitor star of the produced
neutron star had a spectral type between O6/O7 and O9
emission.Ontheotherhand,Fullertonetal.deriveamuch
with a mass rangefrom18to 37M⊙. The X-rayemission
smaller mass-loss rate from fitting the UV PV resonance
ofthepulsarseemstobedominatedbynon-thermalradia-
doublet,theproductofthemass-lossrateandtheionfrac-
tionprocesses(e.g.Beckeretal.2006).Anarc-likenebula
surrounding PSR B1929+10 and extending up to 10′′ was tion of P+4 being only M˙ q(P+4)<∼1.3 × 10−10M⊙yr−1
with q(P+4)<1. The mass-loss rates derived from fitting
identified in Chandra data and interpreted as a bow-shock ∼
thewindprofilesofUVresonancelinesdependlinearlyon
nebula(Hui&Becker2008).Theestimatedmagneticfield
thedensity.Toresolvethisdiscordanceinmass-lossdeter-
strengthintheshockedregionaccountsfor∼75µG,while
minations based on ρ2- and ρ-diagnostics, Fullerton etal.
the typical magnetic field strength in the ISM is about 2–
suggestthatthewindsarestronglyclumpedwithavolume
6µG.
filling factor of ∼10−3–10−5. Marcolino etal. (2009) an-
The presenceof a bow-shocknebula hasalso been de- alyzed optical and UV spectra of ζOph among their sam-
tectedforζOph.Figure1showsanimagebasedonarchival ple of O-type dwarfs. They derive an upper limit on the
SpitzerIRACmaps(AOR17774848).Recently,Kobulnicky mass-lossrateofζOphas1.6×10−9M⊙yr−1 ifthewind
etal.(2010)analyzedasampleofbowshocksaroundmas- wassmooth.ThisvalueagreeswiththeM˙ q(P+4)valueob-
sive stars in Cygnus-X. They used the analytical descrip- tainedbyFullertonetal.(2006).
tion of momentum-drivenbow shocks and dust/polycyclic
Using the example of the O-type supergiant ζPuppis,
aromatic hydrocarbon emission models to estimate stellar
Oskinovaetal.(2007)demonstratedthatthediscordanceof
mass loss rates from the observed properties of the bow
mass-loss rates found by Fullerton etal. can be overcome
shocks. It was found that mass-loss rates in the range be-
byaccountingforstellarwindporosity(seealsoSundqvist
tween 10−7M⊙yr−1 and a few times 10−6M⊙yr−1 are etal.2010).ItwasfoundfortheO5IastarζPuppisthatonly
required to generate the bow shocksaround typicalB2V -
amoderatereductionofthemass-lossratebyafactorof2–3
O5Vtypestars.
(comparedtothesmoothwindmodels)isrequiredtorepro-
The mass-loss rate M˙ of this star was empirically ob- ducebothHαandPVlines.Ifthisresultholdsalsofornon-
tained from different diagnostics by a number of authors. supergiantOtypestars,themass-lossrateofζOphisonly
Repolust etal. (2004) fitted the Hα photospheric absorp- a few times lower compared to the radio-based mass-loss
tionlineandderivedtheupperlimitontheζOphmass-loss determinedbyFullertonetal.,i.e.∼10−7M⊙yr−1.Impor-
rate as 1.8×10−7M⊙yr−1. Fullerton etal. (2006) deter- tantly, this mass-loss rate is in agreement with values that
mined the radio-based mass-loss rate of ζOph as 1.1 × arerequiredtoproducebowshocksaroundOstars(Kobul-
10−7M⊙yr−1. The mass-loss rates determined from ra- nickyetal.2010).
dio depend on the square of the density since the physical An additional aspect, which may hint at the presence
mechanism responsible for the radio emission is free-free of a magnetic field in runaway stars, is that a number of
(cid:13)c 2011WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim www.an-journal.org
Astron.Nachr./AN(2011) 149
individual abundance studies indicate nitrogen enrichment
in the atmospheres of runaway stars (e.g. Boyajian et al.
