Stellar Spectroscopy and Elemental Abundances
DefinitionsSolar Abundances
Relative AbundancesOrigin of Elements
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Definitions
• X, Y, Z: amounts of H, He, and rest (metals) by mass (total of 1; log Z/Zsun in Kurucz models)
• Solar: X=0735, Y=0.248, Z=0.017• Abundance as number density relative to H
A=N(element)/N(H)Usually given as log A or log A +12
• [Fe/H]=log[N(Fe)/N(H)]star – log[N(Fe)/N(H)]sun • [M/H] sometimes reported as mean metallicity
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Solar Abundances
• From spectroscopy and meteorites
• Gray Table 16.3• Scott, Grevesse et al.
2014arXiv1405.0279S
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Relative Abundances
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Origin of the Elementshttp://ned.ipac.caltech.edu/level5/Pagel/Pagel_contents.html
• Hydrogen is most abundant element, followed fairly closely by helium.
• He formed in the Big Bang, with some increase from the primordial He abundance (Yp =0.24) by subsequent H-burning in stars (Y =0.28 here and now).
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Light Elements: Li, Be, B• Li, Be and B are very scarce, mostly destroyed in the
harsh environment of stellar interiors• Li abundance comes from measurements in
meteorites; it is still lower in the solar photosphere because of destruction by mixing with hotter layers below.
• Abundant in primary cosmic rays as a result of fusion and spallation reactions between p and (mainly) CNO nuclei at high energies.
• Deuterium and some Li formed in Big Bang.
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Carbon (6) to Calcium (20)
• Downward progression modulated by odd:even and shell effects in nuclei which affect their binding energy.
• From successive stages in stellar evolution: exhaustion of one fuel is followed by contraction, heating, alpha=He capture fusion.
• Onset of Ca burning leads to Mg and nearby elements; accompanied by neutrino emission(ever faster evolution).
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Iron Group
• Fe-group elements represent approximate nuclear statistical equilibrium at T ≈109 K
• Result of shock that emerges from the core of a massive star that has collapsed into a neutron star (SN II) OR sudden ignition of C in a white dwarf that has accreted enough material from a companion to bring it over the Chandrasekhar mass limit (SN Ia).
• Dominant product is 56Ni, most stable nucleus with equal numbers of protons and neutrons, which later decays into 56Fe.
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s-process: slow addition of neutrons
• Nucleosynthesis beyond the Fe group occurs neutron capture. Captures on a seed nucleus (mostly 56Fe) lead to the production of a β-unstable nucleus (e.g. 59Fe).
• Outcome depends on relative time-scales for neutron addition and decay.
• s-process: slow addition, so that unstable nuclei have time to undergo decay
• Nuclei form along the stability valley to 209Bi. 9
s- and r-process decaysin neighborhood of Tin (Sn)
10β decay
r-process: rapid addition of neutrons
• Many neutrons are added under conditions of very high T, neutron density; build unstable nuclei up to the point where (n, γ) captures are balanced by (γ, n) photodisintegrations
• After neutron supply is switched off, products undergo a further decays ending at the nearest stable isobar (neutron-rich side of the stability valley).
• Some elements from both r- and s-processes.
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Stability Valley
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Abundance peaks occur corresponding to closed shells with 50, 82 or 126 neutrons
Summary of Relative Abundances
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• Metal rich vs. metal poor stars:Frebel et al.2005, Nature, 434, 871[Fe/H]=-5.4
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Abundance Trends• Metals higher in Pop I stars (younger, disk)
than in Pop II stars (older, halo);Galactic enrichment with time
• Metals higher closer to Galactic center• Evolutionary changes:
Li decrease with ageCNO-processed gas in stars with mixingC enhancement in older stars with He-burning
• Magnetic fields can create patches with unusual abundance patterns: Ap stars
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