HST/NICMOS Imaging Polarimetry of Proto-Planetary Nebulae II: Macro-morphology of the Dust Shell Structure via Polarized Light

0.92 MB·English

by  Toshiya Ueta

TOAPPEARINTHEASTRONOMICALJOURNAL
PreprinttypesetusingLATEXstyleemulateapjv.10/09/06
HST/NICMOSIMAGINGPOLARIMETRYOFPROTO-PLANETARYNEBULAEII:
MACRO-MORPHOLOGYOFTHEDUSTSHELLSTRUCTUREVIAPOLARIZEDLIGHT
TOSHIYAUETA1,KOJIMURAKAWA2,MARGARETMEIXNER3
ToappearintheAstronomicalJournal
ABSTRACT
The structure of the dusty circumstellar envelopes (CSEs) of proto-planetary nebulae (PPNs) reveals the
mass-loss history of these sourcesand how such historiesmay differ for elliptical(SOLE) and bipolar (DU-
PLEX)PPNs. TostudythePPNstructuresviadust-scatteredlinearlypolarizedstarlight,wehavecompiledthe
imaging-polarimetricdataforall18evolvedstarsthathavebeenobtainedtodatewithNICMOSon-boardthe
7
HubbleSpaceTelescope(HST). This alternative imagingtechnique providesa uniqueway to probethe dis-
0
0 tributionofdustgrainsthatscatterlightaroundevolvedstars. Thenewperspectivegainedfromtheimaging-
2 polarimetric data has revealed several new aspects to the structures of PPNs. Point-symmetry is a prevalent
imaging-polarimetriccharacteristic resulting fromthe azimuthaldensity gradientin the CSEs. Amongthese
n
point-symmetricnebulae,threedetailedmorphologicaltypescanbedifferentiatedbytheirpolarizedintensity,
a
I ,andpolarizationstrength,P.Whiletheazimuthaldensitygradientisreversedaboveandbelowtheequatorial
J p
planein opticallythicker bipolarnebulae, there is no gradientreversalin opticallythinnerelliptical nebulae.
0
Theequatorialplaneofthesystemdefinedbytheintegratedangleofpolarizationisnotnecessarilyorthogonal
3
totheaxisoftheapparentbipolarstructureinthetotalintensitydata.

1 Subjectheadings:circumstellarmatter—stars: AGBandpost-AGB—stars:individual(IRASz02229+6208,
v IRAS 04296+3429, IRAS 04395+3601, IRAS 06530- 0213, IRAS 07134+1005, IRAS
0 09452+1330, IRAS 10131+3049, IRAS 10178- 5958, IRAS 10197- 5750, IRAS
8 12419- 5414, IRAS 16594- 4656, IRAS 17150- 3224, IRAS 17245- 3951, IRAS
8 17423- 1755,IRAS17441- 2411,IRAS19343+2926,AFGL2688)—stars: massloss—
1 reflectionnebulae
0
7
0 1. INTRODUCTION ever,theCSEstructuremustdeviatefromtheinitialspherical
/ symmetry. Therefore,theremayexistauniversalmechanism
h Proto-planetary nebulae (PPNs; e.g. Kwok 1993;
through which the required symmetry breaking is achieved,
p VanWinckel 2003) are low to intermediate initial mass
- (∼ 1- 8 M ) objects that are rapidly becoming planetary and thus, we have been performing surveys to identify the
o ⊙ mechanisms that generate the initial non-sphericity in the
nebulae(PNs)afterevolvingofftheasymptoticgiantbranch
r CSEsofAGBstars.
st (AGB). A PPN consists of a post-AGB central star and a Using mid-IR(8- 21µm) data, Meixneretal. (1999) have
circumstellar envelope(CSE) of gas and dust that is created
a shownthatalltheextendedPPNsintheirsampleareaxisym-
: by mass loss from the central star during the AGB phase.
v WhenmasslossceasesattheendoftheAGBphase,theCSE metric and fall into one of the two morphological classes:
i toroidal (showing limb-brightened peaks that suggest the
X physically detaches from the central star and keeps coasting
presence of a dusty torus) or core/elliptical (showing an un-
away duringthe PPN phase. Thus, PPNs preserve the AGB
r resolved emission core surrounded by a low surface bright-
a mass-losshistoryimprintedin the densitydistributionof the
ness elliptical nebula). They concluded that the last stage
CSEs. Therefore,PPNsarethebestastrophysicallaboratories
ofAGBmasslossisshort-livedandinherentlytoroidal(i.e.,
inwhichweinvestigatethenatureofdustymasslossduring
equatorially-enhanced),probablycoincidingwithaperiodof
theAGBphase.
enhancedmasslossknownasthe“superwind”phase(Renzini
During the last decade, high-resolution imaging observa-
1981;Iben&Renzini1983).
tions have revealed a plethora of morphologies among PNs
In an optical imaging survey of PPN candidatesusing the
and PPNs (see review by Balick&Frank 2002). One of the
Hubble Space Telescope (HST), Uetaetal. (2000) have de-
mostimportanttasksintheAGBstellarresearchistherefore
tected elongated reflection nebulosities whose morphologies
todeterminethemainphysicalfactorsthatcausesuchagreat
appeartobifurcatedependingonthedegreeoftheequatorial
morphological diversity emerging from AGB mass-loss ge-
densityenhancementintheCSE.SOLE(Star-ObviousLow-
ometry, which is highly spherically symmetric at the begin-
level Elongated) PPNs show a bright central star embedded
ningoftheAGBphase(e.g.Habing&Blommaert1993).The
in a faint, elliptically-elongated nebulosity, while DUPLEX
observedmorphologicaldiversitysuggestsanumberofshell
(DUst-Prominent Longitudinally EXtended) PPNs exhibit a
shaping processes, and various mechanisms have been pro-
remarkable bipolar structure with a dust lane completely or
posedtoexplaintheobservedmorphologies(Balick&Frank
partiallyobscuringthecentralstar. Theseopticalmorpholog-
2002, and references therein). In all possible cases, how-
ical classes appear to possess a one-to-one correspondence
1Department ofPhysicsandAstronomy, University ofDenver, Denver, to the mid-IR counterparts (core/elliptical ≈ DUPLEX and
CO80208,USA;[email protected] toroidal≈SOLE).
2Max-Planck-InstitutfürRadioastronomie,AufdemHügel69,G-53121, Subsequent radiative transfer modeling of some se-
Bonn,Germany lected sources (e.g. Meixneretal. 1997; Uetaetal. 2001b;
3Space Telescope Science Institute, 3700SanMartin Drive, Baltimore,
Meixneretal. 2002, 2004) have shownthat(1) theobserved
MD21218,USA
2 Uetaetal.
PPN axisymmetry emerges during a brief stage of an en- have used data for two PSF calibration sources, HD 12088
hancedmassloss(∼3×10- 5M yr- 1)towardstheendofthe and BD+32◦3739, respectivelyin the NIC1 and NIC2 band.
