Table Of ContentProceedingsofthe7thEuropeanVLBINetworkSymposium
Bachiller,R.Colomer,F.,Desmurs,J.F.,deVicente,P.(eds.)
October12th-15th2004,Toledo,Spain
Hidden bipolarity in red supergiant winds
A.M.S.Richards1,M.R.W.Masheder2,H.J.vanLangevelde3,R.J.Cohen1,M.D.Gray4,J.A.Yates5,P.J.Diamond1,
W.H.T.Vlemmings6,M.Szymczak7,andK.Murakawa8
1 JodrellBankObservatory,UniversityofManchester,Macclesfield,Cheshire,SK11,UK.e-mail:[email protected]
5 2 DepartmentofPhysics,UniversityofBristol,TyndallAvenue,Bristol,BS81TL,UK.
0 3 JointInstituteforVLBIinEurope,Postbus2,7990AADwingeloo,TheNetherlands.
0 4 DepartmentofPhysics,UMIST,POBox88,ManchesterM601QD,UK.
2 5 DepartmentofPhysicsandAstronomy,UniversityCollegeLondon,GowerStreet,LondonWC1E6BT,UK.
n 6 DepartmentofAstronomy,CornellUniversity,610SpaceSciencesBuilding,Ithaca,NY14853,USA.
a 7 Torun´ CentreforAstronomy,NicolausCopernicusUniversity,ul.Gagarina11,PL-87100Torun´,Poland.
J 8 ASTRON,Postbus2,7990AADwingeloo,TheNetherlands.
3
Abstract. Many observations of late-type M stars show large near-spherical circumstellar envelopes, yet planetary nebulae
1 andsupernova remnants arefrequently axisymmetric. Wepresent VLBIand MERLINobservations ofmasers around thered
v supergiantSPerwhichshowvaryingdegreesofaxisymmetryandadynamicallysignificantmagneticfield.Thereisnoevidence
8 forrotationhereorinmostsimilarobjects.Weexaminepossibleoriginsofthemagneticfield.
2
0
1. Introduction
1
0
5 Red supergiants (RSG) with progenitor masses > 10 M are
0 high-massanaloguesofAsymptoticGiantBranch∼(AGB)⊙stars
/
h suchasMiravariables,typicallyoneorafewM .Bothclasses
p ofstarhavesurfacetemperaturesof2000–300⊙0Kandpulsa-
- tionalperiodsof uptoa fewyears.SiO, H O andOH masers
o 2
r arefoundinthewindsandshowthattheoutflowvelocitiesare
t
s similartowithinafactoroftwo.Suchpropertiesappeartobe
a determinedby small-scale physics, such as the opacity of the
:
v stellaratmosphereortheabsorptivityofdust,andhencedepend
i primarilyonthelocalchemicalcomposition.
X
r Otherphenomenadoscalewiththestellarmass.RSGhave
a
stellarradiiofR > 5auandmasslossratesdM/dt 10 5
⋆ −
10 4, at least ten ∼times higher than those of AGB s≈tars. Fo−r
−
all such stars the H O maser zone extends from 5 30R
2 ⋆
∼ −
containingcloudsofaverageradiusR inthisregion(orabirth
⋆ Fig.1.TheH OmaseremissionfromSPer(MERLIN1999)is
radius 0.1R ,iftheyoriginatefromthesurfaceofthestarand 2
∼ ⋆ showningreyoverlaidwithwhitecontoursofOHemissionob-
expandwith the wind). The survival times of maser emission
served(VLBI1997).The7-masVLBIrestoringbeamisshown
fromthecloudsscaleswiththeirsizeandis5–>10yrinRSG.
atbottomleft.
Most evolved stars which support masers have no de-
tectablerotationofeitherthestellarsurfaceorthewindandthe s 1 (Diamond et al. 1987). We compare the results for S Per
−
presenceofacompanionwoulddisruptmasing.Thearchetype withthoseforotherstarsandwithmagneticfieldmodels.
