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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)
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)
(1)
Winter School on Nuclear Astrophysics,
Hirschegg January 15 - 21, 2006
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).
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.
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
[ls/Fe] vs [Fe/H]
envelope last pulse condition
ls =(Y, Zr)
[hs/Fe] vs [Fe/H]
envelope last pulse condition
hs =(Ba, La, Nd, Sm)
[Pb/Fe] vs [Fe/H]
envelope last pulse condition
[hs/ls] vs [Fe/H]
First intrinsic indicator
envelope last pulse condition
[Pb/hs] vs [Fe/H]
Second intrinsic indicator
envelope last pulse condition
A. Intrinsic Halo AGBs
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.
B. Extrinsic Halo AGBs (Dwarfs - Giants)
P ~ 2-3 yr
(13 Gyr ago)
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.
Choice of initial abundances
UPDATED
Light elements
3.5
F
3.0
Ne
[X /F e ]
2.5
C
2.0
N
Na
Mg
1.5
1.0
Al
O
0.5
0.0
-0.5
5
7
9
11
Z
13