Pereyra & Magalhães 2002, ApJS 141, 469

15 Pages·2002·2.93 MB·English

TheAstrophysicalJournalSupplementSeries,141:469–483,2002August E
#2002.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A.
POLARIMETRY TOWARD THE IRAS VELA SHELL. I. THE CATALOG
A.Pereyra1 and A.M.Magalha˜es2
InstitutodeAstronomia,Geof´ısicaeCieˆnciasAtmosfe´ricas,UniversidadedeSa˜oPaulo,CaixaPostal3386,
Sa˜oPaulo,SP01060-970,Brazil;[email protected],[email protected]
Received2001September26;accepted2002March28
ABSTRACT
WehaveobtainedlinearpolarizationmeasurementsofstarsalongthewesternsideoftheIRASVelaShell
towardHD62542.From16CCDfieldsdistributedalongtheionizationfront(I-front)wehavebuiltacata-
logof856objectswithpolarizationsignal-to-noiseratiolargerthan10.Wedetectverysignificantlevelsof
polarizationandhenceanappreciablemagneticfieldthroughouttheregion. Compositepolarization maps
around the I-front are shown. In some regions the polarization vectors are parallel to the I-front, but a
perpendiculartrendisalsoevidentalongpartsofthefront.Inaddition,thepolarizationpatternseemstobe
affectedbygasstreaminginsidethecloud.
Subjectheadings:catalogs — ISM:clouds — ISM:individual(GumNebula,VelaShell) —
ISM:magneticfields — polarization — stars:individual(HD62542)
On-linematerial:additionalfigures,machine-readabletable
1. INTRODUCTION The interstellar medium (ISM) toward the Puppis-Vela
region shows important, large-scale structures within 1–2
Interstellarshellsandassociatedshockwavesandioniza-
kpc from the Sun. Sahu (1992) analyzed the morphology
tionfrontsmaybedrivenbysupernovaexplosions,pressure
andkinematicsoftheISMinPuppis-Vela(l ¼240(cid:1),(cid:2)275(cid:1),
of photoionized gas, stellar winds, and collisions between
jbj<20(cid:1)) and identified three main structures, in order of
fast-moving clumps of interstellar gas (Draine & McKee
decreasingdistancefromtheSun:theVelaMolecularRidge
1993). The interaction between expanding shells and mag-
(r(cid:3)1 kpc), the Gum Nebula (GN, r(cid:3)800 pc), and the
netic field was examined by Troland & Heiles (1982) using
IRASVelaShell(IVS,r(cid:3)450pc).AlsopresentareOBand
Zeeman measurements, and they concluded that the mag-
Rassociations(VelaOB1,VelaOB2,andVelaR2).Early-
netic field severely limits the density enhancement in the
typestarsinclude(cid:1) Puppis(O5Iaf),thebrightestOstarin
cool, postshock gas. This is indeed expected theoretically
the sky, and (cid:2)2 Velorum (WC8+O9I), the brightest and
(Spitzer 1978). It is clear that magnetic fields in ionization
probablythenearestWolf-Rayetbinary.Agreatnumberof
fronts and shocks may be important in the evolution of
cometary globules and dark clouds are distributed in the
these systems. Magnetic ionization fronts have been
region(Hawarden&Brand1976;Sandqvist1976;Zealeyet
modeled for parallel (Redman et al. 1998) and oblique
al.1983;Reipurth1983).