⊙
AGBphase,(2)theenhancedmass-lossrateintotheequato- Table 1 summarizes the major observing parameters for all
rialplaneisverymuchhigher(>∼103)thanthatintothepolar observations. Unfortunately, the data for IRAS 02168 and
directions, and (3) the differences between SOLE and DU- IRAS10131(inthePOL-Lband)areaffectedbyseveresat-
PLEX PPNs stem from the physical distinction between the uration and not included in the subsequent analysis. In the
twotypesandcannotbeattributedsolelytotheinclinationan- following,weusetheIRASdesignationofthetargetstorefer
gleeffects. Furthermore,itappearsthatSOLEandDUPLEX toeachobjectexceptforAFGL2688,whichdoesnothavean
PPNs develop respectively from low and high mass progen- IRASdesignation.
itors. This agrees with a recent discovery of mostly bipolar The standard set of NICMOS pipeline routines, CAL-
structuresamongOH/IR-selectedsources, whicharelocated NICA (Version 3.1), in IRAF/STSDAS4 has been used in
close to the galactic plane (i.e., more massive), observed by datareduction. Additionalcarehasbeentakentoremovethe
HST(Sahai&SánchezContreras2004). pedestal effect (a variable quadrant bias) and the Mr. Stay-
Imaging polarimetry is an alternative technique to probe puft(theamplifierringingandstreakingduetobrighttargets;
theCSEstructure,inwhichthedust-scatteredlinearlypolar- Dickinsonetal.2002). Whenmultipleframesarecombined,
izedcomponentoflightcanbeseparatedfromtheunpolarized wehaveusedthedrizzlepackage(Koekemoeretal.2002)in
componentoflight.Gledhillandcollaborators(Gledhilletal. STSDAS. The resulting images maintain their originalpixel
2001; Gledhill 2005) have found, through UKIRT imaging- scalesof0.′′043pixel- 1and0.′′075pixel- 1respectivelyinthe
polarimetricsurveysofPPNcandidates,thattheaxisymmet- NIC1andNIC2band.
ricstructureofthePPNshellsfallintooneofthetwoclasses, DerivationoftheStokesparametersisdoneviathematrix
shellandbipolar,dependingontheamountofdustintheCSE. inversion method (Hinesetal. 2000; Dickinsonetal. 2002).
TheseclassesrespectivelycorrespondtoSOLEandDUPLEX However, the coefficients are differentfor data taken before
PPNs. Theresultsofthepreviousmid-IRandopticalsurveys and after the servicing mission 3B (SM3B) in 2002 March,
havethusbeenconfirmed. duringwhichtheNICMOScoolingsystemwasinstalled.This
Wehaveperformednear-IRimagingpolarimetrywithHST resultedintwosetsofthephotometricconversionfactorsfor
toinvestigatethedustdistributioninSOLEPPNsbymaking thepre-andpost-SM3Bdata.Wehaveusedthesecoefficients
use of the polarized intensity (I ) maps. Imaging polarime- (listedin Table2 ofPaperI) whereappropriate. A morede-
p
tryis especiallypowerfulforthispurpose,becausethe tech- tailedaccountofdatareductionisfoundinPaperI.
nique isolates dust-scattered light arising from the CSE and
permitsusto probetheCSE structureeveninthevicinityof 3. RESULTSANDDISCUSSION
thebrightcentralstar;CSE structureisoftenobscuredinto-
3.1. Results
tal intensity (I) maps, which are affected by the enormous
The imaging-polarimetric maps of evolved stars are pre-
point-spread-function(PSF) of the central star. Since I de-
p
sented in a uniform format, following the scheme in Paper
pends only on the Stokes Q andU, which are free from the
I(Figs. 1to 14: Readersareencouragedtolookatthecolor
unpolarizedcomponentbydefinition,itcanbeextractedeven
images available on the electronic version of the paper.). In
from the PSF-affected data. We have successfully demon-
eachfigure,weshow(a)thetotalintensity,I,map,(b)thepo-
strated that (1) SOLE PPNs consist of a hollow spheroidal
larizedintensity,I ,map,(c)thepolarizationstrength,P,map
shellwhichhasabuilt-inequatorialdensityenhancementand p
and(d)the polarizationvectorpositionangle(P.A.), θ, map,
(2)thevariabledegreeoftheequatorialdensityenhancement
respectively from top to bottom. When the PSF-subtracted
(i.e., the optical depth of the CSE) would result in a variety
dataareavailable,theyarealsodisplayedintherightcolumn
ofapparentPPN structureevenamongthisspecificgroupof
(panels e–h) in addition to the original data in the left pan-
opticallythinPPNs(Uetaetal.2005,hereafterPaperI).
els, except for IRAS 10131 and AFGL 2688 for which the
Inthepresentwork,weextendourinvestigationoftheCSE
two-epoch data are presented (the earlier/later epoch on the
structure via I into the realm of DUPLEX PPNs. We ap-
p
left/right).
plythetechniqueofimagingpolarimetryininvestigatingthe
TheresultsarealsosummarizedinTable2,whichliststhe
structure of the bipolarlobes, in which the optical depth is
measured coordinates and I and I fluxes of the object, the
generallylowenoughforthemethodtowork. Todate,there p
maximum P (P ), the mean P (hPi) and its standard devi-
havebeenobservationsof18evolvedstarsbyHST/NICMOS max
ation, and description of the I structure. The listed coordi-
in the polarimetricmode. Only a small portionof the entire p
nates are the center of the polarization vector pattern that is
datasethasbeenpublished,andtheI datahaverarelybeen
p
determined by minimizing the sum of the square of the dis-
utilizedtostudytheCSEdustdistribution.Therefore,weana-
tance from the presumed center to a line orthogonal to the
lyzealltheexistingHST/NICMOSimagingpolarimetricdata,
polarization vectors. The pattern center is initially assumed
especially in I , obtainedfromthe archivein orderto obtain
p
to be the I peak (when seen), and the center finding process
more insights into the general mechanism(s) responsible for
is iterated until the shift between the previous and current
the structure development in the AGB wind shells. Below,
centers becomes smaller than the numerical accuracy of the
wedescribethedatasetandreductionprocedurebriefly(§2),
analysis.Thefluxesaredeterminedbyintegratingthesurface
presentanddiscusstheresultsbothindividuallyandingroups
brightness over an aperture in which there is more than one
(§3)andgiveasummary(§4).
σ detection. P and hPi plus its standard deviation are
sky max
2. OBSERVATIONSANDDATAREDUCTION determined by using an aperture which is delineated by the
well-establishednebulaedgeoranapertureinwhichthereis
WehavecollectedNICMOSimaging-polarimetricdatafor
13 evolved stars from the HSTarchive and combined them
4 IRAFisdistributedbytheNOAO,whichisoperated byAURAunder
with our own data for 4 SOLE PPNs that have alreadybeen
cooperativeagreementwithNSF.STSDASisaproductofSTScI,whichis
published (Paper I). When PSF correction is necessary, we operatedbyAURAforNASA
HSTNICMOSImagingPolarimetryofPPNs. II 3
morethan three σ detection. The I structure is described The I map shows a general bipolar shape of the nebula
sky p p
bythe overallshape, dimensions(typicallymajorand minor superposedbyclumpystructuresinthelobes,ashasbeenre-
axislengths),andP.A.measuredfromthenorthtotheeast. vealedintheopticalbyTrammell,&Goodrich(2002). How-
Tocomparethepresentresultswiththepreviousunresolved ever, we do not see the tips of the outflows in I . This is
p
polarizationmeasurements,wehavealsoperformedaperture consistentwiththeirshock-excitednature(Uetaetal.2001a).
polarimetry and summarized the results with the measure- ThemorphologyofthenebulachangesdramaticallyintheP
mentsofPaperIinTable3. Wehavedoneaperturephotome- map. Strong polarization is localized in several clumps dis-
tryonthethreepolarizerimagesandthenusedthemeasured tributed along the south edge of the west lobe and the north
fluxes from each polarizer image to compute the integrated edgeoftheeastlobe(respectivelymarkedasWandEclumps
P and θ. We have varied the aperture size from ×1 to ×8 in Fig. 1c). These clumps are distributed along a curve that
of FWHM in I to determine how the resulting values of P delineatesaninverseS.Theθmapshowsgeneralcentrosym-
andθ behaveasafunctionoftheaperturesize. Ingeneral,P metry,andthevectorpatterncenterandtheIpeakareroughly
andθ becomestableandconvergetoasinglevaluewhenthe coincident.
