The planetary nebula Abell 48 and its [WN] nucleus

20 Pages·2014·1.9 MB·English

MNRAS440,1345–1364(2014) doi:10.1093/mnras/stu198
AdvanceAccesspublication2014March23
The planetary nebula Abell 48 and its [WN] nucleus
David J. Frew,1,2‹ I. S. Bojicˇic´,1,2,3 Q. A. Parker,1,2,3 M. Stupar,1,2 S. Wachter,4
K. DePew,1,2 A. Danehkar,1,2 M. T. Fitzgerald1,2 and D. Douchin1,2
1DepartmentofPhysicsandAstronomy,MacquarieUniversity,Sydney,NSW2109,Australia
2ResearchCentreinAstronomy,Astrophysics&Astrophotonics,MacquarieUniversity,Sydney,NSW2109,Australia
3AustralianAstronomicalObservatory,POBox915,NorthRyde,NSW1670,Australia
4MaxPlanckInstituteforAstronomy,Koenigstuhl17,D-69117Heidelberg,Germany
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Accepted2014January27.Received2014January27;inoriginalform2013January14 n
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ABSTRACT fro
m
Wehaveconductedadetailedmulti-wavelengthstudyofthepeculiarnebulaAbell48andits h
centralstar.Weclassifythenucleusasahelium-rich,hydrogen-deficientstaroftype[WN4– ttp
s
5]. The evidence for either a massive WN or a low-mass [WN] interpretation is critically ://a
c
examined,andwefirmlyconcludethatAbell48isaplanetarynebula(PN)aroundanevolved a
d
e
low-mass star, rather than a Population I ejecta nebula. Importantly, the surrounding nebula m
ic
has a morphology typical of PNe, and is not enriched in nitrogen, and thus not the ‘peeled .o
u
atmosphere’ of a massive star. We estimate a distance of 1.6kpc and a reddening, E(B − p
.c
V)=1.90mag,thelattervalueclearlyshowingthenebulaliesonthenearsideoftheGalactic om
bar, and cannot be a massive WN star. The ionized mass (∼0.3 M(cid:3)) and electron density /m
n
(700cm−3)aretypicalofmiddle-agedPNe.Theobservedstellarspectrumwascomparedto ra
s
agridofmodelsfromthePotsdamWolf–Rayet(PoWR)grid.Thebest-fittingtemperatureis /artic
71kK,andtheatmosphericcompositionisdominatedbyheliumwithanupperlimitonthe le
-a
hydrogen abundance of 10 percent. Our results are in very good agreement with the recent b
s
studyofTodtetal.,whodeterminedahydrogenfractionof10percentandanunusuallylarge tra
c
nitrogenfractionof∼5percent.Thisfractionishigherthananyotherlow-massH-deficient t/4
4
star, and is not readily explained by current post-AGB models. We give a discussion of the 0
/2
implicationsofthisdiscoveryforthelate-stageevolutionofintermediate-massstars.Thereis /1
3
4
nowtentativeevidencefortwodistincthelium-dominatedpost-AGBlineages,separatetothe 5
/1
helium-andcarbon-dominatedsurfacecompositionsproducedbyalatethermalpulse.Further 0
1
9
theoreticalworkisneededtoexplaintheserecentdiscoveries. 1
9
5
b
Keywords: stars:evolution–stars:Wolf–Rayet–planetarynebulae:general–planetaryneb- y
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ulae:individual:Abell48. u
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t o
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1
3
A
temperatureWOstars,almostallCSPNecataloguedtodatebelong p
1 INTRODUCTION tothe[WC]→[WO]sequence(Tylenda,Acker&Stenholm1993; ril 2
0
Classical Wolf–Rayet (WR) stars are massive hydrogen-deficient Crowther,DeMarco&Barlow1998;Acker&Neiner2003),with 1
9
objects with powerful, fast winds and high mass-loss rates up to oneortwoexceptions(e.g.Miszalskietal.2012b,hereafterMC12).
10−4M(cid:3)yr−1(Crowther2007).Theirspectraarecharacterizedby In the course of a spectroscopic survey of detectable CSPNe
strong,broademissionlinesofhelium,nitrogen,carbonoroxygen. (DePewetal.2011),wewerestruckbytheunusualnatureofthe
Asimilarphenomenonisfoundinsomeionizingstarsofplanetary nucleusofAbell48(PNG029.0+00.4),discoveredandcatalogued
nebulae(PNe),whichdescendfromlow-massprogenitorstarsand asaPNbyAbell(1955,1966).Abell48isareddened,low-surface
areunrelatedinanevolutionarysense.Thesecentralstars(CSPNe) brightnessPNwithamorphologyresemblingathicktorusorcylin-
aredenotedas[WR]stars,utilizingthesquarebracketsintroduced der,possiblyviewednearlypole-on(Fig.1).Abell(1966)classified
byvanderHuchtetal.(1985)toavoidconfusionwiththeirmassive itasadouble-ringnebula1andgaveanopticaldiameterof40arcsec.
analogues.WhilemassiveWRstarsarepredominantlynitrogenor Ithasnotbeenwellstudiedopticallyduetoitsrelativefaintness.
carbonenriched(WNorWCtypes,respectively),withafewhigh
1Curiously, Helfand et al. (2006) classified it as a candidate supernova
(cid:2)
E-mail:[email protected] remnant.
(cid:5)C 2014TheAuthors
PublishedbyOxfordUniversityPressonbehalfoftheRoyalAstronomicalSociety
1346 D.J.Frewetal.
Figure1. MontageofimagesforAbell48,alltothesamescaleandorientation(90×90arcsec2withNEattopleft),andwiththecolourpalettesetsothat D
o
RGBchannelsmaplongertoshorterwavelengths,respectively.1.SHSHαimagewiththeCSPNmarked–notetheelliptical,double-ringstructureofthe w
n
nebula;2.SSS/SHScompositeBJ/R/Hαcolourimage;3.UKIDSSJHKscompositeimage;4.IRAC/MIPScomposite4.5/8.0/24µmimage.Acolourversion loa
ofthisfigureisavailableintheonlineversionofthearticle. d
e
d
DePewetal.(2011)alreadyconcludedthatAbell48wasaPNand Higherresolutiongratings (B7000andR7000)were usedona from
gaveapreliminaryclassificationoftheCSPNas[WN]or[WN/C], second run in 2010 April. This observation was aimed at getting h
butsincetheimportantCIVλ5806doubletwasunobservedinthat kinematicdataonthenebula,sotheIFUwaspositionedtocover ttps
work,amoreprecisespectraltypecouldnotbedetermined.Inde- both the CSPN and centre of the nebula. A further exposure of ://a
c
pendently,Wachteretal.(2010)classifiedthecentralsourceasa thesouthernsectionofthenebulawastakenusingWiFeSatlow a
d
WN6star,incontrasttotheobject’spreviouslongassociationas resolution in 2012 August. The night was photometric and con- em
aPN(Perek&Kohoutek1967;Zuckerman&Aller1986;Acker sequentlythesignal-to-noiseratio(S/N)isbestonthisspectrum. ic
.o
etal.1992;Kohoutek2001).Thisuncertaintyledustomoreclosely For the WiFeS data, the frames were bias subtracted and flux- u
p
investigatethisinterestingobjectandtiedownitsidentification. andwavelength-calibratedusingtheWiFeSdatareductionpipeline .c
o
m
In this paper we present a detailed multi-wavelength study of (Dopita et al. 2010) in conjunction with standard IRAF routines. /m
Abell48anditsCSPN,expandingonthepreliminaryanalysisre- The flux calibration was performed using the spectrophotometric n
ported in Bojicˇic´ et al. (2013). As this paper was completed, an standardLTT3864. ras
ihnedreeapfetnedreTnKts1t3u)d,ycoomftihnigsotobjeescstewntaiaslplyubthlieshseadmbeycoTnocdltuestioanl.s(2as01u3s,. WaWchetearlseotualt.ili(z2e0d10th),eosbpteacintreudmwoitfhththeeC5S.0P-NmfiHrastleptreelseesnctoepdebayt /article
Wewillrefertotheirresultsinthecontextofourownpaperwhere PalomarObservatoryon2008September4usingtheDoubleSpec- -a
b
necessary. We have confirmed Abell 48’s CSPN as a hydrogen- trograph.Thisspectrographalsohasatwo-armconfiguration,uti- stra
poor [WN] star, as first suggested by DePew et al. (2011). We lizing a dichroic to split the light into two channels, observed si- c
alsoexplorethenatureandcharacteristicsofthe[WN]classbased multaneously. The 316 line mm−1 grating in first order, dichroic t/44
0
on a comparison of Abell 48 with the other [WN] and [WN/C] D55andaslitwidthof1arcsecwereused.Theresultingspectra /2
stars,theirpossibleprogenitors,andtheirlikelyprogeny(Todtetal. cover4000–5600Åand5800–8300Åwithadispersionof2.0and /13
2010a,b,2012;Werner2012;MC12;TK13).Thispaperisarranged 2.4Åpixel−1ontheblueandredsides,respectively,withaspectral 45
/1
asfollows:wedescribeourobservationsofAbell48inSection2, resolutionof5–7Å.FeAr(blue)andHeNeAr(red)arcswereused 0
1
undertake a detailed analysis of the nebula and its CSPN in Sec- forwavelengthcalibration.Biasandflat-fieldcorrectionstotheraw 91
9
tions3and4,anddiscusstheevidenceforitsnatureinSection5. spectrum images were performed using standard IRAF routines. A 5 b
In Section 6 we review the [WN] or [WN/C] classes, discuss the logoftheobservationsispresentedinTable1. y
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possibleevolutionarypathwaysofthesegroupsinSection7,before u
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givingourconclusionsandsuggestionsforfutureworkinSection8. st o
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2.2 Photometricobservations 1
3
2 OBSERVATIONS A
NewopticalphotometryoftheCSPNwasalsoobtainedwiththe p
2.1 Spectroscopicobservations AMuasyt,raalniadnthNea2ti.o0n-mal FUanuilvkeerssiTtye’lses2c.o3p-meNreoflrtehct(oFrTaNt)SaStOHainlea2k0a1l2a ril 201
9
OpticalspectraofAbell48anditsCSPNwereobtainedontheANU Observatory,Maui,inqueuemodeonthreedifferentnightsin2012.
