spitzer irac observations of newly discovered planetary nebulae from the macquarie-aao ...

20 Pages·2007·1.08 MB·English

TheAstrophysicalJournal,669:343Y362,2007November1
#2007.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A.
SPITZER IRAC OBSERVATIONS OF NEWLY DISCOVERED PLANETARY NEBULAE
FROM THE MACQUARIE-AAO-STRASBOURG H(cid:1) PLANETARY NEBULA PROJECT
Martin Cohen,1 Quentin A. Parker,2,3 Anne J. Green,4 Tara Murphy,4,5 Brent Miszalski,2,6 David J. Frew,2,7
Marilyn R. Meade,8 Brian Babler,8 Re´my Indebetouw,9 Barbara A. Whitney,10
Christer Watson,11 Edward B. Churchwell,8 and Douglas F. Watson8
Received2007April6;accepted2007July6
ABSTRACT
WecompareH(cid:1),radiocontinuum,andSpitzerSpaceTelescopeimagesof 58planetarynebulae(PNe)recently
discoveredbytheMacquarie-AAO-StrasbourgH(cid:1)PNProject(MASH)oftheSuperCOSMOSH(cid:1)Survey.Using
InfraredArrayCamera(IRAC)data,weexaminetherelationshipsbetweenopticalandMIRmorphologiesfrom3.6to
8.0(cid:2)mandexploretheratioofmid-infrared(MIR)toradionebularfluxes,whichisavaluablediscriminantbetween
thermalandnonthermalemission.MASHemphasizeslateevolutionarystagesofPNecomparedwithpreviouscatalogs,
enablingstudyofthechangesinMIRandradiofluxthatattendtheagingprocess.Spatiallyintegratedfluxesare
constructedforallMASHPNeobservedbytheGLIMPSELegacyProject,usingtheH(cid:1)morphologiestodefinethe
areasofMIRandradiocontinuumemissionobservedbytheMidcourseSpaceExperiment(MSX),IRAC,theMolonglo
ObservatorySynthesisTelescope,andtheVeryLargeArray(VLA).Theratioof IRAC8.0(cid:2)mtoMSX8.3(cid:2)mfluxes
providesanassessmentoftheabsolutediffusecalibrationof IRACat8.0(cid:2)m.Weindependentlyconfirmtheaperture
correctionfactortobeappliedtoIRACat8.0(cid:2)mtoalignitwiththediffusecalibrationofMSX.Theresultisinaccord
withtherecommendationsoftheSpitzerScienceCenterandwithourresultsfromaparallelstudyofHiiregionsinthe
MIRandradio.However,thesePNeprobethediffusecalibrationofIRAConaspatialscaleof900Y7700,asopposed
tothemany-arcminutescalefromtheHiiregions’study.
Subject headinggs: infrared: ISM — planetary nebulae: general — radiation mechanisms: thermal —
radio continuum: ISM — space vehicles
1. INTRODUCTION Surveyinthesouth(MGPS2;Green2002)ortheNRAOVLA
SkySurvey(NVSS)inthenorth(Condonetal.1998).
TherecentavailabilityoftheSuperCOSMOSAAO/UKSTH(cid:1)
MASHPNelieinsideGalacticlatitudesofjbj10(cid:2)Y13(cid:2).MSX
surveyof thesouthernGalacticplane(SHS;Parkeretal.2005)
imagedtheentireGalacticplanewithin(cid:3)5(cid:2),whileGLIMPSE
hasledtoasubstantial60%increaseinthenumbersofGalactic
isconfinedto(cid:3)1(cid:2).ForthepresentmultiwavelengthstudyofPNe
planetarynebulae(PNe)identified.Thisincreaseislargelydueto
weinvestigatedbothregionsoftheGLIMPSEsurvey,covering
theexcellentcombinationofresolution((cid:1)100),sensitivity((cid:1)5R),
thelongitudesfrom10(cid:2)to65(cid:2)and295(cid:2)to350(cid:2).Aspartofthe
andarealcoverage(4000deg2)offeredbythispowerfulonline
finalcullingofMASHcandidatespriortopublication,various
survey.12These905newPNehaverecentlybeenpublishedasthe
multiwavelengthcomparisonsweremadebetweenMASHPN
Macquarie-AAO-StrasbourgH(cid:1)PNproject(MASH)byParker
candidates and other extant data from MSX, 2MASS and, for
et al. (2006). In this paper we describe a multiwavelength ex-
theareaofoverlap,GLIMPSE.Therewere85PNecandidates
aminationofasubsetoftheseobjectsforwhichSpitzerSpace
intheMASH/GLIMPSEoverlapzonetobeginwith.Mid-infrared
Telescope (Werner et al. 2004) observations are available be-
(MIR)dataarevaluabletostudythepropertiesoftheseobjects
tween3.6and8.0(cid:2)mfromtheGLIMPSEsurveyofportionsof
aswellastoremovemisidentificationsfromMASH(Cohen&
theGalacticplane(Benjaminetal.2003;Churchwelletal.2004).
Parker2003).Parkeretal.(2006)discussindetailhownon-PN
WecomparemultiwavelengthimagesofPNeinH(cid:1),the8.3(cid:2)m
contaminantswereeliminatedfromtheMASHcatalog.Anenvi-
bandoftheMidcourseSpaceExperiment(MSX;Priceetal.2001),
ronmentwiththeindicatorsofactivestarformationorheavydust
thefourInfraredArrayCamera(IRAC;Fazioetal.2004)bands,
obscuration,oranopticalmorphologyshowingmultiplepatchesof
andtheradiocontinuumfromeithertheMolongloGalacticPlane
nebulosity,possiblylinked,sufficetoremovemanyHiiregions.
AnyobjectthatlacksH(cid:1)emissionwillappearthesameinthe
narrowband H(cid:1)and accompanying broadband red continuum
1 RadioAstronomyLaboratory,UniversityofCalifornia,Berkeley,CA94720;
exposures, precluding reflection nebulae and normal galaxies.
[email protected].
