1983ApJ. . .273. .280K The AstrophysicalJournal,273:280-288,1983October1 © 1983.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A. either asclassicalnovaeorsymbioticstars,andto burst andatquiescence.Thespecificsystemsinquestion explain theirappearancesandbehaviors,bothinout- elaborate amodelforthiscollectionofobjectswhichcan a restrictedclassofobjects,variouslyclassifiedelsewhere AS 239,V1016Cygni,V1329andHMSagittae; Telescopii 1946andthesymbioticstarsAGPegasi, are theclassicalnovaeRTSerpentis1909andRR throughout thispaper,werefertothesesystems to theseseeminglydiversesystemsinclude:(i)their collectively as“symbioticnovae”.Propertiescommon grants AST80-18198,and80-18859. association with(relatively)longperiodbinarysystems; fact thattheyarethetwoslowestclassicalnovae(Payne- which mayextendovermanydecades.Wenote,inthis regard, thatRTSerandRRTelaredistinguishedbythe Gaposchkin 1957). (ii) thepresenceofanormalMgiant;and(Hi)outbursts reviewed indepththeirobservedproperties.Hesuggested from thelate-typecomponentsprovideanattractive similarities exhibitedbythisgroupofobjectsand explanation ofthephenomenaobservedinthesesystems. that hydrogenshellflashesonwhitedwarfsaccretinggas classical novaoutburstsgenerally andthatRTSer runaways provideaverysatisfactoryexplanationof In viewofthefactsthathydrogen-fueledthermonuclear 1 Our aiminthispaperistoidentifyanddistinguish © American Astronomical Society • Provided by the NASA Astrophysics Data System SupportedinpartbytheNational ScienceFoundationunder Allen (1980)haspreviouslycalledattentiontothe 8-1 envelopes ofaccretingwhitedwarfsgiverisetooutburstssimilarinnaturethoseobservedthe rates produceconfigurationsthatevolveintoA-Fsupergiantsatmaximumvisuallightandwhich In principle,thermonuclearrunawaysinvolvinglow-luminositywhitedwarfsaccretingmatteratlow symbiotic starsAGPeg,RTSer,RRTel,AS239,V1016Cyg,V1329andHMSge. resemble theoutburstsofRRTel,RTSer,andAGPeg.Veryweak,nondegeneratehydrogen shell flashesonwhitedwarfsaccretingmatterathighrates(M>10"Myr)donotproduce cool supergiantsatmaximum,andmayexplaintheoutburstsinV1016Cyg,V1329 on low-luminositywhitedwarfsarenotcompatiblewithobservationsof“normal”quiescent white dwarfsinwidebinaries,whichsuggeststhattheoutburstsofclassicalnovaemaybe companion. accelerated bytheinteractionofexpandingwhitedwarfenvelopewithitsclosebinary symbiotic stars.Theextremelyslowoutburstsofnovaeappeartobetypicalaccreting HM Sge.Thelowaccretionratesdemandedforsystemsdevelopingstronghydrogenshellflashes Subject headings:stars:accretion—combinationspectranovae 0 We discusspossibleconditionsunderwhichthermonuclearburningepisodesinthehydrogen-rich I. INTRODUCTION stars: whitedwarfs 1 THE OUTBURSTSOFSYMBIOTICNOVAE Received 1982December21;accepted1983March9 Scott J.KenyonandJamesW.Truran Department ofAstronomy,UniversityIllinois ABSTRACT 280 both reasonableandstraightforward.Ontheotherhand, normal Mgiant,ofthemasswhitedwarf consequence ofthelongperiodcharacterbinary systems inoutburstissoextremelyslow:Isita we mightaskwhyitisthatthedevelopmentofthese RR Telrepresentlimitingcasesofsuchevents,thisseems physical parameter?Thesequestionsaswellwillbe component, orperhapsofsomeotheryetunidentified system, ofthefactthatcompanionisaseemingly lying physicalpropertiesofandtheobservationalcon- explaining the“symbioticnovae”weareconsidering. sequences ofnuclearpoweredoutbursts,associatedwith addressed inoursubsequentdiscussions. accretion ontowhitedwarfs,whicharecapableof