(Williams,Dyson,&Hartquist2000)magneticfields.Inthe
TheVelaMolecularRidgeisagiantmolecularcloudthat
same way, the structure of shocks in a weakly ionized and
sitsbehindboththeGNandtheIVS.Inturn,theGNisan
dustless media depends on the orientation of the magnetic
enormousstructureofH(cid:3)emissionwithanangulardiame-
field with respect to the velocity of the shock (Hartquist,
terof(cid:3)36(cid:1)((cid:3)500pc),occupyingagreatpartofPuppisand
Pilipp, & Havnes 1997; Pilipp, Hartquist, & Havnes 1990;
Vela.Reynolds(1976a,1976b)proposedthattheGNisan
Pilipp&Hartquist1994;Wardle1998).
oldsupernovaremnantheatedbytheionizingUVfluxfrom
Polarization maps of background stars shining through
(cid:1)Puppisand(cid:2)2Velorum.
obscuredregionsmayrevealthemagneticfieldstructureof
Sahu (1992), based on IRAS maps of extended emission
theregions(forreviewsseeJones1996;Lazarian2000).The
andkinematicstudiesoftheionizedgasinPuppis-Velaand
directionofthelinearpolarizationwillprovidethedirection
proper motions in Vela OB2, discovered the existence of
ofthemagneticfieldprojectedontheplaneoftheskyinthe
the IVS, a ringlike structure with sections previously mis-
classical paramagnetic relaxation alignment (Davis &
taken as part of the GN but much closer. The IVS has a
Greenstein 1951; Jones & Spitzer 1967), where the average
radius of (cid:3)7=5 ((cid:3)60 pc) and is positioned asymmetrically
grainprofileisorientedperpendiculartothemagneticfield.
(l,bÞ(cid:3)ð263(cid:1),(cid:2)7(cid:1))withrespect totheGNcenter.While (cid:1)
Thisisgenerallynotdisputedwhenthevisualabsorptionis
Puppis, arunaway fromthe VelaR2association, isindeed
not too high (Lazarian, Goodman, & Myers 1997). Other
primarilyresponsiblefortheionizationoftheGN,theVela
alignment mechanisms, however, may predict grain align-
OB2association,whichincludes(cid:2)2Velorum,isresponsible
mentparalleltothefield,suchasthoseinvolvingsupersonic
fortheformationoftheIVSthroughstellarwindsandradi-
flows(Lazarian1994).
ationpressure.ThepositionoftheIVSontheskyisshown
in Figure 1. A more detailed, 100 lm view is shown in
Figure2.
TheIVSisinterestingforotherreasons.Thelineofsight
1AlsoatFacultaddeCiencias,UniversidadNacionaldeIngenier´ıa,Av. toHD62542interceptsthewestsideoftheIVS(Figs.2and
Tu´pacAmaru210,R´ımac,Casilla31-139,Lima,Peru´. 3).HD62542hasaratheranomalousUVextinctioncurve
2Visiting Astronomer, Cerro Tololo Inter-American Observatory. (Cardelli&Savage1988)withaveryweak2200A˚ extinction
CTIOisoperatedbyAURA,Inc.,undercontracttotheNationalScience
Foundation. bump. Previous works associated the structure toward
469
470 PEREYRA & MAGALHaA˜˜ ES Vol. 141
Fig.1.—GalacticdistributionmapofdarkcloudsfromFeitzinger&Stu¨ve(1984)withtheIRASVelaShellregionindicated.ThepositionofHD62542is
shownbytheasterisk(*).
HD 62542 with the GN (Cardelli & Savage 1988), but its (CTIO), Chile. The polarimeter is a modification of the
spatialdistributionisinfactmoreconsistentwiththatofthe CTIOdirectCCDcameratoallowforhighprecisionimag-
IVS(Sahu1992;Churchwelletal.1996).Theeastborderof ingpolarimetry.Thefirstelementinthebeamisarotatable,
the structure (inside the cavity) shows a well-defined achromatic half-wave retarder followed by a Savart plate.
ionization front (I-front), seen almost edge-on, with This provides two images of each object in the field, sepa-
different curvatures in the north and south sections. The rated by 1 mm (corresponding to about 1800 at that tele-
IVS far-infrared dust emission accompanies the morphol- scope’sfocalplane)andwithorthogonalpolarizations.One
ogy of the I-front (Sahu 1992), implying that the radiation polarizationmodulationcycleiscoveredforevery90(cid:1)rota-
field of Vela OB2 and shocks induced by the expansion of tionofthewaveplate.Thesimultaneousimagingofthetwo
theshellmustheatthedust.Ascenarioofanionizationand beams allows observing under nonphotometric conditions
shockfront(I/S-front)mightprevailinthisregion(Church- andatthesametimetheskypolarizationispracticallycan-
well et al. 1996). Whittet et al. (1993) analyzed the IRAS celed(Magalha˜esetal.1996).
