Magnetic fields in planetary nebulae and post-AGB nebulae

0.56 MB·English

by  L. Sabin

Mon.Not.R.Astron.Soc.000,000–000(2006) Printed5February2008 (MNLATEXstylefilev2.2)
Magnetic fields in planetary nebulae and post-AGB nebulae
L. Sabin1,2, Albert A. Zijlstra1 and J. S. Greaves3
1JodrellBankCentreforAstrophysics,UniversityofManchester,P.O.Box88,ManchesterM601QD,UK
2InstitutodeAstrof´ısicadeCanarias,C/V´ıaLa´ctea,38205Laguna,Tenerife,CanaryIslands,Spain
3SchoolofPhysics&Astronomy,UniversityofSt.Andrews,NorthHaugh,StAndrewsKY169SS,Scotland,UK
7
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0 5February2008
2

n
a ABSTRACT
J
Magnetic fields are an important but largely unknown ingredientof planetary nebulae.
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Theyhavebeendetectedinoxygen-richAGBandpost-AGBstars,andmayplayaroleinthe

1 shapingoftheirnebulae.HerewepresentSCUBA sub-millimeterpolarimetricobservations
v of four bipolar planetary nebulae and post-AGB stars, including two oxygen-rich and two
4 carbon-richnebulae,todeterminethegeometryofthemagneticfieldbydustalignment.Three
5 of the foursources(NGC7027,NGC 6537and NGC 6302)presenta well-definedtoroidal
0 magneticfield orientedalongtheir equatorialtorusor disk. NGC 6302may also show field
1
linesalongthebipolaroutflow.CRL2688showsacomplexfieldstructure,wherepartofthe
0
field alignswith the torus, whilstan other partapproximatelyalignswith the polaroutflow.
7
Italsopresentsmarkedasymmetriesinitsmagneticstructure.NGC7027showsevidencefor
0
a disorganizedfield in the south-westcorner,where the SCUBA showsan indicationforan
/
h outflow.Thefindingsshowaclearcorrelationbetweenfieldorientationandnebularstructure.
p
- Keywords: stars:AGBandpost-AGB–stars:mass-loss–method:polarimetry
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v 1 INTRODUCTION interactionsmaybefoundin25–50percentofplanetarynebulae
i (Zijlstra2006),althoughvaluesof50–100percenthavealsobeen
X
Whether magnetic fields play a role in the shaping of planetary suggested(DeMarcoetal.2004;DeMarco&Moe2006).
r nebulae(PNe)isanopenquestion.Mostofthepost-AGBnebulae
a
appear elliptical, bipolar or even multi-polar (Balick&Frank Ontheotherhand,themagneticfieldmayactasa“squeezer”
2002). These morphologies are amplified by the interaction aroundthecentralstarofthePNandtherebygivethedustitsdi-
betweenaslowAGBwindwithafasterpost-AGBwind.However, rection(inthesenseoftheoutflow).Magneticfieldshavebeende-
this amplification still requires an initial asymmetry in the slow tectedaroundAGBstars(Vlemmingsetal.2006)andafewpost-
wind. This initial shaping has been mainly attributed to two AGB stars (Bainsetal. 2004) using radio observations of masers
possiblephenomena:binarityandmagneticfields. (H2O, SiO,OH). Such observations measure alocal value of the
strength of field, within masing high-density clumps, which may
Inthebinarymodel,aclosecompanionaffectsthemass-losing differ from the global field. The origin of the magnetic field is
AGB star via common envelope evolution (DeMarco&Moe unknown: a dynamo effect resulting from an interaction between
2006), mass transfer and/or tidal forces, and may lead to the a slow rotating envelope and a fast rotating core has been pro-
formation of an accretion disk around the companion star. The posed(Blackmanetal.2001).Asmagnetic fieldsarenow known
binary orbit provides a source of angular momentum which tobepresentintheAGBandthepost-AGBphase,theirimportance
get carried by the wind, and leads to an equatorial disk. The shouldnotberuledoutprematurely.
angular momentum loss by the stars may lead to a merger. The Greaves (2002) found evidence for dust alignment in two
model of shaping due to this binary interaction is quite popular carbon-rich objects, NGC 7027 and CRL2688 which are respec-
(Bond&Murdin 2000), (Ciardulloetal. 2005), partly because tivelyayoungandcompactbipolarPNandastronglybipolarproto-
of the presumed impossibility for a single star to supply the planetarynebula(PPN).ThiswasbasedonpolarimetricScuba850-
energy necessary to create a magnetic field strong enough for micron observations. The data suggests the presence of toroidal
its shaping (Soker 2005). Nevertheless, there is still a lack of collimatedmagnetic fields, aswould be required for theshaping.
observationalevidencefortheoccurrenceofclosebinarysystems Butthepresenceofsuchafieldwasnot conclusivelyproved, be-
duringtheAGBphase.Neithercompanions(Rieraetal.2003)nor causeof theveryfew detectedvectorsand limitedspatial resolu-
high orbital velocities of the AGB stars (Barnbaumetal. 1995) tion.Wepresent herenew 850-microns and thefirst450-microns
are detected in a sufficient amount to establish the role of the polarimetryofpost-AGBstars,whichallowsforbetterresolution
binarity as predominant (Mattetal. 2000). Evidence for binary andreducesangularsmearing.Inadditiontotheobjectsobserved
2 L. Sabinet al.
byGreaves(2002),wealsoobservedNGC6537 andNGC6302. 3 THERESULTS
Thesamplecontainstwocarbon-richandtwooxygen-richnebulae.
