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2/28 Lecture 18.3-4 Astronomy 218 The Spectacular Deaths of Massive Stars End of a Massive Star By the time the helium shell burning ignites, the core is effectively decoupled from the envelope. The star’s remaining life is less than the Kelvin-Helmholtz timescale, τ ~ U/L ~ GM2/RL ~ 104 years. The visible luminosity is largely provided by the helium shell. 0.01 R 1000 R In the core, Tc > 500 MK lead ☉ ☉ to neutrino-pair production, allowing the luminosity of the core to escape primarily as neutrinos. Successive burning stages proceed rapidly to form an iron core with lighter element shells. High Energy Photons γ-rays are photons with wavelengths λ < 10−11 m, carrying energies of 100 keV or more. This corresponds to kT for a temperature of 1.1 GK. At temperatures of 0.5 GK or more, the tail of the photon distribution has sufficient energy (1.02 MeV) to transform into a electron-positron pair. Usually the positron will annihilate with an electron, producing γ-rays, but rarely a neutrino-antineutrino pair is produced. These radiate freely from the core, cooling it. For T > 1.2 GK the tail of the photon field is sufficient to photo-disintegrate neon, 20Ne + γ → 16O + α, igniting neon burning. This occurs at a lower temperature than the T > 1.5 GK required for oxygen burning. Inside a Massive Star This diagram shows the history of a 22 M☉ star. The green and red shadings show convective regions. Blue marks nuclear Rauscher, Heger, Hoffman & Woosley 2002 energy generation. Purple marks nuclear energy loss. Carbon, Neon, Oxygen and Silicon burning, leave a core of Iron. Nuclear burning stages The nuclear burning stages in the life of a 20 M☉ star. Stage Fuel Products Tc Lumin. Duration (100 MK) (100 L☉) H Burning H He 0.3 600 10 Myr He Burning He C 2 1000 1 Myr C Burning C O, Ne, Na, Mg 9 1400 103 yr Ne Burning Ne O, Mg 16 1500 200 days O Burning O Mg to S 20 1500 500 days Si Burning Si Fe peak 30 1500 10 days Collapse up to Th >30 0.3 s Why Stop at Iron? Fission Fusion For T > 3 GK, the photon field is sufficient to photodisintegrate most nuclei. This causes silicon burning. Photodisintegrations balance captures resulting in Nuclear Statistical Equilibrium (NSE). This favors iron, the most tightly bound nucleus. Iron core ⇒ no fusion to generate energy to balance gravitational collapse. Supernova! Sk 202-69 SN 1987a © Anglo-Australian Observatory Supernova Taxonomy Observationally, there are 2 types (7 subtypes) based on their spectra and light curves. Physically, there are 2 3 4 mechanisms, thermonuclear Cappellaro & Turatto (white dwarf) 2001 core collapse (massive star) collapsar (very massive star) pair production (very very massive star) 6 from 1, 1 from another The core collapse Super Luminous Supernova mechanism results in Ia White Dwarf supernovae with quite II-L Red Supergiant varied spectra and light II-P Blue Supergiant curves. Ib Wolf-Rayet Star Each CCSN is unique because of variations in the massive star’s mass and envelope which surrounds the central engine. In contrast, the Type Ia SN are remarkable similar, suggesting a mechanism with little variation. Supernova at 320 Years The supernova Cassiopeia A: Type IIb SN deposits 1044 J (the equivalent of 1028 MTons TNT) of Kinetic Energy into the ISM. X-ray (NASA/CXC/SAO) Optical (MDM Obs.) Supernovae are ~ 10 pc a major heating source in the ISM, responsible for coronal gas and star formation. Radio (VLA) Infrared (ISO) Old Supernovae Supernova remnants sweep up the interstellar medium as they spread, which gradually slows the ejecta. Thus old remnants lose their spherical shape as they encounter irregularities in the interstellar medium, including other supernova remnants. Vela SNR is Vela 8° on the sky. supernova The Vela remnant pulsar sits at the center of the Vela SNR, marking this as the grave of Exploded ~ a massive star. 9000 BCE Pulsar Winds In many supernova remnants that result from core collapse supernovae, the initial 1044 J carried by the blast wave is augmented over the course of thousands of years by 1043 J of pulsar magnetic dipole radiation and pulsar winds. Most CCSN SNRs combine shell and PW nebulae. The Crab nebula is an example where the shell remnant is missing leaving only the PW nebula. Central Source Even with complex morphology of supernova remnants it is generally possible to reconstruct their origin. Positive and negative photographs of the Crab Nebula 14 years apart show the the gaseous filaments moving away from the site of the explosion. Invisible Supernova As bright as supernova are, you might think they can be seen whenever they occur nearby, but this is untrue. As young stars, O & B stars occur predominantly in the Galactic Disk. In