Metallicity dependence of the sprocess in massive stars: theoretical predictions

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Metallicity dependence of the s-process in
massive stars theoretical predictions

• Marco Pignatari
• Universita di Torino - Italy

(GIANTS, Perugia november 27-28 2006)
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Käppeler et al. 1982 ApJ
The weak s component
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The main s component from AGB stars gives a
marginal contribution in the region between Fe
and Sr
The weak s component from massive stars
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The weak s-component Convective
Convective Core He-burning
Shell C-burning
Low neutron density (106 n/cm3) T3-3.5108 K
Classical s-process See Lamb et al. 1977,
Couch et al. 1974, Prantzos et al. 1987, Raiteri
et al. 1991 ......
Peak neutron density (1011- 1012 n/cm3) T
1109 K The convective shell works over the
ashes of the core He-burning (Raiteri et al.
1991)
The final weak s component is an overposition of
two different s(s) components
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The weak s-component Convective
Convective Core He-burning
Shell C-burning
Neutron source 22Ne(a,n)25Mg CNO
cycle ? 14N 14N(a,?)18F(ß)18O in the initial
phase of the He-burning 18O(a,?)22Ne when Teff
gt 2.5108 K Residual 22Ne in the He core ashes
provides neutrons in the next convective C shell
episodes. 22Ne is secondary
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The weak s-component Convective
Convective Core He-burning
Shell C-burning
Other neutron sources 13C(a,n)16O recycles
neutrons captured by 12C 17O(a,n)20Ne recycles
neutrons captured by 16O In the C shell there
are also 21Ne(a,n)24Mg that recycles neutrons
captured by 20Ne. Marginal contribution from the
primary 12C(12C,n)23Mg
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Neutron poisons in the He core, 25 Msun Fe/H 0
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Neutron poisons in the He core, 25 Msun Fe/H
-2
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Neutron poisons In the C shell -Primary 16O,
20Ne, 23Na -Secondary 25Mg (and 17O)
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Weak s distribution at the end of the C shell
solar scaled composotion
Fe/H0
Fe/H-1
Fe/H-2
25 Msun
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Mishenina et al.2000 AA O/Fe-0.370Fe/H0.047
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Mishenina et al.2000 AA O/Fe-0.370Fe/H0.047
Asplund Garcìa Pèrez 2001 AA
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Weak s distribution at the end of the C shell
enhanced distribution
Fe/H0
O/FeFe/H0.5
Fe/H-1
Fe/H-2
25 Msun
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Weak s distribution at the end of the C shell
enhanced distribution
Fe/H0
O/Fe0.4Fe/H
Fe/H-1
Fe/H-2
25 Msun
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Weak s distribution at the end of the C shell
enhanced distribution
Fe/H0
O/Fe0.4Fe/H
Fe/H-1
Fe/H-2
15 Msun
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Weak s distribution at the end of the C shell
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Weak s distribution at the end of the C shell
The effect of several nuclear uncertanties
affect the weak s behaviour with the
metallicity. Ex 17O recycling point
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Constrains from the observationsCopper
Germanium
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5 of solar Copper 12 of solar Germanium
AGB stars
Marginal contribution to solar Copper and
Germanium
SNIa
(Travaglio et al. 2004 AA 425)
The major part of solar Copper and Germanium has
been produced by the Weak s-process
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Copper
Secondary-like contribution
SNIa Fe contribution
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Spectroscopic observations from Halo and
Intermediate stars (very metal poor stars not
included) (Sneden et al. 1991, Mishenina et al.
2002, Reddy et al. 2003...)
Cu/Fe -0.7
Cu(pr.) 7-10 Cu(sun)
SNII produced 1/3-1/2 of Fe(sun)
Assuming that all primary-like Cu component has
been produced by SNII
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Ge Abundances in Halo Stars
Ge Fe
Challenge to theorists.
What happens at higher Fe/H with the s-process?
JC et al. (2005)
A/B log10(A/B)star - log10(A/B)sun
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Spectroscopic observations from Halo stars (e.g.
Cowan et al. 2005 and references therein)
Ge/Fe -0.85
Ge(pr.) 5-7 Ge(sun)
SNII produced 1/3-1/2 of Fe(sun)
Assuming that all primary-like Ge component has
been produced by SNII
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Conclusions

• The weak s-process nucleosynthesis depends on the
  initial metallicity of the star
• The s-process in the C shell adds an important
  contribution at solar metallicity. For
  metallicities lower than solar the s-process
  efficiency in the C shell rapidly decreases and
  the neutron exposure is almost totally given by
  the He core
• At the same metallicity, a different initial
  composition or/and a different initial mass
  change the weak s distribution.

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Conclusions

• Nuclear uncertainties affect the behaviour of the
  weak s component with the metallicity.