Massive expanding torus and fast outflow in planetary nebula

We present interferometric observations of $^{12}$CO and $^{13}$CO $J$=2$-$1 emission from the butterfly-shaped, young planetary nebula NGC 6302. The high angular resolution and high sensitivity achieved in our observations allow us to resolve the nebula i

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aMassiveexpandingtorusandfastout owinplanetarynebulaNGC6302Dinh-V-Trung1InstituteofAstronomyandAstrophysics,AcademiaSinicaP.OBox23-141,Taipei106,Taiwantrung@asiaa.sinica.edu.twValentinBujarrabalObservatorioAstronomicoNacional,Apdo.112,AlcaldeHenares,28803Madrid,Spainv.bujarrabal@oan.esAranchaCastro-CarrizoInstitutdeRadioastronomieMillim´etrique,300ruedelaPiscine,38406SaintMartind’H`eres,Franceccarrizo@iram.frJeremyLimInstituteofAstronomyandAstrophysics,AcademiaSinicaP.OBox23-141,Taipei106,Taiwanjlim@asiaa.sinica.edu.twSunKwok

DepartmentofPhysics,HongKongUniversity,HongKong,China

sunkwok@hku.hkABSTRACT

We present interferometric observations of $^{12}$CO and $^{13}$CO $J$=2$-$1 emission from the butterfly-shaped, young planetary nebula NGC 6302. The high angular resolution and high sensitivity achieved in our observations allow us to resolve the nebula i

Wepresentinterferometricobservationsof12COand13COJ=2 1emission

fromthebutter y-shaped,youngplanetarynebulaNGC6302.Thehighangularresolutionandhighsensitivityachievedinourobservationsallowustoresolvethenebulaintotwodistinctkinematiccomponents:(1)amassiveexpandingtorusseenalmostedge-onandorientedintheNorth-Southdirection,roughlyperpendiculartotheopticalnebulaaxis.Thetorusexhibitsverycomplexandfragmentatedstructure;(2)highvelocitymolecularknotsmovingathighveloc-ity,higherthan20kms 1,andlocatedintheopticalbipolarlobes.Theseknotsshowalinearposition-velocitygradient(Hubble-like ow),whichischaracteristicoffastmolecularout owinyoungplanetarynebulae.Fromthelowbutvariable12CO/13COJ=2 1lineintensityratioweconcludethatthe12COJ=2 http://doc.guandang.netingtheopticallythinnerline13COJ=2 1weestimateatotalmoleculargasmassof～0.1M⊙,comparabletotheionizedgasmass;thetotalgasmassoftheNGC6302nebula,includingthemassiveionizedgasfromphotondominatedregion,isfoundtobe～0.5M⊙.Fromradiativetransfermodellingweinferthatthetorusisseenatinclinationangleof75 withrespecttotheplaneoftheskyandexpandingatvelocityof15kms http://doc.guandang.netparisonwithrecentobservationsofmoleculargasinNGC6302isalsodiscussed.

Subjectheadings:planetarynebulae:inpidual(NGC6302),circumstellarmat-ter

1.Introduction

Lowandintermediate-massstars(1M⊙≤M ≤8M⊙)evolvethroughtheAsymptoticGiantBranch(AGB)phase,whichischaracterizedbycopiousmasslossintheformofdustyslowwind,beforeemergingasplanetarynebulae(PNe).Themass-lossprocessiscommonlyassumedtobeisotropic,resultingintheformationofasphericallysymmetricenvelopearoundthecentralAGBstar.However,asigni cantfractionofthecircumstellarenvelopearoundpost-AGBstarsandplanetarynebulaepossessbipolarandevenmultipolarmorphology.Themechanismsresponsibleforsuchdeparturefromsphericalsymmetryarestillunknown(e.g.Balick&Frank2002).Theinteractionbetweencollimatedfastout owsandthesurroundingenvelopeorthein http://doc.guandang.netrgeexpandingorrotatingdisks/torihasbeenfrequentlyinferredtobepresentinbipolarnebulaesuchastheEggNebula(Sahaietal.1998)andtheRedRectangle(Bujarrabaletal.2005).Thesedisksortorimightcon neandchannelthe

We present interferometric observations of $^{12}$CO and $^{13}$CO $J$=2$-$1 emission from the butterfly-shaped, young planetary nebula NGC 6302. The high angular resolution and high sensitivity achieved in our observations allow us to resolve the nebula i

windfromthecentralstarintobipolardirections.Ifthedisks/toriaredenseenough,thestronginteraction(i.eobliqueshock)betweenthewindandthedisks/toricouldfocusandcollimatetheout ow(Franketal.1996).Thus,moredetailedstudiesofthestructureandkinematicsofsuchdisk/toruscouldprovidebetterunderstandingontheformationofthenebulae.

NGC6302isayoungplanetarynebulaandbelongstotheclassofthehighestexcitationPNe(Pottaschetal.1996).Fromthepresenceofnumerousemissionlinesfromhighlyionizedspecies,Pottaschetal.(1996)concludedthatthecentralstarisawhitedwarforapproachingawhitedwarfanditsexcitationtemperatureisveryhigh,～380,000K.Inopticalimages,NGC6302appearsasabutter yshapednebulawithtwohugebipolarlobesseparatedbyadarkequatoriallane(Matsuuraetal.2005),whichispresumablythelocationofamassivediskortorus.AnexpandingHiiregionisdetectedatthecenterofthenebulaintheradiocontinuum(G´omezetal.1989).ThepresenceofamassivemolecularenvelopeisknownthroughthedetectionofstrongCOrotationallinesinNGC6302(Hugginsetal.1996,Hasegawa&Kwok2003).TheCOlineshaveverypeculiarandcomplexshapes,withadouble-peakedpro leandhighvelocitywings,extendingupto～40kms 1fromthesystemicvelocity.Suchlineshapesuggeststhatthemolecularenvelopeislikelynon-sphericalandcouldcontainafastmolecularout ow,similartofastbipolarout owsobservedinsomeproto-planetarynebulaeandyoungplanetarynebulae.

DetailedobservationswiththeVLTandHSTofMatsuuraetal.(2005)showthepres-enceofalargewarpeddiskorientedperpendiculartothebipolarlobesseenprominentlyintheopticalimages.Thediskismassiveandhasalargeextinction.StrongemissionfromcrystallinesilicatestogetherwithPAHbandshavealsobeendetectedinNGC6302(Kemperetal.2002).Wenotethat,interestingly,OHmaseremission,whichisusuallyassociatedwithoxygenrichcircumstellarenvelopes,hasalsobeendetectedinNGC6302(Payneetal.1988).Thus,NGC6302ischemicallypeculiar,containingamixtureofcarbon-richandoxygen-richmaterial.

ThedistancetoNGC6302isuncertain,withestimatesrangingfrom0.15to2.4kpc.FromtheexpansionpropermotionofthecentralHiiregion,Gom´ezetal.(1989)estimatesadistanceof2.2±1.1kpc.However,Matsuuraetal.(20 …… 此处隐藏：33160字，全部文档内容请下载后查看。喜欢就下载吧 ……