Nucleosynthesis

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Nucleosynthesis

• Trevor Ireland

GEOL 3022 Planetary Geology 2
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Recap

• Big Bang produces H, He
• Other elements caused by stellar
  nucleosynthesis(B2FH, 1957)

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Recap

• H burning
• 1st generation PP
• CNO cycle
• He burning
• triple a

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Recap

• If star is massive enough, successive burning
  reactions
• C, O burning
• Mg, Si burning
• If no fuel (and no mass) star dies
• white dwarf

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Fusion

• Progressive building reactions
• 4p -gt 4He releases 90 of available E
• fusion proceeds to 56Fe (e process)
• e equilibrium
• forward reactions compete with back
• buildup of Fe abundance peak
• progressive increase in T of reactions
• Up to T9 (T 1x 109 K)

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Heavy nuclei

• Protons, Alpha reactions up to Fe peak
• Neutrons allow a nuclear pathway to heavy nuclides

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Nuclear Equilibrium
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Neutrons

• Release of neutrons
• 13C(a,n)16O, 22Ne(a,n)25Mg, 25Mg(a,n)28Si
• Reaction with Fe group elements (e.g. Fe)
• 56Fe(n,g)57Fe(n,g)58Fe(n,g)59Fe(n,g)60Fe(n,g)
  61Fe
• 59Fe-gt59Cob (44 days)
• 60Fe-gt60Co (105 a) -gt60Ni (5.3 a)
• 61Fe -gt61Co (6 min) -gt 61Ni (1.7 h)
• Timescale of reaction important

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S process

• slow addition of neutrons compared with decay
  times
• nuclei stay close to valley of stability
• branching ratios
• indicator of n abundance
• Theoretically up to 209Bi
• Isotope abundances depend on neutron capture
  cross sections

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Chart of the Nuclides
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S -process

• astrophysics
• AGB (assymptotic giant branch) stars
• observation of Tc in stellar atmospheres
• Tc shortlived, extinct on Earth
• also Ba, Zr, Mo lines visible
• AGB stars rapidly lose mass
• dust recycled into ISM

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More Heavy nuclides

• S-process to 209Bi
• Heaviest stable nuclide
• So how do we get to 238U, 232Th?
• Require rapid addition of neutrons before decay
• r-process

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r process

• Rapid addition of neutrons
• Before ß decay
• Fast neutrons
• Reactions on timescale of lt 1 s
• Nuclides driven far to right of nuclear stability
  line
• nuclides then decay back to stable path

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Pathways
Nuclides can have s and/or r contributions
S only 96Mo is shielded by 96Zr from
r-process R only 96Zr is shielded from s by
95Zr decay SR 95Mo has both S R
contributions Branch 95Zr decay or neutron
addition before?
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Shielded nuclei

• r process nuclei decay diagonally back to stable
  nuclei
• first stable isotope takes all of that isobar
• some nuclei protected from r process input
• similarly, some nuclei r process only
• (nuclei isolated to right of stable nuclei)

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R and S abundances

• Structure to s, r abundance curves
• Can be related to magic nuclei
• N50,82,128

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Magic Nuclei

• Chemical analog
• filled electron shells of noble gases
• Electric force
• Nuclear
• Stable configurations of p n
• Strong force
• Magic (p or n) and doubly magic (p n)
• 2, 8, 20, 4He,16O,40Ca (are doubly magic)

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Heavy Magic Nuclei

• s, r abundance peaks evident
• related to magic nuclei number
• s and r offset
• why?
• r process nuclei meet magic numbers off the
  stable nuclei line. They decay diagonally back
  to the stable nuclei curve and hence end up
  before the magic s-process nuclei.

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Magic continued

• R-process contributions get stuck at magic
  numbers, then decay back

s
r
s
r
s
r
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Astrophysical Modelling

• r-process
• numerical codes attempt to build nuclei in their
  observed abundances
• good to factor of two
• s-process
• Primarily neutron capture cross sections
• branching ratios
• resonances of reactions

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Astrophysical Sites

• R-process
• Explosive
• Supernovae
• 1 per 200 yr per galaxy (10-3 solar mass)
• S-process
• Controlled burning
• AGB stars
• 1 per year (10-5 solar masses)

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Explosive nucleosynthesis

• Supernovae (Type I, Type II)
• Type I (no H in spectrum)
• White dwarfs accreting mass from companion
• Type II (H in spectrum)
• Core collapse
• Novae
• Flashing of accreted H on white dwarf

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Nova Cygni 1992
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Type I Supernova
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Supernova (Type II)
25 solar mass star, dying
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Supernova 1987a
Visible in Southern hemisphere in Large
Magellenic Cloud
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Supernovae
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P process

• stable nuclei to left of s-process track
• E.g. 92Mo

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Astrophysics of P process

• p-process
• (p,g) reactions
• photodisintegration (g,n)
• Astrophysical site uncertain
• maybe related to r-process
• supernova with extreme photon density
• and abundant protons

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X - process

• Nuclei blown apart from high energy collisions
• Fragments e.g.
• 28Si (?) ? 21Ne 4He 3He
• 16O ? 9Be 7Li
• Be, Li quickly destroyed in burning reactions and
  so are not remnants of stellar nucleosynthesis

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B2FH
Burbidge, Burbidge, Fowler and Hoyle, 1957

• 8 processes
• H burning
• He burning
• a
• e
• s
• r
• p
• x (spallation)

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Our link to the stars