In §IIofthispaper,webrieflyreviewandcomment distinguish themwithrespecttoothercataclysmic upon thepropertiesofthesesystemswhichserveto we presenttheresultsofhydrodynamiccalculations variable andsymbioticsystems(seeAllen1980).In§III, the progressofthermonuclearrunaways,occurringin exhibiting timescaleandlightcurvebehaviorscom- hydrogen shellsonwhitedwarfs,whichyieldoutbursts patible withtheseveryslownova-likeevents.The characteristics oftheserunawaysandresultingoutbursts flashes onwhitedwarfsundernondegenerateconditions are comparedwiththoseassociatedhydrogenshell which ofthesemodelsmightbe mostappropriatetoone contains adiscussionofour results andconclusions. (Paczynski andZytkow1978; PaczynskiandRudak or anotherofthesystemswe arestudying.SectionV 1980; Iben1982)in§IV,and considerationisgivento We thuswishtoexamineindetailboththeunder- 1983ApJ. . .273. .280K dwarf andareddwarf.ThefillsitsRochelobe short periodbinary(Pelday)consistingofawhite and transfersmattertoitscompanionviaanaccretion disk (Warner1976).Asmaterialisaccreted,adegenerate, hydrogen-rich envelopeisformedatthewhitedwarfs is acceleratedoutward.Underavarietyofconditions reaches acriticalvalue,thematterignitesexplosivelyand surface. Whenthepressureatbaseofthisenvelope Fujimoto 1982a,b;MacDonald1983),thethermonuclear and Truran1978;Nariai,NomotoSugimoto1979; runaway generatessufficientenergytounbinda (Starrfield, Truran,andSparks1978;Sparks,Starrfield, substantial fractionoftheaccretedenvelope.Thisis rather spectacularevent.Inthespaceofafewdays by ~14magtoM~—7.Afterthisabruptrise observed asaclassicalnovaoutburst,whichcanbe period ofmonths.Theratethisdeclinedeterminesthe maximum, anovafadestorelativeinsignificanceover months (cf.Payne-Gaposchkin1957;Gallagherand speed classofanova;typicallyfastnovaefadewithin (sometimes hours!),atypicalnovaincreasesinbrightness few weeks,whileslownovaedeclineoveraperiodof (1980) hasidentifiedfiveadditional“veryslownovae” comparatively leisurelyevent.Payne-Gaposchkin(1957) lists twosuchsystems(RTSerandRRTel),whileAllen Starrfield 1978). or planetarynebulae(e.g.,V1016CygandHMSge). All arecharacterizedby2-7mageruptionsfolloweda v those ofeitherFsupergiants(e.g.,RTSerandRRTel) 20-200 yeardecline.Atmaximum,theirspectraresemble (AG Peg,AS239,V1016Cyg,V1329andHMSge). After maximum,theirspectraallevolvetohigherexcita- tion features(e.g.,Fenand[Fen]emission;Nai becoming hotter(Allen1980).Additionally,lowexcita- tion, suggestingthattheoutburstingstarmaybe TiO absorption)alsostrengthenduringthedecline(cf. emission suggeststhatalate-typegiantispresentinthese Merrill 1951;Thackerary1977;Andrillat,Ciatti,and systems. Insupportofthisview,2.3pmCOabsorption Swings 1982).ThecombinationofTiObandsandFen has beendetectedinallsevenveryslownovae(Allen appear toberelatedthesymbioticstars(cf. giants andMiravariables,thustheseslownovae excitation nebularemissionsuperposedonalate-type Boyarchuk 1969;Allen1979)inthattheydisplayhigh absorption spectrum. 