sources around HD 62542 and concluded that there is no Figure 3 shows the distribution of CCD fields observed
evidence for current or recent star formation along that and Table 1 provides the position of the field centers. The
ridge(Fig.3). datawerecollectedin1991Decemberand1992December.
Churchwelletal.(1996)studiedthisregioninCS,andthis CCDexposuresforeachfieldweretakenthroughtheVfil-
emission accompaniestheI/S-frontandtheemission from ter with the waveplate rotated through four positions 22=5
dust.TheyfoundthatthekineticenergyoftheCScloudis1 apart.Withthe1024(cid:4)1024CCDused(CTIOTekNo.1),
order of magnitude higher than the gravitational potential
energy. This indicates that the structure is not gravitation-
allyboundandwoulddisperseonveryshorttimescaleswere TABLE 1
itnotfortherampressureoftheexpandingIVS,whichcon- CCDFieldPositions
tinuallysweepsupnewinterstellarmatterintothecloud.
In summary, the IRAS Vela Shell encloses a cavity that Field R.A.(J2000) Decl.(J2000) Mission
appearstohavebeenformedbystarsoftheVelaOB2asso-
01........... 74237.08 (cid:2)421344.66 91
ciation through the effects of stellar winds and supernovae 02........... 74201.22 (cid:2)420702.30 91
explosions (Sahu 1992). In this paper we present a catalog 03........... 74125.31 (cid:2)420202.93 91
ofthelinearpolarizationofstarsseentowardawesternsec- 04........... 74312.93 (cid:2)422025.03 91
tion of the IRAS Vela Shell. Our technique provides good 05........... 73859.84 (cid:2)413655.29 92
qualitydataforarelativelylargenumberofobjects.Wealso 06........... 73932.15 (cid:2)412859.42 92
discuss the general properties of the polarization vectors 07........... 74003.96 (cid:2)413659.53 92
throughout the region. The fact that the structure is seen 08........... 74003.70 (cid:2)414459.52 92
09........... 74035.51 (cid:2)415301.63 92
almost edge-on is indeed helpful to the interpretation of
10........... 74211.43 (cid:2)420106.96 92
polarization measurements. In a forthcoming paper, we
11........... 74210.92 (cid:2)421707.95 92
presentafulleranalysisofthedataandrelatethemtoother
12........... 74242.72 (cid:2)422511.05 92
existingdataontheIVSstructure. 13........... 74210.66 (cid:2)422507.94 92
14........... 74106.41 (cid:2)422903.71 92
15........... 74009.39 (cid:2)425659.98 92
2. OBSERVATIONS AND DATA REDUCTION
16........... 73936.94 (cid:2)430858.84 92
TheobservationsweremadeusingaCCDimagingpolar-
Note.—Units of right ascension are hours, minutes,
imeter (Magalha˜es et al. 1996) attached to the 1.5 m
and seconds, and units of declination are degrees, arc-
Telescope of Cerro Tololo Inter-American Observatory minutes,andarcseconds.
No. 2, 2002 POLARIMETRY TOWARD THE IRAS VELA SHELL 471
Fig.2.—IRASimagein100lmofthewestsectionoftheIRASVelaShelltowardHD62542,obtainedfromtheSkyViewWebpage(http://skyview.gsfc.-
nasa.gov/skyview.html).Thefigurecovers14(cid:1)(cid:4)14(cid:1).The(cid:3)1(cid:1)(cid:4)2(cid:1)boxedregionindicatedhighlightsthesectionoftheIVSwhichwestudiedandwhichis
detailedinFig.3.(cid:1)Puppisand(cid:2)Velorumareindicatedbyaplusandanasterisk(*),respectively.