2.3-mtelescopeatSidingSpringObservatory(SSO)usingtheWide AlogoftheobservationsispresentedinTable1.TheSSOobserva-
FieldSpectrograph(WiFeS;Dopitaetal.2007,2010),on2009July tionsusedthestandardimageratthef/18Nasmythfocus.Thisused
1.WiFeShasadouble-beamconfiguration,withspectralresolutions a2048×2048pixelthinnedE2VCCDwithapixelsizeis13.5μm,
of3000or7000,anda38×25arcsec2fieldofviewwithaspatial giving a plate scale of 0.34arcsec pixel2 across a 6.6 arcmin cir-
resolutionof1arcsec.SincetheangularsizeofAbell48islarger cular field ofview. Standard UBVI filters were used. The seeing
c
thanthis,theinstrumentwaspositionedonthenorthernsectionof wasmodest(∼1.5arcsec)andthenightwasphotometric.Standard
thenebula(includingtheCSPN)tomakesurethatsomesuitable extinctioncoefficientswereapplied,andarangeofstandardstars
backgroundskywasavailableforsubtraction.Thiswasanearlyrun fromLandolt(2009)weremeasuredtodeterminethecolourequa-
duringWiFeScommissioningtimesothemoreeffectivenod-and- tionsusedtoconvertfrominstrumentalmagnitudes.FortheFTN
shuffle mode was not used. Low-resolution gratings (B3000 and observations,theMeropecameraatthef/10focuswasusedwith
R3000)wereemployedandtheexposuretimeontargetwas1500s. BVI Johnson–Cousins filters. The camera utilized an E2V CCD
C
Thincloudwaspresentduringtheobservation. with 2048 × 2048 pixels, in 2 × 2 binning mode, giving images
MNRAS440,1345–1364(2014)
A[WN]starinAbell48 1347
Table1. SpectroscopicandphotometricobservationsofAbell48utilizedinthisstudy.
Date Telescope Instrument λRange R=λ/(cid:5)λ Exp.time
2008Sept4 Hale200-in DualSpectrograph 3400–5600,5850–8300 1000 900
2009Jul1 ANU2.3-m WiFeS 3400–5900,5300–9600 2900 1500
2010Apr22 ANU2.3-m WiFeS 4180–5580,5300–7060 6900 1200
2012Aug23 ANU2.3-m WiFeS 3400–5900,5300–9600 2900 1200
Date Telescope Instrument Filter ... Exp.time
2012Feb24 2.0-mFTN MeropeCamera B ... 900
2012Mar14 2.0-mFTN MeropeCamera Ic ... 10
2012May8 2.0-mFTN MeropeCamera BV ... 200,150
2012May18 ANU2.3-m Imager UBVIc ... 300,2×300,2×180,2×120
D
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4.7arcminonasidewithaplatescaleof0.28arcsecpixel2.The lo
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averagedobservedmagnitudesarepresentedinSection4.1. d
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2.3 Archivaldata h
ttp
Tanoesxutpepnlseimveensetaoruchr sfpoercatrrcohsicvoaplicmaunltdi-wphaovteolemnegttrhicddaatatao,fwtheemnaedbe- s://a
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ulaandcentralstarusingtheAladinSkyAtlas,2 andtheSkyView ad
e
VirtualObservatory.3Narrow-bandHα(+[NII])imageswereob- m
tained from the SuperCOSMOS Hα Survey (SHS; Parker et al. ic.o
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2005;Frewetal.2014a)andtheSouthernH-AlphaSkySurveyAt- p
.c
las(SHASSA)(Gaustadetal.2001).Broad-bandimageswerealso o
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obtainedfromtheSuperCOSMOSSkySurvey(SSS;Hamblyetal. /m
2et00al1.)2,t0h0e72)MsuArvSeSys(Sinkrtuhtesknieeaert-IaRl.,2a0n0d6)thaendGULIKMIDPSSES((LBaewnjraemncine Flinigeus,reex2t.racStuerdfafcroembrtihgehtWneisFsemSaopbsseorfvAatbioenllo4f8Ainprth2e01H0α.Nanodte[NtheII]pλa6i5r8o4f nras/a
eatnadl.W20IS0E3;(CWhruigrchhtwetelall.et2a0l1.02)0m09i)d,-MIRIPsuSrGveAyLs.(Careyetal.2009), oatpptoopsilnegftkinnoetsacwhhpicahnealreanpdrotmheinfleunxtiunntihtsea[NreIIi]nλ1605−8145leinrge.cNmo−r2ths-−ea1spteisr rticle-a
Continuum fluxes for the nebula (and the CSPN separately if spaxel.Acolourversionofthisfigureisavailableintheonlineversionof bs
available)wereretrievedfromtheVizieRserviceormeasuredfrom thearticle. tra
c
theoriginalimagesaspartofthiswork.Wealsodownloaded20cm t/4
and90cmhigh-resolutionMAGPISimages(Helfandetal.2006) importantimplicationsfortheoriginofthenebula,asweelucidate 40
to supplement the NVSS radio data (Condon & Kaplan 1998). below. /2/1
Fig. 1 shows a multiwavelength montage of merged-colour im- Soker(1997)hintedthattheCSPNofAbell48maybeabinary 34
ages for Abell 48, all to the same scale. We adopt the best posi- system,basedonnebularmorphology(seelater).Ifthisturnsout 5/1
tionfortheCSPNfromthe2MASScatalogue,α=18h42m46s.92, tobetrue,abinaryevolutionchannelmaybeimplicatedtoexplain 01
9
δ=−03◦13(cid:7)17(cid:7).(cid:7)3(J2000). theunusualsurfaceabundancesoftheCSPN,ashasbeensuggested 1
9
fortheLMCPN,N66(Hamannetal.2003)andforthepeculiar 5 b
CSPNoftheEskimonebula,NGC2392(e.g.Me´ndezetal.2012). y g
3 THE PLANETARY NEBULA Thenebularemission-lineprofiles(seeSection3.3.3)alsoappear ue
s
toruleoutsimplesphericalsymmetry(cf.Lo´pezetal.2012),sowe t o
3.1 Nebularmorphology tentativelyinterpretthedoubleshellmorphologyasduetothegeo- n 1
Abell48hasanapparent‘double-ring’morphologywithalozenge- metricorientationoftwoopposinglobes.Afullmorpho-kinematic 3 A
analysisofthenebula,basedonourWiFeSimagingdata,willbe p
shapedinteriorcavity(seeFig.1),whichmightbetheprojectionof the subject of a future paper, but a preliminary analysis indicates ril 2
athicktorusorcylinderviewednearlypole-on,ashasbeenkine- 0
thisisthelikelyscenarioatplay. 1
maticallydemonstratedforothermorphologicallysimilarnebulae 9
Tothenorth-westofthePNthereappeartobetwofaintclosely
(e.g.O’Delletal.2013).ThemeasureddimensionsfromtheSHS
spaced arcs about 30arcsec in extent, with the outermost being
are44×39arcsec2.Thereisalsoapairofpointsymmetricknots
45arcsecfromtheCSPN(seeFig.3).Thesearc-likestructureswere
whichareprominentinthe[NII]λ6584line(seeFig.2),andwhich
firstnotedbyDePew(2011)andtherealsoappearstobeevidence
areoftenseeninPNe.Thisfact,thePN-likemorphologyseenin
ofaveryfaintellipticalhaloondeepcontinuum-subtractedSHSHα
Hα images, and the emission measure of the ionized nebula all
images,discoveredaspartofthesurveyofFrew,Bojicˇic´&Parker
indicate that it is material recently ejected by the CSPN, and not
(2012).4 The overall dimensions are about 280 × 210arcsec2 in
simplywindsweptinterstellarmaterial(seethemorphologicalatlas
extent,butdeepCCDimagesareneededtoconfirmthem.Together,
ofWRshellsbyGruendl,ChuDunne&Points2000).Thisfacthas
these features may provide evidence for earlier AGB mass-loss
2AccessiblefromtheCentredeDonne´sAstronomiques(CDS). 4TheSHSisanexcellentsearchmediumforfaintstructuressurrounding
3http://skyview.gsfc.nasa.gov/ PNeandsymbioticstars(seealsoMiszalskietal.2012a).