1.00
2005). Nitrogen enrichment was found in ζOph by Villa- x
mariz & Herrero (2005). Recent NLTE abundance analy- Flu 0.95
d
ses(e.g.,Moreletal.2008;Hunteretal.2008)suggestthat e 0.90
z
slowrotatorshavepeculiarchemicalenrichmentsuchasni- mali 0.85
trogenexcessorborondepletion,andthesepeculiaritiesare r
o
N 0.80
linkedtothepresenceofamagneticfield.Ontheotherhand,
0.75
Hubrig et al. (2011c) showed that some magnetic massive

starspreviouslyassumedtobeslowrotators,areinfactfast
rotators,butareviewedclosetotheirrotationpoles. %] 0.2
[
Totestthemagneticnatureofthisparticularlyinterest- V/I 0.0
ing rapidly rotating runaway star, we acquired spectropo- s
e
llaarriimmeettreircFoObsReSrv1ataiotntshewVithLTth.eInlotwhi-srewsoolruktiownesrpeepcotrrtopthoe- Stok -0.2
firstdetectionofamagneticfieldinthisstar.
4840 4850 4860 4870 4880 4890
Wavelength [A˚]
2 Magnetic fieldmeasurements
Spectropolarimetric observations with FORS1 (Appen-
1.00
zeller et al. 1998) were obtained on 2008 May 23 x
u 0.98
(MJD54609.34)using grism 600Band a slit width of 0′.′4 Fl
d 0.96
to achieve a spectral resolving power of R ≈ 2000. The e
z
use of the mosaic detector made of blue optimized E2V ali 0.94
m
0.92
chipsandapixelsizeof15µmallowedustocoveralarge r
o
spectral range, from 3250 to 6215A˚, which includes all N 0.90
0.88
hydrogen Balmer lines from Hβ to the Balmer jump. Six

continuousseriesoftwoexposureswithdurationsbetween
0.3 and 3sec were taken at two retarder waveplate setups %] 0.2
[
(α = +45◦ and−45◦). For allbutthe first exposurepairs V/I 0.0
weachievedasignal-to-noiseratiobetween1000and1500. s
e
Mbeofroeudnedtaeillsseownhethree(oeb.gse.,rvHinugbrtiegchentiaqlu.e20w0i4tha,F2O0R04Sb1, acnand Stok -0.2
references therein). The mean longitudinal magnetic field,
hB i,wasderivedusing 5870 5880 5890 5900 5910 5920
z
Wavelength [A˚]
V g eλ2 1 dI
eff
=− hB i, (1)
I 4πm c2 I dλ z
e Fig.2 Stokes I and V spectra of ζOph in the spectral
where V is the Stokes parameter that measures the circu- regionsaroundHβ (top)andtheNaIdoublet(bottom).
lar polarisation, I is the intensity in the unpolarised spec-
trum, g is the effective Lande´ factor, e is the electron
eff
charge, λ is the wavelength, m the electron mass, c the InFig.3wepresenttimeseriesofStokesI spectracor-
e
speedoflight,dI/dλisthederivativeofStokesI,andhB i respondingtoourdatasetofsixsub-exposuresintheregion
z
is the mean longitudinal magnetic field. The longitudinal around He II 4686A˚ and He I 4713A˚. Although the time
magnetic field was measured in two ways: using only the lapsebetweentheobservationsofthefirstpairandthelast
absorptionhydrogenBalmerlinesorusingtheentirespec- pairisonly13minutes,somesmalllineprofilevariationsin
trumincludingallavailableabsorptionlines.Weobtainfor theHeIlinearealreadydetectableatsuchshorttimescales.
themeanlongitudinalmagneticfieldusingallavailableab- The only other measurements of the magnetic field in
sorptionlineshB i = 141±45Gandforthemeanlon- ζOph have been presented by Schnerr et al. (2008) who
z all
gitudinal magnetic field using the hydrogen Balmer lines used the MuSiCoS spectropolarimeter to derive Stokes I
hB i =123±54G.Ourdetectionusingtheentirespec- andV spectrawiththeLeastSquareDeconvolutionmethod.
z hyd
trum has a significance of 3.1σ, determined from the for- No longitudinalmagnetic field was detected in this star at
mal uncertainties we derive. In the Stokes V spectra dis- a levelmorethan 3σ. However,the measurementerrorsin
tinct Zeeman signatures are well visible at the position of their observations were in the range from 700G to more
hydrogenandmetallines.InFig.2wedisplayStokesI and than3kG.