2 DepartmentofPhysics,MacquarieUniversity,Sydney,NSW2109,Australia. CompactHiiregionswereeliminatedbyopticalspectroscopy
3 Anglo-AustralianObservatory,Epping,NSW2121,Australia. basedontheweaknessof [Nii]kk(6548+6584)relativetoH(cid:1)
4 Schoolof Physics,UniversityofSydney,NSW2006,Australia. emission(hereafter[Nii]/H(cid:1)).Additionalemission-lineobjects
5 SchoolofInformationTechnologies,UniversityofSydney,NSW2006, thatare not PNe were rejected by opticalspectroscopy if they
Australia. lackedthecharacteristicPNlines(e.g.,[Oiii]intheblue;[Oi],
6 ObservatoireAstronomique,Universite´LouisPasteur,67000Strasbourg,
France. [Nii],and[Sii]inthered)orshowedacontinuum.MIRmorphol-
7 PerthObservatory,WalnutRoad,BickleyWA6076,Australia. ogyisoftensufficienttodiscriminatebetweenPNeandHiiregions.
8 DepartmentofAstronomy,UniversityofWisconsin,Madison,WI53706. MASH PNe are generally distinguishable from previously
9 AstronomyDepartmentUniversityofVirginia,Charlottesville,VA22904.
knownnebulaebytheirlargersizeandfaintness.Thisdistinc-
10 SpaceScienceInstitute,Boulder,CO80303.
11 DepartmentofPhysics,ManchesterCollege,NorthManchester,IN46962. tiontranslatestolargeproportionsofhighlyevolvedobjectsof
12 Seehttp://www-wfau.roe.ac.uk/sss/halpha. largeapparentsizeandlowsurfacebrightnessthathavealmost
343
344 COHEN ETAL. Vol. 669
Fig.1.—Left:MIRfalse-colormorphologyoftheIRACcounterparttoPHR1517(cid:4)5751(blue,green,andred:emissionat4.5,5.8,and8.0(cid:2)m,respectively).Image
is40;40.TheobjectconsistsofanalmostcompleteringwhichhastheappearanceofanHiiregionratherthanaPN.Centerofthesmallcircle(1200radius)istheMASH
centroidofthenebula.Right:H(cid:1)imageofthesamePNcoveringthesameareaasshownintheIRACimage,overlaidbywhitecontoursof8.0(cid:2)memissionatlevelsof
50,70,100,200,and300MJysr(cid:4)1.
dissolvedintotheinterstellarmedium(ISM),andofcompactfaint 2. THE SAMPLE OF PNe
PNe.ThiscomplementaritybetweentheMASHandoldPNcat-
2.1. Removing Non-PNe from MASH
alogsisimportantbecauseonlyasmallfractionoftheestimated
(cid:1)30,000GalacticPNe(e.g.,Frew&Parker2005)areknown. Figure 1 compares the H(cid:1) appearance with the IRAC mor-
TounderstandPNeitisvitaltoidentifymoreofthispopulation phologyofaPNcandidatethathasnowbeenreclassifiedasan
andtohaveaccesstoabroadrepresentationofdifferentevolu- Hiiregion.ItwasinitiallyconsideredaPNcandidatebecause
tionarystages.Thishelpsingaugingtherelationshipbetween of its plausible optical morphology and its reasonable optical
morphologyand ageand inquantifyingthegradualchemical redspectrum(Fig.2)with[Nii]/H(cid:1)of0.60,closetothecutoff
enrichmentoftheGalaxy. valueforHiiregionsof0.70(Kennicuttetal.2000).Highextinc-
Carefulconsiderationof MASHopticalspectraandofMSXand tionprecludedbluespectroscopythatmighthaverevealedtheab-
Spitzerimageryof MASHPNcandidatesresultedintherejection senceof PNlines.However,therelativelocationsof H(cid:1)and8(cid:2)m
ofatotalof27candidates:4objectswerefoundtocorrespondto emission(seeFig.1)andwhatappeartobepossiblesecondary
portionsofknownsupernovaremnants;4areinconclusiveiden-
tificationsfollowingconsiderationoftheavailableopticaland
IRdata;1isasymbioticstar;2are‘‘likely’’PNe;andtheremain-
ing16areprobablyHiiregionsand,inanycase,wereneverclas-
sifiedasmorethan‘‘possible’’PNe.Fifty-eightnebulaesurvived
thiscull,composedof28true, 16likely, and14 possiblePNe
(fromtheclassificationcategoriesof Parkeretal.2006).Objects
inconclusiveinnaturehavebeenplacedintheMASHMiscella-
neousEmissionNebulae(MEN)cataloguntiltheirtruecharacter
canbediscovered.SomeofthesemaylaterturnouttobePNeand
willreturntoMASH.
Inthispaperwepresentthesampleof PNeandsummarizetheir
multiwavelengthcharacteristics(x2);describethetoolsweuseto
confirmtrueMIRcounterpartsofthe58objects(x3);comparethe
ratioofIRAC8.0(cid:2)mtoMSX8.3(cid:2)mfluxestoinvestigatefurther
theaccuracyof IRAC’sabsolutediffusecalibrationonsmaller
spatialscalesthanprobedbyCohenetal.(2007)intheirstudyof
Hiiregions(x4);comparetheIRACcolorsoftheMASHPNe
with those found by Hora et al. (2004) to provide a diagnostic
color-colorplotthatdistinguishesPNefromothersourcesinthe
GLIMPSEdata(x5);examinetheratioofspatiallyintegrated
MIR/radiofluxdensityforPNe(x6);andcompareMIRandop- Fig.2.—Redspectrumof PHR1517(cid:4)5751showingtheweaknessofthe
ticalmorphologiesofPNe(x7).Section8givesourconclusions. [Nii]linesrelativetoH(cid:1)thatdefinethisobjectasanHiiregion.
No. 1, 2007 IRAC OBSERVATIONS OF MASH PLANETARY NEBULAE 345
Fig.3.—Left:MIRfalse-colorimage(asinFig.1)ofthebipolarcounterparttoPHR1253(cid:4)6350.Imageis40;40.NotetheseveralbrightMIRfilamentsthatextend
forseveralarcminutes.CircleandimagesizeasforFig.1.Right:H(cid:1)blowupshowingnebulardetailintheformoftwoasymmetricbipolarlobesflankingtheimageof
thesymbioticstar.
starformationregions(severalcompactHiiregionsonthenorth- paper(Q.A.Parkeretal.2008,inpreparation).Althoughwe
westernrim)aremuchmoreconsistentwithanHiiregion. regarditasanon-PN,theinfluenceofenvironmentontheevo-
FurtherexamplesofmoresubtlecontaminantsinMASHare lutionofPNeisstillarelevantissue,asshownbytheworkof
foundamonghighlycollimatedbipolarnebulaethatcontaindusty Villaveretal.(2006)onthepotentialcontributionoftheISM
equatorialdisksproducingthermalemissionfromwarmdustwith toPNmorphology.