MNRAS440,1345–1364(2014)
1348 D.J.Frewetal.
Table2. SummaryofcontinuumfluxmeasurementsforAbell48.
Wavelength Flux Survey References
(µm) (mJy)
3.6 67±7 IRAC C11
4.5 109±10 IRAC C11
5.8 32±5 IRAC C11
8.0 244±24 IRAC C11
8.3 375±15a MSX EP03
11.6 1195±120: WISE Thiswork
14.7 2053±125 MSX EP03
18 3341±42 AKARI I10
21.3 1735±110 MSX EP03
D
22.1 1600±200 WISE Thiswork o
w
24 2065±100 MIPSGAL M10,PM11 n
25 2530±250 IRAS IPAC loa
60 <2070 IRAS IPAC de
d
7605 1219000200±±2507000a MAKIPASRGIAL ThiIs1w0ork from
Figure3. Adeepcontinuum-divided(Hα)SHSimageofAbell48showing 90 30990±2200a AKARI I10 h
twoshortarcs,theouteronearrowed,located35and45arcsecnorth-west 6cm >70 MAGPIS Thiswork ttps
oftheCSPN.Theimagesubtends300 × 240arcsec2withNEattopleft. 6cm <400 NVSS Thiswork ://a
c
11cm ≥220 VLA PB03,thiswork a
d
events. Even though Abell 48 is close to the Galactic mid-plane, 20cm 159±15 NVSS C98 em
where the interstellar medium (ISM) density is highest (Spitzer 20cm 200±20 MAGPIS Thiswork ic
1978), the PN has little signature of an ISM interaction (Soker, 90cm 60±10 MAGPIS Thiswork .ou
p
Borkowski&Sarazin1991;Pierceetal.2004;Wareingetal.2006; References:C11–Cohenetal.(2011);C98–Condonetal.(1998); .co
Wareing 2010; Frew et al. 2011; Ali et al. 2012). Therefore, the EP03–Eganetal.(2003);I10–Ishiharaetal.(2010);IPAC–IPAC m
peculiar velocity of the CSPN is likely to be low (<40kms−1). (1986);M10–Mizunoetal.(2010);PB03–Paladinietal.(2003); /mn
Incontrast,TK13claimAbell48isarunawaystar,basedonthe PM11–Phillips&Marquez-Lugo(2011). ra
s
CSPN’spropermotionof19.5masyr−1 inp.a.222◦ fromthePP- aFluxeslikelyconfusedwithsurroundingemission. /a
MXLcatalogue(Roeser,Demleitner&Schilbach2010).However, rtic
this differs from the USNO-B1.0 catalogue value (Monet et al. within the uncertainties, we measured a flux of ∼200 mJy from le-a
2003)of25.3masyr−1 inp.a.342◦.Owingtothefaintnessofthe theMAGPISimage,atoddswiththecataloguedvalue,butcloser bs
star,thesecataloguevaluesmaybespurious. to the NVSS flux. In fact, some published MAGPIS fluxes need trac
toberevised(D.Helfand,2011,privatecommunication),andthe t/4
4
cataloguedvalueof657mJyiserroneousinthiscase.Abell48is 0
/2
3.2 Integratednebularfluxes notdetectedinthehigh-resolutionCoordinatedRadioandInfrared /1
3
WehavefourindependentestimatesoftheintegratedHαflux.Frew, SurveyforHigh-massStarFormation(CORNISH)SourceCatalog 45
Bojicˇic´&Parker(2013)derivedlogF(Hα)=−11.52±0.09(incgs (Purcelletal.2013),recentlyundertakenat5GHzwiththeVery /10
units)fromaperturephotometryonSHASSAimages,whileFrew Large Array (VLA). Abell 48 is just below the survey sensitiv- 191
et al. (2014a) estimated logF(Hα) = −11.56 ± 0.15 from newly ity threshold and is likely resolved out by the uv-coverage of the 95
calibratedSuperCOSMOSHαimages,ingoodagreement.Wealso VLA B-configuration (C. Purcell, 2013, private communication). by
TheresolutionoftheMAGPISimageallowsustofilteroutthefea- g
usedourWiFeSdata,scalingupthefluxestoaccountfortheportion u
orufntshetonoebbtualiantlhoagtFw(Hasαn)o=to−bs1e1r.v5e9da.nWde−d1id1.6th2i,srfeosrpetwctoivseelyp.aTrahtee tauffreecstcsltohseeotoldAerbNelVlS48Sadnadtas.oHaevnocied,wtheeucsoenfouusrionnewprmobelaesmurwemhiecnht est on
weightedmeanfluxislogF(Hα)=−11.58±0.06ergcm−2 s−1, for further analysis. In addition, we measured the flux density at 13
0.325GHz(90cm)fromanintensityimagetakenasapartofthe A
adoptedhereafter. p
Intheradio-continuumdomainwefoundtwocataloguedbutdis- MAGPIS survey (for more details see Helfand et al. 2006). Both ril 2
fluxvaluesaretabulatedinTable2,alongwithseveraladditional 0
crepant values forthe 1.4 GHz (20cm) integrated flux density of 1
Abell48;S =159±15mJyfromtheNRAOVLASkySurvey integratedfluxmeasurementsofthenebulatakenfromthelitera- 9
(NVSS;Co1n.4donetal.1998;Condon&Kaplan1998)andS ≈ ture.Itisclearlyseenfromthesefluxesthattheemissionfromthe
1.4 nebulaisthermalinnature,typicalofphotoionizedgas.
657mJy,cataloguedintheMulti-ArrayGalacticPlaneImagingSur-
vey(MAGPIS;Helfandetal.2006).Thediscrepancyistoolargeto
beattributedtoconfusionbythecomplexbackground,asnotedby
3.2.1 Infraredfluxes
Condon&Kaplan(1998).Therefore,were-measuredthefluxden-
sityfromtheMAGPISandNVSStotalintensityimages5usingthe Thereareseveralfluxdeterminationsatinfrared(IR)wavelengths.