OH/IR stars or H O envelopesaroundRSG, for example,ap-
2
pear almost spherical at moderate resolutions. This contrasts
2. PastandpresentobservationsofSPer
with the usually axisymmetric shape of planetary nebulae or
into supernovaremnants. Earlysigns of this can be discerned In 1997 Masheder et al. (1999) observed S Per and VX Sgr
in high-resolution studies of AGB/RSG stars (e.g. Bowers et withGlobalVLBI.ThedatawerecorrelatedinSocorrotogive
al. 1989, Bains et al. 2003; Murakawa et al. 2003; Etoka & a velocity resolution of 0.176 km s 1. We took great care in
−
Diamond 2003), implicating the magnetic fields detected by calibrationandeditingtoremoveinstrumentaloffsetsbetween
e.g.Chapman&Cohen(1986);Szymczaketal.(1998,1999). LL and RR and between the 1665- and 1667-MHz bands, on
We use VLBI and MERLIN observations of OH and other all baseline lengths. We then imaged the brightest channel of
maserstoinvestigatetheCSEofSPer.ThisRSGhasadistance thebrightestSPerlinetouseasamodelforiterativephaseand
of2.3kpc(Schild1967)andastellarvelocityV = 38.5km amplitudeself-calibration.Weappliedtheresultingsolutionsto
⋆
−
210 A.M.S.Richardsetal.:Hiddenbipolarityinredsupergiantwinds
Multi-epochVLBAmonitoringofSiOmasers(Ostrowski-
Fukudaetal. 2002)showsthatatsomestellarphasesthehot-
spots trace an elongation similar to that of the H O masers.
2
Thompson & Creech-Eakman (2003) imaged S Per at 2.2µm
andfoundthatthestarappearedelongatedwiththeminoraxis
at a similar position angle of 40 , suggesting that it is limb-
◦
darkenedinthedirectionofstrongestmass-loss.
3. Emergingaxisymmetries
3.1.OHandHOmaserdistribution
2
Fig. 1 shows the OH mainline and H O maser distributions,
2
aligned on the centres of expansion, assumed to be the stel-
lar position. This shows that the OH masers have a very pro-
nounced elongation NNE-SSW, also seen in 1993. The OH
emission is brightest in between the H O clumps. The OH
2
1612-MHzmasershaveanangulardistributionwhichissimilar
in shapeto themainlinesbut> 6 greaterin extent,covering
×
Fig.2.Angularseparationof masersfromSPer as a function nearly 1 arcsec N-S (also seen in 1985 observations). Fig. 2
ofV . shows the angular separation of masers from the stellar posi-
LSR
tionasafunctionofV .
LSR
These Figs. show that the H O masers form a thick limb-
2
alltheSPerdataandthenimagedandself-calibratedtheother brightened shell typical of strong acceleration whilst the OH
lines individually.In this way we were able to retain the cor- masersarebrighterattheextremevelocitiesnearthefrontand
rect relativepositionsand intensities. The final data cubes(in backcapsoftheshellsuggestingthattheyareexpandingmore
LHCandRHCandinStokesparametersI andV) weremade steadily. For all species the extreme velocities are found near
usinga 7-mascircularrestoringbeamandhada typicalnoise the centre of emission and there is no indication of rotation.
σ 7 mJy beam 1 in quiet channels. We fitted Gaussian BothH OandOHmainlineclumpshaveanaverageradiusof
rms − 2
≈
componentsto measure the propertiesof each patch of maser r 9auwithamaximumofalmost30au.
c
∼
emissionandgroupedcomponentsadjacentinpositionandve- TheinterleavingofOH mainlineandH O masersappears
2
locitytoformfeaturesthoughttorepresentphysicallydiscrete toconflictwiththeclassicmodelinwhichpenetrationofinter-
clumps. The individual maser components were mostly spa- stellarUVformsanOHshelloutsidetheH Oshell.Itcanbe
2
tially resolved, as were the features, allowing us to measured explainedinthecontextofthehighdensityoftheH Oclumps
2
theirangularsizesandlinewidths. mentionedinSection2.Thedust-gascollisionrateislowerat
MERLINobservationsmadein1993(Richardsetal.1999) lower density. If the OH comes from surrounding regions at
alsoshowedthatOHmainlineemissionappearedtooriginateat 1/50th the density, they are likely to be cooler and less effec-
asimilardistancefromthestarastheH Omasers.In1999we tivelyacceleratedsincebothheatingandexpansionaredriven
2
made observations in full polarization, to produce data cubes bythedustabsorptionofstellarradiation.
inStokes I, Q,U andV fromwhichwededucedthetotallin-
ear polarized flux P and polarization angle χ. The individual
3.2.Polarization
componentsandfeatureswerespatiallyunresolved.