V spectra in the spectral regions around Hβ and the Na I Massivestarsusuallyendtheirevolutionwithafinalsu-
doublet. pernovaexplosion,producingneutronstarsor blackholes.
www.an-journal.org (cid:13)c 2011WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
150 S.Hubrig,L.M.Oskinova&M.Scho¨ller:MagneticfieldintherunawayOestarζOph
theaimtounderstandwhethertheX-rayemissionofζOph
1.25
isdominatedbymagneticorwindinstabilityprocesses.
1.20 Babel& Montmerle(1997) studiedthe case ofa rotat-
ing star with a dipole magnetic field sufficiently strong to
x 1.15
Flu confine stellar wind. The magnetic field locally dominates
ed 1.10 the bulk motion of stellar wind, when the ratio of mag-
z
mali netic to kinetic energy density, B2/µ0ρv2 > 1, where v
or 1.05 isthesupersonicflowspeed.Acollisionbetweenthewind
N
1.00 streams from the two hemispheres in the closed magneto-
sphereleadstoastrongshockandX-rayemission.
0.95
MHD simulations in the framework of this magnet-
4680 4690 4700 4710 4720 4730 ically confined wind shock (MCWS) model were per-
Wavelength [A˚] formed by ud-Doula & Owocki (2002) and Gagne´ etal.
(2005). Using their notation, the wind is confined when
Fig.3 Six Stokes I spectra corresponding to six sub- η∗ ≡(R∗2B2)(M˙ v∞)−1 >1. New observations are re-
exposures in the region around He II 4686A˚ and He I quired to establish whether the magnetic field of ζOph is
4713A˚.SmalllineprofilevariationsintheHeIlineareal- a dipole. However, for the purpose of this discussion, let
us assume that the field has an average strength of 150G.
readydetectableonthetimescaleofonly13min.
Using the stellar parameters of ζOph as inferred by Mar-
colinoetal. (2009),we estimateη∗(ζOph) ∼ 103,i.e. the
The initial massesof these stars rangefrom∼8–10M⊙ to magnetic field should dominatethe wind motionup to the
100M⊙ or more, which correspond to spectral types ear- Alfve´nradiusthatislocatedat<∼10R∗.Inthiscase,theX-
lier than about B2. Contrary to the case of Sun-like stars, rayemissionshouldmainlyoriginatefromtheMCWS.
the magnetic fields of stars on the upper main sequence The MCWS model predicts that the X-ray emitting
(Ap/Bp stars), white dwarfs, and neutron stars are domi- plasmashouldbelocatedatafewR∗fromthephotosphere;
nated by large spatial scales and do not change on yearly thattheX-rayemissionlinesshouldbenarrow;thattheX-
time scales. In each of these classes there is a wide distri- ray luminosity should be higher and the spectrum harder
bution of magnetic field strengths, but the distribution of than in non-magnetic stars; that in oblique magnetic rota-
magnetic fluxes appears to be similar in each class, with tors the X-ray emission should be modulated periodically
maxima of Φmax = πR2B ∼ 1027−28Gcm2 (Reiseneg- asaconsequenceoftheoccultationofthehotplasmabya
ger2001;Ferrario&Wickramasinghe2005),arguingfora cooltorusofmatter,orbytheopaquestellarcore.