3.1 NGC6537
Weshowthepresenceofwell-alignedtoroidalfieldsinthreeofthe
nebulae. The fourth object shows indications for both a toroidal NGC6537isalsoknownastheRedSpidernebula.Itisabipolar
field and one aligned with the polar outflow. We conclude that planetarynebulawithaveryhotcentralstar(1.5–2.5×105K).The
toroidal fields may be common in bipolar PNe, and could play a nebula suffers extinction by circumstellar dust. This extinction is
roleintheshapingofthenebulae. localized mainly in a compact circumstellar shell. An extinction
map (Matsuuraetal. 2005) reveals a compact dust shell with a
roughlysphericalinnerradiusofabout3arcsec,withaminimum
towardsthecentralstar(Matsuuraetal.2005;Cuestaetal.1995).
2 OBSERVATIONS
The polar outflows (traced by high velocity winds of about
ThepolarimetricdatahavebeenobtainedMay10th2005,withthe 300km/s: Corradi&Schwarz 1993) extend 2 arcminutes along
polarimeterontheSub-millimeterCommon-UserBolometerArray the NE-SW direction. The Scuba 850-µm continuum map (Fig.
(SCUBA),attheJCMT.Theinstrument(nowdecommissioned)is 1) shows elongation perpendicular to the outflow direction, with
describedinHollandetal.(1999).TheJCMTbeamsizeis15arc- amajor-axisdiameterof approximately20arcsec.Theextinction
secat850µmand8arcsecat450µm. mapisnow seen torepresent theinner edge of amore extended,
SCUBAcontainstwoarraysofbolometricdetectors,covering and possibly toroidal, structure. The equatorial plane may be
afieldofviewof2.3′indiameter.The850-µmarrayhas37individ- oriented a little closer to the EW direction, based on the Scuba
ualdetectors,andthe450-µmarrayhas91detectors.Thetwoar- map.
raysareusedsimultaneously.Thespatialresolutionofthedetector
is7.5”at450µm,and14”at850µm.Thegapsbetweenthedetec- The best polarimetric results obtained with SCUBA are ob-
torsarecoveredbymovingthetelescopeinsub-pixelsteps,inthe tainedat850µm.Theconsistentorientationofthepolarizationvec-
so-calledjiggle-mapmode.Thestepsizeneedstobeoptimizedfor tors shows that the magnetic field (hereafter B~) has a dominant
the wavelength used. The polarimeter (Greavesetal. 2003) mea- directionalong theequatorial plane, approximately perpendicular
sures the linear polarization by rotating the half-waveplate in 16 totheoutflowdirection.Thedustalignmentisthereforedirectedin
steps.Weusedthisincombinationwithjigglemapmode.Achop- thesamesenseastheoutflow.Thelengthofthe18differentvectors
throwof45arcsecwasused. doesnotshowlargevariations,withadegreeofpolarizationvary-
Good photometric images cannot be obtained at both wave- ingfrom8to14%,suggestingaconsistentmagneticfield.More-
lengths simultaneously. When the jiggle pattern is optimized one overtheabsenceofsmallerpolarizationvectorstowardthecenter
wave-band, the simultaneous image obtained inthe other band is of the nebula indicates that there is no change in geometry of B~
under-sampled. The size of the pixels in the final image is set towards the core (Greaves 2002). This supports a location some
during the data reduction with the polarimetric package of Star- distancefromthestar(i.e.inadetachedshell),sinceotherwiseav-
link.Forajigglepatterncorrespondingtoa450µmmeasurement, eraging of vectors in different directions within the JCMT beam
the pixel spacing becomes 3 arcsec and for 850µm it is 6 arc- would reduce the detected net polarization at the central position
sec.Thereductionofinfraredpolarimetryisdiscussedindetailby (beamdepolarization).
Hildebrandetal.(2000). These observations indicate the presence of a consistent
The instrumental polarization (IP) of each detector has to toroidalmagneticfield,locatedalongtheequatorialplaneofNGC
be removed in order to have a correct polarization calibration. A 6537,inacircumstellartorus.Comparedtothesizeoftheoutflow
newIPcalibration,providedonsite,wasused:thiscontainsare- lobes,thefieldislocatedrelativelyclosetothestar.Thepresence
measurement of the central detector. For the other detectors, an ofaB~-fieldhadalreadybeensuspected, basedontheoccurrence
older IP calibration was used. The IP should not vary too much offilamentsnearthecentralstar(Huggins&Manley2005).
for detectors close to the center of the array. Some tests done to TheextinctionmapobtainedbyMatsuuraetal.(2005)(Fig.1-
see the importance of the instrumental polarization (by changing Bottompanel)showsanasymmetry,inthattheextinctionishigher
itsvaluebythesizeoftheerrorforexample)andifitcouldaffect onthewesternside(>∼2magversus<∼1.6magtowardstheeast).
thepolarizationvectorslength,showedthatinourcase,theIPwas This asymmetry is also seen in the Scuba data, with a larger ex-
small(at850µm:∼1.20%±0.25%andat450µm:∼3.25%±0.25% tensionandmorepolarizationvectorsonthisside.Thisisafurther
)anddidn’tplayanyroleinmodifyingthevectors.Theresulting indicationthatthemagneticfieldislocatedwithinthedetacheddust
polarimetricimageswerecheckedfordifferentpixelbinning,from shell.