KARMA data analysis package (Gooch 1996). While our estimated Thenebulaisfaintin2MASSandUKIDSSimages.IntheKsband,
fluxdensityfromtheNVSSimageagreeswiththecataloguedvalue whichhasthestrongestdetection,weattributethefluxtomolecular
hydrogen (H ) at 2.12μm, as well as the Brγ 2.17μm hydrogen
2
line (Kimeswenger et al. 1998). The GLIMPSE data resolve the
5TheimageswereretrievedfromtheNVSSandMAGPISpostagestamp nebula well, and it appears brightest in the IRAC2 and IRAC4
servers:http://www.cv.nrao.edu/nvss/andhttp://third.ucllnl.org/gps/ bands.Onthisbasis,theBrα4.05μmlineisprobablythestrongest
MNRAS440,1345–1364(2014)
A[WN]starinAbell48 1349
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Figure4. Individualimageslicesofourlow-resolution(R=3000;toppanel)andmedium-resolution(R=7000;bottompanel)WiFeSredspectrashowing fro
thebroadfeaturesfromtheCSPN,andline-splittingofthenebularlinesduetoexpansion. m
h
ttp
featurecontributingtotheIRAC2flux,andstrongpolycyclicaro- (R = 3000) and medium-resolution (R = 7000) WiFeS red-arm s
matic hydrocarbon (PAH) bands (Allamandola, Tielens & Barker spectra,respectively.ThebroadfeaturesfromtheCSPN,therespec- ://a
c
a
1989) likely contribute to the IRAC4 flux, particularly the bands tive nebular and night sky line strengths, and line-splitting of the d
e
at7.7and8.6μm,withsomeemissionfromPfα7.46μmandthe nebularlinesduetoexpansioncanbeseen.The1Dlow-resolution m
[ArIII]8.99μmfine-structureline.Weranapreliminaryphotoion- spectrum from 2012, binned from the data cube to increase the ic.o
u
izationmodeltotestthesepredictionsusingthethree-dimensional S/N,isillustratedinFig.5.Itshowsthepresenceofabrightneb- p
photoionization code MOCASSIN (Ercolano et al. 2003; Ercolano, ularHαlineandrelativelyweak[NII]λλ6548,84linesinthered .com
Barlow&Storey2005).Thispredictsan[ArIII]8.99μmlineflux ([NII]/Hα=0.28),asteepBalmerdecrementduetotherelatively /m
of around 20 per cent of Hβ. A full analysis will be published highextinction,aswellasfairlyweak[OIII]linesinthebluerelative nra
separately. toHβ.ThereisnoindicationofanebularHeIIλ4686line(toalevel s/a
Abell 48 is prominent in the WISE3 (11.6μm) and WISE4 of1percentofHβ). rtic
(22μm)bands,butonlybarelyseeninWISE2.Featuresthatmay InTable3,wesummarizetheobservedandreddening-corrected le
-a
contribute to the WISE3 band are PAH emission, particularly at linefluxes.TheotherWiFeSspectrawereusedtoconfirmthatthe b
s
11.3μm,thermallyemittingdust,andthe[SIV]10.5μmlinewhich faintestnebularlineswerereal.ThespectrumpresentedbyTK13is tra
c
our photoionization model suggests is comparable in strength to similartoourown,themaindifferencebeingthatTK13estimated t/4
Hβ.Otherpredictedcontributors,inorderofdecreasingstrength, asomewhathigherextinction.Abell48hasalowexcitationclass, 40
arethe[ArIII]8.99μm,[NeIII]15.5μm,and[NeII]12.8μmlines. EC=1.45(Dopita&Meatheringham1990;Reid&Parker2010), /2/1
Similarly,itisamoderatelystrongsourceintheMIPS24μmband soweconcludethattheweakλ7236featurethatwealsodetected 3
4
5
(Mizunoetal.2010;Wachteretal.2010),consideringthatthe[OIV] ismorelikelytobeaCIIrecombinationlineratherthanthe[ArIV] /1
0
fine-structurelineisexpectedtobeweakorabsentinanebulaof lineonthebasisofourphotoionizationmodel. 1
9
this excitation class. In fact our optical spectra (Fig. 5) show no 1
9
emissionlineswithanionizationpotential(I.P.)exceeding41.0eV, 5 b
basedontheweakdetectionofthe[NeIII]λ3869line.Thissuggests 3.3.1 Reddening y g
thatwarmdustisthemaincontributortothe22and24μmbands, ue
withalikelyminorcontributionfromthe[ArIII]21.81μmline.The AinTsuamblme4ar.yWoefdtehreivveadritohueslorgedardiethnminigcdexetteinrmctiionnatiaotnHsβis,apnrdesheenntecde st on
sdpoemctirnaalteednebrygythdiisstdruibsuttcioonm(pSoEnDen)to.fFtrhoemnaebbullaac,knbootdsyhofiwtntohtehree,IRis E(B−V),usingthenebularBalmerdecrementfromourthreein- 13 A
flquuixteestygpiivceanloinfPTNabe.leM2izwuneoeesttiaml.a(t2e0a10d)uasltsotefmoupnedratthuartethoefn1e5b0ulKar, dHeupmenmdeern&t WStioFreeSy(s1p9e8ct7r)a,anwdhtehreertehdedeinntirnignsliacwlionfeHsotrwenargtthhs(1f9r8o3m) pril 2
8μmemissionwasco-spatialwiththe24μmemission.Similarly, winegrtehuesiendt.egWraetiendd1e.p4eGndHezntalynddeHteαrmfluinxeedstfhroemexStienccttiioonn3b.y2c(soemepea.gr-. 019
we find the angular size to be similar at all wavelengths indicat-
Bojicˇic´etal.2011b).WealsoutilizedthePζ/Hαdecrement,again
ingthattheionized,molecular,anddustcomponentsarespatially
mixed.TheIRAC/MIPSfalse-colourimage6(Fig.1)illustratesthis. adoptingthelinestrengthsfromHummer&Storey(1987).
Following Crowther et al. (2006a), interstellar K -band extinc-
AsummaryofallnebularcontinuumfluxesisgiveninTable2. S
tions were also estimated from the observed J − K and H − K
s s
colours of the ionizing star, derived from Table 7. To estimate
3.3 Nebularspectroscopy the intrinsic colours, we take the average of the values for the
weak-linedWN3–4andWN5–6subgroupsfromCrowtheretal.
Our three WiFeS data cubes were used to investigate the nebular
(2006a), i.e. (J − K) = +0.04 and (H − K) = +0.07. Using
conditions. Fig. 4 presents individual slices of the low-resolution s 0 s 0
therecipeofCrowtheretal.(2006a),wecalculateK -bandextinc-
S
tions of AJ−Ks = 0.78 and AH−Ks = 0.84, respectively. Adopt-
Ks Ks
6MIRfalse-colourimagingisalsoapowerfuldiagnosticindicatorforclas- ing the mean value, AKs = 0.81, and using the relation between
sificationpurposes(e.g.Cohenetal.2007,2011;Parkeretal.2012). AKs and E(B − V) from Indebetouw et al. (2005), we derive
MNRAS440,1345–1364(2014)