aperturesize isequalto orlargerthanfivetimestheFWHM The structure of the lobes in I and P suggests that there
p
size in I. Thus, the aperture size is defined to be the mini- is more dust on one side of the lobe than the other, given
mumaperturesize (an integermultipleof FWHM in I) over that the lobes are optically thin at near-IR. The regions
whichtheintegratedPandθaregenerallystable. Thequoted of enhanced surface brightness in I and P spatially corre-
p
errorsinP(0.2to4.0%)andθ(atmost6◦)arethusthemax- spond to the shortest of the multiple outflows seen in Hα
imumdeviationin ourmeasurementswhenthe aperturesize (Trammell,&Goodrich2002)7. Thisindicatesthatthese re-
isvariedbeyondthe adoptedaperturesize. Thesevaluesare gions of enhanced local density are associated with the lat-
consistentwiththeresultsofmorethoroughanalysesdoneby est outflows that channel recently ejected matter away from
Hinesetal.(2000)5andBatcheldoretal.(2006)6. thecentralregion,providedthatmasslossproceedsatacon-
Wehaveadoptedtheintegratedθasameasureoftheorien- stantvelocity. Infact,θ (83◦)isalignedwiththedirections
eq
tationoftheequatorialplanebecauseofthefollowingreason. of these most recent outflows delineated by strong I and P
p
When the optical depth in the equatorialregionis very high (∼85◦), andis notalignedwith thebipolaraxis(94◦). This
(τ ≫1), there is little I because of the dilution effects due impliesthatthecurrentorientationoftheequatorialdusttorus
p
to multiple scattering. In this case, the integrated θ tendsto isobliquewithrespecttothebipolaraxis(byabout10◦).
be parallelto the equatorialplane because of the orthogonal TheclumpinessintheoutflowstructuresoftheenhancedI
p
alignmentof the scattering plane with respectto the equato- andP surfacebrightnesssuggestsmass loss thatis episodic.
rial plane (e.g., Whitney&Hartmann 1993; Gledhill 2005). Trammell,&Goodrich(2002)haveshownmultiplering-like
However,whentheopticaldepthismarginallyhigh(∼1), θ structuresin Hα and[SII] emission as well as an arc in Hα
in the equatorial region can be orthogonal to the equatorial silhouette.Thesestructuresareinlinewiththeepisodicmass
planeduetoscatteringatthewallsofthebipolarlobes(e.g., loss interpretation. These clumps, however, could alterna-
Johnson&Jones1991; Trammelletal. 1994;Gledhill2005; tively imply instabilities in the outflows, as pointed out also
Oppenheimeretal.2005). Eveninthiscase,theintegratedθ byTrammell,&Goodrich(2002).
tends to be parallel to the equatorial plane, because vectors Provided that outflows are associated with episodic mass
inthelobesawayfromtheequatorialregionwouldstilldomi- ejection,theangularseparationbetweenclumpscorresponds
nate.Therefore,theintegratedθismoreappropriateasamea- to the interval of mass ejection. The separations between
sureoftheorientationoftheequatorialplane,andisexpressed clumps are 1.′′1 and 1.′′3 in the west lobe respectively for
asθ inthe following. Note, however,thatθ is definedto thefirstandsecondclosestclumppairsfromthecentralstar,
eq eq
betheangleperpendiculartotheorientationoftheequatorial and 0.′′9 in the east lobe. Using 120 km s- 1 outflow veloc-
plane so that the comparison between θ and P.A. of the I ity(Trammell,&Goodrich2002)andthedistanceof1.7kpc
eq p
surfacebrightnessdistributioncanbedoneintuitively. (Bujarrabaletal. 1988) and assuming a linear outflow, these
separationstranslateto60to90years. Giventheuncertainty
3.2. IndividualSources intheexactgeometryoftheoutflowsandoftheclumps,these
valuesarereasonablyclosetothevalueof40years(scaledto
3.2.1. IRAS04395+3601(AFGL618)
ourchoiceofthedistance)derivedbyTrammell,&Goodrich
IRAS04395(Fig.1)isaDUPLEXPPNthatisontheverge (2002) based on the separation between the ring-like struc-
of becoming a PN (Westbrooketal. 1975; Calvet&Cohen turesseeninHαand[SII].
1978). The I map shows its well known bipolar structure
3.2.2. IRAS09452+1330(IRC+10216)
elongatedinthe east-westdirectiondueto multipleoutflows
(Trammell,&Goodrich 2002). The lobe interior structure IRAS 09452 (Fig. 2) is one of the brightest thermal IR
is more clearly revealed in the I map than in recent high- sourcesinthesky(Becklinetal.1969),andhence,oneofthe
resolution ground-based near-IR maps (Uetaetal. 2001a). best-studiedC-richAGBstar. TheCSE(ofDUPLEXtype)is
PSF subtraction did not improve the quality of the data be- seenasafragmentedbipolarnebulahavingfourmajoremis-
cause no post-SM3B PSF data with a comparable degree of sion components in the optical and near-IR (Skinneretal.
saturationisavailableandthewholePSFstructure(especially 1998; Osterbartetal. 2000; Tuthilletal. 2000; Weigeltetal.
ofthespikes)isenlargedduetothebright,extendedemission 2002;Murakawaetal.2005).
core. The I map shows the main nebulosity that is stretched at
a P.A. of 22◦. We recognize three fragments in the north
5Inverifyingthematrixcoefficientsbymeasuringpolarizationofbothpo-
larizedandunpolarizedstandards,theirmeasurementswereconsistentwithin 7Theeastclumpscorrespondtotheeasternoutflow“d”(thenorthernmost
0.3%inPand5◦inθ ofthethreeoutflows),whilethewestclumpscoincidewiththeshortestwest-
6 Possible instrumental polarization would make thematrix coefficients ernoutflow(thesouthernmostoutflowinthewest)aspresentedinFigure1
uncertainandwasfoundtocausedifferenceof0.1%inPand3◦inθ. ofTrammell,&Goodrich(2002).
4 Uetaetal.
(markedasthe“NE”,“N”,and“NW”inFig.2arespectively In the I map we see the collimated bipolar lobes that are
fromeasttowest[P.A.=22◦,- 19◦,&- 66◦];Osterbartetal. linearly elongated into a P.A. of 73◦. The equatorial region
2000; Tuthilletal. 2000; Weigeltetal. 2002) and an arc be- showsapinchedwaistduetoextinction. Theinnerstructure
yond the south lobe (about 3′′ away from the central peak; intheIRdoesnotappearassharplyasintheoptical.
