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PHYSICSOFSUPERNOVAE∗ 5 0 0 2 D.K.NADYOZHINandV.S.IMSHENNIK n A.I.AlikhanovInstituteforTheoreticalandExperimentalPhysics a B.CheremushkinskayaSt.25,RU-117218,Moscow,Russia J 1 1 Theoriginofcosmicrays(CR)issupposedtobecloselyconnectedwithsupernovae(SNe)whichcre- v atetheconditionsfavorableforvariousmechanismsoftheCRaccelerationtooperateeffectively.First, 2 modernideasaboutthephysicsoftheSNexplosionarebrieflydiscussed:theexplosivethermonuclear 0 burningindegeneratewhitedwarfsresultinginTypeIaSNeandthegravitationalcollapseofstellar 0 coresgivingrisetoothertypesofSNe(Ib,Ic,IIL,IIP).Next,wesurveysomeglobalpropertiesofthe 1 SNeofdifferenttypes:thetotalexplosionenergydistributionofvariouscomponents(kineticenergy 0 ofthehydrodynamicflow,electromagneticradiation,temporalbehavioroftheneutrinoemissionand 5 individualenergiesofdifferentneutrinoflavors).Then,wediscussinthepossibilityofdirecthydrody- 0 namicaccelerationbytheshockwavebreakoutandthepropertiesoftheSNshocksinthecircumstellar / h medium.Thenthepropertiesoftheneutrinoradiationfromthecore-collapseSNeandapossibilityto p incorporateboththeLSDMontBlancneutrinoeventandthatrecordedbytheKIIandIMBdetectors - intoasinglescenarioaredescribedindetail.Finally,theissuesoftheneutrinonucleosynthesisandof o theconnectionbetweensupernovaandgamma-rayburstsarediscussed. r t Keywords:supernovae;neutrino;stellarnucleosynthesis. s a : v 1. BasicPropertiesofSupernovae i X Physically,therearetwofundamentaltypesofsupernovae(SNe):thethermonuclearSNe r a and the core-collapse ones, represented by Type Ia SNe (SN Ia) and by Type II, Ib, and Ic SNe, respectively.The SN Ia show no hydrogenin their spectra and constitute quite a homogeneoussampleofobjects.Thecore-collapseSNearesubdividedintoseveraltypes dependingontheamountofhydrogenhangingaroundthestellarcorejustbeforeitbegins tocollapse.TypeIbandIcSNehavevirtuallynohydrogenleft.TheIcSNeseemtohave lostafairamountoftheirheliumaswell. TypeIISNearerepresentedbythesubtypesIIP(plateaushapedlightcurves, 10M ofhydrogenintheirenvelopes),IIL(linearlydecayinglightcurves, <1M ofh∼ydrogen⊙), andIIn(withsomehydrogeninanextendedenvelopeformedbyde∼nseste⊙llarwind). 1.1. Thermonuclearsupernovae TheSN Ia lightcurvesarepoweredbythe 56Ni 56Co 56Fe beta-decayonaverage → → of 0.6M of 56Ni synthesized as a result of explosive carbon-oxygen (CO) burning ≈ ⊙ inadegenerateChandrasekharmass(M 1.4M )whitedwarf.Thetotalenergyofthe ≈ ⊙ ∗Invitedlectureatthe19thEuropeanCosmicRaySymposium,August30–September3,2004,Florence,Italy 1 2 D.K.Nadyozhin&V.S.Imshennik electromagneticemissionisof 6 1049erg,mostofthisenergybeingradiatedinoptical ≈ × and infrared wavelengths and only a fraction being carried away by X-rays and gamma- photonsthatmanagedtoescapethescatteringoffbytheexpandingenvelope.Theexplosion energyE 1051ergcomesfromthedifferenceinnuclearbindingenergiesoftheinitial exp ≈ carbon-oxygenmixtureandthefinalproductsofthermonuclearburning(mainly56Ni–the most tightly bounded nucleus among all those consisting of equal numbers of neutrons and protons). Finally, almost all E turns out to be converted into the kinetic energy exp of expandingmatter. The white dwarfprovesto be totally disruptedby the explosion,no stellar remnant being left. The mean velocity of the expanding debris is estimated to be u =p2Eexp/M 8,000km/s. h i ≈ Although observationally and theoretically the above concept is considered to be a well-founded conjecture, there remains a big unsolved problem relating to the mode