fossil field whose flux is conserved along the path of stel-
ThelinesofHe-likeionsobservedinX-rayspectraare
lar evolution. According to Reisenegger (2009) the mag-
usefultoderivethelocationofthelineformationregionin
neticfluxeshavepossiblybeengeneratedonorevenbefore
hot stars because forbidden line emission is depressed by
themain-sequencestageandtheninheritedbythecompact
ultraviolet pumping. The latter depends on the distance to
remnants.
the stellar photosphere (Gabriel & Jordan 1969; Blumen-
ThemagneticfieldstrengthofthepulsarPSRB1929+10
thal etal. 1972). The SiXIII line observed in the Chan-
is 0.5129×1012Gauss. Assuming simple conservation of
dra HETGS/MEG spectrumis shown in Fig. 4. prominent
magneticfluxweobtainafieldstrengthofjustafewGauss
forbidden line can easily be distinguished in this fugure,
forthe moremassive pulsarprogenitors.ForζOph,which
while normally forbidden lines are strongly suppressed in
issupposedtobeformedina binarySN,sharingthesame
the spectra of OB-type stars. The presence of the forbid-
environment with the SN progenitor, the expected field
denlineimpliesthatthelineformationregionislocatedfar
strength would be of the order of 10G. This value is no-
from the photosphere,so that the radiative excitation does
tablylowerthanourcurrentmeasurement,possiblyindicat-
notleadtothedepopulationofthecorrespondingmetastable
ingthateitherthemagneticfieldofthismiddle-agedpulsar
energy levels. Waldron & Cassinelli (2008) found that the
hassignificantlydecayedduringthefewMyrsaftertheSN
SiXIIIlineisformedat1.8±0.7R∗inζOphandthatother
explosionorothermechanismsplayaroleinthegeneration
He-like lines are formed even further out in the wind. In-
ofmagneticfieldsinO-typestars.
terestingly,thestrongforbiddenSiXIIIlineisalsoobserved
intheChandraspectrumofthemagneticstarτSco.Cohen
3 Discussion etal.(2005)deriveaSiXIIIlineformaitonradiusforτSco
intherangebetween1.1R∗ and1.5R∗.Theseradiiofline
ζOph has been extremely well studied in all wavelength formation are smaller than those found in the prototypical
ranges, from the X-ray by all major X-ray satellites (with MCWS model object θ1OriC, 3.4 ± 0.8R∗ (Waldron &
theexceptionofXMM-Newton)totheinfraredregionwith Cassinelli2008).
Spitzer.InviewofthedetectionofamagneticfieldonζOph Oskinovaetal.(2006)studiedtheChandraspectrumof
reported in this work, we review its X-ray properties with ζOphamongotherO-typestars.TheyfoundthattheX-ray
(cid:13)c 2011WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim www.an-journal.org
Astron.Nachr./AN(2011) 151
0.06
0.06 SiXIII FeXVII
0.05
m)0.05 m)
o o
gstr gstr0.04
An0.04 An
c/ c/
e e
nts/s0.03 nts/s0.03
u u
o o
C C0.02
x (0.02 x (
u u
Fl Fl
0.01 0.01
λR λI λF λ0=15.01 Ao
0.00 0.00
6.60 6.65 6.70 6.75 6.80 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
Wavelength (Ao) v/v
8
Fig.4 TheSiXIIIlineobservedinthespectrumofζOph
0.10 NeX
(co-addedMEG±1).Verticaldashedlinesindicatetherest-
frame wavelength: λR – resonant line, λI – sum of in- m)
twearcvoemlebnigntahtsioanrelcinoerrse,cλteFd–fofrotrhbeidraddeinallivneel.ocTihtyetraeksetn-frfaromme gstro0.08
n
Hoogerwerfetal.(2001). c/A0.06
e
s
s/
nt
u
o0.04
emissionlinesinthisstararenarrowandthatthesignatures C
ofwindabsorptiononlineprofilesareweak.Figure5shows ux (
Fl
theFeXVIIandNeXlinesasmeasuredbyChandraplotted 0.02
overunitsofthewindterminalvelocity,v∞=1550kms−1. λ =12.14 Ao
0
Thelinesareonlyslightlybroadened,ifatall.