1×1to10×10,andwedidn’tfindanysignificantchanges. Thereisnostrongindicationformagneticfieldsalongthespi-
Thelinearpolarizationismeasuredasapercentagepolariza- derlobes.Thereisaslighttrendforthevectorstocurve,butthis
tion, and a direction. The polarization is typically caused by the iscaused byonly afew of thevectorsandwould need confirma-
alignmentofspinningdustgrains,withtheirlongaxisperpendicu- tion.Wecannotstatewithconfidencewhetherthelobesalsocarry
lartothelocalmagneticfield(Greavesetal.1999).Thus,themea- amagneticfield.
sured angle of polarization is 90 degrees rotated with respect to
themagneticfield.Thedegreeofpolarizationdoesnotgivedirect
informationonthestrengthofthemagneticfield.
3.2 NGC7027
Weobservedfourtargets:NGC6537,CRL2688,NGC6302
andNGC7027.Theywereobservedinjigglemapobservingmode The450µmjigglemapoftheyoung planetarynebulaNGC7027
atbothwave-bands,450µmand850µm.Foreachobject,themean (Fig. 2) and its near environment covers a field of about 40×36
directionandangleofpolarizationarelistedinTable1. arcsec2.Thecentralstarissurroundedbyanionizedareaof∼283
Thefiguresbelowshowthedirectionofthemagneticfield,i.e. arcsec2, which is in turn enclosed by a thin atomic and molecu-
thevectorsareperpendiculartothedirectionofthegrainalignment. larlayerthat isseenforinstanceintheH2 emissionobserved by
MagneticfieldsinPNe 3
Figure1.Scuba850µmresultsonNGC6537.NorthonthetopandEaston Figure2.Scuba450µmresultsonNGC7027.NorthonthetopandEast
theleft.Theaxesgivetheimagescaleinarcseconds.Toppanel:the850µm ontheleft. Theaxesgivetheimagescaleinarcseconds. Toppanel: The
continuummapofNGC6537,withcontoursat1%,2%,5%and10%of 450µmcontinuummapofNGC7027.Contoursaresetfrom1to5%and
thepeak.Middlepanel:Magneticfieldorientation.Thegeneraloutflowdi- 10%of the peak. Middle panel: Magnetic field distribution. Thegeneral
rectionofthenebulaisindicatedbyAandtheequatorialplanebyB.The outflowdirectionisindicatedbyAandtheequatorialplanebyB.Thepo-
polarizationvectorscaleontheleftissetat10%.Bottompanel:Extinction larization vectorscale(showingthedegreeofpolarization) issetat10%.
mappresentedbyMatsuuraetal.(2005).Thehighestlevelsofextinction Bottompanel:TheScubapolarizationvectorsareshownonancontinuum
occurat∼4arcsecfromthecentralstarwithE(Hβ−Hα)>2mag,co- subtractedH2(color-inverted)mapofNGC7027(NorthonthetopandEast
incidentwiththedustemissioninthe850µmmap.Thepolarizationvector ontheleft).Thefieldismostlydirectedalongtheequatorialplane.Thecen-
scaleissetat10%.Themagneticfieldwhichcoincideswiththeareaofin- tralregionshowsmuchreducedpolarizationandthefieldorientationdiffers
ternalextinction,ismostlyalignedalongtheequatorialplane,indicatinga ontheextremewesternside.
toroidalfield.Thedustalignmentisperpendiculartothevectorsdisplayed.
4 L. Sabinet al.
Coxetal.(2002).Athindarkringdelineatestheequatorial plane theemissionhereisfaintandtheuncertaintiesonthepolarization
in optical images. The structure is better seen in the HST image vectorsarelarger.
(Fig.2-bottom),whichshowsboththesurroundingmolecularlayer Thecomplicatedmagneticmorphologymakesitlikelythatthe
(inH2)andtheinnerionizedregion.Latteretal.(2000)showsthat dynamicshaveshapedsignificantpartsofthefield,ratherthanthe
theNWlobeisblue-shifted(closer)andtheSWlobeisred-shifted. magneticfieldshapingthenebula.Thestructuremayshowthesu-
The dominant direction of themagnetic fieldcoincides with perpositionoftwocomponentsovermostoftheareaofthesource.
the equatorial plane. But this behaviour is mainly seen on the Thelackofpolarizationinthecentresuggeststhatthebrightcore
North-East side while the south-west part seems to be disturbed: isnotpolarized,oritspolarizationisaveragedoutovertheJCMT
themagneticfieldmaybe“broken”.Thedegreeofpolarizationis resolution.Theoutermostfieldvectorsinthelongitudinaldirection
8.9%±0.9%)towardstheNEdirection(orlobe)and7.6%±1.3%) are along the outflow, and this may indicate afield carried along
towardstheSW.Thedegreeofpolarisationismuchreducedinthe bytheoutflow.Thisismostapparentinthenorthwhenlookingat
centerofthenebulaandlacksacoherentdirectionhere.Thiseffect theoutervectorsdirectioninthefigure3-bottom-left,inthesouth
wasalsonotedbyGreaves(2002)andmayindicatethatcoherence the outer vectors point towards the corresponding outflow so we
islostintheionizedregion. assumethatthemagneticfieldmaybecarriedbythisoutflow.