of thermonuclear CO-burning. The most important issue is an interplay between the defla- gration and detonation regimes of burning which is controlled by different instabilities of turbulent thermonuclear flame propagating in degenerate matter and by the behavior of the white dwarf as a whole in response to the onset of the burning (pre-expansion,1,2 radial pulsations3,4). Several astrophysical groups are currently engaged in an extensive investigation of the thermonuclear burning in Type Ia supernovae (see Refs. 5, 6, 7 and referencestherein).Thiscomplicatedproblemrequiresasophisticatedapproachbasedon three-dimensionalmodelingoftheCOignitionandpropagationofthethermonuclearflame thatisfraughtwithspecificunsteadinessessuchasconvective,Rayleigh-Taylor,Landau– Darriesinstabilities. SuchaninvestigationisoffundamentalimportancefortheaccuratecalibrationofSN Iaasthecosmologicalstandardcandles(oneneedstopredicttheSNIapeakluminosityat leastwitha10%precision!).Alsofordetailedcomparisonwithobservationsandforstellar nucleosynthesis,aprecisedeterminationofdifferentisotopicyields(apartfromdominating 56Ni)isofvitalimportance. 1.2. Core-collapsesupernovae TypeIISNelightcurvesarepoweredfirstbytheshockheating,thenbyrecombinationof hydrogen,andfinally(attheirtailphase)bythe56Co 56Fedecayof (0.02 0.2)M of56Co(initially56Ni).Thetotalenergyoftheelectr→omagneticemissi≈onisof− 1049er⊙g. ≈ The explosionenergyof the core-collapseSNe is typically of (0.5 2) 1051 erg.It − × comesfromtheshockwavethatislaunchedsomewhereattheboundarybetweenthe“iron” core of a mass M = (1.2 2)M , collapsing into a neutronstar (NS), and the outer Fe − ⊙ envelopetobethrownout.Themeanvelocityoftheexpansionisof3,000 5,000km/s − dependingonthemassoftheenvelopeexpelled. The mechanism of the core-collapse SNe is not yet understood in every detail. The mostdistinctivefeatureoftheseSNeisanenormousenergyof(3 5) 1053erg=(10 − × − 15)%M c2 radiated in form of neutrinos and antineutrinos of all the flavors (e,µ,τ). Fe One would thinkthat it shouldnotbe a problemto extractless than1% fromthe energy of a powerful neutrino flux to ensure the expulsion of the SNe envelope. However, an PhysicsofSupernovae 3 extensive hydrodynamicalmodeling during the last thirty years has demonstratedthat in case of spherical symmetry it is very hard (if not impossible) to simulate the explosion. Basing on this research, an empiricaltheorem can be formulatedtelling that spherically- symmetrical models do not result in expulsion of an envelope;the SN outburst does not occur: the envelope falls back on the collapsed core. Hence, one has to go to two- and, perhaps,three-dimensionalmodelstoconvertthestalledaccretingshockintoanoutgoing blast wave (for the last review see Ref. 8 and referencestherein). One has, nevertheless, to keep in mind that yet undiscoveredelementary particles may be involvedin the core- collapseSNe(e.g.,axion-likeparticles9). Therearethreereasonsofsphericalsymmetrybreakdownwhichcurrentlyareundera closeinvestigation: Large-scaleneutrino-drivenconvection. • Theaccretingshockcanobtainanadditionalenergynecessaryforsuccessfulexplosion fromfast(possiblyjet-like)subsonicstreamsofneutrino-heatedmattercirculatingunder andovertheneutrinosphere.10,11,12,13 Interactionbetweenrotationandmagneticfield. • Hydrodynamicalheterogeneityofthecollapse(centraldenselayersofstellarcorecon- tractincreasinglyfasterthanouterones)resultsinastrongdifferentialrotationthatleads toanamplificationoftoroidalmagneticfield.Underfavorableconditions,anexcessive