each field covers (cid:3)80(cid:4)80. The exposure time at each posi- waveplate position angle ( ) with respect to the expected
i
tion was typically 300 s. Exceptions are fields 01 (10 s), 02 cos4 curveandarequotedinTable2;theyareconsistent
i
(180s),03(240s),and04(240s). with the photon noise errors (Magalha˜es, Benedetti, &
Afterbiasandflatfieldcorrections,photometrywasper- Roland 1984). A set of specially developed IRAF tasks to
formed for both images of stars in each field using the study the polarization data in (eventually crowded) stellar
IRAF3DAOPHOT package,agroup ofprocedures which fields(Pereyra2000,PCCDPACKpackage)wasextensively
createasetofdatafilesforeachimagefield.Aspecialpur- used.Eachfieldwaspainstakinglycheckedforobjectswith
pose FORTRAN routine processed these data files and suspiciouspolarizationthatwereusuallyrelatedtoanover-
calculated the normalized linear polarization from a least- lapping image of a nearby object; these objects were
squaressolution.Thisyieldsthepercentlinearpolarization removedfromthecompilationpresentedhere.
(P),thepolarizationangle(h,measuredfromnorthtoeast), TheinstrumentalQ-andU-valueswereconvertedtothe
the Stokes parameters Q and U, as well as the theoretical equatorial system from standard star data obtained in the
(i.e.,thephotonnoise)andmeasured((cid:4) )errors.Thelatter samenight.Theinstrumentalpolarizationwasmeasuredto
P
areobtainedfromtheresidualsoftheobservationsateach belessthan0.03%,andnosuchcorrectionwasapplied.We
have not purposely applied any correction for statistical
bias(Simmons&Stewart1985)tothepolarizationvaluesin
3IRAFisdistributedbytheNationalOpticalAstronomyObservatory,
Table 2, so that others may handle the data more readily.
which are operated by the Association of Universities for Research in
Astronomy,Inc.,undercooperativeagreementwiththeNationalScience Such correction would be small in any case for the high
Foundation. P=(cid:4) -values(x4)ofthecatalog.
P
472 PEREYRA & MAGALHaA˜˜ ES Vol. 141
CCDfield.Theunderlyingimagesinthefindingchartswere
madefromtheDigitizedSkySurvey(DSS).InTable2,the
firstcolumnistheCCDfieldnumber.Thesecondcolumnis
the identification number (ID) of the object in each field.
Thisnumberlabelsthepositionoftheobjectinthefinding
charts.Thefourthandfifthcolumnsaretheequatorialcoor-
dinates (2000). The estimated error in the position is less
than100.Thesixthandseventhcolumnsarethepolarization
valueintheVfilteranditserror.Thepolarizationangleisin
theeighth column and the estimated visual magnitude (see
below)isinthelastcolumn.
Thepositionsoftheobjectswerecalculatedbycomparing
objectspresentinbothourCCDfieldsandtheDSS.Inthis
waythe(x,y)positionsinpixelsweretransformedto(R.A.,
decl.) coordinates using the plate parameters available in
the digitized image headers. The images used for this con-
versionarethesameusedasfindingchartsforeachfield.