0.00
-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
TheX-rayluminosityofζOph,LX =1.2×1031ergs−1 v/v
and the ratioL /L = 4×10−8 arequite usualamong 8
X bol
Fig.5 FeXVII(upperpanel)andNeX(lowerpanel)lines
late type OV stars (Oskinova etal. 2006). Adopting the
mass-loss rate from ζOph as <∼1.8× 10−7M⊙yr−1, Os- observed in the spectrum of ζOph (co-added MEG ±1).
Vertical dashed lines indicate the rest-frame wavelength,
kinova etal. (2006) noticed that in ζOph the ratio of
X-ray to wind mechanical luminosity Lmech (M˙ v∞2/2), correctedfortheradialvelocity.
L /L >8.5×10−5,isafewtimeshigherthaninother
X mech∼
single O-type stars. Thismay be related to the lower wind
Acknowledgements. WewouldliketothankY.Naze´ fordrawing
opacityinζOph,oritmayhintatsomeadditionalmecha-
our attention to this interesting star and the anonymous referee
nismofX-raygenerationbesidestheintrinsicwindshocks.
forvaluablecomments.LMOacknowledgesthefinancialsupport
FromtheiranalysisofChandraspectra,Zhekov&Palla from grant number FKZ 50 OR 1101. This work used archival
(2007)derivedthedifferentialemissionmeasure(DEM)for dataobtainedwiththeSpitzerSpaceTelescope,whichisoperated
ζOph among other OB stars in their sample. They found bytheJetPropulsionLaboratory,CaliforniaInstituteofTechnol-
thatinζOphtheDEMsharplypeaksatabout6MK.While ogy,underacontractwithNASA.Wealsouseddataobtainedfrom
this is a significantlylowertemperaturethanfoundforthe theChandraDataArchiveandsoftwareprovidedbytheChandra
DEM peak in case of θ1OriC (50MK), it is higher than X-rayCenter(CXC).
found for other stars of similar spectral types (∼3MK).
Thus, considering the X-ray temperature of ζOph, it is
References
not straightforward to attribute its X-ray emission to the
MCWS.Ontheotherhand,recentstudiesofOstarswithde-
Appenzeller,I.,Fricke,W.,Fu¨rtig,W.,etal.:1998,TheESOMes-
tectedmagneticfields(e.g.,HD191612,HD108)showthat
senger94,1
their X-ray propertiesare diverse (Naze´ etal. 2004, 2010)
Babel,J.,Montmerle,T.:1997,A&A323,121
andmay bedifficultto fullyreconcilewith the predictions Balona,L.A.,Kambe,E.:1999,MNRAS308,1117
oftheMCWSmodel. Becker,W.,Kramer,M.,Jessner,A.,etal.:2006,ApJ645,1421
Blaauw,A.:1952,Bull.Astron.Inst.Netherlands11,414
Clearly, new observations are needed to better under-
Blaauw,A.:1961,Bull.Astron.Inst.Netherlands15,265
standthemagneticfieldofζOphanditslinkwiththeX-ray Blumenthal, G.R.,Drake, G.W.F.,Tucker, W.H.:1972, ApJ 172,
emissionfromthisstar. 205
www.an-journal.org (cid:13)c 2011WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
152 S.Hubrig,L.M.Oskinova&M.Scho¨ller:MagneticfieldintherunawayOestarζOph