MERLIN observations of H O masers made in 1994 Figs. 3 and 4 show the OH mainline circular and linear po-
2
(Richards et al. 1999) and 1999 have a similar resolution larisedemission.A1kms 1differencebetweenRHCandLHC
−
to the VLBI OH data. The H O maser shell is consistently peakscorrespondstoamagneticfieldBof0.17and0.28µTat
2
well-filled and shows strong acceleration. At each epoch the 1665 and 1667 MHz respectively (Davies 1974). We found7
thickshellhasasimilarsizeandcontains 100clumps,about possibleZeemanpairsintheEVNdataat1665MHzand2at
∼
halfofwhichcanbecross-identified.Maser theory(Cooke& 1667MHz,bylookingforfeaturesofoppositeStokesV which
Elitzur1985;Yatesetal.1997)suggeststhatattheinnershell coincidedinpositionandweretheclosestmatchesinvelocity.
boundary,55aufromthestar,thesehaveanumberdensityof Thesehadvelocityseparations(oftheStokesI peaks)ofmag-
n 5 1015 which is 50 the wind averageat that distance, nitudesbetween0.5–2.8kms 1,givingamagneticfieldcom-
−
∼ × ×
for a typical temperature T 1000 K. The integrated maser ponentparelleltothelineofsightofmagnitude0.1 B 0.8
emission appears almost sph∼erical using a logarithmic inten- µT. Six out of the 9 pairs showed the magnetic fiel≤d poki≤nting
sityscalebutifthebrightestcomponentsareselectedtheyare awayfromus(B >0).Wherethereweremultiplematcheswe
k
seen to lie in an ellipse elongated NE-SW in 1999. In 1994 comparedthepositionandvelocitypatternmadebythecompo-
the elongation was more nearly E-W. Hotspots in an NE-SW nentswithinthefeatures.Inonlyonecaseanalternativematch
ellipse were also detected in 1998 by Vlemmings et al. 2002 over12kms 1 waspossible,givingB = 3.4µT.Thethermal
−
usingtheVLBA,seeSection3.2. linewidth of OH at T = 300 500 Kkis 0.9–1.2 km s 1; our
−
−
A.M.S.Richardsetal.:Hiddenbipolarityinredsupergiantwinds 211
Fig.3. OH masers observed using VLBI. Circles and squares Fig.4.OHmasersobservedusingMERLIN,symbolscodedas
show total intensity emission at 1665 and 1667 MHz respec- inFig.3.Thelinesshowthelinearpolarizationvectors(thicker
tively. The darker gray symbols show components which are for1665MHz)forcomponentswith>25%linearpolarization.
morered-shiftedwithrespecttoV .The+and symbolsshow ThenumbersarethevaluesofB inµTinferredfromMERLIN
⋆
thesignofStokesV forcomponentswith > 2−5%circularpo- circularpolarisationmeasuremeknts.
larization.
magneticfieldvaluesfornarrowervelocitydifferencesmaybe Some Faraday rotation is present as shown by the change
in χ across individual features, typically 16 , which could be
overestimated(Elitzur1998).Weperformedasimilaranalysis ◦
producedbyaninternalfreeelectrondensityof 5 107m 3
of the MERLIN data and found 8 Zeeman pairs including 6 −
∼ ×
acrossan 18au-deepOHmasingclump(afractionalnumber
with B < 0.Therearealsoanumberofmaserfeaturegroups
∼
k densityof 10 6).IftheOHmasersaredistributedinapartly
whereStokesV isentirelyofthesamesign. −
∼
filledsphericalshell,thered-shiftedemissiontravels 100au
The mean magnetic field is B 0.2 µT and 0.3 µT
≥
k ≈ ≈ − furtherthroughtheinnerCSE whichcould,if inhomogenous,
for the EVN and MERLIN data respectively. There are some
convertlineartocircularpolarizationorproducedepolarization
similaritiese.g.themostred-shiftedpairisseeninalldatawith
averagedoveraMERLINbeam.
B < 0 andthe onlypairdetectedin theNNE limbisat1665
MkHzinbothdatasetswithB >0. Vlemmings et al. (2002) used the VLBA to measure
Significant linear polarizkation is only detected from the Zeeman splitting of the brightest H2O masers. This is only
near (blue-shifted) side of the shell and rarely exceeds 50%, a small fraction of the thermal line width as H2O is non-
strongerat1665MHz.Thedominantdirectionofpolarization paramagnetic,makinganalysismodel-dependent.Thededuced
vectorsχisE-WbuttherearesomeanomaliessuchasintheS magneticfieldcomponentB isintherange15 20µT.
k −
wherethevectorschangedirectionby 90 .Thevectorsmay
◦
∼
beparallelorperpendiculartothecomponentofBintheplane
3.3.BiconicaloutflowmodelofOHemission
ofthesky,B ,dependingonwhetherπorσmasercomponents
⊥
aredetected(Elitzur1998)andalsodependingontheangleof Theelongationofthe1612-MHzmasershell,whichrepresents
B to the line of sight. The high fraction of circular polariza- materialwhichleftthestarseveralcenturiesago,suggeststhat
tion relative to linear polarization could suggest that σ com- the OH axis is stable. The distributionof emission suggestsa
ponentsdominateand B isperpendiculartoχ, implyingthat biconicaloutflowsuchasinthemodelsofZijlstraetal.(2001).