For a given field the visual magnitudes were calculated
usingthefirstofthefourimagesobtained,eachcorrespond-
ingtoagivenhalf-waveplateposition.Foragivenobjectwe
addedthecountsoftheordinaryandextraordinarybeams
within the aperture used in the calculation of the polariza-
tion for the object. The instrumental magnitude obtained
wasthencorrectedtothestandardsystem.Theadditivecor-
rectionwasestimatedineachfieldbyobtaininganaverage
difference between the values of the magnitudes of the
General Star Catalog (Lasker et al. 1990) (GSC) and our
instrumental magnitudes for objects present in both sam-
ples. The GSC magnitudes used in this correction corre-
spond to that of a Schmidt plate with the combination
emulsion/filter=IIIaJ/GG395,centeredin4500A˚ (Lasker
et al. 1990) and which were transformed to the V filter by
Lasker et al. (1990). These magnitude estimates must be
Fig.3.—RegionoftheIRASVelaShelltowardHD62542overwhichwe taken just as indicative because of the impossibility of
haveperformedtheimagingpolarimetry.Theunderlyingimagesinthisand obtainingappropriatecorrectionstoourdataforthecolor
theremainingfiguresarefromtheDigitizedSkySurveyandwereobtained
term and the extinction coefficient as usual. The typical
eitherthroughtheSkyViewWebpage(Fig.15)oritsCD-ROM(thisfigure
and Figs. 5 through 14). The region covers an area of (cid:3)1(cid:1)(cid:4)2(cid:1). The magnitude uncertainty in our catalog is dominated by the
numbersindicatethepositionsofeachoneofthe16CCDfieldsobserved. uncertaintyintheGSCmagnitudesandwasestimatedtobe
Eachfieldcoversanareaof80(cid:4)80.ThepositionofHD62542coincides 0.33magfromtheresultingrmsdeviations.Ahistogramof
withthatofthecenteroffield01. thecalculatedmagnitudesisshowninFigure4.
3. THE CATALOG 4. POLARIZATION MAPS
Thepolarimetricdataarepresentedinonelongtablewith In Figure 5 we show a mosaic of the polarization maps
the data of the 16 CCD fields (Table 2) and the associated acrossthe(cid:3)1(cid:1)(cid:4)2(cid:1)regionstudied.Inthefigureeachvector
finding charts (Figs. 15a–15p, available in the on-line edi- representsanobjectwithpolarizationsignal-to-noiseratio,
tionoftheAstrophysicalJournalSupplement),oneforeach P=(cid:4) , larger than 10. As mentioned earlier, data with a
P
TABLE 2
FieldData
P (cid:4) h V
V PV
Field ID Flag R.A.(J2000.0) Decl.(J2000.0) (%) (%) (deg) (mag)
01....... 9 7 74216.21 (cid:2)421143.30 0.309 0.019 81.9 15.74
01....... 23 7 74220.26 (cid:2)421117.44 0.344 0.029 29.5 15.01
01....... 24 7 74221.96 (cid:2)421543.07 2.088 0.127 122.4 17.04
01....... 30 7 74223.86 (cid:2)421008.11 0.981 0.013 45 14.17
01....... 32 7 74224.56 (cid:2)421124.34 1.948 0.066 62.6 16.42
Notes.—Unitsofrightascensionarehours,minutes,andseconds,andunitsofdeclinationaredegrees,
arcminutes,andarcseconds.Table2isavailableinitsentiretyintheelectroniceditionoftheAstrophysical
JournalSupplement.Aportionisshownhereforguidanceregardingitsformandcontent.
No. 2, 2002 POLARIMETRY TOWARD THE IRAS VELA SHELL 473
lowerS/Nratiomaystillbeusefulbutlesscertainespecially
for faint objects where overlapping with the ordinary or
extraordinarybeamsofnearbyobjectsmayoccur.Thetotal
numberofobjectswithP=(cid:4) >10is856.
P
Nineteen objects have two independent polarimetric
measurements from overlapping fields and are indicated
withtheflaginthethirdcolumnandinfootnotesofTable2.
AplotofthesemeasurementsisshowninFigure6,whereit
can be seen that they are reasonably consistent with one
another. The weighted mean value of the differences
between the pairs of polarization measurements is
0:24%(cid:5)0:01%, comparable withpthe typical error in the
ffiffiffi
catalog, (cid:3)0.1%, multiplied by 2. For the polarization
angles, the weighted mean value of the differences is
5=2(cid:5)0=1.Atypicalpolarizationvalueinthecatalogis(cid:3)1%
which, with the typical error above, gives (cid:3)3(cid:1) as a typical
Fig.4.—Histogramofmagnitudesfortheobjectsinthecatalog uncertaintyforthepositionangle,againcomparabletothe
weighted mean value of the position angle differences. Of
course,polarization,beingapositivedefinitequantity,does
notobeynormalstatistics,asopposedto(Q,U)valueswith
a good polarimetric signal-to-noise (Simmons & Stewart
1985).Usingthe(Q,U)valuesforthepairsofpolarization
measurementsofFigure6weobtainsimilarresults.Inany
case,anyaveragingofpolarizationdataforstarsinthecata-
logbyusersshouldbedoneusing(Q,U)values.