1980; Puetteretal1978).ThisfeatureistypicalofMtype geneous groupofobjects,includingbothverylowexcita- tion binariessuchasEGAndandextremelyhighexcita- tion systemssuchasAS295B(Allen1979;Sahade1982; Boyarchuk 1981).Roughly 20ofthesehavebeen biotic evolutiondisplaysawide varietyoffeaturesas well. Themajorityofthesymbiotics haveA-Fsupergiant observed toundergoanoutburst, andthisphaseofsym- spectra atvisualmaximum, asthehighexcitation II. DISTINGUISHINGPROPERTIESOFSYMBIOTICNOVAE The canonicalmodelforacataclysmicvariableis © American Astronomical Society • Provided by the NASA Astrophysics Data System For asmallsubsetofclassicalnovae,theoutburstis The ~150knownsymbioticstarsarearatherhetero- OUTBURSTS OFSYMBIOTICNOVAE 31 4 The nebularlinesreturnduringthedecline,whichmay emission linesfadeduringtheriseinopticalbrightness. last frommonthstodecades.Typicalexamplesofthis group areClCyg(Belyakina1979)andRRTel excitation andopticalluminosity,inanoutburstwhich brightening inAGDra(J.Kaler1982,privatecom- 1 -8 overwhelms theMtypeabsorptionspectrum.These absorption featuresgraduallyreturnastheoptical (Thackeray 1977).Othersystemsmerelyincreasein examples ofthistypeoutburstareV1016Cyg luminosity fadesovermonthstodecades.Typical munication). (Andrillat, Ciatti,andSwings1982)themostrecent outbursts ofsymbioticstars:accretionontomain- sequence orwhitedwarfstars(Bath1977)andnuclear shell burningonthesurfaceofawhitedwarf(Tutukov and Yungel’son1976).Bothmodelshavebeensuccessful at reproducingoutburstlightcurves.BathandPringle the 1975outburstofClCyg.PaczynskiandZytkow a mainsequencestar(M~10~Myr~)couldexplain (1982), forexample,foundthataburstofaccretiononto biotic stars;theirmodelsproducedveryluminousobjects dwarfs, againusingaquasi-staticcode.Someofhis low effectivetemperaturestypicalofFsupergiants.Iben (1978) firstinvestigatednuclearburningmodelsforsym- (L >10L),buttheywerenotallowedtoevolvethe models evolvetohot,luminoussystems,whileother his Figure20closelyresemblesthatofRRTel. sequences developlargeextendedenvelopeswith (1982) hasrecentlyevolvedanumberofaccretingwhite can bereadilyinferredmerelyfromtheiroutburst likely thattheunderlyingnatureofsymbioticstars properties. Alternativemeansmustbefound.Theadvent Webbink (1983,hereafterPaperI)haveusedtheavailable ö the continuumfromhotcomponent.Kenyonand of theIUEsatellitehasalloweddirectobservation /UE datatoanalyzeasmallgroup(~20)ofsymbiotic dependent onthenatureofhotcomponent.They stars andhaveshownthatthiscontinuumiscritically 0 concluded thatthevastmajorityofsymbioticsareeither accreting mainsequencestars(e.g.,ClCygand YY Her)orhotstellarsources(e.g.,AGPegand RW Hya).Noobviouscandidatesforwhitedwarf M yr“).Intheselattercases,theintrinsicluminosity RW Hya)mightbeaccretingatlowrates(M<10 accretors wereidentified,althoughsomesystems(e.g., luminosity. of thehotcomponentwoulddominateaccretion flashes mayoccuronawhitedwarf:degenerate Paczynski andRudak(1980),twotypesofhydrogen when matterisaccretedonto aluminouswhitedwarf and nondegenerateflashes.Nondegenerateflashesoccur for theseobjects.Eachof these systemscontainsa systems inPaperIarelikely observational counterparts (L æ100Lq).Thesystems identifiedashotstellar luminous, compactstar(L> 100 L,R<1)andan 0 ö0 Two distinctmechanismshavebeenproposedforthe On thebasisofabovediscussion,itseemsun- As notedbyPaczynskiandZytkow(1978) 4 10 K.Infact,themodellightcurveshownin 281 1983ApJ. . .273. .280K -210 -4 4 places itonthecoolingcurveofa0.7Mwhitedwarf. evolved redgiantcompanion.Infact,thesolutionfor Thus, systemssuchasRWHyaappeartobelikely RW HyafromPaperI(L^900L,Tä90,000K) candidates fornondegenerateflashes,suchasthose 282 studied byPaczynskiandZytkow(1978)Iben accreted slowlyontoacoolwhitedwarf(MacDonald we willexploretheirapplicabilitytosymbioticnovaein useful inunderstandingtheclassicalnovaoutburst (1982 andreferencestherein). the nextsection.Itisunfortunatethatobservational been detectedamongthesymbioticstars.However,itis