Mauron&Huggins2000). TheI andPmapsclearlyrevealthephysicallythinnature
p
TheI mapshowstheCSEstructuremoreclearlythanthe ofthewallsofthecylindricallobes. Thedustlanethatsepa-
p
I map,butitsuffersfromthepolarizationghosts(highI re- ratesbothlobesismoreclearlydelineated. Moreover,I and
p p
gionslocatedradiallywithsemi-regularintervalsinthewest Pmapsshow,inthebipolar“skirt”region,thatthenorthwest
and southeastas indicated in Fig. 2; Hinesetal. 2000). The andsoutheastwallsaremoreenhancedthanthesouthwestand
northeastandsouthlobesformthemainbipolarnebulaeata northwestwalls,respectively.Furthermore,thesouthwestand
P.A. of 22◦. However, it is not clear if the north and north- northeast walls appear longer than the southeast and north-
west lobes also have their counterparts in the southeast be- westwalls,respectively.Theθmapindicatesaratheraligned
cause of the ghosts. Murakawaetal. (2005) have detected distributionofpolarizationanglesbyashallowgradientofthe
little(<10%)polarizationinthesoutheastinrecentground- image tone over the bipolarlobes (especially away fromthe
∼
basedimagingpolarimetryin theH bandand suggestedthat centralstar). Nevertheless,theθmapisgenerallycentrosym-
thenorthandnorthwestlobesare causedbyleakageof light metriccenteredattheI peak.
throughfissuresinthewestsideofthecentraldusttorus.The Ourmeasurementsyieldθ =80◦. ThePmapshowsanel-
eq
southeast ghosts are more obvious as in the P map. These lipticallyelongatedregionofweakP(<∼20%ataP.A.of78◦)
ghostsarealsothesourceoftheanomalouspolarizationvec- superposedonthedustlane. ThisweakPregion,therefore,is
torpatternintheotherwisecentrosymmetricθmap. almostorthogonallyorientedwiththe equatorialplane. This
Our aperture polarimetry has yielded θ of 32◦. This is suggeststhatthisweakPregionmayrepresenttheregionof
eq
consistent with past polarimetric measurements in the opti- diluted polarization due to starlight leaking through the bi-
calthroughmid-IR(Shawl&Zellner1970;Dycketal.1971; conicalopeningsofthedusttorusalignedwiththeequatorial
Capps&Dyck1972;Capps&Knacke1976;Trammelletal. plane. However, the cylindrical lobes are aligned at a P.A.
1994;Serkowski&Shawl2001).Therefore,thebipolarlobes of 73◦, which is slanted by 7◦ with respect to the equatorial
are obliquely oriented (10◦ off) with respect to the equato- plane. Inaddition,theeastendofthenorthwestwallandthe
rialplane. Moreover,the southwestlobeappearslargerthan west end of the southeast wall (the regions marked as “en-
thenortheastlobe. Thisisprobablyduetoinclinationofthe hancedskirt” in Fig. 4c) seem to be slightly curvedtowards
lobes with respect to the plane of the sky: the angle of 20◦ thecentralstarwithrespecttotherestofthewalls,whichare
has been determinedbased on radiativetransfer calculations very much linear. It appears as if these curved ends of the
(Skinneretal.1998). wallstracetheweakPregion.
Thepolarimetricdatathusuncover(1)thepoint-symmetric
3.2.3. IRAS10131+3049(CIT6)
distribution of scattering matter in the cylindrical lobes (the
IRAS 10131 (Fig. 3) is an IR-brightC-rich extreme AGB northwestandsoutheastwallsaremoreenhanced)and(2)the
star (Ulrichetal. 1966; Cohen 1979). Schmidtetal. (2002) misalignment between the axes of the equatorial plane and
have studied the CSE structure in detail using a set ofHST the collimated cylindrical lobes. As a cause for the highly
datatoshowthepresenceof(1)largeIRbipolarlobes(about collimatedstructureofthe nebula,García-Larioetal. (1999)
10timeslargerthanintheoptical)inthenortheast-southwest on one hand suggested an extreme density contrast within
directionand(2)multipleconcentricarcs1′′to4′′awayfrom thedusttorusthatwouldpreferentiallychanneloutflowsinto
thecentralstar. Thetwo-epochmapsinFig.3showtheghosts spatially-confined regions, while Sahaietal. (1999) on the
affecting all the quadrantsof the field of view since the roll otherproposedadissipativehydrodynamicinteractionofjets
angleofHSTatthetwoepochsdiffersbyabout180◦. Wedo with the surroundings. Both scenarios, however, still re-
notrecognizeanyinternalstructureintheCSEcoreintheI quireadditionalmechanismstodirecttheoutflows/jetsalong
p
andPmaps. the cylindrical lobes that are not orthogonally aligned with
A series of IR polarimetry between 1967 and 1976 the equatorialplane. Alternatively, the enhancementsin the
showed that θ varied between 110◦ and 180◦ (Dycketal. northwestandsoutheastwallsofthelobesmaybesimplydue
1971; Kruszewski 1971; Kruszewski,&Coyne 1976). Es- totheilluminationeffectsassuggestedfromtheshapeofthe
pecially,Kruszewski,&Coyne(1976)reportedanIRpolari- weakPregion,sincethebiconicalopeningsofthecentraldust
metric variability on the order of several months. Although torus (implied by the orientation of the weak P region) are
Schmidtetal. (2002) have determined that there is no evi- pointedtowardstheseregionsofenhancedI andP. Thepro-
p
dence for temporalchangesin the polarimetricpropertiesof nounceddouble-shellstructureattheeastendofthenorthwest
the CSE structurebetween the two epochs, there is some 6◦ wallandthewestendofthesoutheastwallmayberelatedto
changeinθ . ThismayberelatedtotheintrinsicIRpolari- themisalignment.
eq
metricvariabilityoftheobject. Withrespecttothemeasured
3.2.5. IRAS10197- 5750(Hen3-404)
θ values, the shell elongation is not exactly perpendicular
eq
(8◦to14◦offset).Ifθ variesasθ,itmaysuggestsvariations IRAS 10197 (Fig. 5) is a bipolar DUPLEX nebula sur-
eq
inthedustdensitydistributionneartheequatorialplaneonthe roundinganA2IstarthatisassociatedwithOHmaseremis-
orderofmonths. sion (Allenetal. 1980). The optical reflection nebulosity
has been imaged with HST to reveal its butterfly shape
3.2.4. IRAS10178- 5958(Hen3-401)
(Sahaietal. 1999). The HSTimaging polarimetric data is
IRAS10178(Fig.4)isaPPNofDUPLEXtypeassociated generally consistent with the previous imaging polarimetric
with a Be post-AGB star. HSTobservations in the optical results presented by Scarrott&Scarrott (1995), but reveal
andH 1-0S(1)bandshaverevealedthisobject’shighlycol- muchmoredetailedstructure.
2
limatedstructure(García-Larioetal.1999;Sahaietal.1999). TheImapexhibitssomedifferencesincomparisonwiththe
HSTNICMOSImagingPolarimetryofPPNs. II 5
opticalmap. Near-IRlightdelineatesafigureSratherthana VandeSteene,&vanHoof2003). Theimaging-polarimetric
butterfly shape as in the optical maps, filling the regions of dataofthisobjecthavebeenpresentedbySuetal.(2003)and
heavyopticalextinctionsuch as the equatorialdustlane and us(PaperI).Since thisobjectpresentsaninterestingcase of
thedark“spur”ofdust(Sahaietal.1999).TheI andPmaps a ratheropticallythinDUPLEX PPN, we brieflyre-describe
p
revealthenebula’spoint-symmetrybetterthantheImap:po- the structure of the nebula. This object possesses (1) an el-
larizedlightismoreconcentratedontheeastedgeofthenorth liptical nebula elongated in the east-west direction (P.A. of
lobe and the west edge of the south lobe. The distribution 82◦)withanadditionalprotrusiontowardsnorthwest(P.A.of
ofpolarizedlightemphasizesthefigure Sappearanceof the - 60◦) and (2) the bipolar cusps in I that delineate the in-
p
nebula. Theθmapshowsgeneralcentrosymmetry.However, nermost edges of the equatorial density enhancement in the
the I peak position does not coincide with the center of the shell. The cusps are spatially coincident with H emission
2
polarizationpattern. The location of the central star derived (Hrivnaketal.2004),whichshowsenhancementsattheedges
from the polarization pattern is (α, δ) =(10h21m33.s88, of the equatorial torus in an otherwise elliptically elongated
J2000
- 58◦05′47.′′7). shell (aligned with one of the optical funnels). P is slightly
Theθ measurementyields34◦. Becauseofthecomplex- enhancedonthesouthedgeintheeastlobeandonthenorth
eq
ity of the nebula, we can define a number of axes that may edge in the west lobe. The vector pattern is generally cen-
characterize the nebula structure as indicated in the I map trosymmetric.
