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Group/Chemistry Chemical abundances in closed-box, open-box and infall models; multi-component treatment M. Forusova, S. Pucher, M. Zendel Chemical abundances in closed and open-box Ø Nucleosynthesis: • Chemical Elements • Origin and Abundances • Big bang àToday‘s Universe Overview • Formation Processes ØUniverse's 'element factories' Ø Models: • Parameters • Closed-box and open-box • Multi-component Chemical abundances in closed and open-box 2 Nucleosynthesis Chemical abundances in closed and open-box 3 Periodic Table of Chemical Elements https://en.wikipedia.org/wiki/Periodic_table Chemical abundances in closed and open-box 4 Chemical Elements and Isotopes • Characterized by the number of protons. • 118 elements have been identified. • Some elements are not stable and have only radioactive isotopes. • Isotopes have the same number of protons but different number of neutrons. • Isotopes of the same element have identical chemical properties (Some elements have up to 36 known isotopes, e.g. Xe!). • Most isotopes are radioactive and decay with a specific halflife. • Decay modes: β- ,β+,α , sf Chemical abundances in closed and open-box 5 Abundance of Chemical Elements (1) Abundances in the Solar System https://en.wikipedia.org/wiki/Nucleosynthesis Chemical abundances in closed and open-box 6 Abundance of Chemical Elements (2) • The abundance of a chemical element is a measure of how often you can find it relative to all other chemical elements in a given environment. • Mainly expressed in mass fractions • The abundance of elements in the Sun and outer planets is similar to that in the universe. • All chemical elements except H and He are considered metals. • Metallicity is the mass fraction of all metals. • Metallicity is usually expressed as the logarithm of the ratio of a star's iron abundance compared to that of the Sun Chemical abundances in closed and open-box 7 Big Bang: Nucleosynthesis (1) Chemical abundances in closed and open-box 8 Big Bang: Nucleosynthesis (2) • neutron-proton ratio were set in the first second after the Big Bang. • The universe was almost homogenous, and strongly radiation-dominated. • Within the first few minutes à nucleon gas cooled and p and n fused to D ( 2H) • Still hot enough to fuse D to 4He ( α), and minor amounts of T ( 3H), 3He 7Li • Too cool after expansion to support further fusion to higher elements • mass abundances: Ø 75% 1H (proton) Ø 25% 4He Ø ~0.01% 2H ,3H, 3He and Ø trace amounts (on the order of 10 −10 ) of lithium Chemical abundances in closed and open-box 9 Primordial Nucleosynthesis (3) • Be-Bottleneck due to short halflife of 8 -17 Be ( τ1/2 = 6.7*10 s) • Plasma of 1H, 2H , ( 3H), 3He, 4He (will recombine with electrons much later when temperature falls below 3000 Kàend of dark age) 3 • free neutrons (τ1/2 = 10.2 min) and H (τ1/2 = 12.32 y) will die away. • All other elements are produced by stars using different nuclear processes! Chemical abundances in closed and open-box 10 B2FH • Most cited paper in Cosmology! Chemical abundances in closed and open-box 11 § α – process: • pp – chains • triple α – process • CNO – cycle Element • α – ladder Formation in Stars § shell – burning § s – process § r – process § p – process § fission and spallation Chemical abundances in closed and open-box 12 Proton-Proton Chain Reaction (1) • Dominant fusion process for ! <= !" (majority of stars) • “soup” of electrons and ionized atomic nuclei in the center. • Two protons form deuterium nucleus that fuses further with a proton à He-3 • Two He-3 nuclei will combine à He-4 nucleus + 2 protons (86%) 1 4 + 41H®2 He + 2e + 2n e + 2g + 26 2, MeV https://en.wikipedia.org/wiki/Proton%E2%80%93proton_chain_reaction Chemical abundances in closed and open-box 13 Proton-Proton Chain Reaction (2) • T > 14 million K: He-3 fuses with a pre-existing He-4 nucleus à Be-7 nucleus à electron capture ( reverse of β-decay) à Li-7 nucleus à +proton à Be-8 à split into two He-4 nuclei. • Alternatively, Be-7 can combine with a proton à B-8 à Be-8 à 2 He-4 nuclei. • He-3 nucleus + proton à He-4 + electron and a neutrino (change one proton à neutron). Chemical abundances in closed and open-box 14 CNO Cycle • Dominant process for stars > 1.3 M" • C-12nucleus + proton à N-13 nucleus à positron emission à C-13 à +proton à N-14 à +proton à O-15 à positron emission à N-15 à +proton à C-12 +He-4. • Cycle produces He-4 + 26.7 MeV. • C, N and O act as catalysts (unchanged from their original form at the end of each cycle). • Variations of the CNO cycle à N-15, O-16. https://en.wikipedia.org/wiki/CNO_cycle Chemical abundances in closed and open-box 15 • Fusion of He-nuclei Triple-Alpha