counterparts forthesesystemsinquiescencehavenot possible thatfaintwhitedwarfswithlowaccretionrates (Gallagher andStarrfield1978;MacDonald1983), detected assymbioticstars(seePaperI).Anexampleof 1983; Fujimoto1982a,b).Suchmodelshaveproved 0 performed ontheUniversityofIllinoisCyber175using such anobjectmaybethewhitedwarfcompanionto 0eff Two majorchangeshavebeenmadeinthecodefor an amendedversionofthecomputercodeKutterand by Miraitself.Wewillshowthat,duringoutburst, (L <10L,M10"yr“)wouldnotbe studies presentedinthispaper.Wehavereplacedthe Mira, whichisalow-temperaturewhitedwarf The Ibeninterpolationformulaeprovideamuchmore conductive opacitywithonedevelopedbyIben(1975). Sparks (1972;cf.Starrfield,Truran,and1978). those inwhichnondegenerateflashesoccur. such systemswouldnotbereadilydistinguishablefrom (T ä10,000K)accretingmaterialfromthewindejected which isusuallyfoundtobenegligibleforclassicalnova dwarf coreviaelectronconduction.Thisenergyflow, us bettertodeterminetheenergyflowintowhite accurate representationofthetabulatedvaluesandallow Sweigert (1973)fittotheHubbardandLampe(1969) degenerate hydrogenflashes. models, canbeimportantfortheveryslownovaewe discuss here.Wehavealsoincorporatedthediffusion tends toslowdowntheprocessofconvectivemixing mixing algorithmsuggestedbyDespain(1977).This involve 0.8Mcarbon-oxygenwhitedwarfs.Thefirst of twomodelsfornova-likeoutburstsbothwhich and thustoslowtheprogressofrunaway. 45 masszones,whiletheenvelope has140meshpoints. of roughlysolarcomposition (X=0.7,Y0.27, WD0 Mq) whichisassumedto havebeenenrichedin Z =0.03).Thecarbon-oxygen corehasbeendividedinto (Model 1)hasa1.3x10Mhydrogenrichenvelope Model 2hasaslightlysmaller envelope(1.06x10" eff 0 0 Degenerate hydrogenflashesoccurwhenmatteris © American Astronomical Society • Provided by the NASA Astrophysics Data System The calculationsdescribedinthissectionwere We nowpresentmodelsofoutburstsresultingfrom In thefollowingdiscussions,wedescribeevolution III. RUNAWAYMODELSOFTHEOUTBURSTS b) DescriptionoftheEvolution a) MethodofCalculation KENYON ANDTRURAN 7 -3 2 7_1 8 13- 7 12 -45 necessary toproduceanoutburst,foragivenwhitedwarf abundances ofCNOnuclei(X=0.53,Y0.27, mass andluminosity(cf.Starrfieldetal1982). the minimumamountofhydrogen-richmaterial Z =0.20).Inbothcases,theenveloperepresentsroughly sequence inhydrostaticequilibriumataluminosity massive thantheabsoluteminimummassdueto Generally, theenvelopemassis~5%-10%more determined bythestructureofwhitedwarfcore finite gridsize.Bothenvelopesbegintheevolutionary evolutionary sequenceatatemperatureof1.27x10K and adensityof2500gcm.(Otherinitialconditions electron conductionisthemajorenergytransport the CNOcycleismajorsourceofenergy,while are aslistedinTable1.)Underthesecircumstances, mechanism. Sinceelectronconductionisveryefficientat (~10-L). temperature characterizingtheburningregion(7¡) carrying energyfromtheburningshell,peak rich envelope.Ittakeszone60(atamassof moves outwardinmassfromthebaseofhydrogen- M (Mq) Radius (km) Log T(K) Luminosity (L). runaway begins.Another1.4yearspassesbefore6 to 8x10ergsgs,andanexponentialgrowthinthe M the runaway. above thezonewherepeakToccurs.Thisservestomix The hydrogen-richenvelopeiscompletelyconvective by maximumT=1.37x10Kabout2hourslater. peaks at4.2x10ergsgs,andthisisfollowed 3 x10K.Theenergygenerationrate(6)hasincreased removes energyfromtheburningshell,andraises unburned Cnucleiintotheshellsourceandmaintain burning shellandthendecreasesadiabaticallythrough- temperature increasesabruptlynearthebaseof temperature intheouterportionsofC-Ocore.The of runawayareshowninFigure1.Electronconduction white dwarfcorecanbeseenfromtheluminosity profile: AttheouteredgeofC-Ocoreluminosity out theconvectivezone.Thelargeflowofenergyinto becomes negative. 