temperaturestypicallyintherange250Y400K.Onesuchinter- OnecaveatrelatestotheconstraintimposedbytheGLIMPSE
estingobjectisPHR1253(cid:4)6350,previouslyidentifiedasapos- survey parameters on MASH PNe in this paper. The narrow
siblehighlycollimatedbipolarPNwithabrightcentralstar(CS). latituderange,aroundtheGalacticequator,ofGLIMPSEshould
Figure3presentsbothMIRandH(cid:1)morphologies.(TheMIR resultinrelativelyfewPNebecauseoftheheavyextinction.The
imageisathree-bandfalse-colorimage,asdescribedinx3.2.) MASHsurveyhasfoundnewPNeintheplanebecauseoftheex-
The nebular spectrum (Fig. 4) is PN-like with ½O iii(cid:5)/H(cid:3) (cid:6)10, cellentsensitivityoftheAAO/UKSTH(cid:1)survey,inwhichthe
½N ii(cid:5)/H(cid:1) (cid:6)0:1,butwithanobviousblue,aswellasaweaker longercentralwavelengthoftheinterferencefilter,comparedwith
red,continuum.TheCSisnotbluebutisbrightinthenear-infrared moretraditionalPNsearchtechniquesbasedon[Oiii]detection,is
(NIR),with2MASSphotometryyieldingJ (cid:4)H (cid:1)1:43andH(cid:4) lessaffectedbyextinction.Partofthissuccessalsostemsfromthe
K (cid:1)1:41.Combiningthesevalueswithamagnitudeat3.6(cid:2)mof evidentpatchinessoftheopticalobscuration.However,only6%
7.35andK(cid:4)½3:6(cid:5)of2.87indicatesthatthestellarphotosphereis oftheMASH-catalogedPNeliewithin1(cid:2)ofzerolatitude.Infact,
overwhelmedbythermalemissionfrom(cid:1)700Kdust(seeAllen& onemightwellidentifyanH(cid:1)emissionregionalonganunusually
Glass1974;theirFig.2).Theopticaloutflowscoincideexactly clearline-of-sightasanisolatedPNwhenitismerelyalessob-
withastrongelongatedemissionridgeofamuchmoreextensive scuredpartofalargerentity.Agoodexampleofthisphenomenon,
underlyingMIR structure thatextends infilamentary formfor whichagainemphasizesthevalueof MIRmapsoftheplane,is
severalarcminutesfromnortheasttosouthwest.Thesestructures shownbyFigure5.PHR1841(cid:4)0503’slocationisidentifiedby
areclearlyrelatedandcertainlyatypicalof PNe,asaretherising thecrossinthis8.0(cid:2)mSpitzerimage.Onecannowrecognize
continua.InitsH(cid:1)structureandtheshapeofitsMIRcore,this thatthecandidatecorrespondsonlytoalocalopticalbrighten-
nebulaisstronglyredolentofthehighlycollimated,high-density ingalonganextendedridgeofpolycyclicaromatichydrocarbon
nebulaediscussedbyCorradi(1995).Helinkedthesesystemsto (PAH)emission.Ifthesmallwesterlyoffsetfromthecenterof
interactingbinariescontainingasymbioticstar.Amongtheusual the MIR ridge were significant, then one might argue that the
nebular lines, as well as those from H and He i, one sees four ridge represents a photodissociation region (PDR) created by
[Feii]linesthatareassociatedwithsymbioticspectra.Hence,a ionizingradiationfromsomestar(s)tothewestofthesinuous
morelikelyidentificationwouldbeahighlycollimatedbipolar bright8.0(cid:2)mfilament.
outflow,perhapsfromasymbioticnucleus.CharacterizingPHR
2.2. The Available Data
1253(cid:4)6350asabipolarsymbioticoutflowdoesnotexplainthe
relationshipbetweenthisobjectanditsenvironment.Doesthe Table1containsthefollowinginformation:column(1):source
geometryoftheextendedMIRemissionindicatethepassageof name; column (2): status of the PN as true (T), likely (L), or
thisstarthroughtheISM,representedbythefilaments?Orwas possible(P);columns(3)Y(4):Galacticcoordinatesindegrees;
theobjectborninsidethematerialtracedbythehaloofMIRemis- columns(5)Y(6):J2000.0equatorialcoordinateswithunitsshown;
sion that surrounds it and that is excited by the bright bipolar column(7):opticaldimensionsinarcseconds;column(8):optical
nebulainside?Thisintriguingobjectisthesubjectofaseparate morphologycode,describedinx2.4andinParkeretal.(2006);
346 COHEN ETAL.
Fig.5.—MIRimageat8.0(cid:2)mofaprobablesinuousPDRshowingthelo-
cationofthePNcandidate,PHR1841(cid:4)0503,justofftheridge.Thecrosshas
900arms;thelargecirclehasa7200radius.
tracedbytheH(cid:1)image.IfthisoccurredinthetwoshortIRAC
wavelengths (3.6 and 4.5 (cid:2)m) it would suggest that recombi-
nation lines dominated the emission in these two filters (e.g.,
Pf(cid:4)at3.3(cid:2)mandBr(cid:1)at4.05(cid:2)m,respectively).Alternatively,
if thereis5.8and8.0(cid:2)memissionthatfollowstheformofthe
ionizedgas,butwithagreaterextent,thenwewouldattribute
theMIRemissioninthesetwobandstoradiationfromfluorescing
PAHsinaPDRthatwrapsaroundthePN.Onlyfivenebulaeare
detectedby2MASSasextendedNIRobjects:PHR1457(cid:4)5812,
PHR1831(cid:4)0805,PHR1843(cid:4)0325,PHR1857+0207,andPHR
1619(cid:4)4914(PM5,theonlyknownGalactic[WN]centralstarof
aplanetarynebula;Morganetal.2003).
ThepositionofapotentialCS mightbe closeto theoptical
centroid(i.e.,thecoordinatesprovidedinTable1)foraring,an
ellipticalorabipolarnebula.Forarcuateorpartialringnebulae
therelevantlocationwouldbethecenterofcurvatureofthearc.
TenofthePNe,or17%ofthesample,showopticalCScandidates,
Fig.4.—OpticalspectraofPHR1253(cid:4)6350.Top:Bluespectrumshowing
[Feii]linesat4575and46658,togetherwithnebularlinesofH(cid:3)and[Oiii]on 12%NIR,and24%MIR.