magneto-hydrodynamicalpressurecouldfacilitatetheexpulsionofthesupernovaenve- lope.14,15 RotationalfragmentationfollowedbyaNSexplosion. • Massivefast-rotatingcollapsedcoreundergoesrotationalfragmentationresultinginfor- mationofacloseneutron-starbinarythatevolvesbeingdrivenbytheemissionofgrav- itational waves and mass-exchange and terminates with a low-mass (M 0.1M ) neutron-starexplosion.16,17 ≈ ⊙ Figure1showsageneralviewofelectromagneticandneutrinoluminosityofSNe.The SNIa bolometric light curve is given by a dashed line that includes all electromagnetic spectrum(uvoir+X+γ:ultraviolet,optical,infrared,X-rays,andgammaradiation).In ∼ 40daysafterexplosion,thelightcurvestrictlyfollowsthe56Codecayhalf-lifeof77.1days (111.3daysforexponentialdecaytime).ThereisshownalsothetypicalSNIIPlightcurve with a 100-day period of nearly constant luminosity (plateau) stabilized by a cooling- ∼ and-recombinationwave.18,19 If there were no 56Co in the supernovaenvelopesthe light curveswouldbeofashorterduration(dottedcurves).InthecaseofSN1987AintheLarge MagellanicCloud,about0.075M of56Niwassynthesized. ⊙ Figure 2 shows the SN1987Alight curve on a large scale as observed by two group ofastronomersinChili20 (blackdots)andSouthAfrica21 (opensquares).A20%discrep- ancy between these two sets of observations is a systematic uncertainty connected with reconstructionofthe bolometricluminosityfromluminositiesobservedindifferentspec- tralbands.ThecoincidenceoftheSN1987AbolometriclightcurvewiththeCo-decaylaw att > 140daysgavethefirstdirectproofthatsupernovaejectaareactuallyenrichedwith theoretically predicted 56Co. In a month, this finding was confirmed by the detection of 4 D.K.Nadyozhin&V.S.Imshennik Fig.1. Aschematicillustrationofthelightcurvesandothersupernovaproperties. X-raysbyspace-baseddetectorsKvant22 andGinga23 andsomewhatlaterbydirectmea- surementofgamma-linesfromCo-decay.24DetailedreportontheSN1987Aeventcanbe foundinspecialreviews.25,26,27 ForrecentthoroughstudyoftheSN1987Alightcurveon thebasisofradiationhydrodynamicswith nonthermalionizationfromthe56Niand56Co decaysincludedseeRef.28. 2. ShockWaveBreakout The onset of supernova outburst occurs at the very time when the outgoing shock wave (SW)reachesthestellarsurface.Suchabreakoutresultsinashortpulseofultravioletand softX-rayradiationoftotalenergyupto1047ergandwithcharacteristicdurationof100s– 1h depending on the presupernova radius (Fig. 1). The “tail” of this pulse was actually observedinthecaseofSN1987A(Fig.2).Theshockwavepropagatesthroughstellarinte- rioroutwardinthedirectionofstronglydecreasingdensity.Consequently,theshockenergy turnsouttobeaccumulatedinaprogressivelydecreasingamountofmatter.Asaresult,the SWconsiderablyacceleratesasillustratedinFig.3.Suchacumulativeregimeisdescribed bywell-knownsimilaritysolutionofhydrodynamicequationsthatexistssincethedensityρ typicallyisapowerfunctionofthedistancextostellarsurfaceρ xn(n 3).Formally, ∼ ≈ the velocity tends to infinity at stellar surface (short-dashed curve for t = 0). In reality, theSWcumulationislimitedbyafinitewidthoftheSWfrontwhichopticalthickness∆τ PhysicsofSupernovae 5 Fig.2. Thebolometric lightcurveofSN1987Aexploded onFebruary23,1987.Timeismeasuredfromthe momentoftheshockwavebreakout.Att≈90days,theSN1987Aluminosityattainsamaximumof≈1042erg/s thatbyafactorof2,000exceedstheluminosityoftheprogenitor,bluesupergiantSk−69o202.Theshockwave breakout“tail”isshownbyanearlyverticaldashedlineatt≈0.