1.13 xl0Mo)^10yearstoreachatemperatureof The initiallyrapidincreaseinluminosityfollowsatrack temperature ofthewhitedwarfincreasedramatically. o s env e Q nuc vi ss ss nuc The baseofthehydrogen-richenvelopebegins The luminosityandtemperatureprofilesattheonset (ergsg Following peak£,theluminosityandeffective : nuc Initial ConditionsofDegenerateRunawayModels .(yr). Parameter i) Model1 TABLE 1 4 23 46 1.30 x10"1.06KT 2.4 x10“9.2IO' 9.5 x101.2 Model 1 11.0 11.5 7993 7865 4.33 4.23 0.03 0.20 367 58 Vol. Ill Model 2 1983ApJ. . .273. .280K -1 No. 1,1983 profiles inModel1attheonsetofrunaway. in Figure2.Eventually,thelargeenergyfluxfrom it takesover4monthsforthismodeltoreachvisual With anaverageexpansionvelocityof~5kms, burning shellacceleratestheenvelopeoutward,and magnitudes asfunctionsoftimeareshowninFigure3. evolutionary trackproceedstowardlowereffective of roughlyconstantradiusintheH-Rdiagramasshown escape velocity,andthusnomassisejectedduringthe maximum. Thematerialintheenvelopeneverreaches £ andremainsroughlyconstantastheeffective peaks at~15,500Lnearly1monthaftermaximum temperatures inFigure2.Thebolometricluminosity temperature decreasesslowly. portion oftheenvelopebecomespulsationallyunstable slow visualrise.Duringthenextfewmonths,outer energy generationisattainednear theminimuminbolometric magnitude. nuc 0 Fig. 1.—Temperature(solidline)andluminosity(dashed The behaviorsofthevisualandbolometric © American Astronomical Society • Provided by the NASA Astrophysics Data System Fig. 2.—EvolutionarytrackofModel 1intheH-Rdiagram.Peak OUTBURSTS OFSYMBIOTICNOVAE -1 7 7 7 7 10- 5_1 7-1 with avelocityamplitudeof~10kms.Thisbehavior is notadequatelyresolvedinourmodel,andthusthe tions increasetheopacity.Thisregionofenvelope appears inthephotospherewherehydrogenrecombina- pulsations maybeanumericaleffect.Thetime-averaged shown inFigure3. light curveofthisperiodisverynearlyconstantas since peakenergygenerationforModel1. The energygenerationrateintheburningshelldropsto luminosity remainsroughlyconstantat~16,000L. zones eachflashto8-9x10K,andtheburningshell hydrogen-rich zonesbelowtheburningshell.These shell sourceintothecore,andthisheatsup 8.3 x10K.Energycontinuestobetransportedfromthe shell flashesto~9xIQKandbeginsburnhydrogen moves inwardinmass.Eventuallythelasthydrogen-rich heat upto8x10K,butthistemperatureisnot very rapidly.ThetwooutermostzonesoftheC-Ocore sufficient toignitecarbon. terminated theevolutionarysequencesincecalculation enormous amountofcomputertime.Thebolometricand near thephotosphere,butfallwellbelowlocal of thefurtherevolutionwouldhaverequiredan Eddington valuebeyondthephotospherewhereelectron visual luminositiesareclosetothelocalEddingtonlimit ~6 x10ergsgs,whileTstabilizesat 0 via continuumradiationpressurecannotoccurinour scattering opacitydominates.Thus,sustainedmassloss envelope ofourmodelcloselyparallelthosefoundinthe calculation. However,theconditionsinouter Hubble-Sandage variablesbyGallagher,Kenyon,and supergiants andlosematerial at>10"Myr.A Hege (1981).Theselattersystems alsoresembleA-F simple scalingofMwithL/g (wheregisthegravity) for ournovaremnant.Over a periodof~100yr,then, leads toamasslossrateof 5-10x10“Myr ss 0 0 Fig. 3.