anobviouscontinuum.Bottom:Redspectrumshowingafaintredcontinuum
2.3.GLIMPSE Residual Images
withfaint[Feii]linesat6474and70948;Heilines(5876,6678,and70658);
H(cid:1);therednebularlinesof[Nii],[Sii],and[Ariii]at71368. UnlikewiththeMSXimages,wehaveworkedfromGLIMPSE
‘‘residualimages.’’Theseare3:1(cid:2);2:4(cid:2)imageswith1:200pixels
column (9): presence (y) or absence (n) of a clear MIR false- fromwhichallGLIMPSEpointsourceshavebeenremoved.The
colorcounterpart(fromIRAC)tothePN;column(10):indicates residualimagesareidealforenhancingtherecognitionofdiffuse
whetheranIRACcounterpart(inanyorallfourbands)islinked nebulosityinregionsofhighpoint-sourcedensityandenablea
morphologicallytotheH(cid:1)nebularimage;columns(11)Y(13): farmorereliablephotometryofsuchemission.Theresidualim-
listanyMSX8.3(cid:2)morradiocontinuumcounterparts,withthe agesuseouradaptationofDAOPHOTII(Stetson2000)forall
presence (y) or absence (n) of detections listed and an ellipsis GLIMPSEsourcesdetecteddownto2(cid:5),deeperthanthepublicly
indicatingthatnoimagefromMGPS2orNVSSwasavailable; releasedcatalogandarchivepointsourceliststhatarerequiredto
column (14): notes whether the likely central star of the PN is meethigherreliabilitycriteriabysourcesbeingdetectedmultiple
detected,with‘‘o’’implyinganopticallyidentifiedCScandidate timesata5(cid:5)levelinoneormorechannels.Nomeasurements
inanappropriateposition(forthreePNethatcandidateisoptically lessthan3(cid:5)areeverlistedineithertheGLIMPSEcatalogsor
blueandisfootnoted),‘‘i’’signifyingaCScandidatefoundin archives.Thus,therearefaint2Y3(cid:5)pointsourcesthataresub-
2MASSimages,‘‘m’’indicatingapossiblestellarcandidatein tracted from the residual images but which are not listed in
atleastoneIRACband,and‘‘N’’denotingnebularemissionseen GLIMPSE enhanced products. Residual images may contain
in2MASS.OneoftheMASHPNe,PHR1815(cid:4)1457,designated sourcesnotextractedbyDAOPHOT,suchassaturatedsources
asalikelyPN,liessoclosetotheupperlimitof latitudeinthe andsourcesthatpeakbeyondthenonlinearitylimitforeachband.
GLIMPSEcoveragethatitwasunobservedat3.6and5.8(cid:2)m, Theseobjectswereanalyzedindividually,andtheirintegrated
although images are available in the other band pair (4.5 and fluxdensitiesweresubtracted.
8.0(cid:2)m)becauseofthedifferentfieldsof viewobservedthrough
2.4.Nebular Morphology Descriptors
IRAC’stwodichroicbeamsplitters.
WhenlookingfortheMIRcounterpartmorphologically,one Adetaileddescriptionofthemorphologicalclassificationsap-
mayseearesolvedsourceexactlymatchingtheionizedgasas pliedtothesampleandsomeofthesubsequentanalysisformthe
TABLE1
AttributesofPNeStudiedinH(cid:1),MIR,andRadio
R.A. Decl. Size
NamePHR PNStatus GLON GLAT (J2000.0) (J2000.0) (arcsec) MorphologicalType SpitzerColor SpitzerH(cid:1) MSX MGPS NVSS Star
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14)
1806(cid:4)1956............. T 10.2111 0.3433 180655.3 (cid:4)195618 61 ; 50 Bams n n n ... n ...
1807(cid:4)1827............. P 11.5293 1.0039 180711.7 (cid:4)182754 7 ; 6 E n y y ... n ...
1813(cid:4)1543............. T 14.6575 1.0115 181329.0 (cid:4)154319 27 ; 21 Eas n n n ... n ...
1815(cid:4)1457............. P 15.5185 1.0342 181506.5 (cid:4)145721 9 ; 8 Es N/A y n ... y ...
1818(cid:4)1526............. L 15.5378 (cid:4)0.0195 181859.2 (cid:4)152622 55 ; 11 Br? n y n ... n ..m?
1824(cid:4)1505............. T 16.4158 (cid:4)0.9312 182402.1 (cid:4)150533 30 ; 18 Bps n n n ... n ...
1821(cid:4)1353.............. P 17.2190 0.1272 182143.9 (cid:4)135313 20 ; 6 As n n n ... n ...
1826(cid:4)0953............. T 21.2911 0.9803 182626.1 (cid:4)095326 54 ; 42 Bs n n n ... y ...
1831(cid:4)0805............. L 23.4401 0.7449 183119.6 (cid:4)080543 13 ; 9 Eas y y n ... n N
1834(cid:4)0824............. T 23.5513 (cid:4)0.1362 183441.6 (cid:4)082420 31 ; 26 Ea n y n ... n ...
1842(cid:4)0539............. L 26.8632 (cid:4)0.5529 184217.9 (cid:4)053913 90 ; 65 Ias n y n ... n ...
1843(cid:4)0541............. T 26.9222 (cid:4)0.7630 184310.4 (cid:4)054151 48 ; 39 B?/Eas n y n ... n ...
1844(cid:4)0517............. P 27.4764 (cid:4)0.9616 184454.0 (cid:4)051736 31 ; 26 Es n n n ... n ...
1838(cid:4)0417............. P 27.5860 1.0186 183802.2 (cid:4)041724 15 ; 13 Em y y n ... n ...
1844(cid:4)0503............. T 27.6643 (cid:4)0.8265 184445.7 (cid:4)050354 35 ; 12 Bm?/Em y y y ... n oi
1844(cid:4)0452............. L 27.7721 (cid:4)0.6350 184417.3 (cid:4)045256 38 ; 38 Rar n n n ... n m
1845(cid:4)0343............. L 28.8931 (cid:4)0.2907 184506.0 (cid:4)034333 51 ; 30 As y y n ... n oi?
1843(cid:4)0325............. P 28.9519 0.2570 184315.3 (cid:4)032527 10 ; 9 Ea y y y ... y? N
1842(cid:4)0246............. L 29.5024 0.6246 184257.1 (cid:4)024601 24 ; 13 Em y y n ... n ...
1843(cid:4)0232............. T 29.8197 0.5073 184356.9 (cid:4)023208 61 ; 54 Ear n y n ... n ...
1846(cid:4)0233............. T 30.0485 0.0357 184602.7 (cid:4)023309 36 ; 31 Ear n y n ... n
1847(cid:4)0215............. T 30.5060 (cid:4)0.2200 184747.4 (cid:4)021530 20 ; 14 Bs y n n ... n m?
1856+0028.............. L 33.9770 (cid:4)0.9860 185651.1 +002853 8 ; 7 Er y y y ... n ...
1857+0207.............. L 35.5650 (cid:4)0.4910 185759.5 +020707 11 ;11 Ea y y y ... y N
1150(cid:4)6226.............. L 295.9050 (cid:4)0.4110 115007.0 (cid:4)622632 89 ; 77 R n n n n ... ...
1152(cid:4)6234.............. L 296.2510 (cid:4)0.4580 115255.7 (cid:4)623410 27 ; 24 R n n n n ... ...
1157(cid:4)6312.............. L 296.8490 (cid:4)0.9840 115703.2 (cid:4)631244 15 ; 13 B? y y y y ... ...
1206(cid:4)6122............. T 297.5680 1.0230 120625.5 (cid:4)612244 2 ; 11 E y y n n ... ...