Thegrainalignment,whichisperpendiculartothedirectionof
themagneticfield,isinthedirectionoftheoutflowsonthewestern
3.3 CRL2688
side.Ontheeasternside,thegrainalignmentistoroidal.
Theproto-planetarynebulaCRL2688(orEggNebula)isabright High resolution molecular images in the literature include
carbon-richbipolarobjectcharacterizedbytwopairsofsearchlight IRAMCOJ=2-1data(Coxetal.2000)andHSTinfraredH2 im-
beams superposed on a reflection nebula. The origin of the light ages (Sahaietal. 1998). The continuum-subtracted H2 image in
beams has been suggested to be a very close stellar companion Fig. 4, shows multiple jets, located both in the equatorial plane
(Sahaietal.1998;Kastner&Soker2004),orthepresenceofdust and towards thepolar directions. TheCO datashow these jetsto
layersreflectingthelightofthecentralstar(Gotoetal.2002).The bethe tipof flowsoriginating much closer to thestar. Thesejets
bipolarreflectionnebulashowsadarkequatoriallanewherealarge allfallwithintheScuba850-µmcore,indicatingthatindeedstruc-
amountofdustmaybypresent. tureispresent onscalesmuchsmallerthantheJCMTbeamsize.
The850-µmispresentedinFig.3-Toppanels.Itshowselon- Thus,beamdilutionandbeam-depolarizationofthemagneticfield
gationinthedirectionofthepolaroutflows.Theequatorialextent islikely.
islessbutatthelowestcontourthetorusismoreextendedonthe WenotethattheelongationvisibleintheScubaimage,onthe
easternside.Theextentis∼60”×40”althoughitisnotclearpre- easternside,isinthesamedirectionasthelargestmolecularsub-
ciselywheretheemissionends.Thebrightcoreat850µmshowsa jetE2inFig.4,orA1inCoxetal.(2000)(buttheScubastructure
FWHMof16×14arcsec2,atapositionangleof25degreeswhich ismuchlarger).Thepolarizationvectorsseemunaffected.Onthe
isinthesamedirectionastheoutflow.Wecouldnotmeasurethe westernside,boththemolecular andtheScubaimagesindicatea
coreelongation at 450µm due totheunder-sampling. Thespatial smaller extent of the envelope. The polar outflows as seen in the
resolution is insufficient to separate the two light beams, but the molecularlinesdonotshowthelightbeams,butinsteadshowrel-
widthoftheemissionsuggeststhatthedusttracesthelargerlobes ativelywell-collimatedlobes. Thelobesarebrighter inthenorth.
whichthelightbeamsilluminate,andthatthesebeamsthemselves Thepolar lobesintheScubaimageareconsistent withthis,both
arenotpresentinthefar-infrareddata.(Theelongationofthecore indirectionandinbrightness,butagainaremuchlargerthanseen
isinfactalongthelineofoneofthetwobeamsonly.) in themolecules. The molecular emission shows the wind-blown
CRL2688presentsthehighestnumberofpolarizationvectors cavities,whiletheScubaemissionarisesinthesurroundingshells.
inoursample.Forthisnebula,theunder-sampled450µmmapgives
someadditionalandusefulindications(evenifweobservealackof
3.4 NGC6302
vectorsduetothisunder-sampling).Thepolarizationvectorscover
the full nebular extent (seen at both wavelengths), as they do in Thisoxygen-richplanetarynebula(Pottasch&Beintema1999)is
NGC7027.Thedegreeofpolarizationisnotuniformandstrongly aproto-typicalbutterfly-typenebula.Itshowstwoextendedlobes
decreasesnearthecenter.Thisphenomenonismorevisibleinthe that result from a bipolar outflow, with a dark lane in the center.
850µm map, which suggests that beam depolarization may play It has been studied by Meaburn&Walsh (1980), Meaburnetal.
a role. At 850µm, the mean value of polarization of the region (2005),Matsuuraetal.(2005)andCasassusetal.(2000).
containingthebipolarlobesisabout3.2%,theouterregionhasa The Scuba observations were optimized for 450-µm. They
meanvalueof8.8%,andtheregionofthedarklaneshowsamean show a bright core, of FWHM 14 ×12 arcsec2 (larger than the
degreeofpolarizationof1.4%. beam),withthelongaxisinthenorth-southdirection,withanex-
Ifwedrawalinepassingthroughthenebulainalongitudinal tensiontowardsthesouth-southeast (Fig.5).Weunderlinethefact
direction(NNE-SSWfig.3bottom-leftpanel),wecanseedistinct that the long horizontal linein theeastern part of thenebula and
behavioursofthemagneticfieldoneithersideofthisline:onthe thelongvertical lineintheSouth-West (Fig.5bottom panel), are
easternside,theorientationofB~ ismainlyinthedirectionofthis artifactsfromtheHSTimageandnotScubapolarizationvectors.