—Visualandbolometricmagnitudesasfunctionsofthetime During therisetovisualmaximum,bolometric After nearly4monthsatvisualmaximum,we 283 1983ApJ. . .273. .280K 7 -3 -112 ô 7 9 814- 14 8 -17 9 3 the envelopeofthisnovashouldbeejected.Thisserves retrace itspathintheH-Rdiagram(Iben1982;Truran to extinguishtheburningshellandcausesmodel 284 this sequenceisatatemperatureof1.07x10Kand density of2200gcm.Otherinitialconditionsare energy isinitiallygeneratedsolelybyp-preactions.Over listed inTable1.Thesecircumstancesaresuchthatthe CNO reactionsarenotproceedingrapidly,andthus 1982). g s,andasmallconvectivezonefeedsfreshC the next1.2x10years,CNOcyclebecomes dominant energysourceasTrisesto3x10K.The energy generationratehasincreasedto1.6x10ergs nuclei intotheshellsource.Thisacceleratesrunaway Note thatthisrunawayismoreviolentthanModel1 to reach10K,whene=1.9xergsgs. such thatsubsequentlyittakesonly23daysforT nuclei. Themaximumineof4.75x10ergsg"s~ due tothepresenceofgreaterabundancesCNO is followed312slaterbypeakT=1.29x10K.The behavior ofeandTduringthepeakrunaway is showninFigure4. ss achieved, thehydrogen-richenvelopeiscompletely and Mdecreasesrapidly.Thiscausesarapidexpansion convective. Soonafter,aluminositywavehitsthesurface, begins torecedefromthesurface.Thegradualshrinking of theouterenvelope,andtopconvectivezone nuc ss ergs gs"and5.2x10Kinafewhours. They reachapproximatelyconstantvaluesof1.6x10 cause £andTtodecreaserapidlyasshowninFigure4. similar tothatindicatedinFigure2forModel1. of theconvectivezoneandexpansionenvelope nuc The modelcoolsintherunawaysearlystagesto ss nucss values, theluminosityincreasesrapidly.Ittakesonly L ä3x10~.AfterTand£reachtheirmaximum has reached210,000K.Maximumluminosityof17,000 bol nucss shell sourcetemperature(7¡)asfunctions oftimeforModel2.Note that Treachesmaximumslightlyafter peak£. 0ssnuc ~100 sfortorisefrom+11—5,whileT s ss nuc eff The baseofthehydrogen-richenvelopeatstart © American Astronomical Society • Provided by the NASA Astrophysics Data System At thepointatwhichmaximumenergygenerationis The evolutionofthismodelintheH-Rdiagramisvery Fig. 4—Thebehavioroftheenergy generationrate(£)andthe nuc ii) Model2 KENYON ANDTRURAN 6 -1 -1 -1 9 Model 2.ThetimewasdefinedtobezeroatT=30x10K. 1_ from theabruptluminosityincreaseisfollowedbya Lq occursabout1hourafterpeake.Theinitially re-adjustment intheenvelopestructure,andabrief rapid envelopeexpansion(væ50kms)resulting decrease inluminositywhenTæ100,000K.The pressure cansupporttheouterenvelope,andthis luminosity thenrisesbackto17,000L,onceradiation km s)phasebegins,andMbeginstodeclineas sequence. luminosity ismaintaineduntiltheendofevolutionary shown inFigure5.Thereducedenvelopemassof ss nearly ayearafterbolometricmaximum,when Model 2resultsinageneralyslowerexpansionthan This matterwasejectedduringtherapidbolometricrise, Model 1,andthustherisetovisualmaximumoccurs while theremainingenvelopepulsateswithavelocity nuc pressure shouldejecttheremnantenvelopeonatime- amplitude of10-20kms.AsinModel1,radiation exp eff scale of~100years,andtheluminositywillfade. 0 vis range ofaccretionratesontoawhitedwarfand hydrogen shellflashes.Sincetheiroriginalinterestwas suggested theoutburstsofsymbioticstarsresultedfrom ~ 10"Mofmaterialhasbeenejectedatt;æ50kms"^ expanded pastR=0.5. Their sequenceIfollowsa cataclysmic binaries,most oftheirevolutionary sequences usedaprocedurewhich removedmatterasit low-luminosity whitedwarf accretingmassat 0 Q ~ 1.5x10“Mqyr.The evolutionofthissequence Fig. 5.