1218(cid:4)6245............. L 299.1190 (cid:4)0.1360 121800.9 (cid:4)624538 42 ; 31 Es n y n ... ... ...
1223(cid:4)6236............. T 299.7780 0.0980 122358.0 (cid:4)623621 48 ; 43 E n y n n ... ...
1244(cid:4)6231............. T 302.1330 0.3510 124428.5 (cid:4)623119 300 ; 235 B n y n n ... oa
1246(cid:4)6324............. T 302.3730 (cid:4)0.5390 124626.5 (cid:4)632428 31 ;19 B y y y y ... oim
TABLE1—Continued
R.A. Decl. Size
NamePHR PNStatus GLON GLAT (J2000.0) (J2000.0) (arcsec) MorphologicalType SpitzerColor SpitzerH(cid:1) MSX MGPS NVSS Star
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14)
1250(cid:4)6346............. T 302.7840 (cid:4)0.9080 125004.4 (cid:4)634652 83 ; 74 Ea n y n n ... oa?m
1255(cid:4)6251............. L 303.3725 0.0173 125518.0 (cid:4)625104 185 ; 81 B n y n n ... oi
1257(cid:4)6216............. P 303.6783 0.5923 125751.3 (cid:4)621612 19 ; 13 E y y n n ... m
1408(cid:4)6229............. T 311.7300 (cid:4)0.9500 140847.3 (cid:4)622958 82 ; 46 B n y n n ... m
1408(cid:4)6106............. T 312.1525 0.3741 140851.7 (cid:4)610627 307 ; 264 Es n y n n ... m?
1429(cid:4)6043............. P 314.6780 (cid:4)0.1290 142952.8 (cid:4)604357 167 ; 131 E n n n n ... m
1429(cid:4)6003............. L 314.9220 0.5180 142943.9 (cid:4)600317 141 ;110 Ems n y n n ... ...
1437(cid:4)5949............. T 315.9480 0.3320 143753.2 (cid:4)594925 103 ; 63 Ba n y n y ... ...
1447(cid:4)5838............. P 317.5785 0.8845 144741.8 (cid:4)583841 200 ; 200 I/Ba n n n n ... oai
1457(cid:4)5812............. T 318.9300 0.6930 145735.8 (cid:4)581209 31 ; 25 A y y y y ... N
1507(cid:4)5925............. T 319.5050 (cid:4)1.0140 150750.2 (cid:4)592514 22 ; 17 Ea y y y y ... o
1544(cid:4)5607............. P 325.4480 (cid:4)1.0270 154456.7 (cid:4)560707 14 ; 10 E n n n n ... ...
1552(cid:4)5254............. T 328.3570 0.7670 155256.8 (cid:4)525412 31 ; 26 Es y y y y ... ...
1610(cid:4)5130............. P 331.2780 0.0600 161021.1 (cid:4)513054 20 ; 11 Es n n n n ... m
1619(cid:4)5131............. P 332.3493 (cid:4)0.9814 161957.6 (cid:4)513148 11 ;11 E y y n y ... ...
1622(cid:4)5038............. L 333.2746 (cid:4)0.6547 162240.6 (cid:4)503842 21 ;19 Ear y y y n ... ...
1619(cid:4)4914............. T 333.9279 0.6858 161940.1 (cid:4)491400 36 ; 32 Rs y y y y ... oiNm
1619(cid:4)4907............. T 334.0350 0.7560 161950.1 (cid:4)490652 48 ; 47 Ra n y n n ... ...
1633(cid:4)4650............. T 337.3141 0.6361 163358.0 (cid:4)465007 24 ; 8 B y y y y ... ...
1635(cid:4)4654............. P 337.4831 0.3524 163551.9 (cid:4)465410 77 ; 34 Ia/B? n n n n ... oa
1634(cid:4)4628............. T 337.6825 0.7684 163451.2 (cid:4)462828 22 ; 17 E n y n n ... m
1639(cid:4)4516............. T 339.0980 0.9880 163922.3 (cid:4)451635 38 ; 25 Er n n n n ... ...
1644(cid:4)4455............. P 339.9730 0.5280 164436.2 (cid:4)445523 34 ; 34 E n y y n ... m
1646(cid:4)4402............. L 340.8600 0.8500 164627.6 (cid:4)440225 71 ; 72 A n y n n ... ...
1709(cid:4)3931............. T 347.0320 0.3500 170910.8 (cid:4)393106 51 ;13 B n n n n ... oi
1714(cid:4)4006............. T 347.2000 (cid:4)0.8720 171449.3 (cid:4)400609 20 ; 11 B? y y n y ... m
Note.—Unitsofrightascensionarehours,minutes,andseconds,andunitsofdeclinationaredegrees,arcminutes,andarcseconds.Col.(1):PNname;col.(2):PNstatus(T:truePN;L:likelyPN;P:possiblePN);cols.(3)Y(6):
coordinates;col.(7):apparentsize;col.(8):opticalmorphologytype;col.(9):whetheraPNcanbedistinguishedbyfalsecolorfromitsenvirons;col.(10):whetheropticalandIRmorphologiesarerelated;col.(11):existence
ofanMSX8.3(cid:2)mdetection;cols.(12)Y(13):existenceofaradiocounterpart;col.(14):whetherwehaveidentifiedanopticalorIRcentralstar.
a CScandidateisblue.
IRAC OBSERVATIONS OF MASH PLANETARY NEBULAE 349
basisofaseparatepaper(Q.A.Parkeretal.2008,inpreparation). 3.2. False-Color Spitzer Imagery
However,afterreviewoftheexistingcategories,anadaptationof
MIRspectraofPNearenotalwaysdominatedbyPAHemis-
thecurrentCorradi&Schwarz(1995)schemewasemployed,
sion bands. To eliminate the potential bias against any PN for
whichisbasedonasystemdevelopedbySchwarzetal.(1993).
whichthermalemissionfrom dust orionicfine-structure lines
AbasicERBIASclassifierisusedtoindicatePNethatareellip-
mightbethemajorcontributortoradiationintheIRACbands,
tical,round,bipolar,irregular,asymmetric,orquasi-stellar(point
wemadeasecondcomparison.False-colorimageswereproduced
source).WeaddadistinctionbetweenellipticalandcircularPNe,
usingIRAC’s4.5,5.8,and8.0(cid:2)mbands,showninblue,green,
basedonourinterestintheircanonicalStro¨mgrenspheres(e.g.,
andred,respectively.Thisparticulartrioavoidstheclutterdue
Soker2002)inwhichanobjectisconsideredtoberound(circular)
tothehighstellardensityinthe3.6(cid:2)mband.
ifthedifferencebetweenestimatedmajorandminoraxesis<5%.