p
(Fig. 5b). The lobe axis is oriented with a P.A. of (1) 23◦ θ is found to be 22◦, which is unrelated to any of the
eq
basedontheoverallstructureofthepolarizednebula,(2)- 3◦ observedstructure. While IRAS 16594is of DUPLEX type
basedonthetipsofthefigureSappearance,(3)67◦basedon basedonitsshellstructure,thefactthatthecentralstarisvis-
thestrongestI distributioninthenebula,and(4)- 15◦based ibleintheopticalandnear-IRsuggeststhattheopticaldepth
p
on the protrusions along the western edge of the north lobe in the equatorialregionis nearthe lower limit forDUPLEX
andtheeasternedgeofthesouthlobe.Furthermore,thereisa (aboutunity). Moreover, the two-peakedmid-IRcore struc-
regionofweakP(<∼20%)thatisellipticallyelongatedatthe ture found in the equatorial region (García-Hernándezetal.
center of the nebula (similar to what has been seen in IRAS 2004) also indicates rather low-density enhancement. Thus,
10178). This weak P ellipse is oriented with a P.A. of 12◦ our θ measurement may have been affected by the bright
eq
(indicatedinthePmap,Fig.5c). PSF.
Noneoftheseaxesisexactlyperpendiculartotheequatorial
plane.Ifweassume,asinIRAS10178,thattheorientationof 3.2.8. IRAS17150- 3224
theweakPellipsecorrespondstothedirectionofthebiconi- IRAS17150(Fig.8)isanarchetypicalDUPLEXPPNwith
calopeningsofthecentraldusttorus,thenthetorusisslanted ahighlyequatoriallyenhancedshell(Meixneretal.2002and
withrespectto theequatorialplane. Insucha case, thecen- referencestherein). The I map reveals the equatorialregion
traldusttorusneedstoprecesswithroughly46◦ toshapethe of the nebula, which is very much obscured in the optical
nebula structure by outflows through the biconical openings (Kwoketal.1998;Uetaetal.2000). HSTimagesinnear-IR
ofthetorus. bandshavebeenpresentedbySuetal.(2003). Fromthepo-
3.2.6. IRAS12419- 5414(BoomerangNebula) larizationpattern,wefindthatthepositionofthecentralillu-
minationsourceis(α,δ) =(10h17m19.s81,- 32◦27′21.′′2),
J2000
IRAS 12419is a very large optical bipolarreflection neb- whichis0.′′14±0.′′07offfromthecentralI peak.
ula(55′′×21′′;Wegner&Glass1979),inwhichverystrong TheI andPmapsdisplaythebipolarstructureoftheneb-
opticalpolarizationhasbeendetected(45- 60%inthelobes; ula,inwphichthehollownessofthelobesishighlightedbythe
Taylor&Scarrott1980). TheHSTdatainFig.6showpolar- polarized light delineating the edges and polar caps. In ad-
ization in the central 8′′×8′′ region of the nebula at 2µm. dition, P is even more strengthened in the north wall of the
In the I map, we see an extremely faint nebulosity in the northwest lobe and in the south wall of the southeast lobe.
PSF-dominated central region, which appears to be shaped Thus, there is a preferentialdistribution of scattering matter
in a figurej ata P.A. of0◦. Althoughthe nebulais too faint in a figure S. The measured θ is 120◦, which is consistent
eq
toperformPSFsubtractioneffectively,theI mapshowsthat withthebipolaraxisataP.A.of122◦.
thenear-IRlobesareelongatedalongthegeneralnorth-south
direction(slightlytilted alongthe northwest-southeastquad- 3.2.9. IRAS17245- 3951
rants).
IRAS 17245 (Fig. 9) is a less studied post-AGB star
TheI andPmapsmarginallyshowthedistributionofpo-
p (Hrivnaketal. 1999), whose less evolved nature has been
larizedlightalongfour“streaks”radiallyemanatingfromthe
suggested by the star’s association with OH maser emission
central region (P.A. of - 170◦, - 55◦, 125◦, and 10◦ as indi-
(Sevenster 2002). The polarization maps reveal the bipolar
catedinFig.6b).Thesestreaksseemtodelineatetheedgesof
nature of the nebula that does not possess an apparentbipo-
the bipolarlobes. The θ map pattern appearsgenerallycen-
larnebulosityinthetotalintensitymaps(Gledhilletal.2001;
trosymmetric. Althoughthedegreeofpolarizationissimilar
Suetal. 2003). While the main bipolar lobes are oriented
in the near-IR and optical around the central star (12±8%
alongthenorth-northeasttosouth-southwestdirection,thetail
and15%,respectively),thepolarizationangleisnotthesame
end of the nebula tips appears to be twisted towards oppo-
(147◦ and80◦, respectively). Thisisprobablycausedbythe
sitedirections: thenorthtiptonorthwestandthesouthtipto
matter in the equatorial region that is optically thick in the
southeast. This tendencyis seen more clearly in the P map,
opticalbutopticallythininthenear-IR.
inwhichthestronglypolarizedtipsarefoundataP.A.of7◦.
3.2.7. IRAS16594- 4656 Thebipolaraxisoftheoverallnebulaelongationisorientedat
aP.A.of15◦,whileθ isfoundtobe21◦. Thus,thereis6◦
IRAS 16594 (Fig. 7) is an evolved post-AGB star associ- eq
misalignmentbetweenthebipolaraxisandtheorientationof
atedwithamulti-funneledopticalreflectionnebula,inwhich
theequatorialplane.
shocksstarttoshapetheshellstructure(Hrivnaketal.1999;
6 Uetaetal.
3.2.10. IRAS17423- 1755(Hen3-1475) light is consistent with their proposed shock-excited nature
and suggests verylittle presence of scattering matter around
IRAS 17423 (Fig. 10) is a highly collimated bipolar PPN
theshockedgas.
(of DUPLEX) associated with high velocity outflows (e.g.
The overall polarization characteristics are generally
SánchezContreras&Sahai 2001; Rieraetal. 2003). The
consistent with the previous optical imaging polarimetry
present data cover only part of the brighter northern lobe in
(Schmidtetal. 1978). The present data show that (1) I is
theNIC1band.TheImapmainlyshowsthenorthwestlobeof p
concentrated in the walls of the bipolar lobes, revealing the
thecentralhourglass-shapedbipolarstructurethatisinclined
hollow nature of the lobes, and (2) strong P is found along
with respectto the planeof the sky. The extendedknotsare
the north edge of the northwest lobe and the south edge of
seen in the I map (up to the NW2 knot; Rieraetal. 2003).
the southeastlobe ofthe nebulathatis generallycentrosym-
The I map reveals the physically thin walls of the bipolar
p metric. Thenebulashapedefinesthesymmetricaxis(P.A.of
lobesmoreclearly.
