The s-process in low metallicity stars

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The s-process in low metallicity stars

• Roberto Gallino (1)
• Sara Bisterzo (1)
• Oscar Straniero (2)
• I. I. Ivans (3, 4)
• F. Kaeppeler (5)
• (1) Dipartimento di Fisica Generale , Università
  di Torino, 10125 Torino ( Italy)
• (2) Osservatorio Astronomico di Collurania
  Teramo, 64100
• (3)The Observatories of the Carnegie Institution
  of Washington, Pasadena, CA, (USA)
• (4)Princeton University Observatory, Princeton,
  NJ (USA)
• (5)Forschungszentrum Karlsruhe, Institut fuer
  Kernphysik, Karlsruhe (Germany)
• Winter School on Nuclear Astrophysics,
• Hirschegg January 15 - 21, 2006

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The s-process is characterized by a generally
smooth curve sigma(A)Ns(A) versus atomic mass
number A, but interrupted by steep decreases in
correspondence of magic neutron numbers N 50,
82 or 126, where the neutron capture cross
sections are very small and the resulting
s-process abundances are large. This happens at
the first s-peak at Sr, Y, Zr, at the second
s-peak at Ba, La, Ce, Pr, Nd and eventually at
the termination of the s-process involving Pb-208
(and Bi).
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Three s-process components were anticipated by
the classical analysis (Clayton and Rassbach
1974 Kaeppeler et al. 1982) the weak, the
main, and the strong s-component. The main
s-component is the outcome of many generations of
Asymptotic Giant Branch stars (AGB) polluting the
interstellar medium before the solar system
formed. Actually, the main s-component is far
from being a unique process, depending on the
efficiency of the so-called C13-pocket, the
initial mass, and metallicity.
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Reproduction of the Solar Main Component
(Gallino et al. 1998)

• 13C-pocket choice
• artificially introduced
• ad hoc modulated
• constant Pulse by Pulse
• AND METALLICITY
• Fe/H -0.3

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ls/Fe vs Fe/H
ls (Y, Zr)envelope last pulse condition
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hs/Fe vs Fe/H hs (Ba,
La, Nd, Sm)envelope last pulse condition
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Pb/Fe vs Fe/H envelope
last pulse condition
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hs/ls vs Fe/H First
intrinsic indicator envelope last pulse condition
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Pb/hs vs Fe/H envelope last pulse condition
Second intrinsic indicator
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Today, the typical mass of an intrinsic AGB
HALO STAR is 0.6 Msun (initial mass 0.8 0.9
Msun) NO TDU ? No C or s-process enrichment
observable.
A. Intrinsic Halo AGBs
B. Extrinsic Halo AGBs (Dwarfs - Giants)
P 2-3 yr
(13 Gyr ago)
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Zr over Nb Intrinsic or Extrinsic AGBs
Fig. 2
s-process path
The s elements enhancement in low-metallicity
stars interpreted by mass transfer in binary
systems (extrinsic AGBs). For extrinsic AGBs
Zr/Nb 0. Instead, for intrinsic AGBs Zr/Nb
1.
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Choice of initial abundances
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UPDATED
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Light elements
F
Ne
N
Na
C
Mg
Al
O