The MSX data at 8.3 and 12.1(cid:2)m also measure PAH emis-
Inuncertaincasesadualclassificationmightbeapplied,suchas
sion,but12.1(cid:2)mwasgenerallynotsensitiveenoughtoprovide
‘‘E/B?’’Wethenaddasubclassifier‘‘amprs’’toindicateanasym-
aconfirmingdetection.Bycontrast,theIRACresultsofferthree
metry‘‘a,’’multipleshellsorexternalstructure‘‘m,’’pointlike
sensitivebandscapableofsamplingPAHemission(3.6,5.8,and
structure‘‘p,’’awell-definedringstructure‘‘r,’’orresolvedinter-
8.0(cid:2)m).The3.3(cid:2)mPAHbandlieswithinIRAC’s3.6(cid:2)mband
nalstructure‘‘s.’’Typically,onlyoneERBIASclassifierisgiven,
butitsstrengthistypicallyonly10%ofthatofthe7.7(cid:2)mPAH
butseveralamprssubclassifications,listedalphabetically,may
band,whilethe6.2(cid:2)mPAHfeatureattains56%ofthe7.7(cid:2)m
beapplicable.Oursampleof58PNeiscomprisedofelliptical bandinPNe,reflectionnebulae,andHiiregions(Cohenetal.
nebulae (51%), bipolar (28%), round (9%), asymmetric (7%),
1986;theirTable5).Therefore,3.6(cid:2)mimageryisnotasensitive
andirregularnebulae(5%).Thefractionof bipolarobjectsinthe
tracerofPAHs,andwelookedforsimilarmorphologiesinthe
entireMASHis12.5%.Limitingoursampletotheverylowlat-
5.8and8.0(cid:2)mbandstoconfirmtheexistenceof PAHs,thatare
itudecoverageof GLIMPSEhasmorethandoubledthefraction
oftenthedominantspectralfeaturesinPNe.Furthermore,itis
of bipolarnebulaeinoursample.
well-known that PNe also produce emission from lines of H
ThecorrelationbetweenhighlybipolarPNeandstronglyen- 2
(ro-vibrationalandpurerotationallines;e.g.,Coxetal.1998).
hancedHeandNabundancesisnowwell-known(e.g.,Corradi
H linescancontributetoemissioninseveralIRACbands.They
&Schwarz1995),butthelinkagebetweenchemistryandmor- 2
arechieflyseenat8.0(cid:2)m(Horaetal.2004)butcanberesponsible
phology wasfirstnotedbyGreig (1967,1971).Subsequently,
forover90%ofthefluxdetectedinIRAC’s3.6,4.5,and5.8(cid:2)m
Peimbert(1978)andPeimbert&Serrano(1980)definedtypeI
bands (Hora et al. 2006). This molecular emission in PNe can
PNeintermsofthresholdvaluesofHeorNabundance.Most
ariseeitherinthewarmmolecularzoneofaPDRorinshocks.In
typeIPNewerefoundtobebipolar(Peimbert1978;Peimbert
NGC6302themoleculargasisphotodissociated(Bernard-Salas
&Torres-Peimbert1983),whilesurveysofbipolarPNe(Corradi
&Tielens2005),whileinthemainringoftheHelix90%oftheH
& Schwarz 1995) confirmed their chemical peculiarities and 2
emissionisinshocksandonly10%inPDRs(Horaetal.2006).
addedenhancedNeabundancetothese.Asaclass,typeIPNe
SuchPNecouldappearredbutmighthavedifferentmorphol-
have larger than average diameters and expansion velocities
ogiesat5.8and8.0(cid:2)m,ratherthanidenticalstructuresaswith
(Corradi&Schwarz1995),hotterandmoremassiveCS(Tylenda
purePAHemission.
1989),smallerscaleheights(e.g.,Stanghellini2000),anddeviate
Weintendedthethree-colorimagetobeasimplepragmatic
fromthecircularrotationoftheGalaxy.Theirassociationwith
tool that treats every PN’s trio of images identically. We used
moremassiveprogenitorsthantypicalPNeiswidelyacknowl-
SAOImageDS9initsRGBmode.Eachofthethreecolorimages
edgedontheoreticalgroundsaswell(e.g.,Becker&Iben1980;
foragivenPNwasdisplayedwiththesamestretch:linearand
Kingsburgh & Barlow 1994). This is consistent with the high
zscale.Theseimageswereexaminedforevidenceofanextended
proportion of new type I PNe found by MASH, with its low-
counterparttoeachPN,thathadadistinctivecolorcomparedwith
latitudecoverage.
itssurroundings.Clearly,MIR-brightPNewillstandoutagainsta
3.MIR COUNTERPARTS OF THE MASH PNe darkskybackground,whatevertheirfalsecolor(typicallyred).
Figure7displays10PNethatarerecognizablebythistechnique.
3.1.8.0 (cid:2)m IRAC Imagery
Sixareveryobviousexamplesdemonstratingthepowerofthis
TofindthePAHemissioncommonlyassociatedwithcarbon- identificationtechnique,whilefourothersillustrateitscapability
richPNe(e.g.,Cohenetal.1989)wesearchedforMIRcoun- forrobustnessinthepresenceofcomplicatedMIRbackground
terpartsat8(cid:2)m.Hence,aspartoftherigorouscheckingofall emission.Indeed,weareinvestigatingtheprospectsfordiscov-
MASHcandidatePNepriortofinalizationofthecatalog,aninitial eringnewPNe,tooobscuredtobevisibleintheSHS,byusing
checkagainstavailableMIRdatafromMSXwasmade.Thiswas GLIMPSEthree-colorimagingalone.
latersupplementedatthelowestlatitudesbythenewGLIMPSE PHR1844(cid:4)0503(Fig.7,topimageofsecondcolumnofMIR
data.Asaconsequenceofthiscombinedeffortasignificantnum- composite images), whose position is indicated by the small
berofHiicontaminantswereculledfromtheMASHdatabasebe- greencirclebasedontheopticalposition,epitomizestheobvious
foreitspublication.However,therearealsoasignificantnumber matches,despiteconfusionwithadiffractionspikefromabright
of obviously spatially extended 8.0 (cid:2)m counterparts of PNe. sourceatthenorthernedgeoftheimage.Theorangeperiphery
Figure6illustratestwosuchobjects,bothclassifiedastruePNe: andyellowcoredistinguishthePNfromartifactsandfromthe
PHR1246(cid:4)6324andPHR1457(cid:4)5812.Bothweredetectedby manystarsseeninthefield.