Process • C-12 nucleus by the fusion of three alpha particles • Two-stage process: 2 alpha particles à Be-8 à +alpha à C-12. • Be-8 is extremely unstable à decays quickly to He nuclei. • At T > 100 million Kelvin the second stage proceeds faster than the Be-8 nucleus can decay. • T > 100 million only reached in the cores of most massive stars https://en.wikipedia.org/wiki/Triple-alpha_process • Q/m (C-12) = 5.9 ×1017 erg/g ( ≈ 1/10 of H - burning) Chemical abundances in closed and open-box 16 Alpha Ladder • Once sufficient 12 C has been produced by triple alpha • Heavier elements are formed by ( α ,γ) reactions à O, Ne, Mg , Si, S, Ar, Ca, Ti, Cr and Fe. Chemical abundances in closed and open-box 17 Shell burning (1) • Stars with M > 8 M" create high temperatures and densities • C can fuse with C, thereafter Ne with Ne, etc. à Onion-like structure • Examples: • 2C-12 → Mg -24 # → Ne -20 + α + 4.6 MeV ($ 50%) • 2C-12 → Mg -24 # → Na-23 + p + 2.2MeV ($ 50%) • 2N-20 → O-16+Mg-24 +4.6MeV ( $ 95%) Chemical abundances in closed and open-box 18 Shell burning (2) Chemical abundances in closed and open-box 19 s-process (1) • S-process produces ~ 50% of the isotopes of elements heavier than iron à important role in the galactic chemical evolution. • Rate of neutron capture by atomic nuclei is slow relative to the rate of radioactive beta-minus decay. • Subsequent β- decay moves nucleus up the periodic table of elements. • Low neutron density and intermediate temperature conditions • Process can occur in AGB-stars (late red-giant stage for M > 0.6 M" < 10 M" ) • The elements heavier than iron with origins in large stars are typically those produced by the s-process, which is characterized by slow neutron diffusion and capture over long periods in such stars. Chemical abundances in closed and open-box 20 s-process (2) Chemical abundances in closed and open-box 21 r-process • r-process is a ‘rapid’ version of the s -process • Happens in SN core collapse, maybe in neutron star merger with a black hole in a binary system. • Synthesise atomic nuclei up to Pu-244. • Requires high neutron flux and temperature (10 %& ncm '%, ~10 ( K) • Addition of many neutrons within seconds • Complicated calculations due to the formation of ~ 4000 nuclides • Most of the generated nuclides are unstable and decay (ß -, alpha or fission) to stable nuclides Chemical abundances in closed and open-box 22 r-Process Animation Chemical abundances in closed and open-box 23 r-process: Formation of Transuranium elements Isotop Yield 237Np 0,55 239Pu 0,49 243Am 0,32 248Cm 0,64 249Cf 0,84 253Es 0,44 Vergleich: 238U 0,75 Si 1,E+08 Lingenfelter, R. E., Higdon, J. C., Kratz, K.-L., & Pfeiffer, B. 2003, ApJ, 591 , 228 Chemical abundances in closed and open-box 24 p-process • Formation of neutron-deficient isotopes (Kr to Zr) • Occurs in the early stages of a SN • Capture of protons (p, γ) or by nuclei previously formed by the r-process and the s-process • proton-rich isotopes having low abundances • Process still under debate Chemical abundances in closed-box,open-box and infall 25 models; multicomponent treatment Other processes • Fission: ØTransuranium Elements can undergo neutron-induced or spontanous fission ØSplit in two fission fragments plus lot of Energy ØMay have impact producing seed nuclei for s and r- process • Spallation: ØHigh energy cosmic rays (e.g. high energy proton) splits light element (e.g. oxygen) in ISM or on star surfaces to produce Be ØImportant for synthesis of Li, Be and B (not formed by nuclear fusion inside stars due to instability) Chemical abundances in closed-box,open-box and infall 26 models; multicomponent treatment Universe’s element factories Chemical abundances in closed and open-box 27 • The heavy element content of the universe at any point in its History reflect the integrated nucleosynthesis contributions from earlier stellar generations Universal element formation https://www.sciencelearn.org.nz/image_maps/50-universal-element-formation Chemical abundances in closed and open-box 28 • Stellar nucleosynthesis: First generation of stars • Hypothetical first metal-free Pop III stars • Probably all high mass stars ! = 60 ) 300M" • Pop III stars produce first metals • Dying stars explode in SNs and enrich the ISM with first metals • Basis for Pop II (metal-poor) and Pop I (metal-rich) Artist's impression of the first stars, 400 Myr after the BB, https://en.wikipedia.org/wiki/Stellar_population Chemical abundances in closed and open-box 29 • Comparison of spectra of stars with • Only example so far is at redshift 6.6 : CR7 decreasing metallicity (Evidence for Pop III – like stars, Sobral et al.
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