line,whileonthewesternside,themagneticfieldappearsperpen- Thearcsseenat25arcsecarelikelypartofthediffractionpattern
diculartoit. fromtheverybrightcore.Thepolarimetricdataobtainedat450µm
The450-µmmap(Fig.3-bottom-rightpanel)showsthemag- showsonlyfivepolarizationvectors.Theydonotlineupwitheither
neticfieldathigherresolution,confirmingthebimodaldistribution. thedarklaneortheoutflow.However,theyarefairlywellaligned
Thismapisunder-sampledandonlysomepositionsinthenebula with the ellipsoidal radio source in the center of the nebula. The
are covered. The map gives a suggestion of a superposition of a radionebulashowstheinnerionizedregion,confinedbythedense
toroidal field and one aligned with the polar outflows. The field torus:theelongationisperpendiculartothetorus.
becomeslessorderedtowardsthetipoftheoutflowdirection,but The difference in position angle of the inner torus with the
MagneticfieldsinPNe 5
Figure3.Scuba850and450µmresultsonCRL2688.NorthonthetopandEastontheleft.Theaxesgivetheimagescaleinarcseconds.Topleftpanel:The
850µmmapofCRL2688,withcontoursfrom1to5%and10%ofthepeak.Toprightpanel:The850-µmvectorpolarizationshowingthemagneticfield
orientation.ThegeneraloutflowdirectionisindicatedbyAandtheequatorialplanebyB.Thepolarizationvectorscale(showingthedegreeofpolarization)
is setat10%. Bottom panels: HSTimages (WFPC2,filter F606W)withthe overlaid magnetic field corresponding tothe 850µm map(left) and450µm
map(right).Thedustcontinuumiselongatedalongtheoutflows.Thefieldshowsacomplicatedstructurewithcomponentsalongtheoutflowandalongthe
equatorialplane.Thepolarizationvectorscale(showingthedegreeofpolarization)issetat20%forbothmaps.
dark lane is interpreted as a warped disk (Matsuuraetal. 2005). 4 DISCUSSIONANDCONCLUSION
Furtherfromthecentertheoutflowshavedifferentpositionangles,
and eventually become east-west in the outer bipolar lobes. The Thesub-millimeterobservationsofthefourpost-AGBobjectsre-
450-µmpolarizationindicatesthatthemagneticfieldmaybeori- veal new information regarding the distribution of the magnetic
entedinthedirectionoftheinneroutflow. fields,thedustemissionandthelinkbetweenthetwocomponents.
Extended dust emission was seen for all four nebulae. For
CRL 2688, the large polar lobes have been detected. The orien-
tationofitspolar lobesisalongonlyoneofthetwolightbeams.
Theequatorialemissionisalsoextended,andshowsanasymmetry
Wehavenodataformagneticfieldselsewhereintheoutflows whichiscorrelated withtheappearance of the(much more com-
evenat850µm. pact)equatorialjets.Thesejetsoriginateclosetothestaranditis
6 L. Sabinet al.
Figure4.Continuum-subtractedH2(HST)imageshowingthejetsinCRL
2688.[NorthatthetopandEastattheleft].TheyarenamedasinSahaietal
(1998).Thejetsarealllocatedwithinthecompact850-µmsource,andthe
someperturbationsseenintheScubadatacanberelatedtothe“sub-jets”
E2orA1.
Name Band(µm) Deg(%) Angle(deg)
NGC6537 850 11.2±2.2 26.5±5.7
NGC6302 450 11.4±1.6 32.7±4.6
NGC7027 450 8.2±0.9 -18.5±3.7
CRL2688 850 6.8±1.0 -22.6±4.5
Table1.MeanvaluesofthepolarimetricparametersforthePost-AGBob-
jects,Deg:degreeofpolarization,Angle:positionangleofthepolarization.
Thesevaluesconcernthemainbandstudiedforeachnebula.
Figure5.Scuba450µmresultsonNGC6302.NorthonthetopandEast
ontheleft.Toppanel:Themapshowsthemagneticfieldorientationonthe
unlikelythattheyareaffectedbystructuresatmuchlargerscales.
planetary nebula withcontours from1to5%, 10%,20%and25%ofthe
Instead, thejetsmay affect thedust emission through heating. In
peak.Thepolarizationvectorscale(showingthedegreeofpolarization)is
NGC 6537, an asymmetry in the dust correlates with differences setat20%.Notethatthearcsseen20arcsecoutarepartofthebeamside
intheextinctionmap,suggestingthatheretheScubamapshowsa lobes oftheverybrightcore. Bottompanel: 450µm ScubamapofNGC
trueasymmetryinthedustdistribution. 6302overlaidontheF656NbandHSTimage(WFPC2).Northonthetop
The presence of detectable polarization shows that at least andEastontheleft.Theexternalhexagonalcontoursdonotbelongtothe
someofthedustgrainsarenotspherical. starbutarerepresentativeofthebolometers.ThemainEasternoutflowseen
onthesubmillimeterimagecoincideswiththeoneobservedwiththeHST.
