Transcript Elemental Abundances

Stellar Spectroscopy and
Elemental Abundances
Definitions
Solar Abundances
Relative Abundances
Origin 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 Elements
http://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.
6
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 βunstable 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.
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s- and r-process decays
in neighborhood of Tin (Sn)
β decay
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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
Abundance peaks occur corresponding to closed shells with 50, 82 or 126 neutrons
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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 age
CNO-processed gas in stars with mixing
C enhancement in older stars with He-burning
• Magnetic fields can create patches with
unusual abundance patterns: Ap stars
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