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Nuclear Astrophysics: Supernova Evolution and Explosive Nucleosynthesis

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Nuclear Astrophysics: Supernova Evolution and Explosive Nucleosynthesis Nuclear Astrophysics: Supernova Evolution and Explosive Nucleosynthesis Gabriel Martínez Pinedo Advances in Nuclear Physics 2011 International Center Goa, November 9 – 11, 2011 Introduction Astrophysical reaction rates Hydrostatic Burning Phases Core-collapse supernova Nucleosynthesis heavy elements Outline 1 Introduction 2 Astrophysical reaction rates 3 Hydrostatic Burning Phases Hydrogen Burning Advanced burning stages 4 Core-collapse supernova Neutrinos and supernovae 5 Nucleosynthesis heavy elements Neutrino-driven winds Introduction Astrophysical reaction rates Hydrostatic Burning Phases Core-collapse supernova Nucleosynthesis heavy elements A new Era for Nuclear Astrophysics Improved observational capabilities Improved supernovae models New radioactive ion beam facilities (RIBF, SPIRAL 2, FAIR, FRIB) are being built or developed that will study many of the nuclei produced in explosive events. Hydrostatic burning phases studied in underground labs (LUNA) We need improved theoretical models to fully exploit the potential offered by these facilities. Introduction Astrophysical reaction rates Hydrostatic Burning Phases Core-collapse supernova Nucleosynthesis heavy elements What is Nuclear Astrophysics? Nuclear astrophysics aims at understanding the nuclear processes that take place in the universe. These nuclear processes generate energy in stars and contribute to the nucleosynthesis of the elements and the evolution of the galaxy. Hydrogen mass fraction X = 0.71 3. The solar abundance distribution Helium mass fraction Y = 0.28 Metallicity (mass fraction of everything else) Z = 0.019 Heavy Elements (beyond Nickel) mass fraction 4E-6 a-nuclei Disk solar abundances: Gap 12C,16O,20Ne,24Mg, …. 40Ca general trend; less heavy elements B,Be,Li Elemental 0 10 Halo (and isotopic) 10-1 composition 10-2 + Sun + of Galaxy at 10-3 location of solar -4 n r-process peaks (nuclear shell closures) system at the time o 10 Bulge i t -5 c 10 of it’s formation a -6 r f 10 r -7 e 10 s-process peaks (nuclear shell closures) b -8 m 10 u -9 n 10 10-10 10-11 Fe peak U,Th -12 (width !) 10 Fe Au Pb 10-13 0 50 100 150 200 250 mass number K. Lodders, Astrophys. J. 591, 1220-1247 (2003) Introduction Astrophysical reaction rates Hydrostatic Burning Phases Core-collapse supernova Nucleosynthesis heavy elements Cosmic Cycle 8 4...6 10 y M~10 Mo dense molecular star formation (~3%) condensation clouds interstellar 106-1010 y medium stars star M > 0.08 Mo s infall dudustst winds SN explosion mixing SNR's & ~90% hot bubbles Galactic compact remnant SNIa (WD, halo 102-106y NS,BH) Introduction Astrophysical reaction rates Hydrostatic Burning Phases Core-collapse supernova Nucleosynthesis heavy elements Nucleosynthesis processes In 1957 Burbidge, Burbidge, Fowler and Hoyle and independently Cameron, suggested several nucleosynthesis processes to explain the origin of the elements. νp-process “neutrino-proton process” s-process 184 rp-process “slow process” via chain of stable nuclei through “rapid proton process” 162 via unstable proton-rich nuclei neutron capture through proton capture Pb (Z=82) Proton dripline 126 (edge nuclear stability) Number of protons Sn (Z=50) r-process “rapid process” via Ni (Z=28) 82 unstable neutron-rich nuclei Neutron dripline 50 (edge nuclear stability) 28 Fusion up to iron 20 Big Bang Nucleosynthesis 2 8 Number of neutrons Introduction Astrophysical reaction rates Hydrostatic Burning Phases Core-collapse supernova Nucleosynthesis heavy elements Composition of the Universe after Big Bang Matter Composition 80 76 after Big Bang Nucleosynthesis 70.683 in Solar System 60 ) % ( on i act 40 r F ass M 27.431 24 20 1.886 8.00E-08 0 HHeMetals Stars are responsible of destroing Hydrogen and producing “metals”. Introduction Astrophysical reaction rates Hydrostatic Burning Phases Core-collapse supernova Nucleosynthesis heavy elements Star formation Stars are formed from the contraction of molecular clouds due to their own gravity. Contraction increases temperature and eventually nuclear fusion reactions begin. A star is born. Contraction time depends on mass: 10 millions years for a star with the mass of the Sun; 100,000 years for a star 11 times the mass of the Sun. The evolution of a Star is governed by gravity Introduction Astrophysical reaction rates Hydrostatic Burning Phases Core-collapse supernova Nucleosynthesis heavy elements What is a star? A star is a self-luminous gaseous sphere. Stars produce energy by nuclear fusion reactions. A star is a self-regulated nuclear reactor. Gravitational collapse is balanced by pressure gradient: hydrostatic equilibrium. mdm dF = G = [P(r + dr) P(r)]dA = dF grav − r2 − pres dm = 4πr2ρdr mρ dP G = − r2 dr Further equations needed to describe the transport of energy from the core to the surface, and the change