Fromacursorycomparisonbetweentheobservedalign-
mentandthemorphologyofthissectionoftheIVS,itisevi-
dentfromFigure5thatmorethanonealignmentstructure
ofthepolarizationvectorsalongtheI/S-frontprevails.To
showthisinmoredetail,wepresentsectionsofthecompo-
sitepolarizationmapbygroupingtwoorthreefieldssuper-
imposedontheirrespectiveDSSimages.Inthispreliminary
analysis, we follow the ridge roughly from north to south
andconcentrateonthequalitativepatternsofthepolariza-
tion maps. Some fields are shown more than once to help
follow the patterns along the front from one figure to the
next.
Fig.5.—Mosaicofthepolarizationvectormapsforthe16CCDfields
superimposedonaDigitizedSkySurveyimage.Thepolarizationscaleis
shownatthetopofthefigure.Eachvectorrepresentsanobjectwithpolar-
izationsignal-to-noiseratiolargerthan10.Thetotalnumberofobjectsin Fig.6.—Objectswithtwoindependentpolarimetricmeasurementswith
thecatalogandplottedaboveis856. errors.Thedashedlineisjustthe45(cid:1)linetohelpthecomparison.
474 PEREYRA & MAGALHaA˜˜ ES Vol. 141
4.1. CommentsonIndividualRegions nebula, follows closely that of the nebula border. Field
07 also shows higher polarization values than either fields
4.1.1. Fields05,06,and07
05 or 06; given the otherwise very smooth inferred mag-
In Figure 7 we show the polarization map of fields 05, netic field structure across the area, this is probably
06, and 07. The area shown is 250(cid:4)180 (3:3(cid:4)2:4 pc, at related to the variation of inclination of the magnetic
450 pc). A very smooth structure of the polarization field to the line of sight from one field to the other.
vectors is evident within each CCD field and from one Another possibility is that of field enhancement in the
CCD field to the other. It is also interesting to note the compressed postshock gas. This might translate into a
apparent alignment of the polarization pattern with the smaller dispersion of position angles for field 05 as com-
overall morphology of the I/S-front. The pattern of field pared with that of field 07. Such analysis will be made in
07 in particular, closer to the ridge at that point in the the forthcoming paper.
Fig.7.—Polarizationvectormapforfields05,06,and07.Theareacoveredbyeachfieldisoutlinedbyadashedbox.Thepositionsoftheindividualfields
areshowninthesmallerframeforclarity.Thepercentpolarizationscaleisshownatthetopofthefigure.Thescalesforeachoftheremainingfiguresare
similarlyshown.
No. 2, 2002 POLARIMETRY TOWARD THE IRAS VELA SHELL 475
4.1.2. Fields07,08,and09 following the polarization pattern displayed by the denser
partsoffield04(seeFig.5).
