The sprocess in AGB stars. Models and Observations

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The s-process in AGB stars.Models and
Observations

• Roberto Gallino
• Universita di Torino
• A long-living collaboration started with Maurizio
  Busso in the early 80.ies all the way integrated
  with many very motivated students and friends
  best experts in different research fields,
  stellar evolution, experimental nuclear physics,
  high-resolution stellar spectroscopy,
• short-lived nuclei alive in the early solar
  system, presolar grains
• IX Torino Workshop on
• Evolution and Nucleosynthesis in AGB Stars
• Perugia 22-26 October 2007
• 
  Wednesday, Oct. 24

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Outline

• AGB stars and the two sources of neutrons
• Experimental and observational constraints
  neutron capture cross sections and solar system
  isotopic composition
• The choice of 13C-pockets guided by the Solar
  System main s-process and spectroscopic
  observations of Extrinsic and Intrinsic AGB stars
  at different metallicities
• The impact of Post-AGB stars
• Ba stars and CEMP-s(r) stars
• Primary production of light isotopes
• The Sr, Y, Zr puzzle and the GCE connection

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Dynamical steps are consequence of the magic
neutron numbers at N50 ls(Sr,Y,Zr), at
N82 hs(Ba,La,Ce,Pr,Nd) and at N126 Pb, Bi
sigmaNsconst is valid locally, for unbranched
isotopes, far from neutron magic nuclei
(Clayton et al. 1961, Ann. of Phys. 12, 331
Seeger et al. 1965, Ap.J. Suppl. 11,121
Clayton s book, 1968
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AGB thermally pulsing stars



(Straniero et al. 1997)
TP thermal pulses
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THE AGB ENGINE
( Busso, Gallino, Wasserburg 1999 ARAA, 37, 239 )
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The two neutron sources in AGB
stars 13C(a,n)16O
22Ne(a,n)25Mg
Needs primary 13C from proton capture in the
12C-rich He intershell Major neutron source T8
0.9-1 Interpulse phase (1- 0.4) 105 yr Radiative
conditions Nn 107 cm-3
Abundant 22Ne, from CNO and alpha captures in
thermal pulses Minor neutron source Secondary
source T8 gt 3 (low 22Ne neutron
efficiency) Neutron burst in thermal pulse 6
yr Convective conditions Nn (peak) 1010 cm-3
22Ne becomes primary in low metallicity AGB
stars !
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Reproduction of the main component by an AGB
stellar model (Arlandini et al. 1990 updated
Bisterzo et al, 2006 NIC-IX)

• 13C-pocket choice
• ad hoc modulated
• constant Pulse by Pulse
• Fe/H -0.3
• assumig some proton diffusion at from the bottom
  of the convective envelope into the radiative He
  intershell during the third dredge up

208Pb
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INTRINSIC AGB STARS of Mini1.5Msun Predicted
ls/Fe vs Fe/H ls (Y, Zr)
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INTRINSIC AGB STARS of Mini1.5MsunPredicted
hs/Fe vs Fe/H hs (Ba, La, Nd, Sm)
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INTRINSIC AGB STARS OF Mini1.5 Msun Predicted
Pb/Fe vs Fe/H
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hs/ls vs Fe/H
The first intrinsic/extrinsic indicator
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Comparison with Observations.

Busso et al. 2001 (see also Abia et al. 2002)
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Pb/hs vs Fe/H Second
intrinsic/extrinsic indicator
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• Intrinsic AGBs
• MS, S stars with Tc
• The example of Galactic disc PostAGB Stars
• A few binary Barium stars (with Lead) preserving
  the AGB envelope elemental composition of the
  primary companion
• A few Halo CEMP-s dwarf stars with no dilution

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hs/ls vs s/Fe
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• Extrinsic AGBs
• Barium stars (dwarfs, subgiants, giants)
• CH giants
• dwarf C stars
• A recent sample of 26 Barium stars at high
  resolution spectroscopy with observed lead
• ( Allen and Barbuy 2006 AA 454, 825 ) .

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• Very low metallicity Extrinsic AGB stars
• THE CEMP-s(r) STARS
• (talks by Sara Bisterzo, Inese Ivans, Ian
  Roederer)
• Nb indicator of an extrinsic AGB in a binary
  system
• Na permits an estimate of the initial AGB
  stellar mass
• Lead stars

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The La to Eu ratio in metal-poor AGB stars
AGB of M 1.3 Msun, Fe/H -2.60
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• IN LOW METALLICITY STARS
• 22Ne (primary) source of neutrons
• 22Ne major neutron poison
• 22Ne and primary Light Isotopes
• 22Ne source of s-process iron, bridge
• to s-process elements production

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THE IMPACT OF THE GALACTIC CHEMICAL
EVOLUTION
(GCE) From the Galactic chemical evolution
model (Travaglio et al. 1999) we obtain that
AGB stars produced 94 of the solar 208Pb
(Note Solar Pb is given at 6 uncertainty) The
main component at Fe/H - 0.30 (Arlandini et
al. 1998) only accounts for 43 of the solar
208Pb
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GALACTIC CHEMICAL EVOLUTION AGB CONTRIBUTION AT
THE EPOCH OF SOLAR SYSTEM FORMATION
208Pb
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• SOME QUESTIONS
• 1 - IS THE s-PROCESS TAKING PLACE IN AGB STARS A
  PRIMARY PROCESS?
• ANSWER IS NO
• 2 - IS THE s-PROCESS TAKING PLACE IN AGB STARS A
  SECONDARY
• PROCESS ?
• ANSWER IS NO
• 3 - CAN ONE PREDICT THE r-PROCESS CONTRIBUTIONS
  TO SOLAR ABUNDANCES AS
• r 1 s ?
• ANSWER IS YES, but...

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Predictions updated
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Zr over Nb Intrinsic or Extrinsic AGBs
M 1.2 Msun Fe/H -2.60
Case ST2 Eu/Feini 2.0
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.