Boyajian,T.S.,Beaulieu,T.D.,Gies,D.R.,etal.:2005,ApJ621, Repolust,T.,Puls,J.,Herrero,A.:2004,A&A415,349
978 Schnerr,R.S.,Henrichs,H.F.,Neiner,C.,etal.:2008,A&A483,
Cohen, D.H., de Messieres, G.E., MacFarlane, J.J., et al.: 2003, 857
ApJ586,495 Sundqvist,J.O.,Puls,J.,Feldmeier,A.:2010,A&A510,11
Cranmer,S.R.,Owocki,S.P.:1996,ApJ462,469 Tetzlaff, N., Neuha¨user, R., Hohle, M.M.: 2010, MNRAS 402,
Ferrario,L.,Wickramasinghe,D.T.:2005,MNRAS356,615 2369
Fullerton,A.W.,Massa,D.L.,Prinja,R.K.:2006,ApJ637,1025 ud-Doula,A.,Owocki,S.P.:2002,ApJ576,413
Gabriel,A.H.,Jordan,C.:1969,MNRAS145,241 Villamariz,M.R.,Herrero,A.:2005,A&A442,263
Gagne´,M.,Oksala,M.E.,Cohen,D.H.,etal.:2005,ApJ628,986 Waldron,W.L.:2005,in:R.Smith(ed.),X-rayDiagnosticsofAs-
Hoogerwerf, R., de Bruijne, J.H.J., de Zeeuw, P.T.: 2001, A&A trophysical Plasmas: Theory, Experiment, and Observation,
365,49 AIPC774,p.353
Howarth,I.D.,Prinja,R.K.,Willis,A.J.:1984,MNRAS208,525 Waldron,W.L.,Cassinelli,J.P.:2008,ApJ680,1595
Hubrig,S.,Kurtz,D.W.,Bagnulo,S.,etal.:2004a,A&A415,661 Walker,G.A.H.,Kuschnig, R.,Matthews, J.M.,etal.:2005, ApJ
Hubrig,S.,Szeifert,T.,Scho¨ller,M.,etal.:2004b,A&A415,685 623,L145
Hubrig,S.,Scho¨ller,M.,Schnerr,R.S.,etal.:2008,A&A490,793 Zhekov,S.A.,Palla,F.:2007,MNRAS382,1124
Hubrig, S.,Scho¨ller, M., Kharchenko, N.V.,etal.: 2011a, A&A, Zwicky,F.:1957,MorphologicalAstronomy,Springer,Berlin
submitted
Hubrig,S.,Kharchenko,N.V.,Scho¨ller,M.:2011b,AN332,65
Hubrig,S.,Ilyin,I.,Scho¨ller,M.,etal.:2011c,ApJ726,L5
Hui,C.Y.,Becker,W.:2008,A&A486,485
Hunter,I.,Brott,I.,Lennon,D.J.,etal.:2008,ApJ676,L29
Kambe,E.,Ando,H.,Hirata,R.:1993,A&A273,435
Kobulnicky,H.A.,Gilbert,I.J.,Kiminki,D.C.:2010,ApJ710,549
Leonard,P.J.T.,Duncan,M.J.:1990,AJ99,608
Marcolino, W.L.F.,Bouret, J.-C., Martins, F., et al.: 2009, A&A
498,837
Martins,F.,Donati,J.-F.,Marcolino,W.L.F.,etal.:2010,MNRAS
407,1423
Morel,T.,Hubrig,S.,Briquet,M.:2008,A&A481,453
Naze´,Y.,Rauw,G.,Vreux,J.-M.,DeBecker,M.:2004,A&A417,
667
Naze´,Y.,ud-Doula,A.,Spano,M.,etal.:2010,A&A520,59
Oskinova, L.M., Clarke, D., Pollock, A.M.T.: 2001, A&A 378,
L21
Oskinova, L.M., Feldmeier, A., Hamann, W.-R.: 2006, MNRAS
372,313
Oskinova,L.M.,Hamann,W.-R.,Feldmeier,A.:2007,A&A476,
1331
Reid, A.H.N., Bolton, C.T., Crowe, R.A., et al.: 1993, ApJ 417,
320
Reisenegger, A.: 2001, in: G. Mathys, S.K. Solanki, D.T. Wick-
ramasinghe (eds.), Magnetic Fields Across the Hertzsprung-
RussellDiagram,ASPC248,p.469
Reisenegger,A.:2009,Rev.Mex.Astron.Astrofis.35,139
(cid:13)c 2011WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim www.an-journal.org