sProcess in Low Metallicity Lead Stars

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s-Process in Low Metallicity Lead Stars

• Sara Bisterzo,
• Roberto Gallino
• University of Torino

The Origin of the Elements Heavier than Fe -
25th - 27th September
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• Outline
• Introduction about the AGB models
• CEMP-s
• CEMP-sr stars
• Conclusions

The Origin of the Elements Heavier than Fe -
25th - 27th September
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The AGB engine
Straniero et al. 1995, Gallino et al. 1998
Convective envelope
Neutron source 12C(p,g)13N(b)13C(a,n). Type
primary When interpulse T8gt0.9-1 Time intershell
? 105 y Where He-intershell Density 106-107
(n/cm3)
13C(a,n)16O
He-intershell
TP
During the TDU (third dredge-up) ? p ingestion
in the top of He-intershell (few protons). At
H-shell ignition ? 13C-pocket formation via 12C
p ? 13N ? , and 13N(??)13C At T 108 K ?
13C(a,n)16O in radiative conditions ? s-process.
22Ne(a,n)25Mg
T8 3 (low 22Ne efficiency) Nn(peak) 1011
(n/cm3) Convective conditions Time TP ? 6y
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At very low metallicity

• Today, Intrinsic AGB halo stars
• typical mass is 0.6 M?
• (initial mass 0.8 0.9 M?)
• NO TDU (Straniero et al. 2003, 2005)
• No C or s-process enrichment observed
• Binary systems ? transfer of material C- and
  s-rich on the companion through stellar winds
  (Roche Lobe ).
• The unevolved companion shows the tipical AGB
  composition, while the true AGB star is now a
  White Dwarf.

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AGB models

• 1.2 3 0.3
• 1.3 5 0.3
• 1.4 10 0.3
• 1.5 20 0.3
• 26 0.5
• 35 1

M? n. pulses ?
1.2 M? lt M lt 3 M?
Mass loss from 10-7 to 10-4 M?/yr ? Reimers
13C-pocket ST2 . ST/100 Constant
pulse by pulse where ST 4.10-6 M? , Fe/H
-0.3, Used to reproduce the Solar Main
Component ? Gallino et al. 1998, Arlandini et
al. 1999
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Extrinsic AGB models
Dilution factor used to simulate the mixing
effect in the envelope of extrinsic stars
Note dil 0.0 dex Mstarenv(obs)
MAGB(transf) ? for main-sequence stars the
subphotospheric convective envelope mass is
Mconvenv 10-3 Mo. dil 1.0 dex Mstarenv(obs)
10 MAGB(transf) ? for giants the dilution is
important
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M 1.5 M?, case ST

• El/Fe at
• Fe/H -0.3
• C/Fe 0.7
• Zr/Fe 1.3
• La/Fe 1.3
• Pb/Fe 1.1

• El/Fe at
• Fe/H -2.6
• C/Fe 4.1
• Zr/Fe 1.8
• La/Fe 2.4
• Pb/Fe 4.3

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Various 13C-pockets
M 1.5 Mo similar to M 1.3 and 2 Mo
M 3 Mo Little spread in ls/Fe ls/Fe gt
hs/Fe
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Star sample

• 77 CEMP-s stars analyzed
• BUT only 34 stars have Eu measurements
• 16 are CEMP-sr (1.0 lt Eu/Fe lt 1.8)
• 29 stars have not Pb measurements
• References
• Preston Sneden 2001, Johnson Bolte
  (2002,2004),
• Aoki et al. (2002a,c,d,2006,2007,2008), Van Eck
• et al. (2003), Lucatello et al. (2003), Cohen et
  al.
• (2003,2006), Barbuy et al. (2005), Ivans et al.
  (2005),
• Kipper Jørgensen (1994), Kipper et al. (1996),
• Jonsell et al. (2006), Thompson et al. (2008),
• Roederer et al. (2008), Tsangarides (2005),
  Barklem
• et al. (2005), Goswami et al. (2006), Masseron et
  al.
• (2006), Reyniers et al. (2007).