⊥
themagneticfieldaxisliesalongthedirectionofmaserelonga- TheH Omaserscouldformacomplementarydistributionwith
2
tion.Howeverotherinterpretationsare possible (e.g.Deguchi an equatoriallyenhanced density surroudingthe polar axis of
&Watson1986)includingFaradayrotationordepolarization. the bicone.TheslightE-Welongationofthe H O masersob-
2
In any case it is likely that the 90 jumps in χ are caused servedin1994appearsmorecompatiblewiththispicturethan
◦
∼
bythe detectionof π componentsorby a small changein the the more NE-SW elongation seen in 1999. Projection effects
directionorstrengthofB,notbyafull 90 kink. may be confusing the issue and patience is needed to ensure
◦
∼
212 A.M.S.Richardsetal.:Hiddenbipolarityinredsupergiantwinds
thattheH Oelongationisnotchanceeffects.Over6years,the anism which still requires some some internal rotation to de-
2
projectedoutlineoftheH OmasershelloftheMiraUOrihas fineasymmetryaxis.Arelatedpossibilityisthatthepulsation
2
changedapparentorientation,probablyduetochancedistribu- modes and convection cells in the star create a preferred di-
tionofbrightmasers;severaldecadeswouldbeneededtosee rection for denser or less dense mass loss and a stronger or
ifthehotspotdistributionaroundSPerissystematic. weaker magnetic field is carried in these directions. Freytag
et al. (2002) develop such a model for α Ori without requir-
ingrotation.Asimilarmodelformass-lossinvolvingashock-
4. Polarizationinterpretation:complicationsand
compressedfieldfrozenintoclumpsattheirformationnearthe
outlook
stellar surface was suggested by Hartquist & Dyson (1997).
The magnitudeand directionof BmeasuredfromOH masers The magnetic field strength B √n. The OH-masing gas is
∝
shows no clear pattern nor direction and there are discrepan- 1/50ofthedensityoftheH Oclumpsatthesamedistance
2
∼
ciesbetweentheMERLINandEVNdata.Thismaybedueto fromthestar.Ifn R 2 thenforOHmasersatR = 180au, B
−
∝
falselymatchedZeemanpairsandamorerigorousanalysistak- wouldbe 1/15ofits valueforH O masersat90au, closer
2
∼
ingintoaccountsingle-polarizationfeaturesmaygivea fuller totheobservedstrengths.
picture.Thecombinationofvelocityandmagneticfieldgradi- The supergiant VX Sgr does appear to have an or-
entscanmeanthatonlyonesenseofpolarizationisamplified dered magnetic field measured from OH 1612 MHz masers
inaparticularclump(Cook1976). (Szymczak & Cohen 1997; Szymczak et al. 2001), the axis
The average magnetic field strength measured from OH ofwhichdefinesabiconicalregioncontainingalowerdensity
masers is 0.3 µT at a distance from the star of R 180 of H O masers (Murakawa et al. 2003). The full analysis of
2
∼ ≤
au;thelargestB valuefromareliablematchis0.8µT.Atour OH mainline masers observed by the EVN/Global VLBI and
k
adopteddistanceof2.3kpcthebrightestpartsoftheH Omaser byMERLINwillconfirmormodifythismodel.Themagnetic
2
shellareatR 90auwhereVlemmingsetal.(2003)estimate field configuration appears to evolve with time; OH masers
∼
B 15 20µTor 50 theOHvalue,yetforadipolefield aroundNML Cyg show an old shell with tangentialpolariza-
wkhe∼reB− R 3there∼shou×ldbeatmostafactorof8difference tionvectorssurroundingamorerecentbipolaroutflowparallel
−
∝
betweenthefieldstrengthintheH OandOHregions. toradialpolarizationvectors(Etoka&Diamond2004).Full3-
2
Thediscrepancycouldbeduetoaselectioneffect;onlythe D modelling of S Per is needed, if possible including further
strongestZeemansplittingofH Omaserscanbedetectedbut epochsofobservationtomeasurepropermotionsandexamine
2
a magnetic field of even 10 µT would produce a line split of thepersistenceofthemagneticfield.
59kms 1 forthe1665MHzOHmasers,greaterthanthetotal
−
velocityspanoftheCSE.Ifthemagneticfieldaxiswasparallel
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