131◦), alongwhich CO outflowvelocityis foundto havean
Strong polarization is found close to the west side of the
almostconstantlatitudinalgradient(Bujarrabaletal.1998a).
northlobeandtotheeastsideofthesouthlobe(inthecentral
Thisaxisis,however,notalignedwithθ of151◦.Asinother
“hourglass”part). ThesestrongPregionsarefoundataP.A. eq
of- 54◦ (indicatedinFig.10c). Thisangleisnearlyorthogo- objects,wefindsomePenhancementsinthenebulaalongthe
oppositeedgesineachlobe.
naltotheequatorialplane(θ =128◦). Thereis5◦misalign-
eq
mentbetweentheequatorialplaneandthegeneralorientation
of the bipolar lobes (at a P.A. of 133◦). Thus, these strong 3.2.13. AFGL2688(EggNebula)
Pregionscanbeinterpretedastheparticularlocationsinthe AFGL 2688 (Figs. 13 and 14) is a DUPLEX PPN that is
walls of the bipolar lobesthat are illuminatedby the central consideredoneofthearchetypes(Neyetal.1975;Sahaietal.
star throughthe biconicalopeningsof the centraldusttorus, 1998),anditsstructurehasrecentlybeenreviewedindetailby
whichisnotorthogonaltotheaxisofthebipolarlobes. Gotoetal.(2002). TheI mapsshowtheobject’swellknown
3.2.11. IRAS17441- 2411 bipolarlobesplustheconcentricarcstructureandthe“spin-
dle”structurealongthedustlanebetweenthelobes(seenonly
IRAS 17441 (Fig. 11) is a G0I post-AGB star associated
in the NIC2 band; Fig. 14a). The I mapsdisplaythe point-
p
with an optical nebula of DUPLEX type (Suetal. 1998;
symmetric nature of the lobes with the brightness enhance-
Uetaetal. 2000). The figure S shape of the nebula is more
ment appearing near the southeast corner of the north lobe
pronouncedinthenear-IRthanintheoptical,asseenintheI
andnearthenorthwestcornerofthesouthlobe. Itisremark-
map. TheI peakcoincideswiththepolarizationpatterncen-
able that the spindle structure has almost zero polarization.
ter,andthus,representsthecentralsourceobscuredintheop-
ThisisbecausethespindlestructureislargelyinH emission
2
tical.
(Coxetal.1997;Kastneretal.2001).
AsinIRAS10197,anumberofaxescanbedefinedbased
In the P maps, strong polarization arises near the north-
on various morphological/polarimetric characteristics. The
west corner of the north lobe and near the southeast corner
tipsofthefigureSshellpointtoaP.A.of- 2◦,whiletheinner
ofthe southlobe. Theequatorialregionofthe nebulais op-
region of the nebula, where I is the strongest, is elongated
p tically thick even to near-IR light, but optically thin lobes
alongaP.A.of46◦. ThePmaprevealsthewallsofthebipo-
showagenerallycentrosymmetricpatternintheθ mapfrom
lar lobes, which are generallyorientedtowardsa P.A. of 6◦.
which the position of the illumination source is determined
However, the central weak P region (<∼20%) appears elon- (Weintraubetal.2000). θ (17◦ to19◦ P.A.)isslightlymis-
gated along a P.A. of 20◦. Aperture polarimetry determines aligned with respect to theeq axis of the nebula (P.A. of 12◦;
θ of28◦,whichagreeswithnoneofthecharacteristicP.A.s
eq Weintraubetal.2000;Gotoetal.2002). Proper-motionmea-
describedabove.
surements of the shell structure using the two-epoch NIC-
Oppenheimeretal. (2005) have constructed a detailed
MOS data have yielded the distance of 420 pc to the object
model of this object using evacuated bipolar lobes with an
(Uetaetal.2006).
equatorialdisk by fitting their polarizationdata from optical
tonear-IR.Theirmodelisquitesuccessfulinreproducingthe
3.2.14. IRAS07134+1005,06530- 0213,04296+3429,and
highlypolarizednatureoftheshellaswellastheaxisymmet- (z)02229+6208
ricoverallgeometryoftheobject.However,theirmodeldoes
notaddressthepoint-symmetricaspectoftheshellstructure. Imaging-polarimetriccharacteristicsoftheseSOLEobjects
ThepolarizationstructureispeculiarinIRAS17441because have already been discussed in detail (Paper I). Here, we
I isenhancedintheeastedgeofthenorthlobeandthewest brieflysummarizeourfindings.
p
edgeofthesouthlobe,butPisenhancedintheoppositeside IRAS07134(Fig.3ofPaperI)isaC-richF5Iabpost-AGB
ineachlobe. star (Hrivnaketal. 1989) associated with an elliptical neb-
ula(Meixneretal.1997;Dayaletal.1998;Juraetal.2000a;
3.2.12. IRAS19343+2926(M1-92)
Uetaetal. 2000; Kwoketal. 2002). The shell is a slightly
IRAS 19343 (Fig. 12) is an O-rich PN of DUPLEX type, prolatespheroidwithaninnercavityandabuilt-inequatorial
which has been extensively studied (Trammell,&Goodrich enhancement,andhence,isnotsimply“toroidal”ashasbeen
1996;Bujarrabaletal.1998a,b, andreferencestherein). The interpreted by previous mid-IR observations (Meixneretal.
bipolar lobes that are separated by the dust lane are seen 1997;Dayaletal.1998;Juraetal.2000a;Kwoketal.2002).
in the I maps as have been seen in the previous observa- Inaddition,theI mapconfirmedthatthereismorematteron
p
tionsinatomicandH lineemission(Bujarrabaletal.1998b) theeastsideofthenebulathanthewest(Meixneretal.2004).
2
as well as in CO (Bujarrabaletal. 1994, 1998a). We do IRAS06530(Fig.4ofPaperI)isaC-richF5Ibpost-AGB
not see, however, the chains of spots in the bipolar cavities star(Hrivnak&Reddy2003)withanellipticalreflectionneb-
that have been identified as shock-excited clumps of mate- ula(Uetaetal.2000). Thenebulaisanearlyedge-onprolate
rial(Bujarrabaletal.1998b).Theirnon-detectioninscattered spheroid with a possible inner cavity. The nebula’s density
HSTNICMOSImagingPolarimetryofPPNs. II 7
structure appears such that the equatorial region of the cav- InDUPLEXPPNs,theequatorialregionscanbeveryopti-
ity is filled with some matter, which manifests itself as the callythickin generalasevidencedbythepresenceofa dust
density-enhanced bipolar cusp-like structure. However, the lane. Nevertheless,thebipolarlobesthemselvesareoptically
totalamountofdustalongtheequatorisstillnotlargeenough thin enough that they are seen via dust-scattered starlight.
tocauseself-extinctionwithintheshell(butclosetotheupper This allows us to adopt the same argumentfor SOLE PPNs
limitforaSOLEnebula). that we used in Paper I and to investigate the structure of
IRAS04296(Fig.5ofPaperI)isaC-richG0Iapost-AGB thelobesinDUPLEX PPNs byrelatingtheI andP surface
p
star(Hrivnak1995)thatisassociatedwithanX-shapedopti- brightnessestotheamountofdustalongthelineofsight.Fol-
cal elongation (Sahai 1999; Uetaetal. 2000). While Sahai lowing a simple scattering angle effect (i.e., high degreesof
(1999) suggested that these elongations represent a pair of polarizationareexpectedfromscatteringatnear90◦ angles)
bipolar jets and a collimating disk, we interpreted that the asin PaperI, we can attribute the observedpoint-symmetric
shellhadtwo,mostlikelyhollow,spheroids:one(P.A.of26◦) appearanceinthese10DUPLEXPPNstothecolumndensity
isnearlyedge-onandtheother(P.A.of99◦)isinclinedwith variations along the peripheryof these nebulae. Since there
respectto the planeof the sky. The two-spheroidinterpreta- is more dust on one side of a lobe than the other, we con-
tion was favoredby a recentdust scattering modelingof the cludethatthereisanazimuthaldensitygradientintheselobes.