(AdaptedfromRef.19). fortheradiationdominatedSWcanbeestimatedfromasimplerelation:∆τ c/D,with ≈ D and c being the SW velocity and the speed of light, respectively.The distance x at cut which one has to cut the similarity solution off correspondsto the SW position when its opticaldepthbecomesjustequalto∆τ (seeRef. 29).Withx knownonecanestimate cut the maximum velocity at the SW front emerging from under the stellar surface. During further expansion (curves for t > 0 in Fig. 3), matter undergoesadditional acceleration ina rarefactionwavethatconvertsalmostallthermalenergyintokineticenergyofradial expansionincreasingthelatterbyafactorof 2.6(Ref.30). ≈ ForSN1987A, x isestimated to be 0.02R (R 47R isthe presupernova cut 0 0 ≈ ≈ ⊙ radius).Theresultingfinalmaximumvelocityu relatingtokineticenergypernucleon max ε = 1m u2 (m istheatomicmassunit)andthemass∆M acceleratedtothe max 2 u max u umax maximvelocityu turnouttobe:19 max u 40,000km/s, ε 8.3MeV/nucleon, ∆M 2 10−6M . max max umax ≈ ≈ ≈ × ⊙ TheSWbreakoutwasexpected31tobeanefficientmechanismforaccelerationofcos- mic rays. For SN1987Athis mechanism,however,doesnotlook effectiveenough.Since themaximumkineticenergychangeswithR asR−0.65, onecanthinkoftheSN explo- 0 0 6 D.K.Nadyozhin&V.S.Imshennik stellar surface Fig.3. Velocity uversus distance xfromstellar surface atdifferent points oftimetduringtheshockwave breakout.Itisassumedthatt = 0whentheshockreachesthesurface(short-dashedcurve).Theequationsone mustusetoconvertu,x,andttodimensionalunits(cm,sec)areshownforthecaseofSN1987A.(Adapted fromRef.19). sionsassociatedwithpresupernovaeofsmallerradii.TheSNIbandSNIccanhaveassmall R asafewR andε mayreachabout100MeV/nucleonfortheseSNe.Theexplosion 0 max ⊙ of a white dwarf of typical radius(5,000 10,000)kmwould be just the righteventto − accelerateagoodportionofmattertorelativisticenergies(ε >1GeV/nucleon).How- max ever,theregimeoftheSWcumulationdoesnotoccurinthecaseo∼fSNeIathatcomefrom whitedwarfprogenitors.Thethermonuclearburningbeginsthereinadeflagrationregime causingthestartoexpandsubsonically.EventhoughtheSWdoesappearthishappensat theveryendoftheexplosionundertheconditionsunfavorablefortheSWcumulation. Although, to all appearance, the direct hydrodynamicalacceleration of CR in super- novaeturnsoutto beinefficientthe SWbreakoutcouldprovidefastmovingparticlesfor further acceleration by other mechanisms (e.g., by circum-stellar and interstellar shock waves). 3. ShockWavesinCircum-StellarMedium In a few days after explosion, the SN envelope turns into a state of supersonic inertial expansionwiththevelocitydependingonradiusbythesimplestway:u = r/twherethe PhysicsofSupernovae 7 contact discontinuity SW SW Fig.4. AschematicillustrationoftheinteractionbetweentheSNejectaandambientmedium:generalhydrody- namiclayout(leftpanel)andvelocityversusradiusinarbitraryunits(rightpanel). timetismeasuredfromthebeginningoftheexplosion.Simultaneouslytheouteredgeof theSNenvelopebeginstocollidewiththecircum-stellarmatter.Theinteractionregionis confinedbytwoshockwavesshowninFig.4(leftpanel)byblackcircles.TheSNejectaare decelerated,heated,andcompressedbytheinternal(reverse)SW.Theouter(forward)SW accelerates,heatsandcompressestheambientmedium.Thus,relativetomatterthereverse andforwardshockwavespropagateinaninwardandoutwarddirection,respectively.This isshownbyarrowsinFig.4(rightpanel).Theinterfaceseparatingtheshockedejectafrom shockedinterstellarmattercalledcontactdiscontinuityisshownbya whitedashedcircle inFig.4(leftpanel). ThehydrodynamictheoryoftheSNejecta–ambientmediuminteractionhasbeenelab- oratedwiththehelpofnumericalandsemi-analyticalmethods(seeRef.32andreferences therein).Theinitialstageofthisejecta-dominatedprocessiscontrolledbyasimilarityso- lution33,34 thatbeing combinedwith observationsallowsto describedetailed structureof youngsupernovaremnants(SNRs). A few hundredyearsold remnantsof galacticsuper- novaesuchasCasA,Kepler,Tychoarestillintheejecta-dominatedstage.Observationsof theirX-rayandradiosynchrotronemissioncangivemuchinformationabouttheSNpro- genitor,supernovanucleosynthesis,35ambientmedium,andmechanismsofthecosmic-ray acceleration. 