1350 D.J.Frewetal.
D
o
w
n
lo
a
d
e
Figure5. Extractsoftheintegratedblue(left)andredspectraofAbell48,obtainedfromthehighS/NWiFeSobservationfrom2012.NotethattheHαline d
istruncatedintherightpanel.Theobserved[NII]/Hαratiois0.28. fro
m
h
Table3. ListofemissionlinefluxesforAbell48,adopted Table 4. A summary of the various ttp
s
fromthehighS/NWiFeSIFUspectrumfrom2012. reddeningdeterminationsforAbell48 ://a
whichareallingoodagreement. c
a
Line λ f(λ) F(λ) I(λ) de
Method E(B−V) m
[OII] 3727 0.256 13: 64: ic.o
[NeIII] 3869 0.230 6.8: 28: StellarSED 1.90 ± 0.10 up
Hδ 4101 0.182 6.6: 20: Near-IRstellarcolours 2.3 ± 0.3 .c
o
Hγ 4340 0.127 19.9 43.5 Radio/Hαflux 1.77 ± 0.15 m
[OIII] 4363 0.121 <1.0 <2.1 Hα/Hβdecrementa 2.2 ± 0.2 /mn
HeI 4471 0.095 5.0: 9.0: Hα/Hβdecrementb 1.92 ± 0.1 ra
s
HeII 4686 0.043 <0.8 <1.0 Hα/Hβdecrementc 1.85 ± 0.1 /a
Hβ 4861 0.000 100 100 Pζ/Hαdecrementa 2.2 ± 0.2 rtic
[[OOIIIIII]] 45905097 −−00..002346 141092 130232 Pλ6ζ2/H84αDdeIBcrementc 11..89 ±± 00..25 le-abs
[NII] 5755 −0.195 1.3: 0.40: Adoptedreddening 1.90 ± 0.15 tra
[HSeIIII] 56837162 −−00..221853 627.1.6: 01.377.9: aFrom WiFeS spectrum 1; bWiFeS ct/44
[NII] 6548 −0.318 138 19.4 spectrum2;cWiFeSspectrum3. 0/2
Hα 6563 −0.320 2056 286 /13
[NII] 6583 −0.323 429 58.4 and applying the relation of Friedman et al. (2011). Finally, we 45
[HSeIII] 66677186 −−00..334326 4423..37 55..15 (re2-0e1v0a)lubaytefidttitnhgetheestoimveartaelloSfEADVo=fth5e.s7tamr,aegxcfrluodminWgtahcehutenrceerttaailn. /1019
[H[CASeIrIIII]III] 7776201736336561 −−−−0000....334348094796 1422148.29..109: 11521.0...362: IS0R.e1cA0tCi(ocn[H8β4.0.=4])-.2mW.7ag5e±vada0lou.p1et5,a)tnoheadrveeetaerfartmgeeri.dnTerheEids(dBvean−liuneVgc),aE=n(Ba1l.s−9o0bV±e)c=0o.m110.p9a(0rsee±de 195 by gue
HeI 7281 −0.414 12.1 0.94 withtheestimateofE(B−V)=2.1(cHβ =3.0)fromDePewetal. st o
[[[AOOrIIIII]]II] 777733523100 −−−000...444612790 13703..23.8:: 00..75184.9:: (Mf2ro0Sm1S1Sa)O,Sb2Ea.D3se-mdfit,ospnanectdhtreEu(mnBe,b−wuhlVairl)e=BTaK2lm.113e5rdfedrtoeemcrrmetmihneeendBtaEfl(mrBoem−rdaVe)csr=heaml2l.eo1nw0t n 13 Ap
[ClIV] 8046 −0.497 2.0: 0.1: method. ril 2
P16 8502 −0.540 25: 0.9: 0
1
P15 8545 −0.542 16: 0.6: 9
P14 8599 −0.547 13: 0.4: 3.3.2 Plasmadiagnostics
PP1132 88676550 −−00..555630 2299:: 00..99:: WeestimatedthenebulardensityofAbell48tobene=700cm−3
P11 8863 −0.569 35: 1.0: fromtheobservedratioofthe[SII]doublet.The[NII]λ5755lineis
P10 9015 −0.581 20: 0.6: barelydetectedgivinganuncertainvalueoftheelectrontempera-
[SIII] 9069 −0.585 787 21.3 ture,whilethefarred[SIII]lineswereusedtoestimateTe=7000K,
Pζ 9229 −0.597 94.6 2.4 andanotherlowqualityestimatecomesfromtheratiooftheauroral
tonebular[OII]lines.Twofurtherestimatescomefromtherelative
strengths of the HeI lines (Zhang et al. 2005). Despite the larger
E(B − V) = 2.3 ± 0.3, in fair agreement with the other de- uncertaintiesofsomeofthefainterdiagnosticlines,theindependent
terminations. An independent, albeit lower accuracy estimate is estimates of the electron temperature are in good agreement, and
derived from the equivalent width (2.6Å) of the λ6284 diffuse we adopt an average value of 7500 K for the abundance analysis
interstellar band (DIB) in the red (the only one measurable), (Section3.4).AsummaryispresentedinTable5.
MNRAS440,1345–1364(2014)
A[WN]starinAbell48 1351
Table5. SummaryofplasmadiagnosticsforAbell48. Table6. Ionicabundances,adoptedICFs
andtotalabundancesforAbell48.
Diagnostic Value Result
Line,λ(Å) Ion Abundance
[SII]λ6717/λ6731 0.96 Ne=700cm−3
[NII](λ6548+λ6584)/λ5755 195: Te=7600K: 5876,6678 He+/H+ 0.129
[OII]λ3727/(λ7320+λ7330) 77: Te=6700K: ICF(He) 1.00
[SIII](λ9069+λ9532)/λ6312 198 Te=6700K: He/H 0.129
[OIII](λ4959+λ5007)/λ4363 >210: Te<9950K 6548,6584 N+/H+ 2.21(−5)
HeIλ7281/λ5876 0.17 Te=7300K ICF(N) 2.75
HeIλ7281/λ6678 0.053 Te=7700K N/H 6.07(−5)
Gastemperature(adopted) ... Te=7500K 3727,7325 O+2/H+ 1.76(−4)
4957,5007 O+2/H+ 3.13(−4)
D
3.3.3 Expansionvelocity ICF(O) 1.00 o
w
O/H 4.89(−4) n
Theredonotappeartobeanyexpansionvelocitymeasurementsfor lo
Abell48intheliterature,soweusedtheWiFeSred-armmedium- 3869 Ne+2/H+ 1.10(−4) ad
e
rveesloolcuittyionofsptheectnruembu,ltaakfreonmint2h0e1G0aAupssriial,ntopreosfitilmesatoefththeeexbpriagnhstieosnt INCeF/H(Ne) 11..5773(−4) d from
nebularemissionlines.Theobservedfull-widthathalf-maximum 6717,6731 S+/H+ 5.51(−7) h
(FWHM)foreachlinewasdeterminedusingthesplotfunctionin 6312,9069 S+2/H+ 1.15(−5) ttps
IRAF, and the expansion velocity, corrected for instrumental reso- ICF(S) 1.10 ://a
lutionandthermalbroadening,wascalculatedwiththefollowing S/H 1.33(−5) ca
d
expression(Gieseking,Hippelein&Weinberger1986): 7136 Ar+2/H+ 2.10(−6) em
vexp=0.5(FWHM2obs−FWHM2instr−8(ln2)kTe/m)0.5 (1) IACrF/H(Ar) 13..5370(−6) ic.ou
p
where k is Boltzmann’s constant, T is the electron temperature .c
e o
takenfromSection3.3,mistheatomicmassofthemeasuredion, m
fromtheexpressionsgiveninKingsburgh&Barlow(1994).Note /m
a(eethMxnraepdleaodFnnuiiWsngtieaohHrnte-Mntsvkeaeibynlls.outlrcl2ianii0trsey0srt6.(hi)meeT.wghi(.nheeWsi.Hctgrhe.uWimniSsHbceaehMnro¨tgbaneeilbsrttFeeo1rWrf9nt8epeHr9nroM)e,txatbykduaeefltno.tewr2troemt0h0ibienn5esee)t.edxeqpaWfuadraneoulsmsditoeoensrVetihvvo1ee0-f Fotλhw3ua7rittn2htg7heer[tmOoooxtIhIry]eegd,epotnhoueobarlbenuStenh/oNdanasnoaacfbneutuhnhnedacasλenra3ct8ael6ianr9igsfle[uoNrxnueldynIIucI]eaenrlttioaniitnenhd.teiyTch,haiatgesihvateehbxueetnsinntdeicambtnuiacolteanesr., nras/article-a
iVty10o=f s4a0mkpmles−o1f,[wWhCic]hPaNgree,evswitohfth3e6kavmersa−g1e(ePxepn˜aan,sMioendvinealo&c- adnedrivaeddopatesidgnICifiFcsanatrley phrigesheenrteedlecintroTnabtelemp6e.rWatueren,oatendthtahterTeKfo1r3e bstrac
SEraMtdaSisAaiOnl´,svkwealho2icc0iht0y3ios).flFo+rwo3em6r±tthhae3nskamthmese−evxd1pe,altooabcsittaeyitnwoefde+mus5ei0nasg±utrhe4eakImRhAesFl−ipo1accdekneattergirce- arthelpeornwoedeiurgchoebxdoyiugnreitnnhgeaibHrufiengId.λa84n,4cwe7e1tshleianenen,woweshidgiconh.oRifstehpfeerer[rdOiincIgItIe]tdoλt4ot3hb6ee3spslitenrcoetn,rnugomerr t/440/2/13
byourphotoionizationmodel.Thereforewepreferourownupper 4
minedbyTK13.Ourmeasurementtranslatestoavelocityrelative 5
tothelocalstandardofrestofv =+49±3kms−1. limitforthe[OIII]electrontemperature,andplacemorereliancein /10
This observed expansion velLoScRity also falls in the range seen ournebularabundances. 191
OuranalysisshowsthatAbell48hasnosignificantenrichment 9
fortheshellsaroundlate-WN(WNL)andluminousbluevariable 5
ofnitrogen(N/O≈0.12)anddoesnotbelongtoPeimbert’sTypeI b
(LBV) stars (Nota et al. 1995; Chu 2003), but we point out that y
class(Peimbert1978;Peimbert&Torres-Peimbert1983),forwhich g
nopoint-symmetric,moderate-densityejectashellisknowntosur- u
roundanymassiveWNEstar.Theearliestspectraltypeseenina wInefaadcot,ptthKeinngebsbuularrghab&unBdaarnlocews’sar(e19a9p4p)rodxefiimniattieolnyosfoNla/rO, a>po0i.n8t. est o
high-surfacebrightnesscoherentnebulaisWN7bforPMR5(Frew n
to which we return below. Here we differ from the conclusions 1
etal.,inpreparation).EarlierWNstarspossessincreasinglyone- 3
ofTK13,whofindslightlysubsolarabundancesforAbell48.We A
sided,low-densityshells,fragmentedbyRayleigh–Taylorinstabil- p
itthieesdaifnfdusheaIvSinMg.aAbunnedxaanmcepslediilsutNedGCin6p8a8rt8b(yGsruweenpdtl-ueptgala.s2f0r0o0m; aadttoripbtu.tWeethdisodniosctrdeeptaencctythteo[tOheIIIh]iλg4h3e6r3elleincterownhtiecmhpseetrsataunreupthpeeyr ril 201
limitontheelectrontemperatureof∼10kK,andthemeanofseveral 9
Ferna´ndez-Mart´ınetal.2012)aroundtheWN6b(h)starWR136.