MSXbutnomorphologycouldbediscerned.Thefigurecompares PHR1157(cid:4)6312(Fig.7,topimageoffirstcolumn)liesina
MIRstructures(topimages)withtheircorrespondingH(cid:1)images fieldthatisrelativelysparseinstarsbutsuffersfrombrightMIR
(bottomimages).PHR1246(cid:4)6324hasaclear,tight,bipolarmor- ‘‘cirrus’’emissionthatpermeatesalmosttheentire40;40field
phology,whilePHR1457(cid:4)5812iscompactbutasymmetric.Yet thatweshow.ThePNisthescarletdiffusepatchjustbelowthe
thesimilaritiesbetweentheMIRandopticalareclear.Theangular fieldcenter,stronglycontrastingwiththewidespreadorangedif-
sizesoftheMIRandH(cid:1)imagesareincloseagreement,withiden- fuseemissionandpredominantlygreennoise.Twenty-oneneb-
ticalpositionanglesofthemainaxisofsymmetry. ulae(36%ofthe58)havedefinite,resolvedcounterpartsthatare
350 COHEN ETAL. Vol. 669
Fig.6.—Pairof8.0(cid:2)mIRACimagesofthetruePNePHR1246(cid:4)6324andPHR1457(cid:4)5812(top),comparedwiththeirrespectiveH(cid:1)images(bottom).Dimensions
ofthesmallcrossandlargecircleareasinFig.5.
distinguishable from their environs by false-color imaging. The thedominantspectralfeatures.ThecombinationofBr(cid:1)andPf(cid:3)
dominantcolorsassociatedwiththese21MIRcounterpartssep- (inthe4.5(cid:2)mband)would overwhelmPf(cid:1)and linessuchas
arateintothreegroups:8arered,10areviolet,and3areorange. Hi6Y8(inthe5.8(cid:2)mband).Fluxdensityratios4.5(cid:2)m/8.0(cid:2)m
Redmostlikelyrepresentsdominant7.7and8.7(cid:2)mPAH-band in typical PNe are very small except when H lines are domi-
emission.Orangecouldimplyeitheroftwoexplanations:PAH nant. Ratios of 5.8 (cid:2)m/8.0 (cid:2)m emission without substantial
emissionwithstrong6.2(cid:2)memission(comparableintensities extinctionforPNedominatedby(1)PAHbands,or(2)H lines,
2
inthe5.8and8.0(cid:2)mbandswouldleadtoorangeoryellowin or (3) H recombination would be about 0.5 (Cohen & Barlow
falsecolor)orH emissionlines,whicharestrongestinIRAC’s 2005;Tables2and3),0.3(Horaetal.2006,Table2),and0.1
2
8.0(cid:2)mbandbutalsocontributetothe5.8(cid:2)mbandchieflythrough (usingtheHrecombinationlinesinIC418;Pottaschetal.2004),
the0Y0 S(7) line(e.g., Hora etal. 2006). Violetobjectscould respectively.
also represent two types of PN. In high-excitation PNe, ionic Although the statistics are very limited, we note from the
linesofheavyelements,suchas[Mgiv](4.485(cid:2)m)and[Arvi] availableMASHopticalspectroscopythattheaverageratioof
(4.530 (cid:2)m), are very strong and fall in IRAC’s 4.5 (cid:2)m band. [Nii]/H(cid:1)forthesevenPNewithredthree-colorimages(ex-
However,suchanobjectwouldalsoemitstronglyinthe8.99(cid:2)m cludingPM5,whichhasHeiiratherthanH(cid:1))is2:4(cid:3)0:5,the
[Ariii],7.90(cid:2)m[Arv],and7.65(cid:2)m[Nevi]linesinthe8.0(cid:2)m threeorangePNegive6.3(cid:3)1.7,whilethatfor10nebulaewith
band.Acombinationofhigh-excitationlineswouldincreaseemis- violetimagesis1:0(cid:3)0:5.Itappearsthatthefalse-colorcom-
sioninboththe4.5and8.0(cid:2)mbands,leadingtovioletinthefalse- binationofIRAC bands 2, 3, and4 candifferentiate between
colorimage.Asasecondscenariowesuggestanotherpossibility, PNeof highandlow[Nii]/H(cid:1).Thus,weproposethefollowing
namelylow-excitationPNeinwhichtheHrecombinationlinesare model to explain the MIR false colors and optical excitations
No. 1, 2007 IRAC OBSERVATIONS OF MASH PLANETARY NEBULAE 351
Fig.7.—False-colorimagesofPNefromGLIMPSEmosaics(notresidualimages).PHRdesignationsappearbeloweachPNimage.Inthebottomrightcorneristhe
fieldofPHR1843(cid:4)0232,illustratinggraphicallytheextremedifficultyofextractinganyobjectinthegreencircleagainsttheverybrightandvariable8.0(cid:2)mskyback-
ground(red)emissionofPAHs.Allimagesareof40squarefieldsandanysmallgreencirclethatguidestheeyetothelocationofaPN’sopticalcentroidhasa1200radius.
ofPNe.‘‘Red’’PNearePAHdominatedandofmodestexcitation, 2008,inpreparation).Nosuchpatternsemergeinthesmaller
‘‘orange’’objectsaredominatedbyH orhigh-excitationfine- sampleofPNewithmeasuredratiosof[Oiii]/H(cid:3) intensities.
2
structure lines, and ‘‘violet’’ nebulae are dominated by H re-
3.3.Overlays of MIR on H(cid:1) Images
combinationlinesandareofloworverylowexcitation.
Forthe22PNewith ½N ii(cid:5)/H(cid:1) (cid:7)3,9objects(41%)havea To locate the MIR counterparts, the H(cid:1) images were re-
bipolarmorphology,suggestingthatthislineratiomaybeavalu- griddedtoGalacticcoordinateswiththesameprojectionsasthe
ableproxyforselectingtypeIPNe(seealsoQ.A.Parkeretal. GLIMPSEmapsandeachofthefourIRACimageswasoverlaid
352 COHEN ETAL. Vol. 669
TABLE2
RadioandMIRFluxDensitiesforPNewithTwoorMoreDetectionsofThese
PNNAME Diameter [M]GPS2 Radio MSX8.3 IRAC8.0
(PHR) Status (arcsec) MorphologicalType [N]VSS (mJy) (mJy) (mJy) IRAC/MSX MSX/Radio IRAC/Radio
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11)
1844(cid:4)0503............. T 20 Bm?/Em N 3.2(0.9) 43 78 1.8 13.0 24.0
1843(cid:4)0325............. P 9 Ea N 16.0(2.0) 210 210 1.0 13.0 13.0
1857+0207.............. L 11 Ea N 78.3(2.4) 160 180 1.1 2.0 2.3
1246(cid:4)6324............. T 24 B M 14.3(1.4) 120 140 1.2 8.2 10.0
1457(cid:4)5812............. T 28 A M 79.8(2.7) 78 160 2.1 1.0 2.0
1507(cid:4)5925............. T 19 Ea M 24.2(1.3) 110 100 0.9 5.4 4.8
1552(cid:4)5254............. T 28 Es M 20.9(2.2) 110 150 1.4 4.2 5.9
1619(cid:4)4914............. T 34 Rs M 240.(8.0) 1000 970 1.0 4.3 4.1
1619(cid:4)5131............. P 11 E M 20.2(1.8) 90 99 1.1 4.5 4.9
1633(cid:4)4650............. T 14 B M 26.9(3.1) 55 63 1.2 2.0 2.3
1714(cid:4)4006............. T 15 B? M 9.7(1.3) <31 27 ... ... 2.8
1157(cid:4)6312.............. L 13 B? M 6.4(1.0) <20 30 ... ... 4.7
1437(cid:4)5949............. T 181 Ba M 11.7(1.8) <160 320 ... ... 27.0
1815(cid:4)1457............. P 8 Es N 9.2(0.6) <26 16 ... ... 1.7
1826(cid:4)0953............. T 48 Bs N 8.4(0.6) <160 170 ... ... 20.0
1644(cid:4)4455............. P 34 E M <5 150 150 1.0 ... ...