of composition (nuclear reactions). Supplemented by an EoS: P(ρ, T). Star evolution, lifetime and death depends on mass. Two groups Stars with masses less than 9 solar masses (white dwarfs) Stars with masses greater than 9 solar masses (supernova explosions) Introduction Astrophysical reaction rates Hydrostatic Burning Phases Core-collapse supernova Nucleosynthesis heavy elements Core evolution Hydrostatic equilibrium together with properties equation of state determines the evolution of the star core. red: transition from relativistic to non-relativistic electrons. Introduction Astrophysical reaction rates Hydrostatic Burning Phases Core-collapse supernova Nucleosynthesis heavy elements Where does the energy come from? Energy comes from nuclear reactions in the core. 1 4 + 4 H He + 2e + 2νe + 26.7 MeV ! E = mc2 The Sun converts 600 million tons of hydrogen into 596 million tons of helium every second. The difference in mass is converted into energy. The Sun will continue burning hydrogen during 5 billions years. Energy released by H-burning: 18 1 6:45 10 erg g− = 6:7 MeV/nuc × 33 1 Solar Luminosity: 3:85 10 erg s− × Introduction Astrophysical reaction rates Hydrostatic Burning Phases Core-collapse supernova Nucleosynthesis heavy elements Nuclear Binding Energy Liberated energy is due to the gain in nuclear binding energy. Introduction Astrophysical reaction rates Hydrostatic Burning Phases Core-collapse supernova Nucleosynthesis heavy elements Type of processes Transfer (strong interaction) 15 12 N(p; α) C; σ 0:5 b at Ep = 2:0 MeV ' Capture (electromagnetic interaction) 3 7 6 He(α, γ) Be; σ 10− b at Ep = 2:0 MeV ' Weak (weak interaction) + 20 p(p; e ν)d; σ 10− b at Ep = 2:0 MeV ' 2 24 2 b = 100 fm = 10− cm Introduction Astrophysical reaction rates Hydrostatic Burning Phases Core-collapse supernova Nucleosynthesis heavy elements Types of reactions Nuclei in the astrophysical environment can suffer different reactions: Decay 56 56 + Ni Co + e + νe 15O +!γ 14N + p ! dn a = λ n dt − a a In order to dissentangle changes in the density (hydrodynamics) from changes in the composition (nuclear dynamics), the abundance is introduced: na ρ Ya = ; n = Number density of nucleons (constant) n ≈ mu dY a = λ Y dt − a a Rate can depend on temperature and density Introduction Astrophysical reaction rates Hydrostatic Burning Phases Core-collapse supernova Nucleosynthesis heavy elements Types of reactions Nuclei in the astrophysical environment can suffer different reactions: Capture processes a + b c + d ! dn a = n n σv dt − a bh i dYa ρ = YaYb σv dt −mu h i decay rate: λ = ρY σv =m a bh i u 3-body reactions: 3 4He 12C + γ ! dY ρ2 α Y3 = 2 α ααα dt −2mu h i decay rate: λ = Y2ρ2 ααα =(2m2) α α h i u Introduction Astrophysical reaction rates Hydrostatic Burning Phases Core-collapse supernova Nucleosynthesis heavy elements Reaction rates Consider na and nb particles per cubic centimeter of species a and b. The rate of nuclear reactions a + b c + d ! is given by: rab = nanbσ(v)v; v = relative velocity In stellar environment the velocity (energy) of particles follows a thermal distribution that depends of the type of particles. Nuclei (Maxwell-Boltzmann) ! m 3=2 mv2 N(v)dv = N4πv2 exp dv = Nφ(v)dv 2πkT −2kT Electrons, Neutrinos (if thermal) (Fermi) g 4πp2 N p dp dp ( ) = 3 (E(p) µ)=kT (2π~) e − + 1 photons (Bose) 2 4πp2 N(p)dp = dp (2π~)3 epc=kT 1 − Introduction Astrophysical reaction rates Hydrostatic Burning Phases Core-collapse supernova Nucleosynthesis heavy elements Stellar reaction rate The product σv has to be averaged over the velocity distribution φ(v) (Maxwell-Boltzmann) Z σv = 1 φ(v)σ(v)vdv h i 0 that gives: ! m 3=2 Z mv2 m m σv = 4π 1 v3σ(v) exp dv; m = 1 2 h i 2πkT 0 −2kT m1 + m2 or using E = mv2=2 !1=2 8 1 Z E σv = 1 σ(E)E exp dE 3=2 h i πm (kT) 0 −kT Introduction Astrophysical reaction rates Hydrostatic Burning Phases Core-collapse supernova Nucleosynthesis heavy elements Neutron capture (compound picture) A + n B + γ ! 2 2 2 σn πo B + γ HII C C HI A + n o Tγ(En + Q)Tn(En) ≈ jh j j ih j j ij / T transmision coefficient, E neutron energy, Q = m + m m = S (B), n A n − B n Q En. 2 σn o Tn(En); Tn(En) vnPl(En) / n / Pl(En), propability tunneling through the centrifugal barrier of momentum l. Normally s-wave dominates and P0(En) = 1. 1 1 σn 2 vn = ; σnv = constant / vn vn h i Introduction Astrophysical reaction rates Hydrostatic Burning Phases Core-collapse supernova Nucleosynthesis heavy elements Charged-particle reactions Stars’ interior is a neutral plasma made of charged particles (nuclei and electrons). Nuclear reactions proceed by tunnel effect. For the p + p reaction the Coulomb barrier is 550 keV, but the typical proton energy in the Sun is only 1.35 keV. Cross section given by: 2 r 1 2πη Z1Z2e m b σ(E) = e− S (E); η = = E ~ 2E E1=2 Introduction Astrophysical reaction rates Hydrostatic Burning Phases Core-collapse supernova Nucleosynthesis heavy elements S factor S factor makes possible accurate extrapolations to low energy.
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