object(Oppenheimeretal.2005). Asymmetricilluminationcanpotentiallyinducesimilarpoint-
IRAS02229(Fig.6ofPaperI)isaC-richG8- K0Iapost- symmetricnebulamorphologiesin polarizedflux. However,
AGB star (Hrivnak,&Kwok 1999) surrounded by an ellip- polarizedfluxenhancementscanoccuranywhereanddonot
tical nebula of dust-scattered starlight (Uetaetal. 2000) and havetooccuratthe peripheryofthe nebulae. Thisis incon-
dustemission(Kwoketal.2002). Thisobjectisthemostpe- sistentwiththeobservedtrendinthattheregionsofenhanced
culiaramongthesefourSOLEnebulae. Thisnebuladoesnot polarizationoccursnear the peripheryof the nebulae. Thus,
show the hollow shell structure, but has a greater equatorial weconcludethattheasymmetriceffectislesslikelythanthe
enhancementalongtheminoraxis(P.A.of149◦). Thesepe- densityeffectasthecauseforthepoint-symmetricappearance
culiaritieshavebeenattributedtoahigherdegreeofequatorial ofthenebulaeinpolarizedflux.
enhancementin the shell and to the inclination angle of the Whiletheoverallimaging-polarimetricmorphologyofthe
nebulathatismarginallyedge-on.However,ourθ measure- majority of the sample sources can be described as point-
eq
ment(101◦), whichisnotconsistentwiththisinterpretation, symmetric, there appear to be three distinct CSE structures
appearsaffectedbythebrightstarlightinthismarginallyop- amongthosepoint-symmetricnebulae.Therearenebulaethat
ticallythinnebula. showpolarizedfluxesneartheperipheryofthelobesinboth
I and P (IRAS 10178, 16594, 17150, 17423, and 19343).
p
3.3. Discussion As discussed for SOLE PPNs in Paper I, this structure sug-
geststhatthelobesarehollowshells. Suchhollowlobescan
3.3.1. Imaging-PolarimetricCharacteristicsofDUPLEXPPNs
be created by either the cessation of mass loss in the polar
Having reviewed the imaging-polarimetric characteristics directions or the sweeping up of the mass-loss ejecta into
of individual sources in §3.2, it now appears that the most the polar directions. Recently, García-Arrendondo&Frank
prevalentcharacteristicamongtheobservedDUPLEX PPNs (2004) have shown hydro-dynamically that interactions be-
is their point-symmetric appearance in the polarized surface tweenanAGBwindandajetfromanaccretiondiskcancre-
brightness. This point-symmetricappearanceof these nebu- atea narrow-waistbipolarnebulaif the jetdominates. A fa-
lae in polarized light stems from the fact that one side of a mouswell-collimatedbipolarPPN,M2-9(e.g.Schwarzetal.
lobe shows stronger polarization than the other and that the 1997), whichis verysimilar to IRAS 10178,hasbeen inter-
strongerpolarizationoccursattheoppositesidesoftheoppo- preted as a symbiotic system evolving into a PN based on a
sitelobes. scenario in which outflows emanate from the accretion disk
Among13DUPLEXPPNsinthiscompilation,10sources (Livio&Soker2001). Infact,IRAS19343hasbeenrecently
exhibit a sign of point-symmetry while the degree of point- suggestedtobeasymbioticsystembasedonadetailedspec-
symmetryvariesamongthesources.TheexceptionsareIRAS troscopic study (Arrietaetal. 2005). Furthermore, the pres-
09452, 10131, and 12419. In IRAS 10131 and 12419, the ence of precessing jets has been suggested in IRAS 17423
structureofthepolarizedsurfacebrightnessisnotdetermined and 19343 based on the presence of shock excited knots
well since the polarizednebulais toocompactin the former (Bujarrabaletal. 1998b; SánchezContreras&Sahai 2001;
and the polarizationflux is too weak in the latter. Thus, ne- Rieraetal. 2003). Thus, such systems may reasonably ex-
glectingthetwoinconclusivecases,wehave10outof11ob- plainthemorphologiesoftheseobjectsandeventheoblique
jectsthatdisplaypoint-symmetricimaging-polarimetricchar- angle of the equatorialplane suspected in some of these ob-
acteristics. In IRAS 09452, the main lobe pair (the NE and jects(seebelow)maywellbeexplainedunderthepresenceof
S lobes in Fig. 2) is aligned along a straight line and so do precessingjets.
the otherlobes(the N andNW lobes)andthere isno appar- TherearealsonebulaethatshowPonlyneartheperiphery
entindicationofpoint-symmetricvariationofpolarizationin ofthelobesbutI insidethelobes(IRAS10197,17245,and
p
the lobes. Thus, IRAS 09452providesan examplein which 17441).Thesenebulaearealsocharacterizedbytheverypro-
theimaging-polarimetriccharacteristicsofthenebulaarede- nouncedfigureSshape. Unlikethepreviouscase,thehollow
terminedby the illuminationeffects. In fact, the N and NW Pstructuredoesnotindicatethehollownessofthelobessince
lobes have been shown recently to be due to scattering of Ip isdetectedalloverthelobes. ThereducedPinthemiddle
stray starlight leaking through fissures in the optically thick ofthelobesisdueto increasedunpolarizedlightinthemid-
toroidal core of the CSE (Murakawaetal. 2005). None of dle of the lobes. Since the central star is not very visible in
these 10 DUPLEX PPNs that possess point-symmetry,how- these sources, it is notlikely thatthe centralstar contributes
ever, shows obvious evidence for the illumination effects as to the increased presence of unpolarizedlight. Thus, we in-
doesIRAS09452. terpret that the unpolarized component of light is increased
8 Uetaetal.
in the middle of these nebulae due to multiple scattering in 3.3.3. ObliqueOrientationoftheEquatorialPlane
theiropticallythickerequatorialregions. Suchenhancedop-
Inthe presentwork, we haveadoptedθ , the integratedθ
ticaldepthsinthevicinityofthecentralstar alongtheequa- eq
overanaperture,asameasureoftheorientationoftheequa-
torialplanecanbeattributed,forexample,tothepresenceof
torialplane(modulo90◦tobeprecise).Inconventionalimag-
adiskaroundthecentralstar. Ithasbeenshownrecentlythat
ing,theorientationoftheequatorialplaneisinferredfromthe
awarpedequatorialdensityenhancementcangeneratepoint-
appearanceofthedustlaneand/orofthebipolarlobes.Aswe
symmetriclobesbyatwo-fluidhydro-dynamicalsimulations
havenotedearlier,thewaystarlightgetsscatteredintheequa-
(Icke2003;Rijkhorstetal.2004). Similaritiesbetweenmod-
torialregionscanbestronglyinfluencedbythelocaldensity
els and the nebula structure (especially of IRAS 10197) are
enhancements. Therefore,thesilhouettemorphologyofdust
remarkable.
lanes, especially in highspatialresolutionimagingdata, can
There are yet another nebulae that display clumpiness in
besusceptibletothelocaldensitydistribution. Insuchcases,
thepolarizedsurfacebrightnessdistribution(IRAS04395and
imaging polarimetry can alternatively constrain the orienta-
AFGL 2688). As previously noted, the clumpy structure of
tionoftheequatorialplanebetterthantheconventionalimag-
IRAS 04395 may be due to episodic mass loss or instabili-
ing.
ties in the outflows. The bipolar lobes of AFGL 2688 also
Wehaveseenin§3.3.2thatθ doesnotnecessarilycorre-
showsignsofhollownessbuttheirI structure(especiallyof eq
p spondstotheorientationoftheapparentdustlaneseenintheI
thenorthernlobe)isclumpy,andtherefore,AFGL2688may
data.Among13DUPLEXPPNs,wehavefoundin10sources
belongto the previousgroup. However, noneof the sources
that the discrepancy between θ and the orientation of the
in the previous group shows the presence of the concentric eq
bipolaraxisismorethanthe quotederror,assumingthatthe
arcs as in AFGL 2688. Thus, AFGL 2688 may be a very
uncertaintyinthebipolaraxisorientationis∼2◦.Thetwoan-
uniqueobject. Althoughthe nebulastructureis inconclusive
glesareequalwithinerrorsinonecase(IRAS17150).Intwo
duetoitscompactness,IRAS10131mayberelatedtoAFGL
casesθ isnotreliablyfixedprobablyduetothebrightcentral
2688becauseIRAS10131alsoshowssomearcsintheCSE eq
starandfaintpolarizedsurfacebrightnesses(IRAS12419and
(Schmidtetal.2002).