independent diagnostics suggests a lower temperature of 7500 K
(seeTable5),andhenceahigherderivedoxygenabundance.We
3.4 Nebularabundances willrevisitthenebularabundancesinSections5.2,6and7.
We derived ionic and total nebular abundances from our spectra
using the EQUIB code (see Wesson, Stock & Scicluna 2012), after
4 THE CENTRAL STAR
adoptingthevaluesofT andN giveninTable5.Wedeterminedthe
e e
heliumabundancefromthemeasuredintensitiesoftheλ5876and
4.1 Photometry
λ6678HeIrecombinationlinesusingtheeffectiverecombination
coefficientsfromHummer&Storey(1987).Forthemetallicions, TheCSPNisobviousonSuperCOSMOSB ,R ,I ,andHαimages
J F N
theabundanceswerederivedfromtheobservedlineintensitiesof (Hamblyetal.2001;Parkeretal.2005),aswellas2MASS,UKIDSS
the strong collisionally excited lines. We then derived the total and GLIMPSE images (Benjamin et al. 2003; Skrutskie et al.
abundancesusingtheionizationcorrectionfactors(ICFs)derived 2006;Lawrenceetal.2007).Wesummarizetheavailableliterature
MNRAS440,1345–1364(2014)
1352 D.J.Frewetal.
Table7. Summaryofphotometricmeasurementsfor Table8. PrincipalemissionlinesfoundintheCSPNofAbell48.
the[WN4–5]centralstarofAbell48.Thethirdcol- ThelineequivalentwidthsanduncertaintiesareinÅ.
umngivesthedereddenedmagnitudes.
Species λ −Wλa −Wλb FWHM
Waveband m m0 Source (Å) (Å) (Å) (kms−1)
U 19.8 ± 0.2 10.6 Thiswork NIV 3480 ... 45±10 1300:
B 19.48 ± 0.02 11.7 Thiswork NIV 3748 ... 3.0±1.0 ...
V 17.80 ± 0.01 11.9 Thiswork NIV 4058 ... 23±3 830
V 17.72 ± 0.25 11.8 YB6 HeII+NIII 4100 ... 29±4 1060
I 15.50 ± 0.05 12.0 DENIS HeII+NIII 4200 ... 15±2 800
Ic 15.14 ± 0.05 11.7 Thiswork HeII 4339 ... 11.5±2.0 800
J 13.54 ± 0.09 11.8 DENIS NIII 4379 ... 6.0±1.0 ...
J 13.508 ± 0.027 11.7 2MASS NIII 4518 ... 12±2.5 ... D
J 13.440 ± 0.002 11.7 UKIDSS HeII 4542 n.m. 16±4 1200 ow
H 12.834 ± 0.028 11.7 2MASS NNVV 44660240 nn..mm.. 2129..55±±33..00 566000 nloa
H 12.823 ± 0.001 11.7 UKIDSS NIII 4634-40 ... 12±3 ... ded
KKKsss 111222...332028251 ±±± 000...01001227 111111...666 U2DMKENAIDISSSSS HNHNeeIVIIIIII 4444989606485046 n....m....... 122101.6..0500±±±±1324.0..005 1111....72..00 from https
[3.6] 11.69 ± 0.06 11.3 GLIMPSE NIV 5202 5.4±2.0 5.0±1.4 ... ://a
[4.5] 11.25 ± 0.10 11.0 GLIMPSE HeII 5411 23±4 34±6 900 ca
[5.8] 11.06 ± 0.09 10.8 GLIMPSE CIV 5806 ... 15.2±3.0 1300: de
[8.0] 11.04 ± 0.16 10.8 GLIMPSE HeI 5876 3.8±2.0 n.m. ... mic
Referencesforphotometry:2MASS(Skrutskieetal. HeII 6118 4.0±1.4 2.2±1.0 .ou
2(B0e0n6j)a;mDinENetISal.(E2p0c0h3t)e;inUeKtIDalS.S19(L94aw);reGnLceIMePtSaEl. HNeIVII 66338121 38..80±±22..00 65..84±±12..60 8.0..0 p.com
2007);YB6(Zachariasetal.2004). HeII 6406 7.1±1.0 7.9±1.0 ... /m
HHeeIIII 66552670 41.330±±13.00 6.6n±.m.1.0 10..2.0 nras
pbhyoutso.mInetroyrdinerTtaobcleor7r,eacltotnhgewobitshernveewdUmBaVgnIimtuadgensitaunddescomloeuarssurfeodr HHeeIII+HeII 66688931 11.4.±.. 1.0 10.4.±.. 1.0 b7le5n0d /article
rrSeeeddcddtieeonnniinn3gg.3,la.a1wc.oTolhofeuHrvoeiwsxucaaerlsthsabo(1sfo9Er8p(3Bt)io.−nVis)t=he1n.9A0vm=a5g.9ismadaogputseidngfrothme HNHHeeeIVIIIIII+NIV 7787125197310876 411066..n037.±m±±.325..023 31913..810±.±±..213.00.0 14835.5.00.00: -abstract/4
No time series photometry of the CSPN is available, so infor- HeII 9225 7.9±1.8 ... 700 40
mation on any short-period variability is lacking. However J and /2
aSecondWiFeSspectrum;bHalespectrum;n.m.=detectedbut /1
K magnitudes are available from three different epoch surveys, 3
DsENIS,2MASSandUKIDSS(Epchteinetal.1994;Skrutskieetal. notmeasured. 45/1
2006;Lawrenceetal.2007)andallagreewithintheuncertainties, 01
sowecantentativelysaythestarisnotalargeamplitudevariable, We measured the stellar emission lines from all of our spectra 91
9
unlike the ionizing star of the LMC planetary N 66 (McKibben usingtheSPLOTfunctioninIRAF.Table8presentstheidentifications, 5 b
Nsyasitle&mS(Hhaapmleaynn19et55al).,2w0h0i3c)h.islikelytohostaninteractingbinary tkhmesl−in1e)oefquthiveaplreinntciwpiadltlhinse(sinthÅat)w, aenredctlheearlliynedewteicdttehds.(TFhWeHtyMpicianl y gue
ulinnceesr)taanindti±es50arkem10s−–115repspereccteivnetly(n,obalseesdsothnarnep±e1atÅmefoarsutrheemweneatsk. st on
1
Thenumericalcriteriaweusedtoclassifythestararesummarized 3
4.2 Spectroscopy A
inTable9.BesidesthehighS/NPalomarspectrum,wealsomade p
SpectroscopicclassificationsofthecentralstarofAbell48wereun- use of our low-resolution WiFeS stellar spectrum, which extends ril 2
dertakenbyWachteretal.(2010)andDePewetal.(2011).Wachter to∼9500Å,butveryfewclassificationschemesusethefar-red/NIR 01
9
etal.(2010)classifiedthecentralstarasaWN6accordingtothe wavelengthregion.Aspartofanotherproject,wedownloadedall
classificationschemedevelopedbyConti,Massey&Vreux(1990). the red spectra of the Galactic WN stars presented by Hamann,
ThepreviousshallowMSSSO2.3-mspectrumpresentedinDePew Koesterke & Wessolowski (1995), and measured the equivalent
etal.(2011)showedabroadfeatureatλ7116Åinthered,buta widthoftheλ8237HeIIlineforeachstar,ifavailable.Wetabulated
largesectionofthespectrumbetween5050Åand6300Åwasnot theHeII/HeIratiosandspectraltypefromSmith,Shara&Moffat
observed, so it was unclear whether this was a [WN] or [WN/C] (1996)foreachandfound,asexpected,atightrelationbetweenthe
star.Inourdeeperspectra,weagainfindastrongfeaturecentred twoquantities.Thissuggestsaspectralclassof[WN4]fortheCSPN
around λ7116 Å, attributed to the complex of NIV lines between of Abell 48. In summary, the NIVλ4057 and NVλλ4604,20 lines
λ7103 Å and λ7129 Å. The HeII λ6560 line appears as a broad beingmuchstrongerthantheNIIIλ4634,40blend,withrelatively
(FWHM∼25Å)featurefromwhichthenebularHαlineprotrudes weak CIV, we revisited the classification and now find an earlier
(thenebularlinesareover-subtractedinthePalomardata).TheHeII typethan[WN6](cf.Wachteretal.2010),afterprimarilyfollowing