4. Core-CollapseNeutrinos Thecollapseofstellariron-coresintoaneutronstar(NS)isfollowedbyahigh-powerpulse ofneutrinoemission.Figure5showsthecumulative(bolometric)neutrinolightcurvethat includes all the neutrino and antineutrino flavors. This light curve was calculated for a sphericallysymmetricalcollapseofa2M stellarcore.36Accordingtodetailedmodeling ⊙ duringthepastfewdecadesoftheneutrinotransportin collapsingstellar cores,thislight curveconsistsoftwoparts. 8 D.K.Nadyozhin&V.S.Imshennik Nonthermal ν e Ther∼mal νν Fig. 5. The normalized bolometric neutrino light curve. The time t is measured from the beginning of the collapse.(BasedonRef.36). Thefirstpart(t<0.5s)relatestothenonthermalneutrinoemissionthatisdominated bytheelectronneutr∼inosν producedbythenon-equilibriumneutronization.Fort<0.5s, e the core is still transparent to ν emitted owing to electron captures by nuclei a∼nd free e protons.Themeanindividualν energyε turnsouttobe15 20MeV.Thenonthermal e νe − neutrinoscarryawayonlyasmallfraction(q <10%,Fig.5)ofthetotalavailableenergy ν E =(3 5) 1053erg. ∼ νtot − × About 90% of E is emitted in the regime of thermal emission when the central νtot regionofthecorebecomesopaquetoalltheneutrinoflavors.Theneutrinosaredecoupled fromstellarmatteratasurfacecalledneutrinosphere.Ingeneral,theneutrinosphereradius isdifferentfordifferentneutrinoflavors,atleastonehasto considertwoneutrinospheres —onefortheelectronneutrinosandantineutrinosandanotherforµ-andτ-neutrinosand antineutrinos.Numericalmodeling37,38showsthattoafirstapproximationonecanassume PhysicsofSupernovae 9 Fig.6. AcomparisonoftheneutrinosignalpredictedforSN1987AwiththeresponseoftheKamokaNDEII detector.40(a)Thenumberofcountsversustimeinthedetector(intotal11events,stepline).Differenttheoretical versionsarealsoshown(seeRef.25fordetails).(b)Theenergymeasuredforeverycountincomparisonwitha theoreticalpredictionε¯boundedby±1σuncertaintyband.(FromRef.25). thatE isequallydistributedovertheneutrinoflavors: νtot 1 E E E E . ννe ννµ νντ νtot ≈ ≈ ≈ 3 e e e TheneutrinospectraarereproducedbytheFermi–Diracdistributionsslightlydepressed39 athighenergiesε kT (T istheeffectivetemperatureoftheneutrinosphere).The νph νph ≫ meanindividualneutrinoenergiesandrelatedT are νph ε (10 12)MeV, ε ε 25MeV, h iννe ≈ − h iννµ ≈h iνντ ≈ e e e T 4MeV, T T 8MeV. ννeph ννµph νντph ≈ ≈ ≈ e e e Thecharacteristictimeoftheneutrinopulseturnsouttobeoftheorderof10–20s. These theoretically predictedpropertiesseem to be in a fair agreementwith the neu- trinosignaldetectedfromSN1987AbytheKamiokaNDEII(KII)40andIrvine-Michigan- Brookhaven (IMB)41 neutrino detectors as it is shown in Fig. 6 for K II. The agreement occurs under the assumption that all the events in these water Cherenkov detectors were producedbytheelectronantineutrinosthroughthereaction ν +p n+e+. (1) e e → In contrastto the neutrino-electronscattering,the relativistic positronsfromthis reaction movevirtuallyisotropicallyindifferentdirections.SothedirectionoftheirCherenkovra- diationshouldnotdependonwhereν hascomefrom.However,onecanobservethatthe