—Visualandbolometricmagnitudesasfunctionsoftimefor Once theluminositypeaks,aslowexpansion(pä;2.5 IV. THERMONUCLEARMODELSFORSYMBIOTICNOVAE Paczynski andZytkow(1978)investigatedawide Vol. 273 1983ApJ. . .273. .280K -1 -91 -7 5 during theflashcloselyparallelsourModel1in R and^eff~6300K.Asinourmodels,peak development ofanextendedatmospherewithR~100 No. 1,1983 bolometric luminosityismaintainedfor>50yearsatthe effective temperatureofanFsupergiant. similar innaturetothoseofPaczynskiandZytkow of hot,accretingwhitedwarfs.Iben’scalculationsare may occuronthesurfaceofawhitedwarf.Asin His resultsdemonstrateawidevarietyofshellflashes for >25yearsbeforereturningtominimum.Theflashes to supergiantdimensionsandmaintaintheirlargeradii M yr.Themodelsathigherdonotevolveto low accretionrates(M<10Myr).Theseexpand Paczyñski andZytkow(1978),strongflashesoccurat (1978), exceptthemodelscanevolvetolargeradii. state nuclearburningisachievedatM^afewx10 grow weakerastheaccretionrateincreasesuntilsteady supergiant dimensionsatloweffectivetemperatures. maximum formanydecadesandlosemassinalow for whichthedwarfsremainathigheffectivetempera- hydrogen shellflashesoccuronwhitedwarfs:(i)those visual (M~QasopposedtoM—5for maximum; thisresultsinarathersmalloutburstthe Instead theyremainveryhot(~2x10K)atbolometric e envelope, itevolvesfromarelativelyfaintobjectto that leadtoexpansionA-Fsupergiants,and(ii)those evolve towardhighereffectivetemperatures,theydevelop tures. Whenaflashbeginsindegeneratewhitedwarf low M). high velocitywindsasinclassicalnovae.Paczynski luminous A-Fsupergiant.Thesesystemsremainatvisual from flashesinanondegenerateenvelope.Thewhite dwarf expandsonlybyafactorof10duringthe and Rudak(1980)identifiedtheseastypeIIsymbiotic velocity windatmaximumvisuallight.Asthesesystems stars. IntheirnomenclaturetypeIsymbioticstarsresult outburst andmaintainsahighbolometricluminosityfor most oftheopticalcontinuum,whilehotstaris of novae(Cassinelli1979;GallagherandStarrfield1978). velocity windasseeninOstarsandthedecliningstages usually consideredthesourceofemissionlines(cf. 0 0 this gianttransfersmattertoawhitedwarf;someof remainder formsacircumbinarynebula.Thevisibility material isactuallyaccretedbythedwarf,while and intrinsicluminosityofthewhitedwarf(cf.PaperI): Sahade 1982).Ourmodelforsymbioticnovaeassumes vmaxyj while (ii)ifLorMishigh,nebularemission of thenebuladependscruciallyonaccretionrate degenerate shellflashes(lowM,L)tohaveweaker strong. Thus,wemightexpectsystemsundergoing in themidstofnondegenerate flashes(highMorL). (i) IfLandMarelow,nebularemissionisweak, preoutburst nebularspectrathan thosesystemscurrently During anoutburst,thegiant componentisnotmerely a spectatorasitprovides lowerlimittotheoptical >15 years.Inthiscircumstance,weexpectahigh’ luminosity ofthesystem.In earlystagesofadegenerate wd WD WD wd Recently, Iben(1982)hascompletedanextensivestudy The calculationsdiscussedaboveshowtwotypesof © American Astronomical Society • Provided by the NASA Astrophysics Data System In asymbioticbinary,thegiantcomponentprovides OUTBURSTS OFSYMBIOTICNOVAE _1 evolves upalineofconstantradiusintheH-Rdiagram outburst, nebularemissionincreasesasthewhitedwarf white dwarfevolvestoanA-Fsupergiant.Theoptical and isreplacedbyalongerperiodduringwhichthe pletely overwhelmedbytheFsupergiantcomponent continuum ofthegiant(withM~0to—2)iscom- infrared. Thenebularspectrumslowlyreturnsasthe (cf. Fig.2).Thisphaseisveryshort-lived(