1843(cid:4)0541............. T 43 B?/Eas N <1.5 130 180 1.4 ... ...
1622(cid:4)5038............. L 20 Ear M <28 51 110 2.2 ... ...
1842(cid:4)0539............. L 77 Ias N <2.5 280 370 1.3 ... ...
Notes.—Col.(1):PNname;col.(2):PNstatus;col.(3):representativediameter;col.(4):opticalmorphology;col.(5):radiodetectionbyM(GPS2)orN(VSS);
col.(6):radiofluxdensitywithparenthesizeduncertainty;cols.(7)Y(8):MSXandIRACfluxdensities;col.(9):ratioofIRACtoMSXflux;col.(10):ratioofMSXto
radioflux;col.(11):ratioof IRACtoradioflux.
ascontoursontheH(cid:1)image.‘‘Quartets’’oftheseoverlayswere wavelength,andthechangeinitsmorphologyfromtheasymmetric
inspected.MostoftheseIRACcounterpartsareclearlyresolved, enhancedbrightnessofthesouthernlimbinthethreeshortestbands
andmanyhavesimilarmorphologiestothatshownbythePNe tothelargecircularappearanceat8.0(cid:2)m.Weidentifythe8.0(cid:2)m
inH(cid:1),atleastattheshorterIRACwavelengths.Thelowestcon- structureandtheincreaseinsizeat5.8(cid:2)masduetoasubstantial
toursdisplayedinallquartetsweresetasthemeanoff-sourcelevel PDRthatenvelopstheentireionizedzone.Ionicfine-structure
ofemissionplus3standarddeviations.FilamentsofPAHemis- linesprobablyaccountforthechangingsizefrom3.6to5.8(cid:2)m.
sion (in the 5.8 and 8.0 (cid:2)m bands) that might represent limb- Thesecondquartet,Figure9,offersanopticallyandMIR-bright
brightenedPDRswillappeardisplacedfromtheperipheryofa truePN,PHR1246(cid:4)6324.Thisfieldhasseveralbluestarsat3.6
PN.Inaddition,onecanreadilyrecognizepotentialMIRcoun- and4.5(cid:2)m,inparticularonebrightstarjustbelowthePN’scenter,
terpartstothePNcentralstars.For9ofthe58PNe(16%)we which seems to be unrelated to the nebula. The number of stars
couldidentifyacandidatecentralstarinoneormoreIRACbands. detectedfallswithincreasingwavelength(asexpectedforrandom
Generallytheseareblue;i.e.,theybecomemonotonicallyfainter starsintheirRayleigh-Jeansdomain)andonlythebrightestoneis
withincreasingIRACwavelength. stilldetectedat8.0(cid:2)m.ThePNisbipolarandthebrightH(cid:1)axisis
ToillustratehowthequartetswereusedtodeterminetheMIR filledacrossthepinchedwaist.Thiselongationisalsoseeninthe
counterparts to PNe, we present four such quartets, chosen to MIR. Unlike inFigure 8,the3.6 and4.5 (cid:2)mbands are almost
highlightdifferentfeaturesinthenebulae,their probableMIR identicalinappearanceanddimensions,whichcloselymatchthe
emissionprocesses,andtheircentralstars. distributionofionizedgastracedbyH(cid:1).Thisisdueeithertore-
Thefirstquartet,presentedinFigure8,showsthesmallnebula combinationlinesinthesetwoshortbands(e.g.,Pf(cid:4) andBr(cid:1),
PHR1857+0207thatappearsinanotherwiseemptyfield.Note respectively)ortothermalemissionfromdustgrains,closetothe
theprogressiveincreaseinsizeofthislikelyPNwithincreasing centralstar,whichareheatedbydirectstarlight.Inboththelonger
bands the PN is markedly larger in latitude extent, reflecting
thecontributionbyPAHsinthePDRintheouterportionsofthe
TABLE3 nebula.Theinnercontourssuggestthermalemissionbycooler
ObservedandCorrectedMedianColors(inmag)ofMASHPNe grainsinasomewhattilteddustdisk.Thisdustemissionmustbe
intheFourIRACBands
optically thintoproduce the two peaks,presumablyfromlimb-
brighteningalongtheline-of-sighttothecircumstellardustdisk.
MASHCorrected Ackeretal.Synthesized
TheU-shapedcurvatureofthesecondhighest8.0(cid:2)mcontours
ColorIndex Median(cid:3)sem Median(cid:3)sem
stronglysuggeststhatIRAChasresolvedatilteddustydisk.
[3.6]Y[4.5]................. 0.68(cid:3)0.22 1.16(cid:3)0.31 Figure10presentsPHR1457(cid:4)5812,inthethirdquartet;atrue
[3.6]Y[5.8]................. 2.12(cid:3)0.25 2.37(cid:3)0.20 PNwithaverydifferentMIRstructure,alsoshowninFigures6
[3.6]Y[8.0]................. 3.84(cid:3)0.28 3.91(cid:3)0.28 and7(leftcolumn,thirdimagedown).Onemustignorethetwo
[4.5]Y[5.8]................. 1.25(cid:3)0.16 1.07(cid:3)0.28 unrelatedpointsourcesprojectedagainstthewesternedgeofthe
[4.5]Y[8.0]................. 2.76(cid:3)0.19 2.57(cid:3)0.34 PN.ItisacompactbutasymmetricPNwithastronglyenhanced
[5.8]Y[8.0]................. 1.66(cid:3)0.11 1.62(cid:3)0.23
eastern edge. The curving eastern portion with the bright H(cid:1)
Note.—ColorsarecomparedwithcolorssynthesizedfromISOSWSspectra emission coincides with the peaks of MIR counterparts in the
ofasampleofwell-knownAckeretal.(1992)PNe. threelongerIRACbands.TheseMIRpeaksshiftslightlywith