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HD 196944
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HD 196944
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To reproduce stars with both sr enhancements

• Vanhala and Cameron (1998) show through numerical
  simulations how the supernova eject may interact
  with molecular cloud ? pollution with r-rich
  material
• Likely trigger the formation of the binary system
  consisting in stars with low mass
• Adopted scenario the observed star and AGB were
  formed from the same interstellar cloud, already
  enriched in r-elements

Different choices of initial r- enrichment in
the progenitor clouds r/Feini from 0.0 to 1.5
and 2.0
Spread due to inefficient mixing in the halo
Travaglio et al. (2004)
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CS 29497-030
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Zr over Nb Intrinsic or Extrinsic AGBs
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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Conclusions

• Main-sequence stars usually interpreted with
  lower initial mass (M 1.3 - 1.4 M?) and low or
  negligible dilution factor
• Na as strong constraint for the initial AGB mass
• (Na affected by NLTE effects)
• Low Na, and low ls/Fe observed in main-sequence
  or turnoff stars as indicator of the themohaline
  mixing efficiency
• 47 of stars with Eu meaurement are CEMP-sr

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The ST case for various metallicities
Pb
hs
ls
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CS 22898-027
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CS 22898-027
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Galactic Chemical Evolution

• At the termination point of the s-process
• the main component explain 50 of solar
  208Pb Clayton and Rassbach (1974)

GCE model (Travaglio et al. 2001) considers the
chemical evolution of low and intermediate mass
stars over the galactic history, in three
galactic zones (halo, thin and thick disk) ? To
have the total s-process contribution we can do
an approximation with a model of M 3 M? and
Fe/H -1.3 (Ratzel et al. 2004)
With GCE 94 of the solar 208Pb
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Stellar Nucleosynthesis

• The elements from H and He until 56Fe are
  produced by nuclear fusion
• Beyond 56Fe the strong Coulomb barrier forbids
  the capture of charged particles
• NEUTRON CAPTURE
• s-process (slow) and r-process (rapid).

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s - process
tb- ltlt tn S process tn ? yr nn ? 106 n/cm3
b decay
p
tb- gtgt tn R process tn ? 10-3 s tb ? h nn ? 1023
n/cm3
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s-process
For heavy elements (A gt 60) ? First stars
nucleosynthesis developed by Burbidge, Burbidge,
Fowler, and Hoyle B2FH (1957) and by Cameron
(1957). Based on the sNs curve in the Solar
System (Clayton et al. 1961, and Seeger et al.
1965).

• Multiple irradiations are needed to bypass the
  bottlenecks of nuclei with very small cross
  sections (magic nuclei)
• Far from bottlenecks, steady flow conditions
  prevail where sN const. (for unbranched
  nuclei).
• Fig. from Gallino et al. (1997)

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Where? AGB (Asymptotic Giant Branch) Stars

• C-O core degenerate
• H and He shells burn alternately
• This leads to a thermally unstable configuration
  (Thermal Pulses).

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IMS ??? N 26???
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Thermohaline effect

• Process which can occur in stars when AGB winds
  with higher mean molecular weight (He, C and
  s-rich material) is transferred onto the envelope
  of the observed star
• ? m-gradient
• Time scale and deep of the diffusion depends on
  the adopted model
• Two examples from the literature
• Vauclair 2004 (moderate mixing)
• 10 Mobs corresponding to dil 0.3 dex
• Stancliffe et al. 2007 (deep mixing)
• 90 Mobs corresponding to dil 1 dex
• ? Z 10-4 Z?, MAGB 2 M?, Mobs 0.74 M?,
  Mtransf 0.1 M?

Other mixing processes

• Gravitational settling (Thoul, Bahcall Loeb
  1994 Straniero, Chieffi Limongi 1997),
  contrasted by Cool Bottom Process (Nollett, Busso
  Wasserburg 2003)

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Main-sequence or turnoff stars

• No thermohaline or other mixing effects in the
  models
• Comparing observations and theoretical
  predictions, one can derive the dilution factors
  for each star
• How much does it matter
• the thermohaline effect?
• From the amount of the dilution factor it is
  possible to have an idea of the importance of
  mixing in the envelope of the observed stars