e eventswithenergiesε >20MeV(efoureventsinFig.6andalleighteventsrecordedby νe the IMB detector) demeon∼strate a statistically significant correlation with the direction to the Large Magellanic Cloud. Since the neutrino-electron scattering cross-section is con- siderably lower than that of the reaction (1) it is impossible to explain this observation by addressing to the ν -electron scattering — the required total energy of the neutrino µτ pulse would exceed by an order of magnitude the energy available from the collapse of 10 D.K.Nadyozhin&V.S.Imshennik stellarcores.Thesignificanceofthisproblem(remainingunresolveduptonow!)wasfirst recognizedandanalyzedinRefs.42,43. Another difficulty in the theoretical interpretation of the SN1987Aneutrino signal is the fact that there occurred two neutrino pulses. The first pulse, detected by the Liquid ScintillationDetector(LSD)underMontBlanc,44,45came4.7hoursearlierthanthesecond onerecordedbytheKIIandIMBdetectors.Since4.7hisaverylongtimeincomparison with the duration of both the neutrino pulses ( 10 s), one has to think of a two-stage ∼ collapse (see discussion in Ref. 25). In the next section, we describe a scenario that has beenrecentlyproposed46 to incorporateboththe neutrinopulsesin a self-consistenttwo- stagehydrodynamicalmodelofthegravitationalcollapse. 5. RotationalFragmentation—NeutronStarExplosionScenario Thekeypointforthisscenarioisthepresenceofrotationinthestellarcorethatisaboutto collapse.ThemechanismoftheSNexplosionproposedinRef.16isbasedontherotational instabilityanddevelopsthroughthefollowingstages. First,therotationalenergyofthecollapsingcoreE reachesthelimitofstabilitywith rot respecttofragmentation:47E /E >0.27(E isthecoregravitationalenergy). rot g g | | ThenthecoreofmassM fragmentsintoaclosebinarysystemofproto-neutronstars 0 ofdifferentmassesM andM (M +M =M ;weassumeM <M hereafter). 1 2 1 2 0 2 1 These binarycomponentsbeginto approacheach other due to the loss of total angu- lar momentumandkineticenergyof orbitalmotionthroughthe radiationofgravitational waves(GW):48 32G4(M +M )M2M2 L (t) = 1 2 1 2 1052−55erg/s, (2) GW 5c5a5(t) ≈ where L is the GW luminosity, G is the gravitationalconstant, and c is the speed of GW light.Themutualapproachofthecomponentslastsuntiltheorbitalradiusa(t)reachesa criticalvaluea = a forwhichthelessmassivecomponentfillsitsRochelobe.Contrary cr tonormalstars,thedegenerateconfigurationslikeNSsandwhitedwarfshavearemarkable property:thesmallertheirmass,thelargertheirradius(R M−1/3). ∼ Assoonasabecomeslessthana ,therebeginsarapidmasstransferfromthecompo- cr nentM tothecomponentM .ThemassM israpidlydecreasingdowntotheminimum 2 1 2 possiblemassofaNSM 0.1M .WhenM becomesalittlebitlessthanM , NSm 2 NSm ≈ ⊙ theprocessofthehydrodynamicdestructionofalow-masscomponentbegins.Suchady- namical instability is controlled by the rate of beta-processes, and initially is developing rather slowly. It terminates, however, with a short ( 0.05 s) phase of a violent transfor- ∼ mation of the internalenergyinto kinetic energyand work against gravity.The resulting energyreleaseisexpectedtobeaslargeas 1051erg( 4.8MeVpernucleon).However, ∼ ∼ the sophisticated calculations are still to be done to estimate how much of the energy is takenawaybyneutrinos.Thehydrodynamicsofthelow-massNSexplosionwasstudiedin anumberofpapers(seeRefs.49,50andreferencestherein). The low-mass NS explosion model has no problems with the explanation of the ex- plosionasymmetry(likethatobservedforSN1987A) andtheoriginofthehigh-velocity