16594). Inthe 10 obliquecases, however,the determination
of the orientationof the bipolar axis from the I structure is
3.3.2. SOLEandDUPLEXMorphologies p
somewhatsubjective:the2◦uncertaintymaybeinappropriate
In this section we comparethe morphologicalcharacteris-
forthreecases(IRAS17245,IRAS17423,andAFGL2688).
tics of SOLE and DUPLEX PPNs in this the most complete
Evenso, therearestillsevensourcesinwhichtheequatorial
compendium of imaging-polarimetric data for PPNs taken planeismorethan∼10◦ obliquefromtheperpendicularori-
withHSTincludingdatapresentedinPaperI.InPaperI,we
entation with respect to the bipolar axis, and there are more
have shown that (1) optically thin SOLE PPNs consist of a
thantwiceasmanyobliquecasesasorthogonalcases.
hollowspheroidalshellwith abuilt-inequatorialdensityen-
Evenifwetakeastricterstanceonthesubjectivityindefin-
hancement,whichmanifestsitselfasthetoroidalstructureand
ingthebipolaraxisandonourinformalerroranalysisinde-
(2)thereisarangeofopticaldepths,whichcaninfluencethe
rivingθ ,therearecasesinwhichtheobliqueangleismore
eq
observed morphological traits even among SOLE CSEs. In than 20◦ (IRAS 17441 and 19343). This issue needs to be
the presentwork, we have concludedthat there exist the az-
investigatedfurtherinfutureasitmayberelatedtothemech-
imuthaldensitygradientinadditiontoequatorialdensityen-
anisms to generate the equatorial density enhancement and
hancementsinthebipolarlobesofopticallythickerDUPLEX
bipolarlobes.
PPNs.
The distinction between SOLE and DUPLEX PPNs was 4. SUMMARY
initiallyproposedbasedprimarilyonthepresence/absenceof We have presented a compendium of near-IR imaging-
the optically thick dust lane (Uetaetal. 2000). The present polarimetric data of evolved star CSEs that have been ob-
imagingpolarimetryprovidesan alternativemeansto differ- tained with HST/NICMOS to date, in order to study polar-
entiatethesetwomorphologicaltypes. Aswehavereviewed ization characteristics of these CSEs, especially of optically
in $ 3.3.1, we have founda prevalentmorphologicalfeature thicker bipolar DUPLEX PPNs. The data have shown that
-point-symmetryinthesurfacebrightnessdistributionofpo- theirpoint-symmetricappearanceisprevalent(seenin10out
larizedflux.Thispoint-symmetricappearanceisprobablydue of 11objects). Since the prevalentpoint-symmetrydoesnot
totheazimuthaldensitygradientbeingreversedintheoppos- seemtobeduetotheilluminationeffectsinalmostallcases
inglobesaboveandbelowtheequatorialplane. Ontheother (exceptforone),wehaveconcludedthattheobservedpoint-
hand,inSOLEPPNsthereisnoreversaloftheazimuthalden- symmetryisduetothepresenceoftheazimuthaldensitygra-
sity gradientaboveand belowtheequatorialplane, although dientintheCSEs.
the azimuthaldensity gradientis present. The best example These point-symmetric nebulae can be grouped into three
forthisisIRAS07134presentedinPaperI,inwhichthepres- based on the detailed morphologies. The first group con-
enceoftheazimuthaldensitygradienthasbeenshowninthe sistsofobjectsthatshowpolarizedfluxesneartheperiphery
dustandCOgasdistributioninitsshell(Meixneretal.2004). of the lobes in both I and P (IRAS 10178, 16594, 17150,
p
Thus, it appears that mechanisms that generate a reversal 17423,and19343). ByusingthesameargumentasinPaper
in the azimuthal density gradient in the opposite lobes are I,wesuggestthatthebipolarlobesinthesesourcesarehollow
related to the fundamental nature of the DUPLEX nebulae shells. Thesecondgroupconsistsofobjectsthatare charac-
whilethesemechanismsdonotoperateintheSOLEnebulae. terized by the very pronounced figure S shape and show P
Therefore,wepostulatethattheemergenceofthereversalin only near the periphery of the lobes but I inside the lobes
p
theazimuthaldustdensitydistributionisdirectlylinkedtothe (IRAS10197,17245,and17441). SincethereducedPinthe
origin of the morphological bifurcation between SOLE and middleofthenebulaisduetotheincreasedunpolarizedlight
DUPLEXPPNs. However,itisstillinconclusiveatthispoint inthenebula,weinterpretthatmultiplescatteringtakesplace
whatcausesthisreversal. intherelativelydenseequatorialregionsinthesesources.The
HSTNICMOSImagingPolarimetryofPPNs. II 9
last group is characterized by their clumpy nebulae (IRAS than20◦ (IRAS17441and19343). Therefore,wehavecon-
04395 and AFGL 2688) that may be due to episodic mass cluded that the occurrence of the oblique orientation of the
lossorinstabilitiesintheoutflows. WhilethelobesofAFGL equatorial plane with respect to the bipolar axis is real and
2688 show similarities to the second group, the presence of thatthisissueneedstobeinvestigatedfurtherinfuture.
theconcentricarcsmakesitratherunique. IRAS10131may
berelatedtoAFGL2688becauseofthepresenceofarcs.
We have found that the ways in which the azimuthalden- ThisresearchisbasedonobservationswiththeNASA/ESA
sitygradientismanifestedinSOLEandDUPLEXPPNsare HubbleSpaceTelescope,obtainedattheSpaceTelescopeSci-
verydistinct: the directionof the azimuthaldensity gradient ence Institute (STScI),which is operatedby the Association
inDUPLEXPPNsisreversedintheopposinglobesaboveand ofUniversitiesforResearchinAstronomy,Inc.(AURA)un-
belowtheequatorialplanewhereassuchareversalisnotseen der NASA contract No. NAS 5-26555. Authors acknowl-
inSOLEPPNs. Themostprobableexplanationforthisseems edge financial support by NASA STI 9377.05-A. Ueta also
to be the presence of precessing outflows into the opposite acknowledge partial support from the project IAP P5/36 fi-
directions and/or magnetic field that is reversed its polarity nancedbyFederalServicesforScientific,TechnicalandCul-
above and below the equatorial plane. We have also found turalAffairs of the Belgian State and the Universities Space
indicationsthattheequatorialplaneofthesystem(definedby Research Association SOFIA (StratosphericObservatoryfor
θ ) is notnecessarily orthogonalto the axis of the apparent InfraredAstronomy)office atNASA AmesResearch Center
eq
bipolarstructureinthetotalintensitydata(atleast7outof10 aswellasaUSNationalResearchCouncilResearchAssoci-
sources). Therearecasesinwhichtheobliqueangleismore ateshipAwardandaNASAPostdoctoralProgramAward.
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