λ4686andλ5412linesareprominentandabroadCIVλλ5801,12 thecriteriaofSmithetal.(1996).Bygivingmostweighttotheratio
doubletisalsopresent,asistypicaloftheWNclass. oftheequivalentwidthsoftheHeIIλ5411andHeIλ5876lines,our
MNRAS440,1345–1364(2014)
A[WN]starinAbell48 1353
Table9. Classificationofthecentralstar, The PoWR grid models are normalized to L/L(cid:3) = 2.0 × 105
afterSmithetal.(1996).Thesecondcol- whichisappropriateforamassiveWNstar;consequently,thestel-
umnistheratioofthemeasuredequivalent lar flux of our CSPN is proportional to L(cid:2), and R(cid:2) and M˙ are
widths. proportionaltoL1/2 andL3/4 respectively.WefollowHamann&
(cid:2) (cid:2)
Koesterke (1998) in assuming a wind clumping factor of D = 4
Criterion Ratio Classification
(f=0.25),whichisappropriateforWNstars.Thestellarradius,R(cid:2),
Wλ5411/Wλ5876 9.5 WN4 istheinnerboundaryofthemodelatmosphereandcorrespondsby
Wλ8237/Wλ5876 7.8 WN4 definitiontoaRosselandopticaldepthof20.Thestellareffective
P5411/P5876 2.6 WN5 temperature,T(cid:2),istheeffectivetemperatureatR(cid:2),andcanbeeasily
P4604/P4640 1.6 WN5 calculatedfromtheStefan–Boltzmannlaw:
PP45085078//PP45680746−40 01..86 WWNN45 L=4πR(cid:2)2σT(cid:2)4. (4)
P5808/P5411 0.6 WN4–5 ForAbell48,variouslineratiosusingtheNIII,NIVandNV,and D
o
Adopted ... [WN4–5] HeIandHeIIlines,initiallyconstrainedthetemperaturetobetween w
n
63and79kKandlog(Rt/R(cid:3))between0.6and1.0.Theabsence loa
adoptedclassificationis[WN4–5],whileTK13estimatedaspectral of HeIIλ4686 emission in the surrounding nebula is due to the de
claWsseoffu[rWtheNr5n]o.tethatthereisnoobviousindicationofanoscillat- λop5t8i7ca6lllyintehmicukcWhsRtrownignedr.tMhaondoeblssewrvitehd,Tweffhi<le6m0okdKelsswhoitwhTaeHffe>I d from
90kKareruledoutastheseproducesignificantfluxshortwardofthe h
isfnrtaagcrtPiisoicnhk,yewdrirenogugsedenetcdhreeefim‘Pceiiencnktte(.reTi.ngog.qSiunmadnietxthi’taedtteivafieln.lye1d9e9bst6yi)mS,mastheiothwtheientgahly.tdh(1raot9g9teh6ne; HpreesIIeinotniinzathtieonsuerdroguenadtin2g28nÅeb,usloa.HTeheIIeesmtiimssaitoendstheomuplderabteurceleiasrliyn ttps://ac
broadagreementwiththetemperaturerangeof60–90kKfoundfor a
cf.Oliveira,Steiner&Cieslinski2003): d
massiveWN4starsbyHamann&Gra¨fener(2003). em
NN(H(He+++)) = [W(Wλ4(5λ4418)5×9+Wλ(λ45846111))]0.5 −1. (2) 10-T1h3e)baensdt-rfietptirnogdumcoedseflahiralysTwefefl=lth7e1rkeKlataivnedeRqtu=iv6al.3enRt(cid:3)wi(dmthosdoefl ic.ou
p
Formally,weobtaintheunphysicalvalueof−0.1,whichwereset theheliumlines.Anewsyntheticspectrumcalculated fromare- .co
m
visedmodel10–13,whichutilizedupdatedatomicdata,waskindly
tozero,onthebasisoftheobserveduncertaintiesoftheequivalent /m
waODbiveudnentrnhdaesalf.lne,Icltndhepe&orsafpcA∼eticn1ctde0rru,ipmwlleaeritsac1dse9ionm8pt3itbl;aaaSrswemtoodiratokhPnineotgthpaueulpl.oap1btei9sor9enlr6ivIm)e,WidbtuNfSot/4rNaotshsowetfaehtrhys(edVhrolroiwengueeixsnn,. dappnaraondravcmsiedyseentdsteheretbstytaiocreHsHTp.eeeTf:cfCot=rd:aNt7a:(1Or2ek0=Kp1l3oa,0tnt.ped9dr8lio:vigan1tEReF-tic4g=o:.m600...m80H,1uow5nw:iicte0hav.tt0eiho.re,nTt)dhheeTefahNoueblItIsmIae,broNvudenIeVd-l nras/article
Section5,thepropertiesofthesurroundingnebulaunambiguously and NV line intensities are all weaker in the model, in particular -ab
showthatwearedealingwithlow-massCSPN. theNV4944andNIV7116blends.Thisindicatesthatthedefault stra
nitrogenabundanceistoolow.Wehavenotconvincinglydetected c
anylineduetooxygen,sowemakenomodificationtothedefault t/44
0
4.3 Modelatmosphereanalysis modelabundance.Ourparametersareinexcellentagreementwith /2
Tobegintheanalysis,wedownloadedthesyntheticspectraappro- thatderivedbyTK13(seetheirtable3),themaindifferenceisthat /134
TK13 estimate a higher nitrogen abundance of 5 per cent and a 5
priateforWNEmodelatmospheresfromthePotsdamWolf–Rayet /1
hydrogenabundanceof10percent,aresultthatdoesnotdisagree 0
(PoWR) data base7 (Hamann & Gra¨fener 2004) to compare with 1
ourdata.ThesespectraaredeterminedfromthePoWRmodelat- withourupperlimitforthiselement.Soitisnowapparentthatthe 919
photosphericnitrogenandhydrogenabundancesdifferconsiderably 5
mospheres,describedinGra¨fener,Koesterke&Hamann(2002)and b
between Abell 48 and IC 4663, mimicking the range of H abun- y
Hamann&Gra¨fener(2003),whichaccountforsphericalexpansion, g
dancesseenintheO(He)stars(Reindletal.2013,andreferences u
non-LTEeffects,andmetallineblanketing.ThePoWRmodelgrid e
hastwodimensions,thestellartemperatureT(cid:2)andthe‘transformed therein). st o
The intrinsic luminosity of the star, and hence its distance, is n
rpaadraiums’e,teRr,tb(Heianmgaanfnun&ctiKooneosftethrkeem1a9s9s8-l)o,swshraicteh,Mis˙,aawndintdhedsetnesliltayr not constrained from the model atmosphere analysis. Instead we 13 A
adoptthedistanceof1.6kpcfromSection5.3inthediscussionthat p
radius,R(cid:2)(cid:2).Itisvgivenby(cid:3): M˙ √D (cid:4)2/3 fWolelofwous,ncdf.thTeKb1o3lowmheotraiscsucmorerdecaticoannoonfitchaelpmoostd-eAlGaBtmlousmpihneorseittyo. ril 201
Rt=R(cid:2) 2500k∞ms−1 10−4M(cid:3)yr−1 (3) be −5.4mag, which leads to an estimated luminosity of the star 9
of ∼5.5 × 103L(cid:3). From our adopted value of the temperature
where v∞ is the terminal velocity of the wind and D is a wind T(cid:2), the stellar radius R(cid:2) was calculated using equation (4) to be
clumping factor (or inverse of the filling factor). It has been pre- 0.49R(cid:3).FromthelinewidthspresentedinTable8,weestimate
viously noted that many parameters of massive WRs and [WR]s aterminalwindvelocity,v∞ of1200kms−1.Finally,weestimate
aresimilar(e.g.Crowther,Morris&Smith2006b),aconsequence themass-lossrate,M˙,whichisproportionaltoL3/4 foraconstant
of the scaling law for WR atmospheres first noted by Schmutz, Rt(equation3).WedeterminelogM˙ =−6.3±0.2M(cid:3)yr−1.