Notethatthearcsseen25arcsecoutarepartofthebeamsidelobesand
4.1 Targets thetwolonglines(horizontalandvertical)onbothsidesofthenebula,are
artifactsfromtheHSTimageandnotpolarizationvectors.
Polarizationisdetectedforallfournebulae.Thisinitselfsuggests
thatmagneticfieldsarecommonforthesetypesofobjects.Itisthus
importanttodiscusswhattypesofobjectswereselected. nebulae have had time to expand significantly. Such fast evolu-
First, the four targets are all bipolar nebulae. The least pro- tionischaracteristicforhigh-masscentralstars.Thecoolerobject
nounced morphology is shown by NGC 7027, but even here the (CRL2688)mayhavealowermass,butthefactthatithasbecome
molecularmapsclearlyshowtheunderlyingbipolarstructure.We a carbon star suggests it still has a relatively higher initial mass
donothaveinformationontheexistenceofmagneticfieldsinless (Mi >∼ 3M⊙),aslower-massstarsexperienceinsufficientdredge-
bipolar(e.g.ellipticalorspherical)nebulae. upo to become carbon rich. For NGC6302, aprogenitor mass of
Second, although three of the nebulae have very hot central 4–5M⊙ hasbeensuggested,basedonthemassofthecircumstel-
starsandionizedcores,allhaveextended molecularenvelopes. It larenvelope(Matsuuraetal.2005).Thepreciseinitialmassesare
isarguedabovethatthedetectedfieldsarelocatedinthemolecu- notwelldetermined,buthighmassprogenitorsappearlikelyforall
laroratomicregions.Thepresenceofmolecularenvelopesaround fourobjects.
ionizingstarsindicatesverydensenebulae,andthereforeveryhigh Thus,theresultofthesurveycanbeinterpretedthatmagnetic
masslossratesontheAGB. fields tend to be present for high mass progenitors evolving into
Third,thedensenebulaewithrelativelysmallionizedregions bipolarnebulae,andthatthefieldsaredetectablewhilethenebulae
implythattheirstarshaveevolvedtohightemperaturesbeforethe stillhavemolecularenvelopes.
MagneticfieldsinPNe 7
4.2 Fieldlocationandorientation areseenonlyinirradiatedregions.Thecontinuumradiationfrom
thedustcomponentcontainingthePAHsisonlyseenatshortwave-
Overall, we see fields aligned with the polar directions and/or
lengths(warmdust)andcontributesverylittletothesub-mmflux
toroidalfieldsintheequatorialplane.Assumingthatthedustinthe
(Kemperetal.2002;Matsuuraetal.2005).Thus,thepolarization
polarflowsoriginatesfromthedustreservoirinthediskortorus,
signalisseenintheoxygen-richgrains,notthePAHs.
thissuggeststhatthefieldalsooriginatesthere.Thealignmentwith
Iron needles could be considered as carriersof the polariza-
thepolarflowisinterpretedthatthegascarriesthefieldwithit.
tion:inO-richstars,someormostoftheironisincorporatedinthe
NGC6302istheonlysourceinoursamplewithoutevidence
silicategrains(olivinesandpyroxenes:Ferrarotti&Gail2001),but
foratoroidalfield.Thisnebulahasacomplicatedmorphologywith
for a C/O ratio close to and above unity, solid iron and FeSi be-
awarpeddiskandthenebulaisorientedat45degreeswithrespect
comemoreimportant(e.g.,Ferrarotti&Gail2002).Metallic,non-
totheequatorial dustlane.Itisthereforedifficulttoascertainthe
spherical iron grains may contribute to the NIR opacity in high
alignment of the field with any particular morphological compo-
mass-lossOH/IRstars(Kemperetal.2002).
nent.However,alignmentwiththeradiocoreseemsmostlikely.
The other nebulae show evidence for toroidal field compo-
nents.Wethereforesuggestthattheinitialconfigurationofthefield 4.4 Comparisonlessevolved/moreevolvednebulae
istoroidal,andislocatedintheequatorialtorus.
The polarization in all cases appears to be seen in the neu- Theextentofthenebulaeshowsthattheobjectsdifferinage.The
tral/molecular regions. Towards the central, ionized regions, the nebulaeofCRL2688(Post-AGB)andNGC7027(youngPN)are
percentageoflinearpolarizationismuchreduced.Thismayinpart youngerthanNGC6537(PN)andNGC6302(PN).(Thelobesof
bebecauseofthebeamaveragingoverregionsofdifferentpolar- thelastobjectextendtoover2parsecacross.)
