Ay 20 - Fall 2004Lecture 12:SupernovaeandGamma-Ray Burstsand some of their usesSupernovae (SNe): Exploding Stars• Two basic types and several sub-types, which differin spectroscopic properties, light curves, locations,progenitors, etc.• Previously normal star suddenly (~few days toweeks) becomes much more luminous (up to ~1010L§), rivals entire galaxy in brightness for a fewweeks! Fades over months to years• Most energy (~ 99%, up to ~ 1054 erg) in neutrinos;kinetic energy ~ 1% (typically ~ 1051 erg); visiblelight only ~ 0.1% of the total• Gas expands at v ≥ 10,000 km/s!• Leave a nebular remnant, and a compact remnant(neutron star or a black hole)Supernova ClassificationType I: no lines of hydrogen in the spectrumOccur in all types of galaxiesType II: lines of hydrogen seen in spectrumOccur in star-forming galaxies onlyType I’s are further divided into subclasses (Ia, Ib, Ic)again based on their spectral properties. There arealso “peculiar” cases.Type Ia SNe are believed to result from explosions ofChandrasekar mass white dwarfs. All other types arethought to result from the collapse of massive stars.Note: this empirical classification predates any physicalunderstanding, and so is potentially confusing!Type IIClear lines ofhydrogen inspectrumType IaSN Spectra ComparisonSN Types: Light Curve DifferencesA Considerable Variety of Light CurvesNote: Always anexponential declineat late times: poweredby the radioactive decayof nucleosynthesisproducts, e.g., Ni56, Co56SN Types: Physical MechanismsType Ia SNe: produced by accretingwhite dwarfs in close binariesType Ib SNe occur when the star has lost asubstantial part of its outer layers (the Henvelope) before explodingType Ic SNe: both H and He envelopeslost before explodingType II supernovae are created by thedeaths of massive stars at the end of theirthermonuclear evolutionCore Collapse (Type II, Ib, Ic) SNeFeFe FeHeHeHType II Type Ib Type IcProgenitors: >8M§ Red giantLeave: Neutron star or black holeEndpoints of Stellar Evolution1) Stars with M < 8 Msun:These stars never develop a degenerate core more massivethan the Chandrasekhar limit (for the more massive stars,this requires a lot of mass loss).Endpoint is a white dwarf with a mass smaller than MCh, inwhich the pressure is provided by non-relativistic degenerateelectrons.An isolated white dwarf simply cools off and becomes dimmerand dimmer for all time.2) Stars with M > 8 Msun:Nuclear reactions in these stars cease once an iron corehas developed. Core is too massive to be supported byelectron degeneracy, leading to core collapse.Type II SupernovaeOverwhelming observational evidence that Type II supernovaeare associated with the endpoints of massive stars:Association with spiralarms in spiral galaxiesType II SupernovaeOverwhelming observational evidence that Type II supernovaeare associated with the endpoints of massive stars:Thought to represent core collapse of massive stars with M > 8 Msun. Type Ib and Type Ic are thought to be similarevents in stars that have lost their outer hydrogen envelopesprior to the explosion.Identification of the progenitorsof some core collapse SNCore collapse in massive starsIn a massive star, core temperature can be high enough thatnuclear burning of Si to Fe can occur. Beyond Fe, further fusionis endothermic, and will not occur under equilibrium conditions.As an iron core develops, other reactions still proceed at largerradii:`Onion shell’ structureIron coreSi shellOxygen shellCarbon shellHelium shellHydrogen envelopeSeparated by zones inwhich nuclear fusion isoccurring - shell burning:e.g. Si burning to Fe justoutside the iron coreEventually iron core becomes too massive to be supportedby electron degeneracy pressure:• Can’t explode like a white dwarf (Type I SN) - the coreis already made of iron so no more exothermic nuclearreactions possible• Core collapsesOnce collapse starts, it proceeds very rapidly:Photodisintegration† 56Fe +gÆ lighter elementsInverse beta decay† e-+ p Æ n +neNeeds high energy gamma rays Needs e- and p to have enough energy to overcome mass differencebetween neutron and protonThese processes rob the core of pressure support, accelerate the collapse, and drive the composition toward neutron richmatter.Once the core reaches nuclear densities - r ~ 1015 g cm-3, nuclear forces provide a new source of pressure support.Scale is now:† M =43pR3rnucR ª3M4prnuc3~ 10 kmFormation of a proto-neutron star stops the collapse, and produces a bounce which sends a shock wave back out into the star.Shock wave can explode the star, if it can propagate out through the infalling matter.Core may leave a neutron star, or if it is too massive, collapsefurther to form a black hole.Proven very difficult to ascertain the exact mechanism of Type II supernova explosions:Problem: the bounce launches a shock wave with an energy that is a fraction of the binding energy of the neutron star -typically ~1052 erg.As the shock propagates through the star, high temperatures break up heavy elements into lighter ones, which absorbs someof the energy:heavy elementsshock wavelight elementsneutron starEnergy needed to completely break upheavy elements is about8 MeV per nucleon:† 1.6 ¥1052 erg Msun-1`Prompt’ mechanism forType II SN fails…Neutrino-driven explosions† e-+ p Æ n +neNeutronization reaction in the core:yields a very large flux of neutrinos, with total energy of a few x 1052 erg.For an interaction cross-section of s ~ 10-44 cm2, mean free path near the neutron star is:† l =1snª 2 km(for scattering off nucleons at r = 1015 g cm-3)i.e. smaller than the size of a neutron star. Most neutrinos will interact with matter as they escape, on a time scale much longer than the free fall time of the core (several seconds).A fraction of the neutrinos will be absorbed by the post-shockmatter, heating it and reviving the shock.Detonation or Deflagration• Modeling SN explosions is a tricky business; onlyrecently we have developed reliable models, as acombination of numerical and analytical• If flame travels supersonically (detonation), then entirestar burned at high density, all the way to Ni• We don’t see entirely Ni supernovae, so star must pre-expand• If flame travels subsonically (deflagration), then the starcan pre-expand and burn at lower densities• But deflagrations cannot produce fastest elements; bothprobably occurA nearby supernova 1987A in LMC gaveus the forst and only modern close-up lookat the