the disk, the UV and optical light that carries much of their light is heavily extinguished by interstellar dust, making some supernova invisible. The young supernova remnant G1.9+0.3A lies in the direction of the Galactic Center, in the Sagittarius. From X-ray and radio observations, we know its angular expansion rate and size, we can calculate an age of 140 years, but no SN was seen. Wind Interactions Supernovae begin interacting with their environment almost immediately. The initial interactions are with their own stellar wind. In the case of SN1987A, the first interaction was a light echo illuminating a hourglass shaped wind. The UV flash of the supernova ionized gas in the waist of the hourglass and along both funnels. The supernova shock is now striking the central ring, which was ejected 20,000 years earlier. Textbook Supernova A Core-Collapse Supernova is the inevitable death knell of a massive star. E = 1046 J comes from the creation of a neutron star. Neutrinos carry away the binding energy of the neutron star, heating the stellar envelope and driving the Hillebrandt, Janka, explosion. Müller/Sci. Am. Observing Neutrinos We are now able to observe neutrinos using a number of different methods. These detectors will allow an amazing view of the next galactic supernova. SN 1987a 1045 W Failed Supernova? 2 Because there is a maximum mass for a 1.8 ] NS, we expect collapse . O of a massive enough star to lead instead to a 1.6 black hole. 1.4 Without the neutron 12-WH07 D-Series 15-WH07 star radiating neutrinos, Proto-NS Mass [M 1.2 B-Series 20-WH07 the core collapse 25-WH07 mechanism fails. 1 0 500 1000 1500 2000 Mass loss & metallicity Time After Bounce [ms] are important to this limit. and very Massive stars may lose much of their envelope leaving the He or C/O core visible, re-enabling CCSN. Collapsar About once per day, a bright burst of highly beamed γ-rays comes to the Earth. Observations have recently tied some of these GRBs to peculiar, hyper- energetic SN Ic. One proposed model is a collapsar, where an accretion disk forms around a newly formed black hole in a failed supernova (M > 30 M☉), producing a jet which we see as the GRB. Heavy Element Ejecta Hughes, Rakowski, Burrows & Slane 2000 The ejecta from core-collapse supernovae are rich in many elements from oxygen to calcium and iron/cobalt/nickel. They may also be responsible for half of the elements heavier than iron. Fresh Nuclei Observations of core collapse supernovae reveal freshly made nuclear species. γ-ray telescopes reveal the characteristic γ-ray lines of 56 57 Ni (t½ = 6 days), Ni (t½ 56 = 36 hrs), Co (t½ = 77 57 days), Co (t½ = 272 days) 44 COMPTEL/NASA and Ti (t½ = 60 yrs). NuSTAR NASA/JPL-Caltech/CXC/SAO The supernova’s light curve in the later, linear phase is 56 56 powered by the conversion of ~0.1 M☉ of Ni to Co and ultimately 56Fe. Almost half of terrestrial iron was made in this way, a direct product of a star’s death. rich in heavy elements Not just iron, but many elements are made in these explosions. Others, made during stellar evolution, are released into the interstellar medium. Umeda & Nomoto 2003 Rauscher, Heger, Hoffman & Woosley 2002 Accretion Power White dwarves, neutron stars and black holes that are part of binary star systems exhibit a range of afterlives impossible for their solitary counterparts. The unifying power source for this range of objects is matter accreted from a binary companion. The wind of a companion can be the accretion source. Companions that fill their Roche lobes provide the largest & steadiest accretion rates. Loading a White Dwarf The result of accretion onto a white dwarf Nomoto, Saio, Kato & Hachisu 2007 depends on the accretion rate and WD mass. For high accretion rates, a Red Giant-like envelope re-forms. For the right accretion Novae rate, accreted H and He burn steadily to C & O, causing the WD to grow. For lesser accretion rates, a layer of H builds on the surface. As accretion continues, the pressure at the base of the accreted layer grows until it ignites. Finding Novae T. Credner 12/99 About 40 novae occur each year in our galaxy and they are frequently discovered by amateurs. Novae are classified as cataclysmic variables. The binary systems that make up cataclysmic variables are typically very tight (a ~ R☉) with periods of < 1 day. Classical Nova The ignition of the degenerate hydrogen layer causes a thermonuclear runaway, a 1038 J (1022 Megaton) Hydrogen bomb. The star to rapidly brighten as much as a million-fold. S2006 S1998 P1995 Starrfield, Iliadis, Hix, … 2009 The enhanced luminosity fades 2-3 magnitudes in < 25 days for a fast nova while a slow nova may take more than 80 days to decline.
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