InFigure8weshowthevectorplotsforfields07,08,and
09, spread over an area of 180(cid:4)300 (2:4(cid:4)3:9 pc). The
observedpolarizationpatternoffield07followstheborder 4.1.4. Fields02and03
ofthenebularridgeandseemstojoinsmoothlywiththatof InFigure10weshowthecompositepolarizationmapfor
field08tothe south. Infield09 thepolarization values are the fields 02 and 03. The area shown covers 180(cid:4)150
low and no dominating pattern is evident. Field 09 covers (2:4(cid:4)2:0pc).Bothfieldsareinsidetheemissionregionand
anemissionstructurewhichisseenseparatedfromthemore apatternalmostperpendiculartotheridgeprevails.Never-
northernsectionoftheridgewherefields07and08sit. theless, small distortions in this pattern exist and may be
correlated withthe uneven observed emission. Inaddition,
almostdirectlyeastoffield02(SEoffield03),Churchwellet
4.1.3. Fields10and11 al. (1996) detected a condensation in CS emission (their
clumpC)whichseems,intheirownwords,tobe‘‘eroding
Fields10and11areshowninFigure9.Theareashownis
away’’ due to the expanding nebula. It is possible that the
120(cid:4)280 (1:6(cid:4)3:7 pc). The saturated star near field 11 is
polarization pattern in field 02 is just reflecting this swept-
HD 62542. Like field 09 to its northwest, field 10 does not
upgas.Seealsodiscussiononfield04belowandx4.2.
show a clear polarization pattern. We note that field 10
straddles both sides of the well-defined interface of the
4.1.5. Fields01and04
ridge.Theemissionisalsonotuniformoverthefield.Field
11 is farther from the borders of the ridge and is also Figure11showsfields01and04inanareawhichcovers
immersedinacirrus-likeemissionthatthepolarizationpat- 160(cid:4)160(2:1(cid:4)2:1pc).Field01,centeredinHD62542(the
tern appears to follow. This field issituated downstream of saturated starintheDSSimage),showsfewobjectswitha
field04andclumpAofChurchwelletal.(1996);seex4.1.5 polarization S/N ratio larger than 10, since only an 10 s
below. The polarization pattern of field 11 may well be integration time per half-waveplate position was used. No
Fig.8.—Polarizationvectormapforfields07,08,and09
476 PEREYRA & MAGALHaA˜˜ ES
Fig.9.—Polarizationvectormapforfields10and11
general alignment pattern seems evident. Nevertheless, a beswept-upambientgasbytherampressureoftheexpand-
careful examination shows that objects near HD 62542 ingnebula.Interestingly,thefieldlinesoffield04,pointing
(ID23,73,76,and88infield01ofTable2)presentvaluesof away from the condensation, would seem to be consistent
polarizationpositionanglesimilartothestar(Claytonetal. withsuchpicture.
1992,30=8).
In field 04, a bimodal alignment pattern exists: one at 4.1.6. Fields11,12,and13
(cid:3)40(cid:1) just east of the ridge and another pattern at (cid:3)100(cid:1) The composite polarization map of fields 11, 12, and
toward the denser parts of the nebula. This symmetry 13 is shown in Figure 12. The area shown in this figure
resembles that of field 12 discussed in x 4.1.6 below covers 200(cid:4)200 (2:6(cid:4)2:6 pc). In field 11, as pointed out
(Fig. 12). It may not be totally unexpected that the denser above, the polarization pattern seems to follow the cir-
partsoffield04showneitheraperpendicularnoraparallel rus-like structures seen in emission. The polarization pat-
alignmentwiththenearbyridgesincethelatterchangescon- terns of fields 12 and 13 seem to smoothly join that of
cavityaroundthatregion,inadditiontotheeffectsofapro- field 11. The pattern of field 12 is very interesting and
jection of a three-dimensional surface on the sky. It is, similar in some respects to that of field 04. Northwest of
however,alsoofinteresttonotethattheregionsoutheastof field 12 center, the pattern follows (cid:5)(cid:3)115(cid:1) like fields 11
field 04 is coincident with the condensation labeled V12 in and 13. Northeast the field center, another polarization
thelistofVilas-Boas,Myers,&Fuller(1994)andclumpA direction seems to prevail. South of the field center, a
in the CS study of Churchwell et al. (1996). The latter pattern perpendicular to the southern ridge is observed.
authorssuggestthattheCSemissionaroundtheclumpmay Field 12 includes clump B of Churchwell et al. (1996),
Fig.10.—Polarizationvectormapforfields03and02
Fig.11.—Polarizationvectormapforfields01and04.ThebrighteststarisHD62542.
478