izations, but the ionized regions may also be expected to lose a Our submillimeter polarimetric data show that the magnetic
directionalfield(non-constancyofB~).Thereasonfortheabsence fieldisbetterorganized for theolder nebulae. Assuming anorig-
ofadetectablemagneticfieldintheionizedregioncanalsobeex- inally toroidal field, later carried by the bipolar flows, the best
plainedbythelackofdust(soB~ isnotcarried).ForNGC6302,the organized field is expected if one component dominates. The
fieldis aligned withthe ionized core but is likely located around bipolar flows are most massive early in the post-AGB evolution
ratherthanwithinthecore. (Bujarrabaletal.2001).Astheydiminish,theremainingfieldmay
Polarizationwillnotbedetectedifthefieldisorientedalong becomelessconfused.However,evolvingdustcharacteristicsmay
thelineofsight.Thisisexpectedforatoroidalfieldsnearthetan- alsoplayarole:dustgrainsbecomelargerinolderdisks,andmay
gentialpointsofthetorus.ThismaybepresentespeciallyinCRL showlesspolarizationasaresult.Itisnotpossiblewiththecurrent
2688,wheretwocomponentsarefoundatdifferentlocations. sampletoseparateevolutionaryandchemicaleffects,butbothare
expectedtooccur.
4.3 Chemistry
4.5 Fieldstrengthandorigin
The sample includes two oxygen-rich nebulae (NGC 6537 and
The detected polarization contains information on the direction
NGC6302, both PNewithhot central stars)and twocarbon-rich
of the field, but not its strength. A method to obtain the field
nebulae (NGC 7027 and CRL 2688: the latter isthe cooler post-
strength out of fluctuations in polarization was suggested by
AGBnebulainoursample).Thissampleistoosmalltodrawgen-
Chandrasekhar&Fermi(1953).Applyingthismethodtoourdata
eral conclusions. However, we point out the following points of
leadstofieldstrengthsoftheorderofmG.Thisequationassumes
interest,forfuturestudy.
thatthemagneticfielddispersioniscausedbyAlfvenwaves.The
• TheO-richPNebothpresentcollimatedmagneticfieldscon- applicabilitytooursourcesmaybeindoubt,asthedispersionmay
centratednearthecentralstar.Inbothcases,wehaveanorganized bedominatedstreamingmotions.
magneticfieldinthecentralregion,resistanttodistortioneffects, In neutral-dominated media, Alfven waves contribute if the
butdonotdetectmoredistantfields. collisiontimesbetweenionsandneutralsareshorterthantheperiod
• The two carbon-rich nebulae show extended fields covering oftheAlfvenwave.Thisisthecaseforwaveslongerthan
theirentirenebula.Inbothnebulae,themagneticfieldismoredis-
organizedthaninthetwoO-richstars. 0.3 B 104 3/2 10−7
λ > (1)
A (cid:18)pc(cid:19)(cid:16)0.1mG(cid:17)(cid:18) n (cid:19) (cid:18) fi (cid:19)
The question whether there is a chemistry-related origin to
these behaviours is valid, but cannot be answered with the cur- wherenistheparticledensitypercm3,andf isthefractionalion-
i
rentsample.Dustchemistrycouldplayarole.First,carbongrains ization(Hildebrand1996).Foroursources,f maybehighexcept
i
tendtobesmallerthansilicategrains.Largergrainsmaybemore inthedensestregionsofthetori.Assumingatypicalsourcesizeof
spherical,whichwouldgiveless(orno)polarization.Bothoxygen- 0.05pc,andafieldstrengthofamG,Alfvenwavesmaycontribute
richnebulaeshowevidenceforcrystallineice(Molsteretal.2002) iff >10−4.Thisisaplausiblevaluefortheoutflows.
i
(strongestinNGC6302),indicatingcoatingsonthegrains.Second, PreviousobservationsofOHZeemansplittinghasshownfield
amorphouscarbongrainsmaybeintrinsicallynon-spherical,build strengths in OH/IR stars and post-AGB stars of a few mG at
upfromgraphiticcarbonsheets. 3×1015 –2×1016cmfromthestars(Bainsetal.2004,2003).
For the O-rich nebulae, it is possible that only the hotter Muchlargervalueshavebeeninferredfromwatermasers:several
dust,abovetheiceevaporationtemperature,leadstonon-spherical Gaussat∼2×1014cm(Vlemmingsetal.2005).Thedifferenceis
grains and a polarized signal. Thus, a difference in polarization consistentwithadipolefield,whichgivesanr−3dependence,and
doesnotnecessaryimplyadifferenceinmagneticfieldstructure. lessconsistentwithasolar-typefieldr−2.Watermaserstracehigh
BothO-richnebulaeshowevidenceforweakPAHemissionin densityclumpsandthemeasuredfieldstrengthsmaynotbefully
theircentralregions.TheoriginsofthePAHsisnotclear,butthey representativeofthesurroundingareas.
8 L. Sabinet al.
Figure6..LocationoftheScubadustemission,comparedtotheHSTimages(contours).WepresentrespectivelyNGC6537,NGC7027andCRL2688.
NGC6302isshowninFig.5(bottompanel).
Assuming we look at typical distances of 5 arcsec at 1 kpc, icant factor inthefurther evolutionof thenebulatowards thePN
ourmeasurementsareat∼5×1016cmfromthestars.Compared phase.
totheOHandwatermasers,onewouldexpect afieldof1mGor
lessatthisdistance.Thissuggestthatthedirectionchangesinthe
fieldareduetostreamingmotions,ratherthanAlfvenwaves.