Hamann&Wessolowski(1989).Inotherwords,twoatmospheres
withthesametemperature(T(cid:2))andtransformedradius(Rt)show 4.4 Stellarspectralenergydistribution
verysimilarspectraregardlessofL(cid:2),R(cid:2),M˙ andv∞.
ThestellarSEDfortheCSPNofAbell48isshowninFig.7,along
with the adopted PoWR synthetic spectrum. The model has been
7http://www.astro.physik.uni-potsdam.de/∼wrh/PoWR/powrgrid1.html normalizedtotheensembleofthedereddenedvaluesfromTable7,
MNRAS440,1345–1364(2014)
1354 D.J.Frewetal.
D
o
w
n
lo
a
d
e
d
Fveigryurpero6m.iNneonrtmHaelizIIeλd4s6p8e6ctarunmdN(bIlVacλk71li1n6e)feoafttuhreesCiSnPtNhe,ssppleiccetrdumfro.mOvoeurrplWotitFedeSisanandPupaldoamteadrmdaotdae.lN1o0t–e1th3e(rperdesleinnec)eforfomthethNeVPoλWλ4R60g4r,i4d6(2p0rolivniedsedanbdytHhe. from
Todt),whichfitstherelativestrengthsoftheHelineswell.Therelativelypoorfittothenitrogenlinessuggeststhedefaultnitrogenabundanceistoolow.A h
colourversionofthisfigureisavailableintheonlineversionofthearticle. ttps
://a
c
5 THE NATURE OF ABELL 48 a
d
e
m
Asaprecursortoanydiscussionontheevolutionaryimplications ic
ofthe[WN]class,itisimportanttotrytoproperlyassessallthe .o
u
observationalevidenceforAbell48sothatitsstatusasatruePN p.c
oracircumstellarnebulaaroundamassivestarcanbemadeclear. om
CircumstellarnebulaecanbemorphologicallysimilartoPNe,and /m
n
arefoundaroundseveraltypesofmassivestars(Chu2003;FP10; ra
s
Gvaramadze,Kniazev&Fabrika2010;Mizunoetal.2010;Wachter /a
etal.2010;Bojicˇic´etal.2011a). rtic
le
-a
b
s
5.1 Diagnosticplots tra
c
In Fig. 8 we present the ‘SMB’ or logF(Hα)/F [NII] versus t/44
log F(Hα)/F[SII] diagnostic diagram of Sabbadin, Minello & 0/2
Bianchini(1977),asextensivelyupdatedbyFrew&Parker(2010, /13
4
Figure7. TheSEDofthecentralstarofAbell48.Theredlineshowsthe hereafter FP10), and further refined here (see also Sabin et al. 5
syntheticspectrum(PoWRmodel10–13),withthephotometricmeasure- 2013). Individual PNe are shown as small red dots and Galactic /10
mentsdereddenedwithE(B−V)=1.90plottedasredcircles.Acolour HIIregionsassmallblacksquares.Intheleftpanelthefieldsshow- 191
versionofthisfigureisavailableintheonlineversionofthearticle. 9
ingHIIregions,SNRsandPNearemarked;notetheconsiderable 5
b
overlapbetweenthem.ThebluetrianglesshowthePNewithcon- y
g
firmed[WN]or[WN/C]stars,Abell48,IC4663,PB8,alongwith u
e
excludingtheJohnsonUandIRAC8.0μmmagnitudes,whichare NGC6572,whichhasacandidate[WN/C]CSPN(Todtetal.2012). st o
both uncertain. We applied the optical and IR extinction laws of TheseallplotsquarelyinthePNdomain,whiletheorangediamond n
1
Howarth(1983)andIndebetouwetal.(2005),respectively,using isN66,theonlyPNintheLMCwitha[WN]ionizingstar.Itshows 3
E(B−V)=1.90. substantialnitrogenenrichmentcomparedtothemeanLMCabun- Ap
There is no evidence from our spectra for any signature of a dance. ril 2
companion star, nor from the SED for any NIR excess due to a The right panel shows the domain of LBV/WR ejecta, and the 01
9
cooler companion (Douchin et al. 2012; De Marco et al. 2013), greentrianglesrepresentseveralejectanebulaebasedonliterature
thoughinthiscaseanylimitisnotstrong.The[WN]starisrela- data.TheseareallnitrogenenrichedfromCNOcycling,andmostly
tivelyluminousandtheNIRmagnitudesincludeanon-negligible overplottheTypeIPNregiondefinedbyFP10.Clearly,Abell48
contributionfromwindfree–freeemission.Asaresult,anycom- fallsinthePNdomainawayfrombothTypeIPNeandtheCNO
panionlaterthan∼F0Vwouldnotbedetected.Thetentativeab- processed ejecta around LBV and WNL stars. This confirms our
sence of variability (Section 4.1) suggests the star does not have abundance analysis (Sections 3.4 and 5.2) showing the nebula is
anirradiatedcompanion(DeMarco,Hillwig&Smith2008;Mis- not strongly nitrogen enriched, and unlikely to be produced by a
zalskietal.2009;Hajduk,Zijlstra&Gesicki2010),sotimeseries massive star. While this part of the SMB diagram has extensive
spectroscopic analysis is probably the best way to determine if overlap between PNe and HII regions, Abell 48 is plainly not a
Abell 48 is a binary (Jorissen & Frankowski 2008). However, at HIIregion,asitsmorphologyandMIRcoloursreveal.Inparticular
orbital periods greater than a year or two, the expected velocity thefluxratios,F /F =4.9,F /F =5.5,andF /F =9.2(see
12 8 24 8 70 12
shiftsoftheemissionlineswouldbelikelytoosmalltobedetected Table2),stronglyindicateaPNratherthanaHIIregion(Anderson
againstthestochasticvariationsofthestellarwind. etal.2012).
MNRAS440,1345–1364(2014)