The grains become aligned with the magnetic field with the
Davis-Greenstein mechanism. The time scale for this to occur is
(Kruegel2003,Ch.11)
trel∝B−2 4.7 Summary
and, for fields of the order of a few mG, is typically 106yr. The Wepresent the discovery of magnetic fieldsin four bipolar post-
ageofthenebulaeis∼ 104yr.Forthistimescale,thealignment AGB stars (NGC 6537, NGC 7027, NGC 6302 and CRL 2688).
ofthedustwiththemagneticfieldrequiresfieldsinexcessofthose ThisconfirmstheearlierworkofGreaves(2002)fortwoofthese.
seenfromOHmaseremission.Thealignmentisthereforelikelyto Thefieldsaremappedathighresolution(forsub-mm),usingeither
originateclosetothecentralstar,atr<1015cm,andismaintained 450or850µm.Thesub-mmemissiontracestheextendedemission.
whilethenebulaexpands. InCRL2688, wefindevidenceforthepolarlobeswhichcontain
thewell-knownsearchbeams.
All objects show polarization indicative of grain alignment
bymagneticfields.Toroidalfieldsarefoundforthreeobjects,and
4.6 Evolution
poloidal fieldsfortwo(CRL2688showsevidence forboth).The
For OH/IR stars, evidence for dipole fields has been presented alignment of thefieldwithnebula isleastcertain for NGC6302,
by, e.g., Vlemmingsetal. (2005). For more evolved stars, in the where the nebula shows a multipolar structure. Our results sug-
early post-AGB evolution, the presence of both toroidal (IRAS gests that magnetic fields are common in these types of targets:
20406+2953) and poloidal (OH17.7-2.0) fields are inferred from bipolarnebulaewithintermediate-massprogenitors.Thefieldsare
OHobservations(Bainsetal.2004,2003).Thepoloidalfieldsarise longlivedastheyareobservedoverdifferentevolutionarystages.
fromastretcheddipole.Fortheevenlaterevolutionarystagestud- The data also show evidence for elongated grains. The po-
iedhere,astrongtoroidalfieldcomponentisfound,combinedwith larizationisstrongerandmoreextendedforthecarbon-richnebu-
apoloidalfield. lae.Thismayshowthatamorphouscarbongrainsareintrinsically
Atoroidalfieldcanformoutofadipolefieldbyrotation.This non-spherical,e.g.thesheet-likestructureofgraphite.Theoxygen-
alreadymakesitlikelythattheformationofthetorusandthefor- richnebulaeshowpolarizationonlyintheircentralregions.Thisis
mation of the toroidal field are related. A binary companion can interpretedin that thedust grainsin thetorii arelarger and more
bethesourceof therequiredangular momentum. Rotationofthe spherical.
staritselfislesslikely,asitsangularmomentumcanbeexpected Thepoloidalfieldsinthepolarflowssuggestthatherethefield
to be lost early on in the mass loss. A binary companion, on the iscarriedalongbytheflowandthattheseflowsarenot magneti-
otherhand, candeposit itsangular momentumduringthetimeof callyconfined.Thetoroidalcomponentsinthetorusmaybemore
thepeaksuperwind,attheendoftheAGBmassloss. important,forthedynamics.AsthefieldsinAGBstarsaredipole-
ThefieldsdetectedviaOHandH2Omasershavebeenclaimed like,thetoroidalfieldstructureappearstobealaterphaseofevo-
to be strong enough to dominate the dynamics of the nebula lution. Wesuggest that this transitionoccurs when theequatorial
(Bainsetal. 2004; Vlemmingsetal. 2005). Soker (2006) argues fieldiswound upthrough theinteractionwithacompanion. Fol-
thatfieldsofthismagnitudecannotbegeneratedbythestaritself, lowing Soker (2006), we suggest that the original shaping agent
andshouldbeattributedtoacompanion. Basedonthis,thebasic isabinarycompanion. Oncethefieldhasbeenwoundup,itmay
shaping mechanism of the nebula is found in binarity. However, howeverbecomedynamicallyimportantforthesubsequentnebular
oncethestrongfieldshavebeengenerated,theywouldbeasignif- evolution.
MagneticfieldsinPNe 9
ACKNOWLEDGEMENTS KastnerJ.H.,SokerN.,2004,inASPConf.Ser.313:Asymmetri-
calPlanetaryNebulaeIII:Winds,StructureandtheThunderbird
L.S.thanks Mikako Matsuura for many useful comments andfor
TheEggNebula(AFGL2688):DeepeningEnigma.pp57–+
providingthereducedHSTdata.TheJamesClerkMaxwellTele-
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scopeisoperatedbyTheJointAstronomyCentreonbehalfofthe
A.G.G.M.,2002,A&A,384,585
Particle Physics and Astronomy Research Council of the United
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Kingdom, the Netherlands Organisation for Scientific Research,
394,679
andtheNationalResearchCouncilofCanada.
Kruegel E.,2003, The physics of interstellar dust. IoP Seriesin
astronomyandastrophysics(IoP:Bristol,UK)
LatterW.B.,DayalA.,BiegingJ.H.,MeakinC.,HoraJ.L.,Kelly
D.M.,TielensA.G.G.M.,2000,ApJ,539,783
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